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Lecture Notes in Earth Sciences Edited by Somdev Bhattacharji, Gerald M. Friedman, Horst J. Neugebauer and Adolf Seilacher
30 Erie G. Kauffman Otto H. Walliser (Eds.)
I[I
Proceedings of the Project216: Global Biological Events in Earth History
Extinction Events in Earth History
Springer-Verlag Berlin Heidelberg NewYork London ParisTokyo Hong Kong
Editors Prof Dr Dr. h.c. Erie G. Kauffrnan University of Colorado Department of Geological Sciences Campus Box 250 Boulder, C O 80309, USA Prof Dr. Otto H. Walliser University of Gottlngen Institute and Museum for Geology and Palaeontology Goldschmidt-Str. 3, D-3400 Gothngen, FRG Layout' Gabrlela Meyer, G~)ttlngen
ISBN 3-540-52605-6 Sprmger-Verlag Berhn Heidelberg New York ISBN 0-38?-52605-6 Sprlnger-Verlag NewYork Berlin Heidelberg
This work is subject to copyright All rights are reserved, whether the whole or part of the material ts concerned, specifically the rights of translation, reprmhng, re-use o{ ~llustrat~ons,rec~tahon, broadcasting, reproduction on microfilms or in other ways, and storage m data banks Duplication of this pubhcatlon or parts thereof is only permitted under the provisions of the German Copynght Law of September 9, 1965, ~nits version of June 24, 1985, and a copyright fee must always be paid Violahons fall under the prosecution act of the German Copyright Law © Springer-Verlag Berhn Heidelberg 1990 Printed in Germany Printing and binding. Druckhaus Beltz, Hemsbach/Bergstr 2132/3140-543210 - Printed on acid-free paper
PREFACE
This volume presents results from members of the Project 216 "Global Biological Events in Earth History" of the International Geological Correlation Programme (IGCP). The project, initiated by the elder editor (O.H.W.) within the framework of the International Palaeontological Association (IPA) in the late 70s, was officially established in 1984. Subsequently, it led to the first three conferences on Global Bio-Events, and their respective symposia volumes: 1) In G6ttingen, West Germany in 1986 (WaUiser, O. H., Ed., 1986, Global Bio-Events, Springer-Verlag); in Bilbao, Spain in 1987 (Lamolda, M. A., Kauffrnan, E. G., and Walliser, O. H., Eds., 1988, Paleontology and Evolution: Extinction Events; Rev. Espafiola de Paleont., n. extraord.); and in Boulder, Colorado, U.S.A. in 1988 (this volume). The next meeting, on Innovations and Revolutions in the Biosphere, is planned in Oxford, England in 1990, to be hosted by Martin Brasier. During the history of this project, the focus of our research has shifted significantly. Initial focus was on specific global mass extinctions (e.g. the Precambrian/Cambrian, Frasnian/Fammenian, Cretaceous/Tertiary, and Eocene/Oligocene events) to a broader treatment of Phanerozoic mass extinctions, their differences or unifying factors, and their causal mechanisms. Subsequent meetings have attempted to focus attention on a fuller spectrum of global bio-events in Earth history. The Boulder Conference, and this volume, although still strongly influenced by the excitement of mass extinction research, expresses these new trends in bioevent studies. The Boulder conference, held on May 16-23, 1988, focused on a broad spectrum of Abrupt Changes in the Global Biota. Over 100 participants from 13 nations attended this meeting, representing diverse disciplines of palaeobiology, palaeoclimatology, palaeoceanography, sedimentology, geochemistry, and a broad spectrum of the stratigraphic and geological sciences. Four days of talks were supplemented by field trips to the continental Cretaceous/Tertiary boundary in the Raton Basin, New Mexico, and to the Cenomanian/Turonian mass extinction interval exposed near Pueblo, Colorado. The Conference itself was characterized by a great diversity of approaches to bio-event research, and the phenomenon of mass extinction. In particular, interactive causes involving both extraterrestrial and earthbound (tectonic, oceanographic, climatic) forces were discussed, and each major Phanerozoic mass extinction was treated by specialists in the field. In addition, many presentations focused on the causal mechanism and patterns of bio-event development that were not restricted to mass extinction intervals, but which could cause regional to global biotic response at any time in Earth history. Thus, both the conference, and this volume, focus attention on climatic and oceanic perturbations from anoxia, advection, rapid thermal change, toxic chemical enrichment, and energy shock from impacts and giant tsunamis as forcing mechanism for regional to global bio-events. The delicate balance of perched ocean/ctimate~fe systems under typical warm equable non-glacial Phanerozoic conditions, and their susceptibility to shock from even small perturbations, was a philosophical theme that ran throughout the meeting. The case for extraterrestrial forcing of tectonic, volcanic, and biological events was greatly strengthened by new data presented at this conference, with special concern for the effects of small comet/meteorite impacts in the oceans, and their chemical/physical/biological signature which might be used, in the absence of shocked minerals, microspheres or trace metals, to identify extraterrestrial events associated with global and regional bio-events. The conference benefitted from the introduction of much new data at high levels of resolution, especially from poorly studied mass extinction intervals. Interactive discussions, and many new ideas characterized the meeting. The new scientific results of this meeting are exciting; they are reviewed in the Conference Report published in Episodes (1988, v. 11, n. 4, p. 289-292). Most of the key papers presented at the Boulder meeting appear in this volume.
IV What lies ahead in bio-event research? Clearly, a great deal of excitement and an age of discovery. We have only touched the surface of this new and dynamic field. We are starting to comprehend the dynamics of global mass extinctions, integrating detailed geochemical, physical and biological data into scenarios of cause and effect. But in the years ahead lies the job of understanding the whole spectrum of regional bioevents preserved in the ancient record, and especially the application of this research to solutions of the critical problems inherent in global change and the modern biotic crisis. Future directions for research at this conference include the investigation and modeling of abrupt chemical and thermal shifts in the ocean, the effects of impacts at deep ocean sites, the documentation of successful survival strategies and repopulation patterns following biotic crises, the deep ocean record of bio-events, and focus on alternative forces other than impacting to account for mass extinction events. This volume introduces some of these new pathways in bio-event research.
ACKNOWLEDGEMENT
We are extremely grateful for the encouragement and support of our colleagues, the International Palaeontological Association, the International Union of Geosciences (IUGS) and the UNESCO. We further thank the University of Colorado and its Department of Geological Sciences for hosting the 3rd International Conference on Global Bio-events in Boulder. We received valuable help from Mrs. Dipl.Geol. L. Alberti and from Mrs. C. Kaubisch. Special acknowledgement is due to Gabriela Meyer who made any effort to type and to design this volume.
Erie G. Kauffman Host of the 3rd International Conference of IGCP Project 216
Otto. H. WaUiser Leader of IGCP Project 216
CONTENTS
PREFACE
...........................................................................................................................................................
GENERAL
III
ASPECTS
WALLISER,O. H.: HOW to define "global bio-events". ................................................................................. 1 BOUCOT, A. J.: Phanerozoic extinctions: How similar are they to each other? ....................................... 5
KITCHELL,J. A.: Biological selectivity of extinction .................................................................................. 31 LEARY, P. N. & RAMPINO, M. R.: A multi-causal model of mass extinctions: increase in trace metals in the oceans .................................................................................................................................
45
LEARY, P. N. & HART, M. B.: Important considerations in the investigation of global bioevents ...... 57 RICE, A.: Shock pressures in igneous processes: Implications for K/T events ....................................... 59 WILDEr P., QUINBY-HUNT, M. S. & BERRY, W.B.N.: Vertical advection from oxic or anoxic water from the main pycnocline as a cause of rapid extinction or rapid radiations ........................ 85 YANG, Z.: An astronomical explanation of anomalous concentrations of iridium element during catastrophic extinctions ............................................................................................................... PALAEOZOIC
99
EVENTS
BIERNAT, G. & BEDNARCZYK,W.: Evolutionary crisis within the Ordovician acrotretid inarticulate brachiopods of Poland .................................................................................... 105 BERRY, W. B. N., WILDE, P. & OUINBY-HUNT, M. S. : Late Ordovician graptolite mass mortality and subsequent Early Silurian re-radiation ........................................................................ 115 BOUCOT, A. J.: Silurian and pre-Upper Devonian bio-events ................................................................ 125 TRUYOLS-MASSONI,M., MONTESINOS,R., GARCIA-ALCALDE, J. L. & LEYVA, F.: The Kacakotomari
event and its characterization in the Palentine domain (Cantabrian Zone, NW Spain) 133
LOTrMANN, J.: The Middle G i v e t i a n p u r n i l i o - E v e n t s
- A tool for high time resolution
and event-stratigraphical correlation ...................................................................................................
145
SCHINDLER,E.: The late Frasnian (Upper Devonian) Kellwasser Crisis ............................................. 151 MC GHEE, G. R. Jr.: The Frasnian-Famennian mass extinction record in the eastern ....................... 161 United States RACKI, G.: Frasnian/Famennian event in the Holy Cross Mts, Central Poland: Stratigraphic and ecologic aspects .......................................................................................................
169
KALVODA, J.: Late Devonian - Early Carboniferous paleobiogeography of benthic Foraminifera and climatic oscillations .................................................................................................
183
SIMAKOV, K. V.: Major evolutionary events among the spiriferids at the DevonianCarboniferous boundary ........................................................................................................................
189
PALAEOZOIC/MESOZOIC
EVENTS
TEICHERT,C.: The Permian-Triassic boundary revisited ........................................................................ 199 BENTON, M. J.: Mass extinctions in the fossil record of late Palaeozoic and Mesozoic tetrapods ...239
VI
MESOZOIC
EVENTS
WHATLEY,R.: The relationship between extrinsic and intrinsic events in the evolution of Mesozoic non-marine Ostracoda .........................................................................................................
253
COLLOM,C. J.: The taxonomic analysis of mass extinction intervals: an approach to problems of resolution as shown by Cretaceous ammonite genera (global) and species (Western Interior of the United States) ................................................................................ 265 HAR~ES, P. J. & KAUFFMAN, E. G.: Patterns of survival and recovery following the Cenomanian-Turonian (Late Cretaceous) mass extinction in the Western Interior Basin, United States ............................................................................................................................... 277 RoY, J. M., MCMENAMIN, M. A. S. & ALDERMAN, S. E.: Trophic differences, originations and extinctions during the Cenomanian and Maastrichtian stages of the Cretaceous .................. 299 JOHNSON, C. C, & KAUFFMAN, E, G.: Originations, radiations and extinctions of Cretaceous rudistid bivalve species in the Caribbean Province ............................................................................ 305 MESOZOIC/CENOZOIC
EVENTS
HART,M. B. & LEARY,P. N.: Periodic bioevents in the evolution of the planktonic foraminifera..325 CALDEIRA, K., RAMPINO, M. R., VOLK, T. & ZACHOS, J. C.: Biogeochemical modeling at mass extinction boundaries: atmospheric carbon dioxide and ocean alkalinity at the K/T boundary ...............................................................................................................................
333
FLEMING, R. F. & NICHOLS, D. J.: The fern-spore abundance anomaly at the CretaceousTertiary boundary: a regional bioevent in western North America ................................................. 347 NICHOLS, D. J., FLEMING, R. F. & FREDERIKSEN,N. O.: Palynological evidence of effects of the terminal Cretaceous event on terrestrial floras in western North America ............ 351 KAUFFMAN, E. O., UPCHURCH, G. R. & NICHOLS, D. J.: The Cretaceous - Tertiary boundary interval at south table mountain, near Golden, Colorado .................................. :............ 365 LANOLDA, M. A.: The Cretaceous-Tertiary boundary crisis at Zumaya (Northern Spain). Micropaleontological data .....................................................................................................................
393
NANSEN,T. A. & UPSHAW, B.: Aftermath of the Cretaceous-Tertiary extinction: Rate and nature of the early Paleocene molluscan rebound ............................................................................. 401 SWAIN, F. M.: Species duration and extinction patterns in Cenozoic non-marine Ostracoda, Western United States .......................................................................................................
411
AGUSTI,J. & MOYA-SOLA,S.: Mammal extinctions in the Vallesian (Upper Miocene) ................... 425
HOW
TO DEFINE
"GLOBAL
BIO-EVENTS"
NE~U O~ A
contribution to Proiect
GLOBAL IJ
W.~LLISER, Otto H. *)
BIO EVENTS
Abstract : Regarding the manifold pattern of global bio-events, the corresponding terms should not be defined too restrictive.
I N T R O D U CT I O N :
Sciencen e e d s
a common
vocabulary and definitions of t h e
used terms. But a
definition must not be necessarily extremely restrictive. Sometimes a relatively wide definition is more adequate or even necessary, e.g. if the term has to describe a large variety of parameters or patterns or, as another example, if the knowledge of the investigated subject is still on a low level. On account of these reasons, the need of a wider definition also applies for those happenings which we call biological or geological events. Therefore I plead for a non-restrictive usage of the term 'global bio-event', as explained in the following.
G 1 o b a I : Extinctions are serf-evidently global, even if the last representative of the extinct taxon lived in a res~icted area for a longer time. Therefore, the problem is to prove whether the disappearance of a certain taxon, recognized in a certain area, is due to local or regional mortality, to migration or to real extinction. The disappearance of certain taxa in the Devonian often was thought to be only of local nature, but later turned out to be a real extinction, i.e. global. Another, but even more important question in connection with global bio-events is whether a globally distributed taxon disappears contemporaneously elsewhere. Up to now, normally we only can confine the corresponding time interval of extinctions in the order of a biozone, i.e. in general about 1 - 0,25 million years. In some other cases (e.g. the Kellwasser Event = KW Event; see Schindler, 1990, this volume) the time interval of stepwise extinctions can be globally restricted to the order of 100 ky and less. Even in the first mentioned case, we may call it contemporaneous with regard to the geological time scale.
E v e n t : Of course, each change, disappearance, new occurrence, interruption, origination etc., i.e. each happening, is an event. In connection with global biological or geological events we should use this term for exceptional happenings which took place in a relatively short time span; 'relatively' in comparison with the tong time intervals between the events. But what does exceptional mean? A few examples may help to elucidate this: significant higher ex~nction rate or origination rate compared with the average rate of background extinctions or background origination respectively; sudden changes of facies; intercalation of an anoxic layer in a sequence with well developed communities. Now I want to take up again the question of the relative shortness of an event. A tsunami-caused layer (compare Lottmarm, 1990, this volume), or most probably also the main Iridium layer at the K/T boundary, are products of an extremely short happening of days or even hours. In relation to that, the KW Event with a duration of several 100 Ky is extremely long. However, comparing the time-span of the KW Event with the
*) Institut und Museum fiir Geologic und Pal/iuntologie, Unlversit~it G~ttingen, Goldschmidt-Str. 3, D - 3400 GiSttingen, F.R.G.
preceding Frasnian time, in which over several million years the changes have been much less, the time interval of the KW Event is relatively very short and justifies the term 'event'. The above mentioned bio-events are connected with high extinction rates or even with mass extinctions. As pointed out earlier (e.g. Walliser, 1986), an extinction event often is followed - after a certain time interval - by radiations. If this phase is short in comparison with the following time of nomismogenesis (during which a "normal" background rate of origination can be observed), we may call it an event, too, in this case a r a d i a t i o n e v e n t . The sudden occurrence and spreading-out of a new biological construction ("Banplan", e.g. the ftrst ceiling of bactritids, leading to goniatites) is an event, too, in this case abiological i n n o v a t i o n event. Another difficulty with the term event may be demonstrated by the otomari Event (compare TruyolsMassoni et al., 1990, this volume), which also has been called Kacak Event by House (1985). The cephalopod limestone sequences of the Rheiulsche Schiefergebirge (Germany) show intercalated dark to black sediments (shales and sometimes limestones), called Odershausen Formation. At the very base of that formation occur Nowakia otornari (sensu lato; it is an early form) and Cabrieroceras sp., formerly called C. rouvillei. The base of the Odershausen Fro. has been taken as the base of the upper Middle Devonian ("Givetian") in the pelagic realm since the beginning of this century. On account of the observation, that the occurrence of "anoxic" sediments can be recognized world-wide at this time level, I called it otornari or rouvillei Event. In the meantime I recognized the real C. rouvillei (v. Koenen, 1886) (from the Montagne Noire, France) is a younger synonym to C. crispiforme (Kayser, 1879), but not identical with the Cabrieroceras from the Odershansen Fro. C. crispiforrne occurs already within the late australis Zone and disappears at the end of the kockelianus Zone. T k. kockelianus and a early form of the P. ensensis (or P. xylus group, respectively) overlap in one limestone layer in the sections Bou Tchrafme (SE-Morocco) and in the Eifel Mountains (Germany; Weddlge, 1988). On account of holostratigraphic considerations this layer represents a time-span in the order of 10 to 20 Ky. In both mentioned regions, also the end of the "anoxic" intercalation is time-equivalent: P. hemiansatus occurs very short after the first limestone layer above, the black shales. What is the otornari Event? Is it at position a of Fig. 1 (corrsponding to Truyols-Massoni et al., 1990, this volume), or at b or c, or is it the time-span between, e.g., b and c? The original intention was to designate the beginning of the black shales (b in Fig. 1) as 'otomari Event', most probably being connected with a transgressive pulse. Of course, also the end of the black shale (c in Fig. I) is not only a lithological, but also a biological event, which could be named separately. Current investigations on the changes between australis and late ensensis Zones are promising in respect to a better knowledge of the taxonomy and ranges of the involved taxa. Therefore, closer definitions will be made after completion of these investigations. Until then the terra 'otomari Event' should be used in the mentioned original, not restricted sense.
C r i s i s : If the extinction rate is higher than the rate of originations over a longer period of time, we shouldn't call this process an event but rather a crisis. Again we shouldn't make too restrictive definitions, because there are all possibilities and transitions between a relatively short crisis and a relatively long event. A good example again is the KW Event (Schindler, 1990, this volume): It is preceded by a crisis, lasting several million years; the KW Event itself has a stepwise pattern, i.e. it can be subdivided in several special events, of which the last, culminating one, may have been extremely short. The sequence 'crisis - stepwise event - final culmination' seems to occur often in earth history such as at the end of the Cretaceous.
Category/Order: The fact that global events of quite different impact on biosphere or sedimentary regime appeared in earth history, allows us to distinguish different categories or orders of events. Boucot (1990, this volume) lists such different orders, fortunately without giving strict def'mitions. Fortunately, because as soon as we would base the def'mitious on exact percentages of extinctions or originations per time-unit, we would no longer be able to rank the events, unless having reached an utopian high
h~
0
standard of knowledge. 0
0
~
0
~
0
"
~
0
':
Figure 1 :
Range of fossils near the
otomari Event (for explanation see text).
REFERENCES Boucot, A. J. (1990): Phanerozoic extinctions: How similar are they to each other? - This volume. House, M. R. (1985): Correlation of mid-Palaeozoic ammonoid evolutionary events with global sedimentary perturbations. - Nature, 313, 17-22. Lottmann, J. (1990): The Middle Givetian pumilio-Events - A tool for high time resolution and eventstratigraphical correlation. - This vohune. Schindler, E. (1990): The late Frasnian (Upper Devonian) Kellwasser Crisis. - This volume. Truyols-Massoni, M., Montesinos, R., Garcia-Alcalde, J. L. & Leyva, F. (1990): The Kacak-Otomari event and its characterization in the Palentine Domain (Cantabrian Zone, NW Spain). - This volume. Walliser, O. H. (1986): Towards a more critical approach to bio-events. - In: Walliser, O. H. (ed.): Global Bio-Events. A Critical Approach. - Lecture Notes in Earth Sciences, 8, 5-16. Walliser, O. H. (1988): Proposal for an Eifelian/Givetian Boundary Stratotype. - Document submitted to the Subcommission on Devonian Stratigraphy (ICS, IUGS), Rennes, August 1988, 4 pp. Weddige, K. (1988): Eifel Conodonts. - In: Ziegler, W. (ed.): 1st International Senckenberg Conference and 5th European Conodont Symposium (ECOS V), Contributions I. - Courier Forsch.-Inst. Senckenberg, 102, 103-110; Frankfurt.
PHANEROZOIC THEY TO EACH
EXTINCTIONS: OTHER?
HOW
SIMILAR
ARE
A contribut;on to Project
N? II
GLOBAL BIO EVENTS
BOUCOT, Arthur J. *)
Abstract: A brief account of the major Phanerozoic extinction events suggests that each one has a flavor all
its own. Major extinction events involving varied combinations of different taxonomic levels, such as many classes, or many orders, or many families, and the varied impacts on community structure in varied combinations are what is referred to here by "flavor". Most of the evidence discussed is taken from the relatively well sampled, marine, level bottom environment. Recent evidence developed by Kauffman and a few others suggests that extinctions take place over a geologically brief interval of time in which more stenotopic taxa are eliminated, followed by an abrupt termination of the more abundant, higher biomass eurytopes, during community collapse. Adaptive radiations too, appear to take place during a geologically brief interval (up to a few million years) rather than "instantaneouslf' in ecologic time (less than a thousand years). This suggests that "instantaneous" extra-terrestrial events are not likely to have influenced the gradual phase of either extinctions or adaptive radiations in any very important manner, unless they are in need of ecological amplification in order to be recognized. Long term extra-terrestrial events that potentially resulted in terrestrial climatic gradient changes might have been significant. However, the more instantaneous community collapse, high biomass effects, might involve other factors, including certain types of extra-terrestrial events. The decoupling, non-coincidence in time of the extinction events and adaptive radiations in varied parts of the global ecosystem, such as the marine and nonmarine environment, does not support a concept of "instantaneous" extra..terrestrial influences. It is concluded that we still have little understanding of the specifics involved in any major extinction event or adaptive radiation on land or sea. Many of our paleontologic e u m geologic data have been collected in such an inadequate manner as to make new sampling mandatory if we are to attain a better understanding of these problems. "Those who cannot remember the past are condemned to repeat it." George Santayana (The Life of Reason)
INTRODUCTION
D'Orbigny (1849-1852, 1850-1852) compiled the original biostratigraphic synthesis that made him the Father of Biostratigraphy, and provided the foundation for future studies on extinctions and adaptive radiations. Following Cuvier, d'Orbigny employed a catastrophist interpretation (1849-1852, 2: 252-254) of the biostratigraphic facts. We no longer subscribe to this view, for which many paleontologists have substituted a neocatastrophist-evolutionary attitude. However, we are still faced with trying to understand what are basically the same data. Most of the major extinction events have, of course, been recognized for a century or more (see Newell, 1956, 1963, 1967, for incisive summaries). The relative ease with which the earlier paleontologists and biostratigraphers had recognized the major extinctions was largely based on the presence of major biofacies and biomass (communities of modern parlance) changes across the boundaries combined with significant taxonomic changes; there was much more to this recognition process than a mere change of taxa at the boundaries (Newetl, 1967). After most of the Treatise on Invertebrate Paleontology and Osnovy Paleontologii volumes became available, as well as "The Fossil Record" (Harland et al., 1967), it was *) Department of Zoology, Oregon State University, Corvallis, OR 97331-2914, U.S.A.
reasonable to compile stratigraphic range data for varied taxonomic levels in order to test earfier conclusions about the major extinction events. Valentine (1969) and Ranp (1972) performed this compiling service, although arriving at somewhat different conclusions about the significance of the compilations, as might have been predicted given the nature of the basic information. Furthermore, it was reasonable to continue to try to refine these data with both additional information and varied statistical techniques. The authors of these later papers eventually were able to recognize most of the major extinctions noted previously by biostratigraphers and paleontologists well before the advent of the Treatise and the Osnovy. We owe a debt of gratitude to these compilers for having called these basic paleontologic, evolutionary problems to the attention of the biologists. But, the recent spate of papers alleging periodicity of extinctions in the post-Paleozoic marine fossil record (see Sepkoski & Raup, 1986, for a typical example), based on statistical treatments of similar basic data, involves a large measure of defective data while at the same time not agreeing with the major biostratigraphic breaks (see Patterson & Smith, 1987, for a critique and summary; also, Ager, 1988; House, 1987; Boucot, 1988). Patterson and Smith reject the alleged periodicity in the Late Permian to present family data. As is discussed below, additional to the basic criticisms provided by Patterson and Smith, I conclude on a paleoecologic basis that most data points compiled by the extinction-periodicity advocates result from taphonomic "noise" on the one hand, and the basic inadequacy of most fossil samples for recovering rare taxa on the other hand. Jaeger's (1986) thoughtful review of the extinction problem raises serious doubts about both the alleged "instantaneous" and "periodic" aspects of the problem. As I have suggested before (1986) it is crucial that we now advance our understanding of the extinction and adaptive radiation problems involved if we are to fred any real solution for them. To accomplish this, we must employ sampling methods that emphasize the ecostratigraphic rather than just the stratigraphic. By "ecostratigraphic", I mean the study of extinctions and adaptive radiations from within individual community types (community groups; see Boucot, 1983). We need to fred out whether communities as a whole are subject to extinction and subsequent adaptive radiation; or whether it is only the more "sensitive" taxa in a community that are subject to these processes. Mere compilation of miscellaneous taxa will not achieve this goal if the taxa are divorced from their community context. We must analyse the system, part by part. We must keep separate the data of the level bottom community from those of the reef complex of communities, of the pelmatozoan thicket, the bryozoan thicket, and the sponge forest. In fact, we must separate all data bearing on the shelf communities from those of the bathyal, planktonic, and pelagic communities. Finally, we must - obviously - keep nonmarine data - both freshwater and terrestrial - separate from marine data. It needs to be stated that the term "extinction" is employed by paleontologists in a manner distinct from the dictionary definition. We refer to a statistical phenomenon in which varying percentages of the biota, never the total biota in the Cuvieran sense, became extinct at different times in the past. Because of this situation Newell (1963) and Teichert (1985) preferred the term "crisis" rather than "extinction". Some workers prefer the term "extinction event". We are not, of course, referring to the terminal extinction of individual lineages, or to name changes within individual, evolving lineages.
HOW
NOT
TO RECOGNIZE
EXTINCTIONS
The best way to obscure real extinction events, and to totally overlook the lower level extinction events, is to continue to prepare summaries featuring the "known" stratigraphic ranges of ecologically miscellaneous taxa - f~rnilles, genera, and species. It is also necessary to keep in mind that most generic and specific "extinctions" are merely bookkeeping changes reflecting the evolution of one taxon into another, not true
7 extinctions. The "normal" times between bio-events, times when nomismogeusis dominates, are the reverse of extinction events, and are the times when one species evolves into another of the same genus, and in less common instances into another, closely related genus. Uncritic~ range snmmaries assume that the "known" ranges of most, or even all published taxa are of equal value, a position that the experienced taxonomist finds unrealistic (Newell, 1986, for example). It has been clear for many years, even during the last century, that the published stratigraphic ranges of the presently described fossils are of highly variable quality. For example, Teichert (written comm., 1987) points out that 25% of the pelmatozoan families are monogeneric; i.e., they probably represent a very fortuitous sample for which the "known" stratigraphic ranges are obviously far below the true ranges, because they are also largely endemic, relatively uncommon units whose reported ranges tend to be minimal. Excluding monotypic taxa and those restricted to a single stage cannot eliminate this problem. Enlarging one's sample size does not improve reliability. The skeletouized echinoderms, for example, have a very low preservational potential that almost guarantees that the known stratigraphic ranges of most taxa will be very minimal. It is, therefore, amafing that many compilers make no serious attempt to evaluate the reliability of the varied components in their samples. This approach defeats a p r i o r i the purpose of the study! The first rule of statistical analysis is to prepare if possible, where some understanding is available, a proper evaluation of the sample and its components, and then to consider how best to weight these components. One should decide on the basis of this body of knowledge how best to discard certain components because of their statistical unreliability. This is particularly true when dealing with natural history data of the biostratigraphic sort, where we have more than a little real understanding of varied controls over the nature of the data. These samples should not be viewed in some sort of statistical vacuum; to do so would not be objective, it would be to discard intentionally the experience and understanding gained by many generations of paleontologists. I have emphasized (1986) that the term "Background Extinction Rate" is largely a sampling artifact. It is largely a combination of taphonomie "noise" plus the all too common small size of most fossil collections. The taphonomic "noise" results from the uncritical use of many well-skeletouized taxa that are so poorly articulated that they are most commonly preserved as indeterminate fragments. A significant fraction also results from the amalgamating of stratigraphic range data obtained from ecologically unlike parts of the ecosystem (lumping level bottom data in with reef complex of communities, bryozoan thickets, pelmatozoan thickets, and so forth), because the non-level bottom units commonly undergo adaptive radiations at times distinct from those undergone by the level bottom, although they commonly share the same times of extinction. As an example of taphonomie "noise" consider the Echinodermata. The overwhelming majority of the generic and fzmitlal stratigraphic ranges within this phylum are obviously gross underestimates, because of the ease with which most taxa disaggregate into unidentifiable particles. The compilers have made no attempt to deal with this problem. Because of this taphonomic defect, as well as related ecologic questions, most echinoderm taxa probably should be eliminated from the compilations, with the exception of a few taxa such as the numerically abundant, cosmopolitan, infaunal, irregular urchins (present in Mortensen's old Subclass Irregularia). Paul (1988) cleverly addresses part of the echinoderm preservation c u m stratigraphic range problem. The sampling bias part of the problem involves the fact that with both living and extinct taxa the great majority belong to the endemic category (Boucot, 1975a), which is characteristically represented by relatively uncommon specimens. The compilers have uncritically lumped together the taxa represented by these poorly sampled, commonly endemic, rare taxa, whose reported stratigraphic ranges are commonly far
short of what community paleoecology suggests should be their true ranges, with the far more numerous, more cosmopolitan taxa whose known ranges approximate their true ranges (Boucot, 1983). Marshall (1988) has even reviewed the statistical basis indicating why the reported ranges of rare taxa will be gross underestimates of their true ranges. Ignoring these facts produces both noise and artifact. Furthermore, the compilers make no attempt to evaluate their data in terms of what is now known about community evolution (Boucot, 1983). This may happen because most paleontologists do not subdivide their data ecologically. I am also referring here to the all too common practice of lumping together stratigraphic data from evolutionarily largely decoupled parts of the marine ecosystem such as the reef community complex and the level bottom environments (an example of this practice is McKinney, 1987). Workers who would never think of combining stratigraphic range data from the noumarine and marine environments for the purpose of recognizing extinctions and adaptive radiations seem to feel free to lump together the data of the level bottom shelf with those of the reef community complex, the pelmatozoan thicket, the bryozoan thicket, and the sponge forest, not to mention the pelagic and planktonic complexes. This failure, or refusal, to make an effort to analyse the distinctly different parts of the ecosystem, in terms of independent times of adaptive radiation and terminal extinction, needlessly blurs the record, giving a false sense of what might actually have occurred. Additional to this failure to consider sample reliability in terms of community paleoecologic understanding is the failure to consider the biogeographic reliability of the sample, i.e., whether a complete community complement has been recovered from the known biogeographic units, as well as whether the available communities have been adequately sampled. It is clear that overall diversity at lower taxonomic levels, in some cases extending up to and including the familial, is positively correlated with numbers of provincial units (I have commented on this some time ago: Boucot, 1975b). Softbodied taxa should not be considered for the purpose of recognizing extinction events or adaptive radiations, and indeed few include them in their compilations. However, rare taxa for which stratigraphie sampling is still woefully inadequate (Boucot, 1986), and for which it is clear that "known range" seldom even approaches "true range", are still being included by most compilers in their summaries (McKinney, 1987, is a typical example). Needless to say, such a major sampling error introduces substantial noise and potential artifacts into the conclusions - leading in no small part to the fallacious concept of a "Background Extinction Rate". However, use of the ecostratigraphic concepts of community evolution provides a more balanced means of evaluating all taxa, whether rare or abundant, in both slowly evolving and rapidly evolving lineages that have similar stratigraphic ranges within the many coeval community groups belonging to each ecologic-evolutionaryunit (Boucot, 1983, Fig. 1). From what we have learned about community evolution it is clear that there will be a continuing "extinction" rate of both genera and species, but particularly species, within the phyletically evolving taxa characteristic of every community group (this type of "extinction" is mere name changing, not extinction in the commonly accepted sense [Boucot, 1978, 1983, 1986]). Within any major portion of the global ecosystem, such as the level bottom, the plankton, the reef complex of communities, pelmatozoan thickets, and the like, there is good evidence that there is no background extinction rate at the family and higher levels. New fzmilies are NOT being continually introduced and removed from extant community groups. There are inevitably some families that appear later and disappear earlier than the bracketing ecologicevolutionary unit boundaries, but most of these are fzmilies represented by rare genera and species (this is an artifact caused by inadequate sampling). The stratigraphic ranges of the benthic non-level bottom ecologic units are overall far less well known than those of the level bottom units..For example, the stratigraphic ranges of the varied pelmatozoan thicket communities are very poorly known, and little attempt has been made to synthesize the currently available
data. If one adds to this the rarity of the taxa within such communities (largely because of the ease with which most of these organisms are disarticulated beyond recognition), it is little wonder that they contribute a great deal to the artifact of "Background Extinction Rate". Sepkosld (1986) and Ranp and Boyajian (1988) suggest that genus-level compilations through time are a far more sensitive means of detecting extinctions than is the case with familial and higher level units. I strongly disagree. In many cases genera within a family tend to grade into each other, as anyone who has ever tried to assign a species to a genus is painfully aware. I am indebted to Sepkoski for providing me with a copy of his articulate brachiopod generic data. Consideration of his data shows that the known disappearance time for each genus is treated as an "extinction", which involves the assumption that there has been a very low level of evolutionary change within families from one genus to another. Examination of Sepkoski's data convinces me that well over three-quarters of these "extinctions" are merely name changes within evolving lineages (i.e., these are not true extinctions). This is not surprising. Sepkoski (1986) apparently ignored generic changes in evolving lineages as contrasted with terminations of generic lineages which seriously flaws his approach. Anyone involved with the study and description of genera and species understands that most genera are merely convenient "cubbyholes" for groups of relatively similar species that are parts of continuously evolving groups. The futility of trying to employ generlc-level compilations for the recognition of minor extinctions is exemplified by Sepkoski's (1986) alleged minor extinction event at the Penusylvanian-Permian boundary. This is an artifact produced by employing generic data based on the all too common tencency to change generic and specific terms at "major" stratigraphic boundaries. Also involved here is the fact that this boundary in the classic regions of Northern Europe involved a noamarine, Coal Measures boundary below with overlying marine beds, i.e., artifactual absence of transitional marine beds. Anyone familiar with the dellberatious involved in the selection of the Pennsylvanian-Permian boundary decision made at the 1937 International Geological Congress in Moscow would be puzzled to learn that a well defined extinction event was supposed to have been present (see Mudge & Yochetson, 1962, for a mass of information dealing with the boundary). One should also note Sepkoski's (1986) inability to reeo£nize the minor extinction events within the Silurian-Devonian (Boucot, this volume). I (1986) have discussed the tendency for a slightly higher level of generic terminations at the end of ecologic-evolutionaryunits rather than within them. There is also a tendency for taxonomists to employ more generic terms than needed just above and below period, series, and stage boundaries. That is, the normal human tendency for some of us to "see" things where they "should" be results in different names for the same genus above and below a boundary. Generic compilations also introduce a biogeographic~ artifact in that times of increasing provincialism, themselves commonly decoupled from extinctions or adaptive radiations (Boucot, 1983), see a marked increase in generic and specific diversity (Boucot, 1978, diacladogeuesis) owing to intensive allopatric speeiation. Still, the compilers have certainly put forward a number of provocative ideas, as well as triggering a lot of additional research. They have encouraged taxonomists and biostratigraphers to pay more attention to the aggregate character of their stratigraphic data, and to its evolutionary implications. Hopefully, the compilers will next begin to examine the available data in a more biostratigraphie, more ecologic manner. If they are willing to do this they will begin to reliably reco~niTe the welter of lower order events, and the problems they pose. Additionally, they have been able to show that the major extinctions and adaptive radiations recognized since the mid-19th Century also show up in their raw statistical complications despite the noisiness of the data. This further emphasizes the monumental nature of the major extinctions.
10 MARINE
HISTORY
&
RECOGNITION
OF EXTINCTION
EVENTS
The marine fossil record is by far our most complete. Therefore, any account of extinction, and the companion problem of adaptive radiation, must emphasize the marine. There is, of course, a spectrum of both extinction events and adaptive radiations, from the major global events, to minor events, as well as to regional events. In this brief summary I stress the major events. In terms of numbers of specimens, the marine record is largely one of the level bottom environments. However, in terms of numbers of taxa, many with poorly known stratigraphic ranges, the non-level bottom environments contribute a very significant fraction to the taxic total (think here of the pelmatozoan and bryozoan thicket taxa from the family to the species). Therefore, it is natural that most conclusions should be based on level bottom data, because of the signgicantly higher level of sample reliability, and from the more common taxa within the level bottom sample. First, I review the major extinctions recognized to date in the level bottom marine record. I attempt to summarize their major characteristics. I then comment on some of the potential correlations of these extinction events with varied geological phenomena such as regression-transgression, global climatic gradients, intervals of orogeny, levels of provincialism, and the like. I also emphasize the major adaptive radiations that follow some, but not all, of the major extinction events. Community continuity and termination at times of extinction are discussed. Finally, I discuss briefly some of the minor extinction events affecting the level bottom environment, pins the adaptive radiations generating the reef environments d e n o v o during the Phanerozoic, following extensive intervals when the reef environment was absent (also see Newell, 1971, and Sheehan, 1985). In 1983 I discussed briefly the well known Phanerozoic extinction events that have affected the marine level bottom environments. They are the end of the Ediacarian, end of the Early Cambrian, end of the younger Cambrian biomeres (including that at the end of the Cambrian), end of the Early Ordovician (about the end of the Arenigian), end of the Ordovician (the "old" Ordovician-Silurian boundary point for which the Zone of Glyptograptuspersculptus formed the base of the Silurian, rather than the "new" boundary point for which the overlying Zone of Akidograptus acuminatus forms the base), mid-Late Devonian (Frasnian-Fameunian boundary), end of the Permian, end of the Triassic, and end of the Cretaceous. In the 1983 paper I set up what were termed "Ecologic-Evolutionary Units" that conform in many respects to the E t a g e s of d'Orbigny (1850-1852) insofar as both reflect the points in the stratigraphic record where major, geologicallyabrupt changes in the level bottom environment biota are obvious. END O F THE EDIACARIAN (Fig. 1): Thanks largely to Glaessner's pioneering studies (summarized in
1979 and 1984), we now have a considerable body of information concerning the enigmatic level bottom, softbodied fauna of the Ediacarian. There has been a great deal of concern about the phyletic affinities of many of the puzzling taxa. We are clearly far from resolving these concerns today. Yet, nearly everyone involved is in agreement that the species, genera, f~milies, probably most superfamilies, and possibly most orders, appear to have no direct, post-Ediacarian continuations. Whether or not these extinctions were rapid or slow on a geologic time scale is unknown, owing to the small number of localities; in fact, it may never be known. However, the magnitude of the potential event is obvious. This extinction event does not give any insight into the behavior shown by varied communities and biogeographic units just before the extinction event. There are still far too few data even to begin to think seriously about Ediacarian biogeography or communities (Glaessner, 1984; Donovan, 1987a). The reason for citing this potential event is that it might just be the greatest of the Phanerozoic extinctions.
11 LEVEL BOTTOM COMMUNITIES EcologicalEvolutionary Time Intervals Units
Major* Extinctions
Major* Adaptive Radiotions
Major* Community Reorganizations
AI
Ct
AI
CI
A3
C3
A5
C3
A2
C2
Major* Dispersal Event
Ediocoron El I
L{3
El
1I
M-U~
E5
ili
LO
E5
M-UO v
E2
%LS
C3 C4
~'ZLS' ~zUD E2 ~zUD C+P
C4 El
A2
X
M-UTr
E3
XI
J +K
E4
Cenozoic
C2 C4
L Tr
](]l
DI
A2
CP
A2
C2 C4
Ranks indicatod,hightolow, os follows, Ei-ES, AI-A3, C1-C4, DI
Figure i : The Ecologic-Evolutionary Unitsof the Phanerozoic (from Boucot, 1983). The abbreviations (E -- major extinctions; numerals same as in "Summary & Rankings"; A = major adaptive radiations; numerals same as in "Summary & Rankings"; C = major community reorganizations; numerals same as in "Summary & Rankings"; D = major dispersal event) refer to material discussed in the text.
END OF THE EARLY CAMBRIAN (Fig. 1): Biotically, the Early Cambrian is in many ways the most distinctive Phanerozoic time interval. It has yielded about three to four times as many phylum to subclass level units as any post-Early Cambrian interval. I mention "phylum to subclass" because of the intrinsic difficulty in deciding how best to classify so many of the Cambrian higher taxa. The archaeocyathids are a typical example, having been assigned at one time or another to the Porifera and Coelenterata, as well as to a unique phylum (Hill, 1972, provides a useful account). The skeletal morphology of the Archaeocyatha is clearly too distinctive to permit their easy placement in any post-Cambrian higher taxon, whereas total ignorance of their soft anatomy and tissues probably precludes certainty on this point for the forseeable future. Yet, all agree that they represent a high level taxon. Brasier (1986) has provided a brief summary of
12 the many taxa appearing near the basal Cambrian boundary. This prominent earliest Early Cambrian, Tommotian interval adaptive radiation is still essentially unexplained. Whether or not there was an extinction event of any magnitude at the end of the Tommotian is still undecided because of taphonomic factors and ecologic shifts. Many of the "unique" Tommotian smaller shelly fossil taxa are now beginning to appear in the post-Tommotian (Brasier, 1986). Many of these "unique" taxa still have poorly known upper ranges owing to varied sampling problems, chiefly being the lack of enough acid treatment of the younger rocks. The fact that the Archaeocyatha have lingered on in a few very restricted locales into the Late Cambrian does not detract from their essential terminal Early Cambrian extinction nearly everywhere. It is common to consider the Early Cambrian as a time of "experimentation" insofar as higher taxa are concerned. In terms of lower taxa, such as the family, syntheses of the Early Cambrian are inadequate for making detailed comparison with later intervals; the sample also is very small. This fact is reflected in no small part by the overwhelming Lower Cambrian dominance of trilobite taxa (not to mention specimens). The Cambrian relative abundance of so-called "small shelly fossils" is still hard to estimate. In addition to the many higher taxa, chiefly represented (with the exception of the Archaeocyatha) by very few genera, species, or even specimens, most lower level taxa involved in the extinction event are trilobites. This may be partly a sampling artifact owing to the fact that the abundant inarticulate brachiopods of the Cambrian as a whole are still poorly known. Palmer (written comm., 1987) points out that "In the Lower Cambrian it is relatively easy to recognize olenellids, agnostoids, corynexochids and ptychoparioids (ordinal or superordinal groups), or species. Some fzmily or superf~mily divisions are becoming recognizable by the end of the Lower Cambrian (Zacanthoididae, Dorypygidae), and the ptychoparioids on the different continents are developing their own biogeographic aspects. (The biogeographic differentiation is there for most groups, except for maybe small shelly fossils about which we are just be~nnlng to develop knowledge, right from the start.) Within the regions, however, there are few very clearcut intermediate level taxa. The fzmily, superfamily and generic classification is thus relatively messy. During the Middle Cambrian, genera become increasingly clearcut, but many supergeneric groupings are commonly disputed. During the Late Cambrian, family-level taxa become increasingly clearcut. In Ordovician and younger Paleozoic rocks the superfamily level taxa (asaphids, phacopids, calymenids) become quite clear. The impression is that through the early Paleozoic differentiation of superspecific taxa becomes increasingly better at increasingly higher taxonomic levels with time. The exception is the ordinal or superordinal levels, which are distinct from the very beginning ...". Most survivors of the Early Cambrian extinction event are represented in the shelf margin equivalent region by the well known agnostid and specialized polymeroid facies, commonly preserved in black shales, Palmer's "Outer Detrital Belt" (Palmer, 1977). This shelf margin to offshelf survival phenomenon continued through the Cambrian and Early Ordovician, and even to the end of the Ordovician insofar as these major trilobite groups are concerned. There are hints in the post-Ordovician record that this phenomenon of selective survival, which takes place close to the shelf margin, continued into the Late Devonian (Pedder, 1982). Sepkoski (1987) also considered this possibility. In fact, the differential survivorshlp of shelf-margin to offshelf taxa may provide clues about evolutionary decoupling of this part of the ecosystem from the level bottom benthos on the rest of the shelf (in fact, most of the shelf). This tendency for greater offshetf survivorship may be correlated with the greater likelihood for elimination of varied shelf environments (and
13 their stenotopic taxa) during intervals of major regression. But, it must be kept firmly in mind (Boucot, 1983) that there is no correlation between widespread regression and extinction. For example, the global regression characterizing most of the Early Devonian occurs at a time of great increase in generic and specific level diversity brought on by greatly increased provincialism. The correlation between regression and extinction is selective, suggesting that factors additional to regression are heavily involved here. (Hallam, in press, suggests that when regression is combined with widespread basin anoxia they become effective in causing extinction). CAUSATION: The cause of the terminal Early Cambrian extinction event is still unknown. At about this time, the global climatic gradient increased (Boucot, 1983), with a parallel increase in the level of provincialism. Globally, little important, well-dated orogeny, took place at this time. The factor(s) involved in the appearance of mineralized skeletons at the beginning of the Cambrian are as unknown as ever; they are equal in significance to the appearance of the soft bodied metazoans in the Ediacaran, and second only to the appearance of life itself in terms of the fossil record. COMMUNITIES: It is very clear that communities, trilobite communities in particular, were almost completely reorganized immediately following the Early Cambrian, except for the shelf margin agnostid and polymeroid facies, and that the first skeletonized communities were organized at the beginning of this interval.
END OF THE CAMBRIAN (Fig. 1): The Middle Cambrian-Late Cambrian ecologic-evolutionary unit opened with a period of adaptive radiation, at least as far as the numerically and t a x o n o m i ~ y dominant shelf trilobites were concerned. Regardless of how one vievcs Palmer's (1984) four Middle and Late Cambrian biomeres, four major extinction events are clearly recognizable during this interval (Palmer, written comm., 1987, comments that most of the biomere boundaries have not yet been recognized outside of North America, and that high levels of platform provincialism during the Cambrian may be partly responsible), at least on the North American Craton (Ludvigsen & Westrop, 1985, regard them as stages in the traditional sense). They seem, however, to be extinction events prominent mostly at the family to species level, with few superfamilies or higher taxa involved (Palmer, 1984). The terminal Late Cambrian event also affected the newly arrived nautiloid cephalopods (Chert & Teichert, 1983) at the family and lower levels; keep in mind, however, that the Cambrian nautiloids are poorly known at present outside of China. The newly diversified and numerically more abundant articulate brachiopods saw the bulk of their genera become extinct, as was also the case with the gastropods. (We still have inadequate information about the inarticulate brachiopods.) Many other higher taxa lived during the Middle-Late Cambrian, but their stratigraphic ranges and overall record are too poorly known to shed much light on the extinction question. CAUSATION: There are no good correlations of the terminal Cambrian extinction event with global regression-trausgression (Ludvigsen et al., in press, make a good case for the absence of major regressive activity in the Cambrian-Ordovician boundary interval), orogeny (an essentially anorogenic interval), and with changing levels of global provincialism or climatic gradient. No evidence favoring extra-terrestrial "events" has been produced at any of the Late Cambrian biomere boundaries (Orth et al., 1984). Palmer (1981) emphasizes that the two important North American Cambrian regressions do not correlate with any of his biomere boundaries. It is still unclear whether the North American Cambrian biomere boundaries, and their extinctions, are global in extent. COMMUNITIES: The benthic communities of the latest Cambrian were largely replaced in the earliest Early Ordovician, along with the lower level taxa (Ludvigsen & Westrop, 1983). Community reorganiTation
14 also occurred with the earlier biomeres. END OF TIlE EARLY ORDOVICIAN (Fig. 1): The important extinction event occurring at the end of the Early Ordovician has, strangely enough, received very little publicity from those who seek major extinctions. The event is well known to the biostratigrapher. A number of families in certain groups terminate (gastropods, articulate brachiopods, nautiloids, and trilobites being among the most prominent). However, the principal characteristic of this extinction event was an almost total communityreorganiTation that affected the shelf depth level bottom benthos shoreward of the shelf margin region with its agnostid-olenid type faunas. After this event, trilobite numerical abundance became subdominant for the first time since the post-Tommotian appearance of the group. This was also the event following which several nearshore environments previously dominated by gastropods and nantiloids, shared that position with other groups, including the first numerically abundant bivalves, as well as a few brachiopods. The abundance of stromatolites (Cryptozoon) of the Cambrian-Early Ordovician nearshore region decreased drastically at this time, never again to assume their former dominance. In many ways the Early Ordovieian EcologicEvolutionary Unit III (Boucot, 1983), is a "halfway station" between the overwhelmin~y trilobite-dominated benthos of the Cambrian and the more varied benthos of the post-Early Ordovician, although very distinct from both. In fact, after Early Ordovician time, no one group ever dominated again, except for the bivalvedominated Early Triassic interval (Ecologic-Evolutionary Unit IX) (Boucot, 1983). The Early Ordovician began with an adaptive radiation involving several new family level units affecting such groups as the trilobites, brachiopods, gastropods, and nautiloids, but very little above that level. A few new higher taxa such as the conulariids (Brasier, 1988 letter, informs me that these are now known from the Tommotian) and the pelagic graptoloid graptolites were involved in this adaptive radiation event. CAUSATION: The cause is difficult to pinpoint. A prominent, probably worldwide regression did take place near the be~nning of the Llanvirnian (Lindstr6m & Vortiseh, 1983). Orogeny, provincialism, and global climatic gradients do not correlate very well, high or low, with the terminal Early Ordovician extinction event. COMMUNITIES: An almost total community reorganization took place at the Early-Middle Ordovician boundary, with the appearance of far more varied communities above, as well as the extinction of most older community types below. Massive community reorganization of this magnitude also took place at the be~nning of both the Early and Middle Cambrian, the beginning of the Carboniferous, the beginning of the Middle Triassic, and the be~nning of the Jurassic. END OF THE ORDOVICIAN (Fig. 1): The faunal composition of Ecologic-Evolutionary Unit IV (Boucot, 1983) is markedly different from its predecessor. Within that unit were the first appearance and subsequent adaptive radiation of the corals (both tetracorals and most tabulates), as well as the radiation of the bivalves, gastropods, stony bryozoans ostracodes, crinoids and other major pelmatozoan groups, and articulate brachiopods. The overall relative abundance of the trilobites decreased again, as did the inarticulate brachiopods. Almost all the community types are new. The terminal Late Ordovician extinction event was one of the three most profound to affect the level bottom environment. A very large number of families, as well as many orders and superfamilies, from most of the benthic megafossil groups were involved. The plankton, including graptolites and acritarchs, also were greatly affected. The reef community complex of the Middle and Late Ordovician was totally e~inguished. Most of the genera and species terminate. However, during this extinction event in about the Benthic Assemblage 4-5, outer slielf to upper bathyal position, some taxa survived very well, particularly
15 within the Dicoelosia-Skenidioides Community Group. The more cosmopolitain and commonly more eurytopic taxa (as judged by presence in numbers of communities, and shoreline to shelf margin distribution) generally survived, as is commonly the case in most extinctions. The taxonomic uniqueness of each major extinction event is attested to by Teichert's (written comm., 1987) comment that "The end Ordovician is not a big extinction event for the cephalopods.". He points out that many higher nautiloid taxa pass through this interval, whereas others disappeared well before. For many other benthic and planktonic groups, however, it is a major event. Whether or not such taxonomic uniqueness of each of the major extinctions has a "cause or causes" as contrasted with being merely a chance event is currently unknown. However, there is no denying the taxonomic uniqueness of each major extinction event in terms of relative effects at different taxonomic levels. This e~inction event was not instantaneous, at least regarding brachiopods (Cocks, 1988), trilobites (Briggs et al., 1988) and pelmatozoans ~ckert, 1988), and probably involved several million years or so for the affected groups. CAUSATION: In terms of cause, attention has commonly been directed to the glacial maximum within the late Ashgillian. The cause of the extinction, however, is unlikely to have been glaciation because many taxa survived well into the glacial interval, within their appropriate climatic zone. Just as with the Pleistocene there is no 1:1 correlation between rapid extinction and rapid regression correlated with the onset of widespread continental glaciation that reaches sealevel. In fact, this Ordovician extinction was delayed considerably after the onset of glaciation. However, it must be pointed out that here, as with almost all extinction events, we lack a careful, stratigraphically controlled account (centimeter-by-centimeter work in many sections that include as many community types as possible) of the timing of the varied extinctions. Moreover, the relations between the extinctions and specific communities have not been well documented. Evidence for extraterrestrial (Orth et al., 1986; Wilde et al., 1986) and orogenic causes is lacking. It is true that the global climatic gradient decreased, but not precisely at the Ordovician-Silurian boundary. Levels of provincialism decreased at this time (Wang Yu et al., 1984, correct Boucot's, 1975a, earlier misconception of widespread Llandoverian cosmopolitanism), but did not approach the high level cosmopolitanism of the Late Devonian or Triassic. COMMUNITIES: The rich communities of the latest Ordovician were taxonomically impoverished by the extinction event, and the reef community complex of the Middle-Late Ordovician totally eliminated. The surviving earlier Silurian communities are fractions of their former, Ordovician, selves. There is widespread, although not total, community reorganiTation. In this regard, the terminal Ordovician event is similar to what happened in the tropical-subtropical world following the end Cretaceous event, as well as following the mid-Late Devonian event. It is worth commenting that at the end of Ecologic-Evolutionary Unit V, within the late Llandovery, there is a marked increase in numbers of taxa/community group owing to the dispersal of so many taxa from the Uralian-Cordilleran Region into the North Atlantic Region (Wang Yu et al., 1984). MID-LATE DEVONIAN ~RASNIAN-FAMENNIAN) (Fig. 1): Be~nnlng with McLaren's (1970) Presidential Address to the Paleontological Society, and followed by his subsequent Presidential Address to the Geological Society of America (1983) great attention has been devoted to the mid-Late Devonian extinction event (e.g., Walliser, 1986). McLaren emphasized the geologically instantaneous nature of the event. It is now clear that this was one of the three major extinction events which affected the level bottom shelf benthos during the Phanerozoic. (However, the extinction at the end of the Permian was the greatest.) Many units at the class to ordinal level and below were terminated at this time. Recent work suggests, however, that not all aspects of this extinction were geologically instantaneous (Becker, 1986; Steam, 1987),
16
but rather extended over a period of as much as a million years. Brasier (1988) summarizes the benthic foraminlferal data. The type of decline described by Becker extends through some millions of years, and is not considered to have been stepwise (see Boucot, in prep., for consideration of the "stepwise" problem in ecologic terms). Copper (1986) reviews the later Devonian record of the atrypacean brachiopods to arrive at a similar conclusion, although suggesting an even longer, more extended time interval. However, Copper's interpretation suffers from the absence of the necessary benthic community data that might have allowed us to decide whether the extinctions he reported are potential artifacts based on collecting failures (samples too small to recover uncommon taxa), or collecting failures based on local absence of particular community types for particularly stenotopic atrypid genera (see Boucot, in prep., for consideration of this environmental sampling problem). Information about this question of time interval duration during which extinction occurs is still very preliminary. Sorauf & Pedder (1986) following up on Pedder (1982), emphasize that most of the shelf depth rugosan corals terminated at this time, but that many of the offshelf, shelf margin-upper bathyal equivalent corals continued on relatively unscathed; this is more evidence for decoupling; their data indicates a relatively rapid event insofar as rugosans are concerned. It is clear that this particular extinction was not followed immediately by an adaptive radiation affecting the level bottom benthos, a character shared with both the earliest Silurian-post-Late Ordovician and Permian extinction events, although a few groups did radiate near the very beginning of the Silurian (including such things as the stricklandiids and eospiriferids). Enough is known about pelagic organisms during the Late Devonian to make it clear that, although the zmmonites were greatly affected by the mid-Late Devonian extinction event, they subsequently underwent several Famennian (Late Devonian) adaptive radiations (House, 1985; Korn, 1986), unlike the shelf benthos. The marine fish, on the other hand, were not too greatly affected by the mid-Late Devonian extinction event, but did undergo a major terminal extinction event at the end of the Famennian, in contrast to the shelf benthos (Harland et al., 1967; Hansen, written comm., 1985). The reef community complex and its taxa were totally terminated in the mid-Late Devonian extinction event (McLaren, 1970). CAUSATION: A cause for the mid-Late Devonian extinction event is obscure. The Late Devonian was biogeographically very cosmopolitan, had a very low global climatic gradient, was relatively but not completely anorogenic, and was a time of moderately high global transgression. No good evidence has been found for an extra-terrestrial cause (McGhee et al., 1984; McGhee et al., 1986a, b; Donovan, 1987b; McLaren, 1985; Geldsetzer et al., 1987). COMMUNITIES: The bulk of the Silurian-Devonian benthic, level bottom community groups terminated at this extinction boundary. Only remnants of some of them straggled through the Famennian, prior to the massive adaptive radiation affecting the level bottom benthic world at the opening of the Carboniferous. Famennian level bottom benthic faunas and communities can be taxonomically described in one word impoverished. END OF THE PERMIAN (Fig. 1): The be~nnlng of Ecologic-Evolutionary Unit VIII, the Carboniferous, was marked by a major adaptive radiation affecting level bottom shelf benthos, and by major community reorganiTation. Within the Carboniferous a minor extinction event took place at the MississippianPennsylvanian (NOT the Early Carboniferous-Late Carboniferous!) boundary (Ramsbottom et al., 1982), but there was no event at the Peunsylvanian-Permian boundary. Sepkoski's (1986) generic data indicating minor extinction events at both the Mississippian-Pennsylvanian and Pennsylvanian-Permian boundaries I regard as coincidence in the first and artifact in the second case, owing to the all too common tendency for
17 taxonomists to change generic names at Period boundaries; remember that the "placement" of the Pennsylvanian-Permianboundary was a difficult question that was fmally "decided" by a committee (see Mudge & Yochelson, 1962, for references, and a mass of stratigraphic range data). The community evidence at the Peunsylvanian-Permianboundary strongly indicates the absence of an extinction event, as everyone knows who has ever collected fossils adjacent to the boundary in Kansas and Nebraska with their supremely repetitive series of cyclothem community types in environments ranging from the nonmarine to the deeper shell. The situation on the Russian Platform at the same horizon is little different for either stratigraphic ranges of varied taxa or for community types. The absence of a similar generic level, artifactual "extinction" event at the Silurian-Devonianboundary may reflect the relatively recent redefinition of that boundary as contrasted with the Pennsylvanian-Permianboundary. The greatest Phanerozoic extinction event took place at the end of the Permian (Newell, 1967, 1973). Class after class was eliminated, together with most orders, superfnmilies, and lower taxa. This event was an order of magnitude greater than any other, including the next two greatest at the end of the Ordovician and in the mid-Late Devonian, at every taxonomic level from the Class on down. This extinction was not immediately followed by an adaptive radiation affecting the level bottom shell benthos; a major adaptive radiation did take place near the beOnning of the Middle Triassic. It is fascinating evolutionarily, as well as biogeographicaUy, to attempt to understand where group after group of organisms underwent the high taxonomic level changes during the Early Triassic that must have occurred in order to explain the presence of distinctly different orders of the same classes in the Permian and Middle Triassic, with no Early Triassic representation known (Boucot & Gray, 1978, for a summary). It is interesting to emphasize that the pelagic ammonoids, just as after the mid-Late Devonian extinction, did radiate in the Early Triassic (i.e., another example of decoupling of the pelagic from the benthic). However, even with this greatest of all extinction events, the evidence suggests that the actual extinctions were not instantaneous or simultaneous in all taxa (Newell, 1967; Teichert, 1968, 1985; Ager, 1988), because there is good evidence for many benthic taxa lingering on for a very brief interval into the earliest Triassic (the Permian-Triassic transition in the marine environment is best preserved in South China). Additionally, there are a few taxa, such as the gastropods (Batten, 1985) and nautiloids (Teichert, 1985) which are characterized by having many taxa that passed through the Permian-Triassic, only to become extinct near the end of the Triassic. Newell (1973) emphasized the apparently stepwise decline in later Permian diversity (this decline now needs to be further examined in an ecostratigraphic manner in order to see if any parts can be ascribed to inadequate sampling; see Boucot, in prep.). CAUSATION: Reams of paper have been devoted to trying to establish a cause for this extinction event which wiped out the reef community complex and its taxa i n t o t o, most of the level bottom taxa, and the bulk of the pelagic organisms. However, when all is said and done, most of the phenomenon has not been explained. A great regression, possibly the greatest in the Phanerozoic, does correlate very well with the extinction event (Newell, 1967), but alone is inadequate to explain the event, because vast shoreline regions and shallow depths are always present (unless one wishes to totally eliminate the oceans); it is the bulk of the epicontinental seas that are eliminated during major regressions, not the continental shelves (although, the latter may be narrowed). Not all Phanerozoic regressions correlate by any means with extinctions (consider the lack of any global extinction during the Quaternary with its many regressive events), and some actually correlate with increases in taxic diversity (e.g., the later Silurian and earlier Devonian). A Permian lowering of the global climatic gradient did take place, but most of the lowering was completed by the middle of the period, many millions of years before the extinction event. Orogeny was moderate at the very end of the Period. A marked change took place from the high provincialism of the
18 later Permian to the very cosmopolitan Early Triassic. That, however, again can explain only a part of the extinction, even combined with the lower global climatic gradient. Boucot and Gray (1978) have commented critically on some of the more extravagant suggestions for causality. We dearly still lack the ability to explain this event satisfactorily. Clark et al. (1986) provide evidence for discounting an iridium anomaly at the boundary. COMMUNITIES: The complex and varied level bottom community groups of the Permo-Carboniferous were abruptly terminated at the end of the Permian, together with the reef community complex, sponge complexes, and peimatozoan plus bryozoan complexes. The level bottom benthic communities of the succeeding Early Triassic were the fewest in number and taxonomically the most impoverished of the entire Phanerozoic.
END OF THE TRIASSIC (Fig. 1): The extinction event at the end of the Triassic has only recently begun to receive the attention its non-cephalopod portion deserves, although biostratigraphers have been welt aware of this event since before the middle of the last century. The event witnessed the end of many molluscan groups (Hallam, 1981; Teichert, 1986), many foramlnlferans (Brasier, 1988), as well as such groups as the conularlids (Babcock & Feldmaun, 1986), and conodonts (Clark et al., 1986). The latter two may not, however, have been strictly benthic, although a demersal, epibenthic possibility for some is reasonable. The bryozoans (Schiller & Fois-Erickson, 1986), ostracodes (Whatley, 1986, 1988), brachiopods (Ager, 1988), cephalopods (Teiehert, 1985), bivalves (HaUam, 1981), gastropods (Batten, 1985), and many other groups suffered massive extinctions at the ordinal, superfamily, and family levels. Much of the information, unfortunately, has not yet been synthesized. The Upper Triassic reef eemmtmity complex terminated at this time; this is another example of the sensitivity of the reef complex to extinction as at the end of the Ordovician, Frasnian, and Cretaceous. In many ways the Triassic as a whole is distinct from the Paleozoic and the Mesozoic in terms of its level bottom biota - it belongs to neither, although possibly having more affinity with the Mesozoic than with the Paleozoie. CAUSATION: This was a time of marked global regression, although no more marked than during the midEarly Devonian, a time for which no evidence of a major extinction event exists. Global climatic gradients remained low prior to and following the event. The interval is not notably orogenic. COMMUNITIES: The Triassic level bottom community groups came to an end at this time, with the later Jurassic-Cretaceous units being totally different in their organization and including many new, higher level taxa - this is not just a question of family and lower level extinctions. This almost total community extinction and subsequent reorganization is comparable in extent to the geologically abrupt events following the Early Ordovician and Permian extinctions, as well as that beginning in the earliest Carboniferous. End Triassic and earliest Jurassic commlmities have not been formally described as such, but there is a rich literature, be#nning in the 19th Century, that deals with this major biofacies change. END OF THE CRETACEOUS (Fig. 1): The modern marine level bottom benthic fauna and communities really began with the Jurassic. This was the time interval when the planktonic globigerine Foraminifera appeared; diatoms were shortly to appear in the record, together with post-Paleozoic larger Foraminifera; in addition the first prominent regular and irregular echinoids, the first modern crustaceans in abundance (the crabs and lobsters of this world), most of the modern superfamilies, and many of the families of bivalves and gastropods were to appear, as well as some demersal fi~es such as the batoids. This "modern" biota was greatly affected, particularly the tropical-subtropical part, by the end
19
Cretaceous extinction event. Many important groups (see Kauffman, 1984, 1986, for incisive summaries) terminated, but many others continued. Insofar as the marine level bottom benthic world was concerned, the terminal Cretaceous event was much less important than the terminal Permian, terminal Ordovician, and terminal mid-Late Devonian events. Kauffman emphasized that higher latitude, cooler climate, marine faunas were not as severely affected as the tropical-suptropical realms, where the reef community complexes, for example, terminated totally (Newell, 1971; Sheehan, 1985). CAUSATION: I will not belabor the end Cretaceous event, because so many others contributing to this volume, far better qualified than I, have worked at the task. However, it is clear from the work of others (WaUiser, 1986, for example) that we still have no well agreed-upon explanation for the end Cretaceous extinction event, although a heightened global climatic gradient appears to be the most pleasing at the moment (see Officer et al., 1987, for a persuasive permutation; Patrusky, 1987, for an incisive review of the overall questions; and Jaeger, 1986, for a forceful, well documented evaluation). COMMUNITIES: The terminal Cretaceous event was not followed by a thorough reorganiTation of the level bottom, benthic communities. However, it did see the disappearance of certain community types, such as the outer shelf inoceramid types, level bottom rudistids, and certain gryphaeid dominated types. Rather it featured the surviving taxa remaining in more or less the same community groups. In this regard it was similar to the situation following the mid-Late Devonian extinction events; it was unlike the others.
MINOR
EXTINCTION
EVENTS
I have said nothing about the many minor extinction events. A fair amount, however, has been published on the later Mesozoic (see Kauffman, 1984, 1986, for examples; Jarvis et al., 1988, for an excellent account of many aspects of the Cenomanian-Turonian anoxic event [Brasier, 1988, comments on some of the foraminiferal aspects]). Hallam (1986), Jenkyns (1985) and Brasier (1988) consider the earlier Jurassic Plieusbachian-Toarcian event. Elsewhere in this volume I have discussed a number of the minor events present in the Silurian and Devonian. These Silurian and Devonian minor events are recognizable only with the aid of the most susceptible groups of organisms (and remain undetected by Sepkoski, 1986), and only after a great deal of attention to many groups of unrelated organisms, to try to uncover trends having some real significance. There was a low level of community reorganization at the Silurian-Devonlan boundary that correlates well with a minor extinction event and subsequent adaptive radiation also present at that boundary (Boucot, 1985). This minor event is most easily detected by paying careful attention to the obvious changes in overall community composition (Boucot, 1975a, Fig. 35; Boucot, 1982), whereas numerical compilations do not easily reveal its presence or significance. It is clear from consideration of the many minor biostratigraphic breaks in the Phanerozoic that there are a host of minor extinctions to be dealt with.
REEF
AND OTHER
NON-LEVEL
BOTTOM
COMMUNITY
COMPLEXES
Throughout this paper I have mentioned briefly the occurrence of reef community complexes. They dearly appeared and disappeared through time (Newell, 1971; Boucot, 1983; Sheehan, 1985; Fagerstrom, 1987; Talent, 1988). It should be emphasized that the reef community complexes present in each distinct reef interval are almost totally distinct from each other in a Darwinian sense, presumably having been derived independently from distinct level bottom taxa~ The arehaeocyathid-algal reefs (Selg, 1986) of the Early Cambrian were followed by a lengthy interval containing no reef community complexes (the biohermal community complexes of the later Cambrian and Early Ordovician are not thought to have had a
20 wave resistant organic framework). Then came the reef community complexes of the Middle and Late Ordovician, followed in turn by a reef-barren interval until the early part of the late Silurian (late Wenlockian). From the late Wenlockian on there was a coral-stromatoporoid reef community complex until the end of the Frasnlan (There may be a reef barren interval between the Pridolian, latest Silurian, and the Pragian part of the Early Devonian, which separates the stromatoporoid-tabulate type reefs of the Silurian from the stromatoporoid-colonlal tetracoral parts of the Devonian reef complex.). A reef eommtmity complex was absent from the Famennian (some algal complexes present), as well as the Mississippian (the Waulsortian mounds are community complexes but not true reef complexes), but a new one appeared in the Pennsylvanian and Permian. Reef community complexes were lacking in the Early Triassic, but reappeared in the later Triassic and terminal Triassic (Stanley, 1988). The Jurassic (Stanley, 1988, indicates reef absence in the lowest Jurassic which separates the Triassic type reefs from the younger types) and Cretaceous contained reef community complexes, but the Paleocene had almost none (they came back in force with the Eocene and then persist to the present). The lesson here is that the extinctions of the reef community complexes coincided with the major extinctions affecting the level bottom environments, but that the reinitiation of the reef community complexes (i.e., the adaptive radiations giving rise to the reef community complexes) commonly do not coincide with the imtiation of the level bottom community groups - the reef complexes generally appear later in time. Hallock (1987, 1988) has discussed the adaptation of zooxanthellate organisms to nutrient deficient waters. She then goes ahead to make the suggestion that the removal of nutrient deficient waters from the system, if corresponding to extinction events involving zooxanthellate or potentially zooxanthellate organisms might be still another extinction mechanism worthy of serious consideration. We have no reliable summaries for the stratigraphic ranges of the many other, important, non-level bottom community complexes. One can predict, however, that they follow an initiation and extinction pattern similar to that for the reef community complex (i.e., extinctions in common with the level bottom world, but initiations commonly later than those of the level bottom community groups).
COMMUNITY COLLAPSE In the section dealing with the mid-Late Devonian extinction event I mentioned evidence for an abrupt event noted by McLaren (beginning with his 1970 paper), and of evidence for more gradual extinction, taxon by taxon during some millions of years prior to the event emphasized by McLaren. How can one reconcile these data? I suspect that what gives rise to these apparently conflicting observations is the influence of a gradually increasing set of extinction influences that gradually eliminate the more stenotopic, more endemic, less abundant taxa earlier from the communities in which they existed, followed by ultimate community collapse (McLaren's event) in which the abundant, higher biomass, eurytopie, more cosmopolitan taxa are eliminated. Just why community collapse should occur suddenly in geologic time is uncertain. Possibly there are certain "keystone" taxa that when f'mally eliminated cause the whole community structure to crash. If so, we have no understanding for the dynamics of the process, nor for which taxa might most likely fill the keystone role, nor whether taxie types regarded by modern day ecologists as keystone are what is involved here. We clearly need to collect additional data adequate for testing these possibilities not only near the mid-Late Devonian extinction event, but also at others, such as those within the Cretaceous discussed by Kauffman (1984, 1986) who has noted more gradual extinction phenomena prior to the Cenomanian-Turonian (see also Jarvis et al., 1988) and Maastrichtian-Danian extinction events.
21 INITIATION
OF ECOLOGIC-EVOLUTIONARY
UNITS:
COMMUNITY
GENERATION
Sheehan (1985) suggested that each ecologic-evolutionary unit is characterised by an initial, geologically brief interval (I have suggested [1983] no more than a few million years) of adaptive radiation. He also views units V, VII, and IX (lower 2/3 Early Silurian, Famennian, Scythian) as such intervals, rather than as proper ecologic-evolutionaryunits. I am sympathetic to his suggestion that there were such early, geologically brief intervals of adaptive radiation and community group formation, but am unf~millar with evidence suggesting that units V, VII, and IX have this character. We clearly need to gather data adequate to test his proposal. The largely North American Cambrian biomeres have been described in a manner conforming to Sheehan's suggestion. Jarvis et al. (1988) describe data from the Cenomanian-Turonian extinction and subsequent adaptive radiation that conforms to Sheehan's suggestion. What is really being considered here is whether the generation of new community groups, ancestral to varied community groups, takes place over a few million years or so. If so, we currently lack the information to properly describe and understand the process. Carefully collected samples from the many ecologicevolutionary unit boundaries will be needed to answer these questions.
DECOUPLING
I have used the term "decoupling" (1983) to refer to the fact that different parts of the global ecosystem undergo massive adaptive radiations at distinctly different times. This fact makes it unlikely that extraterrestrial influences have had a very profound effect in initiating such adaptive radiations. The noncoincidence in time of many of these major marine adaptive radiations with those occurring at different times in the nonmarine environment is involved here, as well as distinctly separate parts of the marine (for example, shelf and bathyal) and uonmarine environments. For example, the paleobotanical time terms Palaeophytic, Mesophytic, and Cenophytic refer, of course, to the beginnings of the Paleozoic-type vascular flora in the Paleozoic, to the following flora in the later, but not latest, Permian, and to the angiospermdominated flora be~nning in the mid-Early Cretaceous. These three time intervals (Middle Ordovicianlater Permian; later Permian-earlier Cretaceous; earlier Cretaceous-present) do not correspond very well with any major events in the marine realm. The earliest, Palaeophytic phase of the higher land plants invasions occurs in the Middle Ordovician (Gray, 1985), followed in the later Llandoverian, by the appearance of the vascular plants (Gray, 1985), followed in turn by woody plants near the Silurian-Devonian boundary. Gray (1988) emphasizes that there is no hint in the spore record of the higher land plants for any extinction event at or near the Ordovician-Silurian boundary, i.e., a free example of decoupling of the marine from the nonmarine. However, Richardson & McGregor (1986) point out that the major, late Paleozoic change in spore floras occurs at about the Frasnian-Famennian boundary, i.e., an exception to the apparent non-coincidence in time of major terrestrial and marine events. The well documented rise of the angiosperms also accords with an accompanying insect radiation, as well as the initiation of the placental, presumably insectivorous mammals, but no first rank, major adaptive radiation in the marine environment. The poor correlation in time between major extinctions and adaptive radiations in the marine and nonmarine environments is what really provides the basis for avoiding extraterrestrial factors as heavily involved in such cases. The adaptive radiations of the reef community complex have already been reviewed briefly, and as stated to not accord very well with major adaptive radiations in the level bottom realm. If we knew more about the other marine community complexes in terms of their initial adaptive radiations we might have still more evidence of this kind with which to emphasize large scale decoupling. There are, of
22 course, any number of lower level decoupling phenomena, such as the probable rise of grasslands in the Eocene of South America as contrasted with their much later origins in North America. Comments have also been made about the decoupling in time for several elements of the pelagic as contrasted with the level bottom environment, such as zmmonite radiations vs. benthos quietude during the Famennian. OTHERS: The radiations of the marine reptiles in the Mesozoic are notable, as is the initial radiation of marine mammals in the Eocene. The rise to importance of the diatoms in the mid-Mesozoic is still another example. Note that the cited vertebrate examples are very decoupled from the marine, level bottom story, and provide still more evidence for a poor correlation between extra-terrestrial events and such phenomena.
SUMMARY ~
RANKINOS
EXTINCTIONS: 1) The Lower Cambrian, and possibly the Ediacarian, are first, at the Subclass to Phylum levels, whereas the Permian is first, at the Phylum through species levels; 2) The end Ordovician and midLate Devonian are second, at the Class or Ordinal through species levels; 3) The end Triassic is third, at the Class through species levels; 4) The end Cretaceous is fourth, at the Ordinal through species levels; 5) The end Cambrian, and earlier, post Early Cambrian biomeres, are fifth, together with the end Early Ordovician, at the Family through species levels, among events reviewed here. ADAPTIVE RADIATIONS: The be~nning Early Cambrian, and possibly beginning Ediacarian, are first, at the Subclass to Phylum levels; 2) The beTnnlng Middle Ordovician (larger than the others at the ordinal level [Sepkoski & Sheehan, 1983]), beginning Carboniferous, beginning Middle Triassic, and beginning Jurassic are second, at the Ordinal through Familial levels; 3) the beginning Middle Cambrian, through the b%~nning Early Ordovician are third, at the Familial level. COMMUNITY REORGANIZATIONS: 1) First in magnitude are the Ediacarian and Early Cambrian; 2) followed by the Middle Ordovician, Early Carboniferous, Middle Triassic, and Jurassic; 3) followed by the Middle Cambrian through Early Ordovician (including the biomeres), and Early Silurian; 4) followed by the Famennian, Early Triassic, and Cenozoic. DISPERSAL: A major dispersal event occurs between the C2-C3 parts of the late Llandoverian (later Early Silurian).
CAUSATION
We have little understanding of the cause or causes for either extinctions or adaptive radiations. The gradual aspect of some extinctions suggests that instantaneous extra-terrestrial events are ~mlikely to have been prime causes. Kyte (1988) has summarized the current data about iridium anomalies, which suggests that only for the K-T boundary is convincing data available. However, extra-terrestrial events manifesting themselves over a lengthy time interval, if responsible in large part for major climatic shifts, or for the change in position of major water masses might be involved (although it would probably be most difficult to recognize such extraterrestrial factors unless they happen to show undoubted periodicity over a statistically very significant part of the Phanerozoie). As our understanding of paleogeography and p a l e o o ~ o g r a p h y improves we may be able to recognize the coming and going of important "gateways" in the marine world that are involved with surface current circulation patterns, and possibly in turn with climatic changes. However, the relatively instantaneous community collapse, major biomass events, might have involved instantaneous physical events of one type or another, IF unconnected with the prior, gradual type extinction
23 of the more stenotopic taxa. Meyerhoff (written comm., 1987) points that out the later Ediacarian-Early Cambrian, Late SilurianFrasnian, and later Permian-Carnian highs in evaporite deposition and preservation, suggesting that these highs might be related to changes in seawater concentration and composition, which produced (at least in part) the extinctions. Holser (oral comm., 1987) suggests that the overall changes in seawater concentration brought about by these evaporite highs was probably not more than about 3-4 parts per rail. Relying on the importance of selenium as a poison in the terrestrial ecosystem (Trelease & Beath, 1949) Meyerhoff (written comm., 1987) suggests that minor changes in selenium concentrations in the oceans, since selenium substitutes for sulfur, paralleling changes in seawater concentration, might have been the critical factor in the major extinctions, although other elements might also have contributed. Wilde & Berry (1984; see also Berry & Wilde, 1987) propose destabilization of oceanic density structure as another potential cause of major extinctions. Their suggestion is worthy of careful, further investigation, particularly in terms of the "anoxic events~ suggested for certain time intervals in the deep oceans. Within the interval Jurassic-Cretaceous HaUam (written comm., 1988) points out that the only two globally significant extinctions are the anoxie events "... in the early Toarcian and at the Cenomanian-Turonian boundary.". Note well that in all of the discussions of extinction causation there has been little or no attention paid to potential biologic rather than physical causes. This may reflect the interests and trzining of those concerned rather than the real situation. For example, Emslie (1987) provides a convincing account of why the virtual extinction of the California Condor is probably due to the earlier extinction of its probable food sources (large Pleistocene and Holocene mammals). Discussions of extinction causes almost never consider the potential role of diseases, and their role in disrupting the food chain, or of other biologic possibilities. Haldane (1949), in a little noticed paper, comments tellingly that some extinctions may result from the diseases spread during dispersal events by the invaders, while commenting on the Neogene disappearance of many of the South American endemic ungulates - obviously, though, such a possibility would need to be supported by a mass of paleopathological evidence that we currently are in no position to provide.
QUESTIONS The following basic questions are raised by the data: 1) Why do the major extinctions appear to affect the tara of many unrelated major groups during a geologically very brief interval (up to a few million years)? Is it merely a symptom of collapse affecting major parts of the ecosystem that had fairly obligate, coevolved relations, or is something else involved, such as the more or less coincidental exceeding of the physical tolerances of varied, unrelated taxa, with eoevolution not being involved to any serious extent? Why, immediately prior to the extinction horizon, do the more abundant, higher biomass taxa (those emphasized by MeLaren, 1985) belonging to varied, unrelated groups contemporaneously become extinct? Keep in mind that these abundant, higher biomass taxa are relatively few in number. 2) why do the major adaptive radiations affecting taxa belonging to many unrelated, major taxa occur at about the same time over a geologicallyvery brief interval (up to a few million years)? Is this a symptom of geologically almost instantaneous, developing eoevolution, or it merely a time coincident reaction to newly appearing physical parameters to which the taxa of these unrelated major groups happen to be sensitive? 3) why do the major adaptive radiations invariably occur at some time following major extinction events? Does this indicate the presence of vacant niches, or are other factors involved?
24 4) Why after certain of the major extinction events (end of the Ordovician, end of the Frasnian, midLate Devonian, end of the Permian) are there geologically lengthy intervals during which there is no evidence for extensive adaptive radiations in the level bottom environment, as contrasted with the other major extinction events? Is there an environmental "lack" involved in a physical sense, or is this merely a stochastic phenomenon? In the same sense, why do most generations of the reef community complexes, presumably from level bottom antecedents, occur significantly later in geologic time than the adaptive radiations that affected the co-occurring level bottom biota? Is this, too, merely a stochastic phenomenon, or are there unrecognized factors involved? 5) Why is there such a melange, in terms of extinction magnitudes and adaptive radiation magnitudes, immediately before and after the major extinction-adaptive radiations? Is this adequate evidence to indicate that extinctions and adaptive radiations are unrelated to each other in a causal fashion? Does this situation provide any evidence about whether or not competition is involved in any of the major extinction events? 6) Why do some taxonomic groups persist through major extinction events, why do some community types persist through major extinction events, are there specific morphologies that are most resistant to extinction, are there certain communities that are more resistant to extinction, with the same questions being asked for adaptive radiations, or is all of this evidence for varied stochastic processes? 7) Summing up, it is obvious that the mechanism(s) controUing both extinctions and adaptive radiations are poorly understood at this time. Hopefully, population biologists will eventually direct their attention to this puzzling area. Until we have a suitably biological explanation for these phenomena our understanding of organic evolution will be deficient insofar as these phenomena are concerned.
CONCLUSIONS It is clear from this brief summary that no two major terminal extinction events affecting the widespread level bottom environment are identical. By "identical" I mean: (1) whether or not community structure was greatly changed, (2) which taxonomic levels were most affected, and (3) which major taxa were most affected. In view of this variety of extinction types, it seems unlikely that the cause or causes of any two are the same. Teichert (1968) arrived at essentially the same conclusion. The overall conclusions made here need to be carefully reviewed with quantitative data obtained after the stratigraphic range data have been properly selected by removing thoroughly unreliable data, and appropriately weighting the remaining data. The selection and weighting process needs to consider the various aspects of sampling such as changing levels of global provincialism, size of individual fossil collections, and the taphonomic biases implicit in the widely differing preservational characteristics of varied fossil groups. A f t e r this has been carried out these compilations should provide a far more realistic account of the distinctive nature of the extinctions and adaptive radiations insofar as their purely numerical characteristics are concerned. But, their community characteristics must also be taken into account if we are to have any real hope of understanding these enigmatic events by means of paleontologic data. One could take the view that the reason for the apparently differing natures shown by many of the extinction and adaptive radiation events is due to the unique taxa involved in each one. In other words, one would not expect similar reactions from trilobite-dominated events as contrasted with brachiopod- or coraldominated events. There is certainly a possibility that distinct taxa will react in distinctive manners to the same forcing factors, but I suspect that there is much more to the matter than the mere nature of the taxa involved; i.e., it is more likely that varying mixes of physical factors have been responsible for the unique
25 flavor of each event rather than that the unique taxic compositions have been the dominant control. The basic reason why both extinctions and adaptive radiations are far more easily recognized on an ecostratigraphic basis than by merely counting taxa, is as follows. The ecostratigraphic method pays careful attention to changing abundances, large biomass changes, of individual taxa within each community group (biofaeies of the geologist, narrowly construed). The ecostratigraphic method brings out the fact that rare genera and their species are commonly unrepresented in the all too common small samples on which so much of our past work has been based. One must keep in mind (Boucot, 1986) that a very large percentage of most families contain only one or a few genera, and are commonly very provincial. The ecostratigraphic method, therefore, takes advantage of both taxic presence and absence (fully understanding that many, possibly the majority of absences, probably reflect sampling deficiencies) and relative abundances of all taxa. The other approach pays attention only to known, reported taxic ranges, and makes no attempt to weight the samples properly in view of what is knowable about community evolution or changing biogeography's effect on diversity. It is little wonder that the ecostratigraphic approach has proved far more sensitive in detecting both extinctions and adaptive radiations, particularly with the minor events, just as in an analogous manner attempts to sort out individual fossil collections by means of similarity indices are most effective when abundances are considered in addition to taxic presences and absences. It is merely a question of employing the most penetrating statistical approach, which in this instance happens to be the ecostratigraphic. We need to emphasize that organic evolution is not a smooth flowing river, as Darwin's work would suggest, but that it has been a stream interrupted by some major waterfalls; the leading adaptive radiations and extinctions preserved in the fossil record, as pointed out for us by d'Orbigny long ago. tt is worth noting (Boucot, 1983) that the major Phanerozoic community reorganizations on a global scale, excluding regional phenomena such as affected the Arctic and North Atlantic Oceans during the Quaternary and Pliocene, occur either immediately following major extinction events and subsequent adaptive radiations, immediately following second and lower rank extinction events followed by adaptive radiations of varying rank, or in a single instance (Ecologic-Evolufiona~J Unit V, Boucot, 1983) following a major dispersal event (a few other, smaller dispersal events are also immediately followed by community reorganizations, such as that affecting the well known later Middle Devonian Hamilton Group faunas of eastern North America in which many Rhenish-Bohemian Region organisms dispersed into and mixed with descendents of preexisting Appohimchi Subprovince taxa [Boucot, in press]). The differing levels of community reorganization are dearly just another facet of the quantum evolution phenomena involved in the production of new families and higher taxa. Note that the major community reorganizations are invariably involved with the production of new fzmilies and higher taxa. In other words, the communities to which taxa belong are just one more facet of their taxonomy, although a statistical property rather than a morphological, cytologic, biochemical, or behavioral character. An ecologically meaningful measure of extinction and adaptive radiation might be compiled by making a comparison between genera/family/community group for specific ecologic-evolutionary units as contrasted with the changes in the numbers of genera/family/community group following specific extinctions and adaptive radiations. When compilations are prepared that begin to ask specific questions having ecologicevolutionary significance we will be in a much better position to take advantage of the compiling possibilities. It is also clear that we have far to go in collecting adequate paleontologic c u m stratigraphie data of a community and biogeographic type adequate even to fred out just what are the major problems facing us. This conclusion merely repeats that published recently by Teichert (1986)! It is the conclusion favored by experienced paleontologists everywhere.
26 Undoubtedly, many surprises lie ahead!
ACKNOWLEDGEMENTS
I am very grateful to the following colleagues for having reviewed a draft of the manuscript, although I am responsible for any misconceptions or errors: Derek Ager, University College of Wales, Swansea; W. B. N. Berry, University of California, Berkeley; Martin D. Brasier, University of Oxford, Oxford; Preston Cloud, University of California, Santa Barbara; N. L. Gilinsky, Virginia Polytechnic Institute and State University, Blacksburg, Virginia; R. E. Grant, National Museum of Natural History, Washington, D. C.; Jane Gray, University of Oregon, Eugene; Anthony HaUam, University of Birmingham, Birmingham, England; Max Hecht, Queens College, Hushing, New York; Hermann Jaeger, Museum fiir Naturkunde der Humboldt-Universit~it zu Berlin; John F. Lance, Washington, D. C.; Rolf Ludvigsen, University of Toronto, Ontario; Digby J. McLaren, Geological Survey of Canada, Ottawa; A. A. Meyerhoff, Tulsa, Oklahoma; Norman D. Newell, American Museum of Natural History, New York City, New York; W. A. Oliver, Jr., U.S. Geological Survey, Washington, D. C.; A. R. Palmer, Geological Society of America, Boulder, Colorado; J. J. Sepkoski, Jr., University of Chicago, Chicago, Illinois; Peter M. Sheehan, Milwaukee Public Museum, Milwaukee, Wisconsin; Curt Teichert, University of Rochester, Rochester, N. Y.; O. H. Walliser, Iustitut fiir Geologic und Pal~iontologie, G6ttingen.
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30 (Organizers): Mollusks, Notes for a Short Course. - Univ. Tenn., Dept. Geological Sci., Studies in Geology, 13, 202-214. Teichert, C. (1986): Times of crisis in the evolution of the Cephalopoda. - Pal/iontologische Zeitschrift, 60, 227-243. Trelease, S. F. & Beath, O. A. (1949): Selenium, its geological occurrence and its biological effects in relation to botany, chemistry, agriculture, nutrition, and medicine. - The Champlain Printers, Burlington, Vermont, 292 pp. Valentine, J. W. (1969): Patterns of taxonomic and ecological structure of the shelf benthos during Phanerozoie time. - Palaeontology, 12, 684-709. Walliser, O. H. (1986): Towards a more critical approach to bio-events. - In: Walliser, O. H. (ed.): Global Bio-Events. - Lecture Notes in Earth Sciences, 8, 5-16; Springcr-Verlag. Wang Yu, Boucot, A. J., Rong Jia-yu & Yang Xue-chang (1984): Silurian and Devonian biogeography of China. - Geological Society of America Bulletin, 95, 265-279. Whafley, R. (1986): Biological events in the evolution of Mesozoic ostracoda. - In: Wa/liser, O. H. (ed.): Global Bio-Events. - Lecture Notes in Earth Sciences, 8, 257-265; Springer-Verlag. Whafley, R. (1988): Patterns of rates of evolution among Mesozoic Ostracoda. - In: Hanal, T., Ikeya, N. & Ishizaki, K. (eds.): Evolutionary biology of ostraeoda, Elsevier, 1021-1040. Wilde, P. & Berry, W. B. N. (1984): Destabilization of the oceanic density structure and its significance to marine "extinction" events. - Palaeogeography, Palaeoclimatology, Palaeoecology, 48, 143-162. Wilde, P., Berry, W. B. N., Quinby-Hunt, M. S., Orth, C. J., Ouintana, L. R. & Gilman, J. S. (1986): Iridium abundance across the Ordovician-Silurian stratotype. - Science, 233, 339-341.
BIOLOGICAL
SELECTIVITY
OF EXTINCTION
A contribution
U•?
to Proiect
GLO BAL BIO EVENTS
KITCHELL,Jennifer A. *)
Abstract: Selective survival across major extinction event horizons is both a bothersome puzzle and an opportunity to delimit the biologically interesting question of causality. Heritable differences in characters may have predictable consequences in terms of differential species survival. Differences in magnitude and intensity of extinction are insufficient to distinguish background from mass extinction regimes. Biological adaptations may establish links of causality between abnormal times of mass extinction and normal times of background extinction. A current hypothesis, developed from a comparison of extinction patterns among Late Cretaceous molluscs, is that biological adaptations of organisms, effective during normal times of Earth history, are ineffectual during times of crises. A counter example is provided by data from highlatitude laminated marine strata that preserve evidence of an actively exploited life-history strategy among Late Cretaceous phytoplankton. These data illustrate a causal dependency between a biological character selected for during times of background extinction and macroevolutionary survivorship during an unusual time of crisis.
PROGRESS
AND THE
SELECTIVITY
OF EXTINCTION
What is the relationship between progress and the selectivity of mass extinction? The question can be rephrased, are the survivors of a mass extinction "oetter" than the victims of the same mass extinction? It is true that surviving a mass extinction event prolongs the expected time to extinction. In a review of fitness measures, Cooper (1984) showed that expected time to extinction is the fundamental measure of fitness, all other measures being derived from it. In this sense, survivors are more "fit" (i.e. durable) than non-survivors. The issue, however, revolves around the question of whether or not directional trends in the history of life are powered by the selectivity associated with mass extinction bottlenecks. Heritable characters that enhanced survivability to one biotic crisis would predictably enhance the probability of survival to a recurrent perturbation. The Darwinian theory rejects any innate tendency toward progress. Mass extinction has been portrayed as an extrinsic sorting process that nullifies extrinsically generated progressive trends (e.g. Gould, 1985). It may (Gould & Calloway, 1980). Or it may not. McKinney (1986) has recently argued that the directional trend of increased diversity among erect unilamlnate bryozoan species represents not only an example of "progressive evolution" but an example of a directional trend unaffected (or even enhanced) by mass extinctions. The general question, ultimately, must escape an empirical resolution. The phenomenon does not fit into the mutually exclusive categories required of strong inference hypothesis testing. Examples of enhancement, ineffectiveness and neutrality are all expected by the theory. Evolutionary trends are not simple functions of time, but interactive functions of time, character, and environment. As discussed below, extinctionresistance does not accumulate merely with age.
*) Department of Geological Sciences and Museum of Paleontology, University of Michigan, Ann Arbor, MI 48109. U.S.A.
32
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Figure 2 : The metric of extinction is the probability (i.e. normalized for standing diversity) of extinction per Myr for the Phanerozoic stages of geologic time. The observed series is plotted in the dashed line and the predicted series (see Kitehell & Pena, 1984 for details of the analysis) in the solid line. Data are from Raup and Sepkoski's (1984) compilation of marine family records.
33 THE
PROBLEM
OF DISTINGUISHING
BACKGROUND
FROM
MASS EXTINCTION
In what way(s) are mass extinctions, interpreted both as crises in the history of life and a potential motive force in evolution, distinguishable from background extinctions, interpreted as the inevitably normal fate of all species? The traditional means of differentiating these two extinction regimes has relied on relative differences in magnitude, but magnitude may be assessed using a variety of measures. In the midnineteenth century, Phillips (1860) showed that diversity declined abruptly at the close of both the Paleozoie and Mesozoic eras, framing the Mesozoic "within deadly brackets of time" (McPhee, 1980). Raup and Sepkosld (1982) similarly identified principal mass extinction events over Phanerozoic time by relatively large and abrupt declines in standing (familial) diversity. Diversity decline, as a metric of mass extinction, however, is ambiguous. The temporal trajectory of diversity must decline whenever extinction rate exceeds origination rate. The implication of equating mass extinction with diversity decline is that extinction rate rises during these intervals, but this is not the necessary case. A decline may occur with no change in extinction rate whenever there is a sufficiently large drop in origination rate. The number of originations generally has exceeded the number of extinctions on a per stage basis, as shown in Fig. 1. which displays the relative rise and fall of originations and extinctions for the marine familial data of Raup and Sepkoski (1984). Exceptions include the Late Permian and Late Cretaceous, eonftrming Phillips' interpretation: here there are sharp rises in the number of extinctions, made more problematic, however, for the Late Cretaceous where the number of originations falls as abruptly. As a consequence of this ambiguity, most recent analyses of mass extinction have focused directly on the behavior of rate (or probability) of extinction. The metric of extinction rate is also potentially equivocal as a means of differentiating times of background from times of mass extinction. Raup and Sepkoski (1982), Van Valen (1984) and Kitehell and Pena (1984) showed, using different analytical methods, that the Phanerozoie record of marine familial extinctions is characterized by an overall decline(s) in rate. The observed pattern of Phanerozoic marine familial extinctions expressed as probabilities of extinction per Myr, normalized for standing diversity, is plotted in Fig. 2. The observed (in dashed line) is compared to that predicted (in solid line) by a covariance analysis, such an analysis being appropriate to highly autoeorrelated data (see Kitchell & Pena, 1984). As is apparent, the absolute magnitude of some mass extinction events, such as the well-recognized Late Cretaceous crisis, is lower than the magnitude of some earlier background rates. The Late Cretaceous peak, however, is significantly above the predicted curve. More recently, Ranp and Sepkosld (1984) replaced absolute magnitude as a measure of extinction intensity with the criterion of reversal from an increase in extinction intensity to a decrease (see also Kitchell & Estabrook, 1986). Such reversals, based on relative magnitude, define "peaks" of extinction intensity. Some of the peaks have absolutely high levels of extinction intensity as well, but this is not a necessary criterion, as is evident in Fig. 3. Peaks of local maxima vary in number depending on whether reversal is the only criterion of recognition, or whether peaks are evaluated relative to either mean extinction rate over time or an a priori criterion of absolute magnitude. Another approach has been to ask whether or not the distribution of magnitudes of extinction rate supports a separation of rates into two distinct regimes. Stigler (1987) found the distribution to support a continuum of extinction rates; mass extinctions represent the tail of a relatively smooth distribution. He reported that "we are not dealing with a single list of large catastrophes set off against a mundane background but with a gradual spectrum of extinction rates, from the mild to the more severe". Raup (1986) similarly concluded that there is a continuous range of variation. The dual classification of mass versus
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background extinction is not supported to date by the evidence of magnitude and intensity.
A PROPOSAL
FOR
DISTINGUISHING
BACKGROUND
FROM
MASS
EXTINCTION
The obvious problem with both absolute and relative magnitude as criteria that distinguish background from mass extinction regimes is that these criteria beg the question. Background extinction is expected to vary in intensity with time, and it would be a mistake to label the high end of aggregated but independent extinctions a mass extinction. Biotic crises in the history of life that attain mass extinction status must be (i) absolutely and (ii) relatively large in extent and intensity, and (iii) the consequence of the same (or cascading) causal mechanism(s). The equivocal nature of magnitude may be better resolved by an additional criterion, that of selective extinction. Differential survival to closely related causal factors, in conjunction with magnitude, better distinguishes mass extinction events from background extinction than any single measure of magnitude (Kitehell et al., 1986). A biotic crisis labeled a mass extinction should be supported by evidence of differential survival to a series of closely related causes whereas there is no such restriction on background extinction. The phenomenon of selective survival necessarily involves the biological characters of organisms, expanding the study of mass extinction beyond measures of magnitude to the more informative measure of differential effect. Although most analyses of extinction report only a measure of the intensity of extinction,
35 an account of the score between "victim" and surviving taxa, the more relevant query is, What decided the score? The research focus is shifted from analyses of individuals toward analyses of classes, namely surviving and non-surviving taxa, the biological properties held in common within each, and the interactive effects of these in the context of a "survival filter". A current opinion, however, is that "mass extinction is probably blind to the exquisite adaptations evolved for previous environments of normal times" (Gould, 1984). This opinion reflects the hypothesis that evolutionary processes operate at levels ('tiers') arranged in a nested hierarchy (Gould, 1985). Higher levels constrain the behavior of lower levels in nested hierarchies. Mass extinctions are described as occupying the highest tier status, thereby constraining macroevolutionary trends (at the second tier), both of which in turn constrain processes acting "at the ecological moment" of the first tier. Using this view of scale-dependent constraints, Gould (1984) has argued for "an irreducible randomness" to life's history. The examples that follow show - contrary to the prediction that mass extinctions are indifferent to biological adaptations - that a biological trait may instead determine a pattern of differential survival. These biological traits operate in the framework of the ecological moment. Such examples show that biological characters (of the frrst tier) may play a role in second- and third-tier level processes. As a consequence, these biological adaptations establish links of causality between normal times of background extinction and times of mass extinction.
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YEARS IN MILLIONS Figure 4 : The technique of plotting extinction-decay trajectories for simultaneously extant species (pseudocohorts) as a function of geologic time is used to identify periods of time when extinction intensity is high (slope steepens) relative jEo other periods of time, and to assess the degree of synehroneity of response across cohorts. The bold arrow identifies the 'peak' of extinction recognized as a global phenomenon in the analyses of Ranp and Sepkosld (1984, 1986).
36 LINKS
BETWEEN
NORMAL
(BACKGROUND)
TIMES
AND
TIMES
OF MASS
EXTINCTION Age
dependency:
An
example
Extinction intensity varies over Phanerozoic time. Any hypothesis that extinction is random with respect to absolute time is rejected by the non-uniform pattern of extinction intensity as a function of geologic time. This pattern is a robust feature of the history of life. A means of exzmlning this phenomenon is to follow the extinction-decay trajectories of species that originate simultaneously (true cohorts) or species that are simultaneously extant (pseudocohorts). Some periods of time are characterized by horizontal decay slopes, indicating no extinction within these cohorts whereas other periods of time are characterized by steep decay slopes, indicating increased extinction (e.g. for species, Fig. 9, Hoffman & KitcheU, 1984; for families, Fig. 3, Raup, 1986). The pattern becomes increasingly more continuous when species within a taxonomic group and paleoenvironment are examined. Fig. 4 illustrates the extinction-decay trajectories of species pseudocohorts for planktonic foraminifera of the tropical Atlantic and Pacific. The arrow marks the time (11.3 Myr BP) of a significant peak of extinction at the level of genus, recognized by Raup and Sepkoski (1986) from a compilation of 9250 extinct genera, and interpreted as a global disturbance. As is evident from the tack of a
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37 significant and simultaneous break in slope across cohorts at this time, these species are relatively insensitive to the perturbation. Another way of analyzing the continuous versus pulsed hypotheses of background and mass extinction is to examine the pattern of first and last appearances, arranged in order of the time of each species' last record. All (extinct) species from the same data set used to construct Fig. 4, as cohort trajectories, are shown in Fig. 5 as individual cases. The overall right-hand margin is not linear, its upward slope indicating an increased probability of extinction as one approaches the Recent. A character inherent in both survivorship and ordered data is age or duration time. Is extinction independent of the age of a taxon and, more specifically, are times of background and mass ex~nction similar or different with regard to this character? An interesting pattern of age-dependency emerged from a recent analysis of marine familial extinctions by Boyajian (1986). The supporting data are the differences in frequency distributions of duration times for taxa going extinct during mass extinctions (Fig. 6C) compared
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Figure 6 : Frequency distribution of species durations (A, B) compared to family durations (C, D). In A and B, the species are planktonic foraminifera; A represents the low- to mid-latitude species of the Pacific known from 111 DSDP sites; B represents the tropical species of the Pacific and Atlantic known from 116 DSDP sites; the shaded portions of histograms distinguish extinct species (with known durations) from extant (open histogram) species. C and D are the frequency distributions of ages of marine families becoming extinct during times of mass extinction (C) versus times of background extinction (D); redrawn from Boyajian (1986).
38 to taxa going extinct during background extinctions (Fig. 6D). During times of mass extinction, survival was more random with respect to age distributions than during background times. During background stages of time, the frequency distribution shifted toward young-aged families (Fig. 6D). Such a selectivity difference was considered by Boyajian to be consistent with distinguishing between background and mass extinction regimes on the basis of age-selectivity rather than magnitude alone. It is informative to compare these f, mily-tevel data with species-level data, on the frequency distributions of duration time. Fig. 6A shows the distributions of duration times for extinct species of low- to mid-latitude Pacific planktonic for-mlnlfera during the past 40 Myr of background extinction time. The frequency distributions of duration times for extinct (stippled) and extant (open) species of tropical Atlantic and Pacifle planktonic foraminifera over the same past 40 Myr are shown in Fig. 6B. Both distributions are similar in that frequency decreases with increasing duration. These distributions for species during background times, however, more closely resemble the familial distributions for mass extinctions. Survival analyses of species-level extinction patterns are remarkable in the degree to which they are ageindependent (e.g. Hoffman & Kitchell, 1984; Kitchell & Hoffman, in press). Such independence is interpreted to represent the consequences of many, unpredictable stresses (associated with background extinction) such that age does not provide a margin of safety in the way that experience is a s',ffeguard. One must not be misled, however, in drawing similar inferences at the family-level. The difference, of agedependency within familial data and age-independency within species data, is consistent with the two consequences of 'age'. At the species level, increased age indicates increased persistence time. The fit of the species-level data to a Poisson process indicates a lack of memory in the system: experience does not accumulate. The element of surprise sometimes associated with the lack of a decrease in extinction probability among older species is tied to the expectation that longer persistence time means, on average, greater success in meeting an increasing number of ehaUenges from a changing environment. The biological traits of older species are more 'tested' and thereby more extinction-resistant, according to this reasoning. Extinction-resistance associated with increasing age of a family, however, is quite a different expectation. Here the intermediaries are species-richness and breadth of ecology and geographic range of constituent species; age is a proxy measure. As a consequence, the finding of age-dependency lacks the element of surprise associated with age-independency of species. Such age-dependency of families is an intuitive prediction.
Geographic
r a n g e : An e x a m p l e
Species age or duration time is not a character that belongs to an individual within the species; it is an aggregate character of the species (as individual). The same holds for geographic range of a species. Assuming ceteris paribus, a straight-forward prediction is that the probability of extinction will decrease with increasing geographic range. Not all environments are affected equally by the same catastrophe. Such a positive relationship between evolutionary longevity and geographic range has been supported by studies of Early Tertiary volutid gastropods (e.g, Hansen, 1978, 1980). Jablonskl (1986) extended the question by asking whether geographic range was effective in aiding survivorship both preceding and during a mass extinction regime. Examining the distribution frequency of species longevities, he found that for both geographically widespread and geographically restricted molluscan species frequency decreases with increasing longevity. The modal values, however, are significantly different with widespread species having longer longevities, as predicted, during background time. During the Late Cretaceous mass extinction, endemic genera of both gastropods and bivalves had a significantly higher extinction rate than widespread
39 genera, again as expected. At the genus level, however, survival across the Late Cretaceous mass extinction horizon did not depend on the geographic range of constituent species.
Life history
strategy:
An e x a m p l e
What of heritable characters of organisms: to what extent may a biological, heritable trait of individual organisms causally influence a maeroevolutionary pattern of selective survival? A current hypothesis, developed from a comparison of extinction patterns among Late Cretaceous molluscs, separated into two classes on the basis of larval development, is that the adaptation of larval development, effective during normal times preceding the Late Cretaceous mass extinction, is ineffectual during the time of crisis (Jablonski, 1986). A counter example (KiteheU et al., 1986) of causal dependency between a biological character selected for during times of background extinction and macroevohifionary survivorship during an unusual time of crisis is provided by data of an actively exploited life-history strategy among Late Cretaceous phytoplankton. One of the best known selective features of the close Cretaceous mass extinction is the high intensity of extinction of marine plankton groups. Among Late Cretaceous marine organisms, plankton experienced severe levels of extinction. But, among plankton groups, there was differential extinction. For coecolithophorids, radiolaria, and foraminifera, the genetic extinction intensities exceed 70, 80, and 90%, respectively (Thlerstein, 1982). In sharp contrast, the current best estimate for diatoms is between 20-25% (Gombos, unpubl, ms; Harwood, 1986). As Tappan (1982) noted, "of the phytoplankton, coccolithophore, diatom and dinoflagellate pigment complexes are similar (all have chlorophyll a and c, and accessory photosynthetic pigments such as fucoxanthin), yet coccolithophores were nearly completely eliminated, dinoflagellates much less affected, and diatoms merely continued their rapid diversification across the boundary". This peculiarity is made even more pn~llng in that suppression of photosynthesis, as a cousequence of large dust loadings, is suggested by several lines of evidence (Alvarez et al., 1980, 1984; Smit & Kyte, 1984; Wolbaeh et al., 1985) as the proximate cause of the Late Cretaceous crisis. Yet the plankton group 1 e a s t susceptible to extinction (the centtie diatoms) are obligate photoautotrophs and thereby most susceptible to the putative mechanism. One line of evidence seems to contradict the other. Why did diatoms fail to experience the Late Cretaceous boundary to the same extent as other plankton groups? Kitchell et al. (1986) recently reported that this dissimilarity may be a predictable consequence of a dissimilarity of life history strategies. A well-preserved assemblage of Late Cretaceous diatoms was recovered from a high-latitude site within the Arctic Ocean. Sections of the core are laminated. Macrosampling even the laminated sections of the core showed a high proportion of diatom resting spores within art assemblage of planktonic diatoms (Fig. 7A). Microsampling individual laminae revealed, however, that alternating layers are comprised of resting spores (without vegetative cells; Fig. 7C) or vegetative cells (without resting spores; Fig. 7B). The laminae provide evidence of an actively exploited biological adaptation, that of a meroplanktonic life history of alternating planktonic stages (the vegetative and sexual phases) and non-planktonic stages (the resting spore phase). Studies of the ecological and physiological aspects of diatom resting spores have led to an understanding of resting spores as a population-level "survival strategy" (Fryxell, 1983). The formation of' resting spores is induced by locally unfavorable environmental conditions, including nutrient-depletion in oceanic upwelling systems. Paieoclimate modeling has shown that open-ocean upwelling would have been seasonally discontinuous at these high Arctic latitudes during the Late Cretaceous (Parrish & Curtis, 1982; Barron,
40 1985). The local environmental conditions, of nutrient depletion and high light levels, necessary for maintenance of a meroplanktonic life history today (Hargraves & French, 1983), were met seasonally in this environment during the Late Cretaceous. All diatom species, however, do not form resting spores. Consequently, both the presence of the character of spore-forming and its absence must be explained. In particular, families that evolved more recently than the Miocene, an approximate time of division between spore-forming and non-spore forming families (Simonsen, 1979), do not form resting spores. Instead, these families have the characteristic of
Figure 7 : Smear-slide samples of material from Arctic core F1-437. A, from macrosampling across laminae showing mixed vegetative cells and resting spores of Late Cretaceous centric diatoms; B, from microsampllng lnmlnae rich in vegetative cells; C, from mierosampling laminae rich in resting spores.
41
forming a physiologically resting cell (Hargraves & French, 1983). A comparison of these alternate strategies has shown that the more primitive trait entails both an increased energetic cost and a reduced number of potential cell divisions. Retention of the primitive trait in modern species within older genera has been given both a genetic and ecologic explanation: spore formation may be genetically linked to other indispensable aspects of cell metabolism; there may be selective advantages of retention of the strategy (Hargraves & French, 1983); or unfavorable environmental conditions may be sufficiently frequent on an ecological time scale that many phytoplankton have retained the primitive life history strategy. Equatorial upwelling, for example, is driven by the poleward divergence of equatorial surface waters, a more continuous phenomenon. Correlative Maastrichtian diatom assemblages within the subtropics are conspicuously devoid of resting spores (Gresham, 1985), confirming the modern-day observation that discontinuity of the favorableness of the photic zone is the probable causal factor linking the life history trait to the local environment. The biological property of a meroplanktouic life history is an adaptation (Kitchell et al., 1986). As Sober (1984) has emphasized, the term is meant to imply (i) an historical process of selection for possessing this (heritable) trait, and (ii) that the trait contributes to survival in the local environment. Resting spores contribute to adaptedness (Sober, 1984) by providing a mechanism of survival during times when the conditions of the planktonic environment exceed either the physiological tolerances (Sandgren, 1983) or competitive abilkies (Garrison, 1980) of the vegetative cells. This adaptation resulted in what is termed "effect macroevolution", an evolutionary process driven by upward causation from the ecological realm and the level of the individual organism to the macroevolutionary realm of differential survival and extinction. As Kitchell et al. (1986) concluded, "the adaptation of a meroplanktouic llfe cycle among populations of centric diatoms in the Late Cretaceous increased the probability of survivorship during normal times of local environmental deterioration, as it does today. This innovation of a meroplanktonic life history may have been differentially enhanced during the Late Cretaceous mass-extinction regime". The chance interaction of this locally-adapted life-history trait and the close Cretaceous extinction mechanism(s) turn an otherwise enigmatic pattern of differential survival among marine plankton into a predictable pattern. In so doing, the postulate that mass extinction is indifferent to organismal attributes is qualified. There is, in this example, a causal dependency between evolutionary survivorship during a time of global perturbation and a biological character selectively relevant in normal times. As a consequence, the differential selectivity of a mass extinction event has been reduced, in terms of explanation, to the organismal level of selection and adaptation.
S UMMARY
Differential survival across major extinction boundaries is a means of understanding causality. Data on the differential magnitude of extinction are insufficient both to distinguish mass extinction from background extinction and to address the context-dependent relationship of cause and differential effect. In the case of Late Cretaceous phytoplankton, the interaction of a proximate ecological adaptation, selected for in a local environment, and the postulated extinction mechanism resulted in the fortuitous but macroevolutionary cousecluence of differential survival.
42 A CKNOWLEDGEMENTS
This research was supported in part by grant BSR-8605310 from the National Science Foundation.
REFERENCES
Alvarez, L. W., Alvarez, W., Asaro, F. & Michel, H. V. (1980): Extraterrestrial cause for the CretaceousTertiary extinction. - Science, 208, 1095-1108. Alvarez, W., Alvarez, L.W., Asaro, F. & Michel, H. V. (1984): The end of the Cretaceous: sharp boundary or gradual transition? - Science, 223, 1183-1186. Barron, E. J. (1985): Numerical climate modeling, a frontier in petroleum source rock prediction: results based on Cretaceous simulations. - American Association of Petroleum Geologists Bull., 69, 448-459. Boyajian, G. F. (1986): Phanerozoic trends in background extinction: consequence of an aging fauna. Geology, 14, 955-958. Cooper, W. S. (1984): Expected time to extinction and the concept of fundamental fitness. - Journal of Theoretical Biology, 107, 603-629. FryxeU, G. A. (1983): Survival Strategies of the Algae. - Cambridge University Press, 144 p. Garrison, D. L. (1980): Monterey Bay phytoplankton I. Seasonal cycles of phytoplankton assemblages. Journal of Plankton Research, 1, 241-265. Gombos, A. M., Jr. (unpubl. ms): A review of the record of Late Cretaceous diatom extinctions. Gould, S. J. (1984): The cosmic dance of Siva. - Natural History, 8, 14-19. Gould, S. J. (1985): The paradox of the first tier: an agenda for paleobiology. - Paleobiology, 11, 2-12. Gould, S. J. & Calloway, C. B. (1980): Clams and brachiopods - ships that pass in the night. - Paleobiology, 6, 383-396. Gresham, C. W. (1985): Cretaceous and Paleocene siliceous phytoplankton assemblages from DSDP sites 216, 214 and 208 in the Pacific and Indian Oceans. - Univ. of Wisconsin-Madison M.S. Thesis, 233 p. Hansen, T. A. (1978): Larval dispersal and species longevity in Lower Tertiary gastropods. - Science, 199, 885-887. Hansen, T. A. (1980): Influence of larval dispersal and geographic distribution on species longevity in neogastropods. - Paleobiology, 6, 193-207. Hargraves, P. E. & French, F. W. (1983): Diatom resting spores: significance and strategies. - In: Fryxell, G. A. (ed.): Survival Strategies of the Algae. - Cambridge University Press, Cambridge, 49-68. Harwood, D. M. (1986): Upper Cretaceous and lower Paleocene diatom and silicoflageUate biostratigraphy of Seymour Island, eastern Antarctic Penin.~ula. - In: Feldmann, R. M. & Woodburn, M. O. (eds.): Geological Society of America Memoir Series. Hoffman, A. & Kitchell, J. A. (1984): Evolution in a pelagic planktic system: a paleobiologic test of models of multispecies evolution. - Paleobiology, 10, 9-33. Jablonski, D. (1986): Background and mass extinctions: the alternation of maeroevolutionary regimes. Science, 231, 129-133. KitcheU, J. A., Clark, D. L. & Gombos, A. M., Jr. (1986): Biological selectivity of extinction: a link between background and mass extinction. - Palaios, 1, 504-511. KitcheU, J. A. & Estabrook, G. (1986): Was there 26-myr periodicity of extinctions? - Nature, 321, 534-535. Kitchell, J. A. & Hoffman, A. (in press): Rates of origination and extinction: age-, time-, and diversitydependence. - In: Stenseth, N. (ed.): Coevolution in Ecosystems. - Cambridge University Press, Cambridge. Kitchell, J. A. & Pena, D. (1984): Periodicity of extinctions in the geologic past: deterministic versus stochastic explanations. - Science, 226, 689-692. McKinney, F. K. (1986): Evolution of erect marine bryozoan faunas: repeated success of unilamlnate species. - American Naturalist, 128, 795-809. McPhee, J. (1980): Basin and Range. - Farrar, Straus, Giroux, New York, 216 p. Parrish, J. T. & Curtis, R. L. (1982): Atmospheric cLrculation, upwelling, and organic-rich rocks in the Mesozoic and Cenozoic eras. - Paleogeography, Palaeoclimatology, Paleoecology, 40, 31-66. Phillips, J. (1860): Life on the Earth: Its Origin and Succession. - Cambridge and London. Raup, D. M. (1986): Biological extinction in Earth history. - Science, 231, 1528-1533.
43 Raup, D. M. & Sepkoski, J. J., Jr. (1982): Mass extinctions in the marine fossil record. - Science, 215, 15011503. Raup, D. M. & Sepkoski, J. J., Jr. (1984): Periodicities of extinctions in the geologic past. - Proc. of the National Academy of Sciences USA, 81, 801-805. Raup, D. M. & Sepkoski, J. J., Jr. (1986): Periodic extinction of families and genera. - Science, 231,833-836. Sandgren, C. D. (1983): Survival strategies of chrysosphycean flagellates: reproduction and the formation of resistant resting cysts. - In: FryxelL G. (ed.): Survival Strategies of the Algae. - Cambridge Univ. Press, Cambridge, 23-48. Simonsen, R. (1979): The diatom system: ideas on phylogeny. - Bacillaria, 2, 9-71. Smit, J. & Kyte, F. T. (1984): Siderophile-rich magnetic spheroids from the Cretaceous/Tertiary boundary in Umbria, Italy. - Nature, 310, 403-405. Sober, E. (1984): The Nature of Selection. - MIT Press, Cambridge, Mass., 383 p. Stigler, S. M. (1987): Testing hypotheses or fitting models? Another look at mass extinctions. - In: Nitecki, M. (ed.): Neutral Models in Biology. - Oxford Univ. Press. Tappan, H. (1982): Extinction or survival: selectivity and causes of Phanerozoic crises. - In: Silver, L. T. & Schuh, P. H. (eds.): Geological Implications of Impacts of Large Asteroids and Comets on the Earth. - Geol. Soc. of America, Spec. Pap., 190, 265-276. Thierstein, H. R. (1982): Terminal Cretaceous plankton extinctions: a critical assessment. - In: Silver, L. T. & Schuh, P. H. (eds.): Geological Implications of Large Asteroids and Comets on the Earth. - Geol. Soc. of America, Spec. Pap., 190, 385-399. Van Valen, L. M. (1984): A resetting of Phanerozoic community evolution. - Nature, 307, 50-52. Wolbach, W. S., Lewis, R. S. & Anders, E. (1985): Cretaceous extinctions: evidence for wildfires and search for meteoritic material. - Science, 230, 167-170.
A MULTI-CAUSAL INCREASE
IN
MODEL
TRACE
OF MASS
METALS
IN
LEARY, Paul N. *) & RAMPINO,Michael R. **)
EXTINCTIONS: OCEANS
THE
I•
A contribution to Proiec!
GLO BA L BIO EVENTS
Abstract: Mass extinctions of life may be the net result of multiple, related causes. The correlation among
mass extinctions, large-body impacts, ocean anoxic events, and flood-basalt volcanism suggests some common thread of cause and effect. Links between these phenomena have been proposed, and although the interactions are most likely complex, one common effect might be an increase in trace metals in the earth's surface systems. Increased trace metals could lead to severe effects on the biota, and would exacerbate the general environmental crises at times of comet or asteroid impacts. INTRODUCTION
Mass extinctions of life are most likely multieausal events. Bolide impacts may be the primary triggers of major mass extinctions (Alvarez et al., 1980; Alvarez, 1986), but although impacts may be the initiators of large-scale perturbations in the ocean/atmosphere systems, secondary effects may be important as the proximate causes of extinctions among various groups of organisms (Kauffman, 1986, 1988). The recognition of a worldwide iridium anomaly and shocked mineral grains at the Cretaceous/Tertiary (K/T) boundary, coincident with mass extinctions, has increased the plausibility of an extraterrestrial cause of mass extinctions in general (Alvarez, 1986). Subsequent papers have argued that increased volcanism may play a part in extinction events (e.g. Officer & Drake, 1985), and that flood basalt volcanism may be in some way related to the large body impacts (Rampino & Stothers, 19°o8). The postulated effects of a bolide collision with the earth have concentrated on the likely physical and chemical stresses that would have been imposed on the biota. These include darkness and cold from dust and smoke clouds, nitric acid rain (Prinn & Fegley, 1987), the release of potentially toxic substances (Hsu, 1986), trace metal pollution from the bolide (Eriekson & Dickson, 1987) or from volcanism (Vogt, 1972), carbon dioxide or water vapor greenhouse warming (Emiliani et al., 1981; Caldeira et al., 1988), and destructive tsunamis (Bourgeois et al., 1988). The current impact scenario involves multiple impacts during comet showers, which may be periodic or quasi-periodic (Rampino & Stothers, 1984), leading to stepwise extinctions over a period of a few million years (Hut et al., 1987). It has also been noted that major oceanographic changes, such as the development of so-called Ocean Anoxic Events (OAEs) (although probably not representing true anoxic conditions) (Schlanger & Jenkyns, 1976), seem to correlate with many mass extinction events (Kauffman, 1986, 1988). OAEs are marked by the widespread deposition of sediments rich in organic carbon, and have been proposed as an independent cause of the marine extinctions (e.g., Hallam, 1986; Kulmt et al., 1986; Hallock & Schlager, 1986; Roth, 1987; Jarvis et al., 1988). If mass extinctions are triggered by extraterrestrial impacts, and they are also correlated with flood basalt volcanism and OAEs, then it is reasonable to ask whether conditions favorable for the development of OAEs are somehow related to impacts and/or volcanism, or if the occurrence of impacts or enhanced *) **)
Department of Geological Sciences, Polytechnic South West, Drake Circus, Plymouth, Devon PIA 8AA, United Kingdom Earth Systems Group, Department of Applied Science, New York University, New York, NY 10003, U.S.A.
46 volcanism during times of oxygen-depleted oceans leads to secondary effects which tend to increase environmental stress on marine life. Here we examine some of the correlations between impact events, flood basalt volcanism, mass extinctions and oxygen-depleted oceans, and discuss a possible multi-causal model of mass extinctions. Increase in trace metals from a number of sources provide one possible mechanism for selective mass extinctions in the marine realm.
OCEAN
ANOXIC
EVENTS
Ocean Anoxic Events (OAE) are large-scale oceanographic perturbations characterized by abnormally high rates of organic carbon deposition and/or preservation in the sedimentary record (Schlanger & Jenkyns, 1976; Arthur et al., 1987). OAEs cut across major facies boundaries, over a wide range of paleobathymetry-intermediate and deeper water environments are marked by dark-colored, laminated, organic-rich mudstones. Benthonic faunas are usually impoverished or absent in these sediments, which locally contain high concentrations of planktonic microfossils, especially radiolarians. Shallow shelf environments are also marked by organic-rich sediments, and shelf carbonates deposited during OAEs show abnormally high carbon stable-isotope signatures (Kauffman, 1986, 1988). These features have been interpreted as indicating that significant parts of the world Ocean were oxygen deficient, and OAEs apparently reach a maximum stage with a rising and expanding oxygen minimum zone (Jarvis et al., 1988). OAEs of variable intensity and areal distribution are recognizable in the geologic record throughout the Paleozoic and Mesozoic Eras; Jurassic and Cretaceous OAEs have been especially well-documented from sections on land and Deep-Sea Drilling sites. Table 1 lists the major OAEs of the last 250 million years compiled from various sources. The duration of OAEs is quite variable; for example, the Cenomanian/ Turonian OAE lasted about 500.000 to 800.000 years (Leary, 1987; Jarvis et al., 1988), and the Lower Toarcian OAE lasted about 500.000 years. The onset of the C/T Boundary event seems to have been very rapid and nearly synchronous at all localities (Kauffman, 1986, 1988; Arthur et al., 1987). During the Aptian to Middle Albian, by contrast, widespread conditions of oxygen depletion occurred intermittently over a total period of about 15 million years, producing rhythmically bedded sequences of black shales and carbonates, which show Milankovitch periodicities (Herbert & Fischer, 1986). A number of studies suggest that the duration o f anoxic periods was typically a few thousand years to a few tens of thousands of years, with rapid transitions in the order of a few thousand years or less. Sediments that apparently accumulated under conditions of low dissolved oxygen are interbedded with bioturbated, normally oxygenated sediments. At peak times of OAEs, the rapidly rising oxygen minimum zone can be traced through the selective extinctions of marine plankton that inhabited decreasing depth intervals (Hart & Ball, 1986; Jarvis et al., 1988). It has been shown for the Cenomanian-Turonian OAE event, for example, that the deeper dwelling plankton were affected before the surface-dwelling species (Jarvis et al., 1988). OAEs have been attributed to a number of factors including changes in ocean circulation patterns and water temperatures, periods of transgression, increases in nutrient supply and productivity, changes in bottom water salinity, influx of organic matter from terrigenous sources, and the enhanced preservation of organic matter due to anoxia and/or rapid burial (see Meyers & Mitterer, 1986). Models of OAEs have thus far relied largely on ocean-derived causes (e.g., Wilde & Berry, 1984, 1986; Kulmt et al., 1986; Summerhayes, 1987; Jarvis et al., 1988). Sarmiento et al. (1988), for example, suggest that individual anoxic events were too brief to involve long-term controls on climate and ocean circulation, such as continental drift or changes in atmospheric CO 2. Nitrate and phosphate residence times in the ocean also appear to be too long to control periods of anoxia. Thus, Sarmiento et al. (1988) proposed that internal features of the
47 ocean-climate system, namely deep-water formation and the consumption of nutrients in the source region of deep waters. In their model, a shift towards greater consumption of nutrients in source regions, or reduced deep-water formation would promote oxygen depletion. De Boer (1983) presented a complex integrated atmosphere/ocean model of OAEs connected to the acceleration of sea-floor spreading and sea-level rise, with related atmospheric and oceanic changes. Sheridan (1987) and Roth (1987) have also discussed OAEs in the context of global tectonic pulsations, correlating with times of rapid sea-floor spreading and increased volcanism, leading to increased atmospheric CO2 and warm climate, marine transgression, and warm stagnant bottom waters. Hallam (1984) supports a correlation between anoxic events and global transgressions. Kauffman (1986, 1988) proposed that OAEs might be related to catastrophic ocean turnover events, and mixing of anoxic waters throughout the water column, possibly caused by oceanic bolide impacts (see also Sandberg et al., 1988). Several models of OAEs have as their central feature high surface water productivity. Modern ocean productivity is limited primarily by the availability of nitrates, phosphates and trace metals, and to some extent temperature, light levels, salinity, and oxygen levels. Jarvis et al. (1988), for example, proposed that the C/T Boundary event involved rapid upwelling and ocean turnover, increasing the normally low ocean productivity, and leading to deposition of radiolarian bearing, organic-rich muds in deeper water areas. By contrast, Bralower and Thierstein (1984) have calculated low ocean productivity for the mid-Cretaceous OAE. They proposed that the primary cause of the OAE was extremely low rates of deep-water renewal, leading to anoxia. As mentioned above, the recent box model studies by Sarmiento et al. (1988) suggest that reduced deep-ocean oxygen levels can be induced by high productivity in deep-water source regions, and/or by stagnation of horizontal circulation cells. The primary cause (or causes) of OAEs is thus still a matter of considerable debate. Certain trace metals may also act as effective limiting agents on phytoplankton productivity (e.g., Fe, Co, Mn, Mg, Mo, Se) (Martin & Fitzwater, 1988; Martin & Gordon, 1988). A model of OAE development involving an increased influx of trace metals into the ocean might explain initial periods of increased productivity, and eventual dieback and extinctions as metal concentrations reached levels toxic to marine organisms (Vogt, 1972; Hsu et al., 1982; Erickson & Dickson, 1987).
MASS
EXTINCTIONS
AND
OCEAN
ANOXIC
EVENTS
Recognized Oceanic Anoxic Events during the Mesozoic show a good correlation with the marine mass extinction record (Ranp & Sepkoski, 1984) (Table 1). Widespread OAEs and mass extinctions occur at the Cenomanian/Turonian (C/T) boundary, the Valanginian/Hanterivian (minor), in the Aptian, the Tithonian (Kimmeridgian), CaUovian/Oxfordian, the Pliensbachian/Lower Toarcian, Lower Sinemurian (minor), and the end Permian (Zechstein). The end-Triassic extinctions are also associated with a less widespread blackshale event on the European epi-continental shelf (Schger & Fois-Erickson, 1986). A number of these intervals are also marked by extinctions of non-marine tetrapods (Benton, 1987; Rampino, 1988), indicating that the changes in the ocean were approximately coincident with crises in terrestrial environments. Evolutionary changes in planktonic organisms also coincide with the peaks of anoxic conditions and biogenic silica sedimentation (Thurow & Kuhnt, 1986; Roth, 1987). Calcitic benthic foraminiferal diversity is reduced in the Toareian, Oxfordian, Kimmeridgian, and the C/T boundary, leaving only impoverished diminutive, agglutinated faunas (Brasier & Young, 1988). A euxinic shaly facies is also widespread at the Frasnian-Fzmmenian extinction. Geldsetzer et al. (1988) suggest that the F/F anoxic event represents a sudden flooding of the shelf margin by an "old" anoxic water mass in a major ocean turnover event. The
48 Frasnian/Fammenian event seems to have been one of recurrent anoxia (Hladil et al., 1986). Ocean turnover events might bring anoxic waters into the photic zone, which could lead to selective extinctions among ocean plankton as seen, for example, at the C/T boundary (Jarvis et al., 1988), and would have affected other organisms such as reef corals (Halloek & Sehlager, 1986). Several major reef diebacks occur during periods of apparent oxygen deficiency and nutrient excess in the oceans (e.g., the Toarcian, Mid-Cretaceous, and in the Frasnian/Fammenian). FLOOD
BASALT
ERUPTIONS
Flood basalt eruptions involve large accumulations of basaltic magma (up to about 2 million cubic kilometers), with the bulk erupted intermittently over a peak period ranging from about 500.000 to 2 million years (Jaeger et al., 1989). Rampino and Stothers (1988) have recently compiled data on flood basalt eruptions, and have shown a good correlation between flood basalts and mass extinction events during the last 250 million years. For example, during the Mesozoic, the Aptian extinctions may coincide with the Rajmahal Traps of India, the Tithonian extinction event correlates with the eruption of the Serra Geral (South America) and the coeval Southwest African Basalts, the CaUovian/Oxfordian event may correlate roughly with the Antarctic Basalts, the Plieusbachian event correlates with the Karroo Basalts of South Africa, the Late Triassic extinctions seem to correlate with the eruption of the Eastern North American Basalts, and the Late Permian extinctions with the Siberian Traps (Table 1). Most workers have related flood basalts to internal perturbations of the earth's mantle possibly associated with the formation of hotspots and mantle plumes (Morgan, 1981). A plausible model connects
Table 1 : Correlation of Mesozoic Anoxic Events, Mass Extinctions and Initiation Dates of Flood Basalt Eruptions (all dates in Myr BP). Anoxic Event
Mass Extinction
Continental Flood Basalt
---
*K/T
65-+- 1
Deccan Traps Brito-Arctic
*C f r Aptian
91 __.0 110 +-3
None known Rajmahal Traps 110 - 5
66_ 2 62 -+3
CenomanianfI'uronian Aptian/Lower Albian
91 -+0 110 -+3
Valanginian/Hauterivian (minor) KimmeridgianfI'ithonian
119 -+5 137 -+7
Tithonian
137 ---7
Serra Geral 135 --.5 Southwest Africa
Callovian/Oxfordian
148 _+8
*Callovian
157 ---5
Antarctic
170 - 5
Pliensbachian/ Lower Toarcian
- 190
Pliensbachian 191 -+3
Karroo
190 ---5
Valanginian 119 --+5
Lower Sinemurian (minor) 196 -- 2 Norian black shale 211 -+8
Rhaetian/Norian 211 -+8
East. N. America 200 ---5
Tatarian(Zechstein)
*Dzulfian/Guadelupian249-4
Siberian Traps
249-+4
250---10
*Direct evidence of impacts, some still questionable. Dates of anoxic events and extinctions are averages of 3 recent timescales. (Refs. A n o x i c e v e n t s : Sch/ifer & Fois-Erickson, 1986; Roth, 1987; Summerhayes 1987; Jenkyns, 1988. E x t i n c t i o n s : Raup & Sepkoski, 1984. F 1 o o d B a s a 1 t s : Rampino and Stothers, 1988)
49 flood basalts and hotspot activity with large-body impacts (Rampino, 1987). Volcanic effects on the atmosphere from large flood basalt eruptions may have contributed to environmental changes at the times of some mass extinction (Stothers et al., 1986; Rampino et al., 1988). These atmospheric perturbations could have led to further climatic instabilities that affected ocean circulation and oxygenation of bottom waters, leading to and/or intensifying the development of OAEs.
POSSIBLE
CONNECTIONS
BETWEEN
OAES
AND
IMPACTS
Several of the geological stages associated with OAEs and mass extinctions are marked by evidence of possible impacts. Five iridium anomalies have been found at the Cenomanian/Turonian boundary in Colorado associated with elevated concentrations of Mn, Fe, Co, Ni, As, Sc, Ti, Cr, Pt and Au; four have a composition suggesting a deep mantle (hotspot) source, but the fifth and largest spike has a different makeup suggesting the possibility of an extraterrestrial source (Orth et al., 1988; Kauffman et al., 1988). In Europe, iridium has been recorded from the C/T boundary in Italy (Wetzel et al., 1981), notably in the Bonarelli horizon (Asaro, pers. comm.). Recently, Brochwicz-Lewinski et al. (1988a) reported evidence of iridium and shocked quartz at the C/T boundary in southern Poland. Iridium and shocked quartz have also been reported at the Mid-Upper Jurassic Boundary (Callovian/Oxfordian) in Polish and Spanish sections (Brochwicz-Lewinski et al., 1988b). However, these latter studies have yet to be rechecked and evaluated critically. These recent discoveries support a general connection between large-body impacts and mass extinctions (Rampino & Stothers, 1984; Hut et al., 1987), and the correlations give credence to the idea that OAEs might be related in some way to impacts as well. Impacts (and related aftereffects) could cause widespread climatic destabilization, such as seen in detailed sections at the C/T boundary (Kauffman, 1988), and lead to an intensification of fluctuations related to the normal Milankovitch cycles of climatic change. The unstable climate system might lead to oceanographic perturbations that could cause changes in deep-water formation and oxygenation, and the eventual production and overturn of anoxic waters. In one scenario, multiple impacts in the oceans could have triggered ocean turnover events through tsunamis and/or major perturbations of climate (Kauffman, 1986, 1988). These overturns of anoxic waters could have had further effects on marine biota and climate, and such ocean impacts may terminate (at least temporarily) anoxic ocean conditions by mixing deep and surface waters.
TRACE
METALS
AND
OAES
Good correlations between fluxes of organic carbon and various trace metals (A1, Cu, Fe, Mn, Ni, Pb, V and Zn) provide evidence that the settling of particulate organic matter regulates the sedimentation of these and other trace elements (Sigg et al., 1987; Moore & Dymond, 1988). The flux of organic matter to the deep oceans during OAEs might act to scavenge trace metals from the ocean surface; these trace elements may then be released by the partial destruction of the organic material in the water column. In anoxic waters, oxygen is initially replaced by nitrate and nitrite as the oxidant, with ammonia being the reduced nitrogen end product (Wilde & Berry, 1986). Trace metals would build up in such waters until sufficient ammonia (as ammonium ions) was available to produce soluble metal amines. As anoxicity increases, sulfate becomes the favored oxidant; dissociated H2S would then react with many of the released metals to form particulate sulfides, removing the metals from solution (Wilde & Berry, 1986). De Graciansky et al. (1986) report that Fe, Pb, Cu, U and other trace elements are commonly accumulated in
50 anoxic layers, probably as insoluble sulfides. Iron and manganese-rich deposits are associated with the Early Toarcian organic-rich shales (Jenkyns, 1988), and Mn-carbonates were deposited during the C/T boundary event (Frakes & Bolton, 1984). The C/T boundary black shales in southern Europe and North Africa show a great enrichment in trace metals including Ag, Ba, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Sb, Sr, V and Zn (Brumsack, 1986). All of these species form stable sulfides or are strongly associated with organic matter, and pronounced maxima of some trace metals (especially Mn and Co) are associated with the oxygen deficient waters of the present oxygen minimum zone in the eastern North Pacific (Jones & Murray, 1985; Burton & Statham, 1988). Ocean turnover events during anoxia could bring oxygen depleted waters and trace metals into the photic zone. In relatively low concentrations, trace metals might have the effect of stimulating productivity (Apte & Howard, 1986), but at higher concentrations trace metals could lead to poisoning of ocean plankton (Erickson & Dickson, 1987). The association of rising oxygen minima and possible turnover of anoxic waters during peak times of OAEs, periods of fluctuating productivity, and mass extinction events could therefore come through a link involving trace metal pollution of surface waters. Mass mortalities can also be caused by toxic blooms of dinoflagellates (Noe-Nygaard et al., 1987). Recent studies have shown that certain trace elements essential for particular types of dinoflagellates, such as cobalt and arsenic might be a factor in causing such toxic blooms, and arsenic seems to be especially soluble in acidic and reducing waters (Apte & Howard, 1986).
SOURCES
OF TRACE
METALS
The above discussion suggests that trace metal enrichment detected at bioevent boundaries can thus come from a variety of sources. These include: 1) E x t r a t e r r e s t r i a 1 : Erickson and Dickson (1987) have calculated the input of metals to the ocean from a 10 km bolide impact, such as hypothesized for the K ~ boundary. They ftud that the mass of certain elements such as Fe, Co, Mn, AI, Zn, Ni, Ge, Cr, Su, Cd, Pb, Ag, Cu, Hg and Se are comparable or greater than the world ocean burden, and that the concentrations of some elements such as Cu, Ni and Pb might reach levels lethal to open-ocean phytoplankton and other organisms. Metals from atmospheric fallout are known to be especially concentrated in the sea-surface microlayer (Hardy et al., 1985). Trace metal enrichments at other mass extinction/OAE boundaries may also be related to bolide impacts (Kauffman, 1988). 2) V o 1 c a n i s m : The Cenomanian/Turonianboundary is marked by evidence of violent volcanism in the form of numerous bentonite layers in the continental interior of North America (Kauffman, 1986, 1988; Elder, 1988), but none are enriched in trace metals or correlated with mass extinction steps (Orth et al., 1988). Possible votcanogenic sediments occur in the Plenus Marls in Great Britain, and in NW Germany (Pacey, 1984; Hambach, 1988). A large literature exists on the occurrence of various trace metals in volcanic emanations, including Ag, As, Au, Cu, Cd, Bi, Fe, Mn, Mo, Sn, Sb, Pb, V, Zn (see e.g., Angus & Davis, 1976; Buat-Menard & Arnold, 1978; Menyialov & Nikitin, 1977; White & Waring, 1962). Thus, an explosive volcanic source for some trace metals is possible. Hotspot and possibly impact-induced volcanism may be important sources of trace metals. As pointed out, flood basalt volcanism is also correlated with a number of mass extinctions/ocean anoxic events (Rampino & Stothers, 1988). Trace-metal rich effluents from basaltic volcanism have been reported (ZoUer et al., 1983), and these include high levels of iridium, so that hotspot volcanism may be partly responsible for some iridium anomalies detected in the geologic record (Orth et al., 1988).
51 3) G 1 o b a 1 w i 1 d f i r e s : Wolbach et al. (1988) have reported evidence of global wildfires at the Kfr boundary. The fires may also have ignited fossil carbon (oil, coal, black shales). Some terrestrial plants are known to be enriched in Ir and other trace metals (Valente et al., 1982), and these could be released into the atmosphere and surface ocean during fires. Combustion of organic-rich rocks might add further trace metals to the environment (Cisowsld, 1988). Therefore, impacts at other mass extinction boundaries may have led to fire-induced trace metals. 4) A e i d r a i n : Prima and Fegley (1987) have proposed that severe acid rain caused by large body impacts could mobilize trace metals from soils, and these would be carried into the oceans in river runoff. The increase in acidity in terrestrial and marine environments could significantly increase the mobility and residence times of various trace elements. Destruction of vegetation could supply increased organic material, possibly enriched in trace metals, to the oceans, where it might contribute to reducing the oxygen content of deep waters. Eruption of basaltic magma would also release acids in the form of HC1 and H2SO4; these would also act to produce acid rain which would further contribute to mobilization of trace metals from terrestrial environments. 5) O c e a n W a t e r s : Upwelling of anoxic and partially oxic deep waters could bring increased trace metals into the surface ocean. Wilde and Berry (1986) suggest that waters from the tapper anoxic and sub-oxic zones would carry trace metals and ammonia/trace-metal complexes into the photic zone. Furthermore, turbulent mixing of the anoxic and oxic waters during ocean turnover events could lead to i, remobilization of trace metals (Holmes, 1986; De Luca Rebello et al., 1986). The OAEs themselves may therefore be a significant source of trace metals to the surface ocean. These data suggest that impact-related, volcanic, and oceanic sources are plausible mechanisms for high inputs of trace elements into the atmosphere/surface-ocean system. In impact and volcanic scenarios, metals would be deposited in both marine and terrestrial environments. Trace-metal enrichment might contribute to periods of high surface water productivity, and it might account for a temporary rise in the oxygen minimum zone during peak times of Ocean Anoxic Events. Trace metal poisonifig from these various sources may account in part for mass extinction events. Metal-biotic interactions are known to be complex (Erickson & Dickson, 1988), with variable levels of tolerance at different concentrations and durations of exposure, and various synergistic relationships among metal toxius. Certain organisms are more susceptible to high metal concentrations (e.g., diatoms). Tolerance also depends upon the stage of development of the organisms involved; juveniles and individuals undergoing gametogenesis are apparently more vulnerable to trace metal poisoning. It is thus possible that continued release of trace metals over periods of several hundred thousand years by volcanism, increased weathering and/or ocean-turnover events contributed to the "Strangelove Ocean" conditions of extremely low surface water productivity that persisted for up to 500.000 years at the K/T boundary and perhaps at other mass extinction boundaries (Hsu, 1986; Zachos & Arthur, 1986).
CONCLUSIONS
The correlation among mass extinctions, bolide impacts, ocean anoxic events, and flood basalt volcanism suggests that these major geologic events may be causally related. The complexity of mass extinctions, and the environmental perturbations that seem to accompany the extinctions, supports the idea that extinctions are multi-causal phenomena. Models of mass extinctions must take into account the possible relationships and interactions among the numerous components of the coupled ocean/atmosphere systems. These would
52 all be affected by large impact events, and the perturbations caused by impacts on the land and in the ocean would reverberate throughout the Earth's systems. A number of possible inter-related phenomena with significant feedbacks come into play during mass extinction events. Increased trace metals in the surface ocean are suggested here as one possible common factor among the various geologic events that occur at times of mass extinctions. A multi-causal model of mass extinctions, involving the effects of impacts, increased volcanism, and major changes in ocean chemistry has the advantage of explaining the correlations at mass extinction boundaries, and the complexity of interactions expected at these times of global crisis.
A CKNOWLEDGEMENTS
We thank K. G. Caldeira, U. Hambach, M. Hart, E. G. Kauffman, R. B. Stothers, O. H. WaUiser and J. C. Zachos for helpful discussions. MRR was partially supported by the Center for Global Habitability at Columbia University.
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CONSIDERATIONS OF GLOBAL BIOEVENTS
IMPORTANT
GATION
IN THE
INVESTI-
N•
A contribution to Project
GLOBAL
U
BIO
-
EVENTS
LEAP,Y, P.N. & HART, M.B.*)
This communication stems partly from the post I.G.C.P. meeting in Boulder, Colorado and from our own views on the subject. We hope to outline a more effective m o d u s o p e r a n d i for the investigation of the mechanisms and manifestations of global faunal/floral changes. The importance of the Raup & Sepkoski curve in focusing palaeontologists attention on biotic changes through Geological time cannot be undervalued. Although the peaks in extinctions are slightly artificially high due to the lumping of data sets from time intervals into data points, there are times when extinction rates are anomalously high. Any study across one of these peaks should include sufficient background cover to fully establish what the pre-crisis standing biotic stock is comprised of in terms of diversity and abundance. In this vein we should move towards a more honest and representative method of displaying our data other than a straight line from the first occurence of a taxon to its last. Here the moving populations need to be documented so we know whether we are dealing with the gradual decline of a species or the abrupt demise of a large population. The search for Ir has demonstrated that the sample spacing needs to be on the freest scale possible. Palaeontologists should adopt the same level of resolution where feasible. More importantly the sequencing of biotic changes against the geochemical signatures is crucial. For any given section all the analyses (of whatever nature) should be attempted from common samples. Only then will we be truly confident of the subtleties of the biotic interactions. Finally, it may well be worthwhile changing our emphasis away from the formulation of "simple" explanations for the fossil record and concentrate on the manifestations of each crisis. Even though the responses may vary in detail, they will hopefully lead us to the elucidation of overridding forcing mechanisms, where present. Thus we call for more co-operation across all the fields of the Earth Sciences working with common, tightly spaced samples from sections with the widest geographical and facies setting with detailed analysis of the intra-group and inter-group population dynamics against the geochemical data. We believe the I.G.C.P. provides the most likely vehicle to provide the requisite interdisciplinary/International co-operation, planning and funding to achieve these goals.
ACKNOWLEDGEMENTS
We would like to thank those in Boulder who provided ideas and thought-provoking comment and Ms. S. Braley for her useful comments in the preparation of this communication.
*) Department of GeologicalSciences,PolytechnicSouth West, Plymouth, England, PL4 8AA
SHOCK PRESSURES IMPLICATIONS FOR
IN IGNEOUS K/T EVENTS
PROCESSES:
A contribution
N?
to Project
GLO BA L
U
BIO
-
EVENTS
RICE, Alan *)
Abstract: The seismicity initiating the May 18, 1980 catastrophic eruption at Mt. St. Helens indicates an
explosion occurred at depth generating an average pressure of about 500 kbar. Such pressures fall off with distance from the magma chamber although jointing, fractures, etc. may act as stress concentrators to extend the radius of formation of shocked minerals as far as a kilometer. Shocked minerals are not to be expected from the magma itself as high temperatures would anneal such features but temperatures fall away rapidly enough from the chamber wall to allow retention even of such possible exotics as stishovite. The subsequent kinetics of the failure of the north slope support these pressures as do thermodynamic considerations and nucleation kinetics of CO 2 exsotution from magmatic melt. Confining pressures (e.g., overburden head) are not a limiting factor. Unconfmed detonations in open air yield pressures to several megabars although some recent arguments asserted to be volcanological would indicate open air bursts greater than one bar to be impossible. Further, it has been indicated that pressure estimates from ballistic considerations have been too high and large phenocryst content in the discharge material argues against high pressure explosions. In the first instance, sonic choking and volatile diffusion time constraints make these assessments implausible and in the second instance, both theoretical and geological considerations provide for the phenocryst distributions under explosive situations. These results and recent discoveries of high levels of iridium in volcanic ash in the Antarctic blue ice have implication for K/T boundary events, mass extinctions and endoexplosions. The geographical breadth of volcanic activity attending the K-T transition (e.g., Antarctic volcanism as well as the Deccan Traps) indicates a global mechanism and suggests a large portion of the mantle experienced convective surge as occurs at high Rayleigh number flow. Scaling to mantle conditions yields episodicities of the same order as the 30 my intervals.
INTRODUCTION A number of peculiarities attending the 18 May 1980 eruption of Mt. St. Helens signal the possibility that volcanic processes can yield pressures high enough to form shocked minerals such as those associated with the Cretaceous - Tertiary (K-T) mass extinctions. These peculiarities will be succinctly discussed here. The specific arguments for high pressures in volcanic processes that attend these peculiarities have not yet themselves been called into question. Appeal has been made, however, to other phenomena to argue against such pressures. These appeals will be discussed and shown on correction to indicate high pressures are available. Shocked minerals have assumed a singular importance in the K-T extinctions debate and the indication that volcanism may be source of these minerals also lends a coherency to the apparent interdependency between much of the K-T phenomena. Although the major portion of this paper deals only with high pressures in volcanic processes, it is obviously important to at least outline some of the other K-T phenomena that can also be related to volcanic processes. This is done in the conclusion.
VOLCANISM
AS A S O U R C E
OF S H O C K E D M I N E R A L S
Inferences from Mt. St. Helens
The U.S. Geological Survey (Lipman & Mullineaux, 1981) reported Mt. St. Helens to have been quiet *) Department of Physics, University of Denver, 1100 14th Street, Denver, Co, 80202, U.S.A.
60 the early morning of 18 May 1980 until the "magnitude 5+ earthquake at 0832:11.4 PDT - started minor rock and ice falls from the south crater wall -. Following an interval of a few seconds, a major fracture propagated rapidly along the apex of the bulge north of the summit crater. North of this fracture, the rock rippled and churned, apparently in place, for an interval of several additional seconds. The north face then slid down in a gigantic rockslide". Before collapsing, the surface of quarry benches similarly ripple and churn when shock from an explosion below impinges on the air-rock interface (e.g., high speed filming of quarry blasting, Martin - Marietta, courtesy of S. Winzer). Such behavior then indicates Mt. St. Helens was subject to a shock from below. The onset of the rockslide at Mt. St. Helens was taken as 0832:21 PDT with a possible several second error associated with the estimated delay time (e.g., Voight, 1981, p. 81). The seismicity of this initiating earthquake recorded at far field stations indicated a massive explosion at depth some tls before the failure of the north slope of the mountain and subsequent eruption. (Kanamori et at., 1984, reported complete azimuthal uniformity in far field P wave arrivals, first motion up and small S wave amplitude. This is the seismic signature of an explosion at depth, e.g., Bolt, 1976; Rodean, 1970). In addition, the north slope of the volcano failed "retrogressively" in three sections: that is, from the outside in (Voig,ht et al., 1981). Underground explosions induce failure in earthen materials by spalling the confining material into slabs from the outside in, i.e., "retrogressively". Failure occurs only through the shock accompanying the detonation and not by the expansion of the gases generated by the explosion (e.g., Hino, 1959). Fig. 1 depicts an idealized triangular shaped shock moving out from the detonation point to the surface of the material in which the explosion occurred. The shock is reflected back toward the detonation center from the interface with a reversal in phase. That is, the shock reflected back inward is a tension wave as opposed to the outgoing compression wave. As the compression and tension shocks pass each other proceeding outward and inward respectively, the resultant of their stresses will at some point exceed the tensile strength of the material and at this point the material fails and a slab of it will separate from the main body. Further slabbing will continue back in toward the detonation point. Although an explosion does comminute and plastically deform material in a relatively confined region about the detonation center, it is important to emphasize that an explosion does not generate failure from the detonation point outward but from the surface back in toward the detonation center. The thickness of the spalled slabs is about one half the shock length which in turn has length close to the crater depth (e.g., Hino, 1959). Taking the crater depth at Mt. St. Helens to be I kin, then the north slope should have slabbed into sections approfimately 500 m in width if it failed under explosive attack from a detonation at depth. The U.S. Geological Survey reports that the north slope of Mt. St. Helens fell in three consecutive sections each approximately 500 m thick. The failure followed the reflected tension wave back toward the detonation point (see Rice, 1985, 1987, for additional detail). In short, the mountain collapsed because of shock. In later studies (Moore & Rice, 1984), it was determined that the landslide velocity immediately after the north slope failure at Mt. St. Helens had a magnitude of 50 to 80 m/s. Assuming a slope angle of ca. 20* and no friction, it would take 15-25 s to acquire these velocities under the action of gravity alone. The maximum run of the slide would then have to be about 900 m. The landslide is reported to have advanced about twice as far by this time (Moore & Rice, 1984). This establishes that the north slope had an initial "throw veloci~'. Explosions impart momentum to the surroundings, i.e., the burden, by shock alone and not by the expansion of the gases of explosion themselves (e.g., Hino, 1959). Therefore a shock front itself had to originate at depth beneath the volcano in order that there be a throw velocity to north slope material. The throw velocity will be twice the particle velocity engendered by the shock. Taking a throw velocity of ca. 40 m/s would be consistent with particle velocities of ca. 20 m/s observed in tufts under explosive attack
61
O'CM IS MAXIMUM COMPRESSIVE STRESS OF THE ROCK
s;,ooi, WAS"O,~,O;;A.;•R~K> • ;1 AIR INTERFACE AT SPEED UC
" " ' ' " "" t I
l
....
'.. '"-
c:..
." . " . : ' : ' . ' . : '
7" :
"
----~uc l
. . . . . .
"
i
i
WHEN SHOCK HITS ROCK/AIR INTERACE A TENSION WAVE OF MAXIMUM STRENGTH 0 " N r IS REFLECTED BACK TOWARD SOURCE
'
BOTH OUTGOING COMPRESSIVE STRESS AND INGOING TENSION STRESS YIELD RESULTANT
er
UTH • .....
•
.
•
•
• .'- : '. : . '. " .ac
.,
t
SEVERAL SLABS MAY BE THROWN OFF. IF THE SLABS ARE JOINTED OR INCOMPETANT THEY MAY BREAK UP
WHEN THE COMBINED STRESSES OF THE OUTGOING AND REFLECTED SHOCK EXCEED THE TENSILE STRENGTH GTS OF THE ROCK, THE ROCK SLABS AND MOVES OFF WITH THROW VELOCITY UTH
Figure 1 : Depiction of an idealized shock wave proceeding from detonation point to surface of, say, a quarry wall. To conserve momentum a tension wave is reflected from the rock/air interface back toward the detonation point. When the combined compressive and tensile stresses of both outgoing and reflected waves exceed the tensile strength of the rock, the rock fails at that point, This process keeps repeating itself back into the rock, i.e., the quarry wall fails retrogressively; from the sur-face inward. Mt. St. Helens failed in a similar fashion. Relationships describing shock failure indicate Mt. St. Helens should have fallen in sections about 500 m thick which the volcano did, Mt. St, Helens fell due to shock disruption indicating explosive volcanism to be a possible source of shocked minerals.
(e.g., Larson, 1977). Such a particle velocity implies a shock velocity less than sonic (e.g., Teller et al., 1968) by the time the shock reached the surface of the mountain. In solids shock may stow to speeds less than that of sound and is often accompanied by an elastic precursor (see Rice, 1985, 1987 for details a~d references). Immediately before the failure of the north slope, the mountain "rippled and churned" for a number of seconds (Lipman & MuUineaux, 1981). This would be indicative of an elastic precursor which further indicates that shock induced the failure of the north slope. Sound speed in alluvium is ca. 0.5 km/s and picking this value for the shock speed yields an estimate of the shock pressure p at the mountain's surface from p =
90Uv
ca. 150 bar
where v is the particle velocity, U the shock velocity and the mountain material density is taken as 9 0 ca. 1.5 x 103 kg/m3 (e.g., Hino, 1959). This pressure is somewhat larger than the expected tensile strength of the material of which the north slope is composed and provides a boundary condition to obtain an estimate of the detonation pressure Pd from P = Pd (a/r)2 sin2 ~ where we take the cone angle Ct of the crater to be ca. 20°, the reduced radius a ca. 200 m and the depth of the detonation point r = 4.5 kin. This yields a detonation pressure ca. 700 kbar. This pressure estimate is
(52 conservative because 1) some workers are placing the depth of the explosion as far down as 7 km (e.g., Rutherford et al., 1985) and 2) the estimation of north slope throw velocity herein assumed no friction in the slide kinetics. Carter et al. (1988) report multiple shock planes are formed in quartz at shock pressures of 28 kbars and 20* C but only single lamellae at temperatures of 400° C. Such pressures are then more than sufficient to form lameUae structure in shocked minerals. It should be noted however that shock features are not expected to form in the magma or in wall rock that is hot enough to anneal out these features. The source of shocked minerals must be from the surrounding country as indicated in Fig. 2. The above results were obtained by accessing Hugoniots that have been established from considerable experimental work on materials of a geologic nature (e.g., see Teller et al., 1968). Some modes of analysis have become practice in the science of explosives and these also provide thermodynamic relations to yield pressure. The pressure history of a detonation is separated in several stages for treatment. The most extreme pressure in a detonation is termed the spike pressure Ps which proceeds into the undecomposed explosive to ignite it. The spike pressure def'mes the front of the advancing detonation wave and the front of the reaction zone within which decomposition takes place. The specific volume of the charge is smallest at the spike pressure but recovers, i.e., expands through the reaction zone. The end of the reaction zone defines the "Chapman-Jouguet" or C-J plane at which decomposition is complete. The pressure at the C-J plane is defined as the detonation pressure Pd = Ps/2. Pressure continues to drop, however and the specific volume continues to expand to the "explosion" pressure Pe = Pd/2. The explosion pressure is defined by the return of the volume of the charge and the volume of the inerts to their pre-detonation values. The volume of the inerts here constitute the covolume (e.g., Cook, 1968). The explosion pressure is then given by Pe (v- Ct ') = nRT where v is the pre-detonation specific volume, Ct' the pre-detonation covolume, n the number of moles, R the gas constant and T temperature. A CO 2 content of 10 w/o yields an explosion pressure of about 60 kbar. Regardless the equation of state, the detonation pressure is about twice the explosion pressure (e.g., Hino, 1959), i.e., about 220 kbar in this case. The spike pressure should be about 240 kbar. A concommitant water content of 3 w/o would add another 75 kbar or so to the detonation pressure, raising the spike pressure to about 400 kbar. The independent results of the slide kinetics given above and the seismic inferences given below suggest the volatile content of the explosive section of the Mt. St. Helens magma chamber to be higher than employed here. A CO 2 content of 30 w/o would be more in line with these other results. The Fig. 2 : Shown above is the industrial experience of quench supersaturation which occurs in solidifying ingots. A quench margin is formed upon pouring the ingot which impedes rapid heat transfer from the remaining melt. Note that the top of the ingot does not founder even though the solidified portion is heavier than the underlying melt; this because rapid cooling and formation of many small crystals at the top so increase the viscosity near the top that the top becomes competent before it has had a chance to pull away and fall through the melt. As the ingot continues to solidify inward from the sides, volatiles are forced into the remaining melt as their solubility is many orders of magnitude less in the solid. Eventually the last of the melt becomes supersaturated in volatiles and supercooled. The initiation of a nucleation site causes volatiles to come out of solution creating a local pressure spike that provides activation energy to form more nucleation sites. This autocatalytic reaction sweeps through the melt as a shock front, leaving gas and solids behind. Pressures to 10 kbar are seen in industry in ingots that are dimunitive in comparison to magma chambers. The carry over of this phenomena to the magma chamber environment is shown on the left and is brought about in a different fashion as the chamber is zoned with more silicic material at the top and more marie material at the bottom. The compositional variation through the chamber as well as the temperature variation, cooler at the top, warmer at the bottom, dictates double diffusive layering develop in the chamber. As explained in more detail in Figs. 5 and 6, this layering provides a stepwise distribution in temperature
63
through the chamber, this because each convecting layer is well mixed. Temperature is indicated to increase to the right. The variation of melting point with depth is also depicted. The more mafic material at the bottom of the magma chamber has the highest melting point and is generally always close to its liquidus in nature. As the chamber cools the effectiveness of the convection as a heat transfer agency sees to an undercooling of the marie material before the melting point of the more silicic material at the top of the chamber is approached. The lowermost layers are the layers with the propensity to explode. See Figs. 5 and 6 for further detail.
INDUSTRIAL MELT
MAGMATI ANALOG M E L T | NG
C
CQNVECTING L A ~'~~ R TEMPERATURE DISTRIBUTION
POINTS
t--
= TOP MOR~ SILICIC
DEFICTED
i
BOTTOM
FROM TOP
IN T H I S
TO
)
COLUMN
IS T H E S O L I D I P I C A T I O N O F A N I N D U S T R I A L ~,~ELT A N D A T T E N D I N G B U I L D UP ol ~ V O L A T I L E P R E S S U R E tN S H R I N K I N G M O L T E N CORE
SOLUBILITY OF GASRS MUCH LF.SS IN S O L I D P H A S E T H A N IN LIQUID FREEZING PUSHES GAS INTO REMAINING MELT
MAGMA CHAMBER
BOTTOM :::::::::: :: : : :::::-* *~
h
::::::::::::::::::::::::::::::
)
:.:~P::.~L:.;:.~P'. T ~D':'~P'.~:.~'":.:::.~!:...~
MORE
MARIC
AS TH~ MAGMA C~AMBER COOLS THE TEMPERATURE DISTRIBUTION MOVES TOWARD THE LIQUID AND ACTUALLY CROSSES OVER IN THE LOWER DEPTHS TO SUP~RCOOt. MORE M^~C
T.~
MELT TO " r . E P O ~ T O~ ~UC~.E^TIO~ OF
IHiiiiii iii!HHiiiiiiiiil H liiHiiH l *:*:.::::~
SOLID
FHASE
AND I~XPLOSIV~
OFFOASS|N(~
PRESSU RE SPIKE AT ORIGINAL NUCLEATION
MANY
CENTIMETERS
OF
~.~.~
i
'~
~'.i;IC
~",~ji" ..
COUNTRY
,~.,~.~ ~
~'~'~
~'::
not to scale
ROCK
64 above exercise independently provides pressures similar to those previously determined. Another completely independent assessment is given below confirming these pressures. Little of the energy of an underground explosion is expended in seismic energy. The coupling coefficient between explosive energy and generated seismic energy runs about 10-3. Most of the energy of an underground explosion goes into inelastic deformation, local brecciation, phase changes. Kanamori et al. (1984) report the magnitude of the Mr. St. Helens event to be M s = 5.2. There are numerous relations in the literature equating magnitude of earthquake with energy (e.g., see Kasahara, 1981). That employed by the U.S. Geological Survey seems to provide the most conservative values of energy, i.e., logEs = 1.5 Ms +4.4 For the 18 May 1980 Mt. St. Helens seismicity, this yields E s ca. 1.6 x 1012 J. An estimate of the give of the magma chamber can be had from E = FdR. Kanamori et al. (1984) indicate the source of the 18 May 1980 quake to be a downward vertical force of 2 x 1012 N from which dR ca. Im. From these approximations p = E/dV = E/4r~ R 2 d R = 103Es/4iI: R2 dR ca. 6 kbar Bath's relationship seems to yield the largest energy for a given earthquake magnitude, i.e., Log E s = 1.44 M + 12.24 This provides an E s of 5.4 x 1012 J for Mt. St. Helens. Assuming a cylindrical magma chamber which pistons the downward directed 2 x 1012 N force of Kanamori et al. (1984), then with dV = I~ r 2 dx, a 0.3 m displacement of a magma chamber floor about 0.5 km wide yields pressures approaching a megabar. The discovery of shocked minerals in volcanic discharge (e.g., Carter et al., 1986), in particular the Mt. St. Helens' ash that fell near Pasco, WA, would favor the relationship of Bath as do the results from the slide kinetics. As a frame of reference, it is to be noted that 1 kiloton of TNT in wet tuff yields pressures of about 1 mbar and in granite, 2 mbar. The U.S. Geological Survey reports the energy release of the 18 May 1980 Mt. St. Helens explosion to be equivalent to 24 megatons ofTNT (Lipman & Mullineaux, 1981). The great majority of this energy, however, can be attributed to the fall of the north slope. In general, it isn't until the collapse of the broken material such as a quarry wall that the gas generated by the explosion begins to escape. As events proceed rapidly to the eye, the initiation of gas release may be incorrectly identified with the detonation itself when in fact the detonation may have taken place many seconds beforehand and the surrounding material already comminuted. The north slope of Mr. St. Helens had pretty well collapsed before there was appreciable venting of gases (Rice, 1985). The characteristics of the failure of the north slope were in complete keeping with breakout of burden by a buried explosion and are totally within the confines of the mining engineering experience.
A MECHANISM
FOR ENGENDERING
HIGH
PRESSURE
VOLCANIC
EXPLOSIONS
There is little doubt from the observational properties of Mt. St. Helens that pressures high enough to form shocked minerals attend explosive volcanism. This holds regardless any lack of knowledge of the mechanism that engendered these pressures. It would be satisfying to have in hand, however, a mechanism that might yield such pressures. Chemical processes seem forbidden because of the low oxygen fugacity available in the chamber (e.g., A. Huffman, 1988, personal communication). The conditions under which melt coolant interactions can occur are so stringent as to forbid them from the volcanic environment. These
65 conditions are discussed in the appendix which also includes an example of their misapplication to explain peculiar seismic phenomena thought associated with a melt - coolant explosion. These difficulties suggest examining another phenomena which has the advantage that it commonly occurs in the industrial environment and has support from experience in solidifying igneous melts. It is common industrial experience that solidifying melts often develop very high volatile pressures within them and to the point of bursting through several centimeters of solid steel. This phenomenon is depicted in Fig. 2 which shows also the postulated carry over to the magmatic environment. When an ingot of steel is poured and set to cool, a quenched layer is immediately frozen to the sides, bottom and top of the container. The quench margin impedes heat flow and slows further cooling and freezing of the interior. Note the solid cap at the top of the ingot container which does not founder on forming even though it is heavier than the underlying melt. Quenching the surface melt of the ingot greatly increases the temperature dependent viscosity of the melt at the top and also generates innumerable small crystals at the top which so thickens and immobilizes the surface melt that it solidifies and becomes competent before it can sink. Industrial melts invariably contain volatiles. However, volatile solubility is many orders of magnitude smaller in the solid phase then in the liquid phase. As the ingot freezes inward, the volatile content is pushed into the melt remaining in the interior, supercharging the remaining melt with gas. The final melt becomes supercooled before freezing (as do all melts). Hence the last melt ends up supersaturated in gas and in liquid phase. Further cooling causes loss of this metastable state with the formation of a crystal nucleation site that initiates the freezing. Volatiles are ejected from this site into a high temperature environment too quickly to be diffused into a new phase (the situation is even worse in silicic melts whose diffusion times are on the order of many, many years). This generates a local pressure spike which provides activation energy to generate more nucleation sites which in turn generate more gas and an autocatalytic chain reaction sets in driven by the pressure associated with the dumping of the gases from the melt. This pressure sweeps through the melt as a shock which spawns solidification and gas release in its wake. The volatiles in the melt are dumped within the time it takes for the shock to traverse the remaining melt. This is in the same time frame as observed in industrial melts (Rice, 1985, for references to the industrial literature). It is the common observation that fluids possessing both a temperature gradient and a concentration gradient break up into layers of different density, the density of each layer increasing with depth. The conveeting system in the fluid is then one of many layers, each layer well mixed such that they are of uniform composition and temperature throughout. A fluid of uniform composition would comprise a single layer only bounded by the top and bottom of the fluid. In the case of varying composition there are gradients only at the boundary layers separating each layer of convection. This gives the vertical variation in temperature and composition a staircase like distribution, both temperature and mafic content increasing with depth. A representative temperature profile is shown in Fig. 2 as well as a representative melting point distribution in the magma chamber. The more silicic material at the top of the chamber has a lower melting point than the more mafic material near the bottom, the variation in composition shown by the grading in the idealized representation of the magma chamber. As the magma chamber cools, the staircase temperature distribution shifts to the left, i.e., to lower temperatures. Note that the lower portions of the chamber become cooled below their melting point, setting up a situation similar to that in the ingot depicted on the right. In the magma chamber case, however, it is a layer in the lower portion of the magma chamber that explodes. Offgassing will flash cool other layers in the vicinity which can lead to subsequent explosions by these layers also. There were apparently two explosions at Mt. St. Helens, about two minutes apart. In industry, mechanical disturbance such as vigorous stirring of supercooled melts does not lead to precipitation of the
66 solid phase whereas crystal seeding will. For instance, a supercooled metallic melt can be vigorously stirred with a ceramic rod without initiating freezing. However, pitching a nail into the melt will instantaneously lead to massive precipitation of solid phase and explosive exsolution of volatiles (see Rice, 1985). Rayleigh number considerations suggests the convecting layers in the magma chamber to be on the order of 10 m thick which is the same estimate provided by the blasting literature for the thickness of the exploding layer. The above mechanism which is suggested to apply in magma chambers is an outgrowth of earlier considerations (e.g., Rice & Eichelberger, 1976; Rice, 1981). Once open to atmospheric conditions, the magma chamber may continue to offgas in a "boiling" mode rather than an explosive one and continue to boil for some time after the explosion which exposed magma to atmospheric conditions. It is unlikely that shock features would be retained in phenocrysts or other material in the magma chamber due to the elevated temperatures of the magma which would anneal out such features. The same applies to the magma chamber walls. However, further out into the country rock where it is cooler, such features could be retained. Several tens of meters away from the magma chamber wall the temperature will have fallen off to several hundred degrees (vivid evidence of such poor thermal conductivity of rocks is demonstrated by those scientists who sample magma by poking through the thin solid crust across which they have walked in ordinary shoes). Rice (1987) estimated the shock features to be derived from surrounding country rock to distances of the order of 500 meters out from the magma chamber. Equilibrium estimates of the stishovite transition indicate its formation could run to several hundred meters from the magma chamber, well into a region cool enough for its retention. However, nothing is known of the effect of impurities or load rate on the formation of stishovite. It may well occur even further out. Jointing cracks or angularities in country rock can serve as stress concentrators which would extend this region considerably further. If the country rock is not of magmatic composition, shocked minerals from it would not reflect magmatic origins. The mass transfer associated with the offgassing of solidifying melts can be quantified from relations developed in industry. For example, the transport coefficient Kw is given by Kw0C j1/2 VbD1/2 where Vb is the bubble volume, D is the diffusion coefficient and the rate of bubble formation is given by J = z [exp(- A H/kT)
"~f6y / (3-b) r~ m ] [exp {-16r~ 5/ 3/3kT(Pe_ pa)2}]
where b = (Pc - Pa)/Pc (e.g., Richardson, 1974), z is the number of exsolving species per cm3, A H is the heat of formation of one molecule of vapor from the melt, m is the mass of the vapor molecule, k is Boltzmann's constant, Pc is the pressure in a bubble of critical size such that the partial pressure of the gas in equilibrium with the fluid exceeds the sum of the ambient pressure Pa and the pressure due to the bubble surface tension, 2 5/ /r. A 10" K drop in temperature can lead to an increase in the rate of bubble formation of 1032 times (e.g., Katz & Sliepovich, 1971). Gases are known to disassociate in industrial melts, e.g., CO2 ----> CO + O. For magmatic melts and assuming a heat of formation of C + CO ---->CO 2 from the melt of about 70 kcal/mole, the overpressure above ambient is about 500 kbar. The proposed mechanism then yields pressures as inferred from the dynamics of the Mt. St. Helens eruption. Further, shock in industrial melts can also disassociate CO into C and O (e.g., Nellis et al., 1981) which may provide explanation for the association of soot with the K/T boundary. It is known that strong impact (as opposed to stirring) can induce sudden freezing in supercooled fluids (e.g., Lovett et al., 1982). The reaction rate equations that describe freezing phenomena are general and
67 also describe explosive kinetics which, too, can be initiated by impact. For detonating explosives, the reaction rate K is coupled both to the energy equation and to the shock velocity. Assuming a planar reaction front normal to the x direction, the shock coupling has form (e.g., Courant & Friedrichs, 1967; Fickett & Davis, 1979) Ude/dx = K where e is the percent of reacted material in the reaction zone. K has the usual form for reaction rates,
K = A(kT/h)exp [- a G/kT] where A is a transmission factor, k is Boltzmann's constant, T is temperature and A G is the activation energy which is the sum of the surface energy of the nucleation site and the difference in the free energy in the volume of the nucleation site between the liquid and solid state (e.g., Knight, 1967). The free energy has the usual form dH - SdT where it is useful to note the form of the enthalpy dH = dQ + vdP which contains change in pressure. The factor A is related to the inverse of the viscosity and a geometric factor (e.g., Willnecker et al., 1986). Taking the effect of viscosity and G = 161% 3( 3 T2 m/3(L3 m A T 2) (e.g., Davies, 1973), an overpressure on the order of kbars as computed above will be sufficient to drive explosive crystallization of mafic magmatic melt.
TEMPERATURE*C I0 A~DRY
r
f b
!7!/ :1;1
:1:!
i I: I
r-,,,o,,,
I---
Iiquidus
I0
0L , t , t tlI, ~00 600 800 I000
= I 7 51-
go
Ill ~ 1200
400
3O
1 /,
i',;
irl
',. !
i ',
',1
, ,
600
£
,,
longed build up of supersaturation does occur in the industrial foundry if the sides of the retort are
I- if? I
oL,
0 ~
800
Here T m is the melting point, L m is the latent heat and A T is the degree of undercooling. It is to be noted that the reaction rate equation yields an exponentially increasing pressure curve as temperature drops below the freezing point. Experimentally derived pressure versus undercooling relations show a similar response for solidifying magmatic melts, pressures exceeding the confinement strength of the experimental apparatus, i.e., 35 kbars (Yoder, 1976). The above equation more properly describes homogeneous nucleation which may occur if the magma walls are glassy. Pro-
II
-~10 I000 1200
'
10
Figure 3 : Phase relations for wet and dry magmas are shown above. The arrowed line running along the bottom of the lower diagram depicts supercooling at possible magma chamber depths. Many melts can be supercooled many tens of degrees below their solidus. If this occurs for either rhyolite or basalt as shown above, an apparent unbounded oressure is available.
glassy and do not provide nucleation sites to bleed of volatiles. Such a situation is extremely dangerous and care is taken to prevent the sides of the retort from becoming smooth. Although undercoolings of over 300° have been obtained in metallic melts (e.g., Thompson & Spaepen, 1983) it is much easier to attain such deep uudercoolings in silicic material which will go to glass unless cooling times are on the order of 103 to 106 years depending on their viscosity (volatile content can alter this). However, such cooling times are not out of reason in the magma chamber context. The
68 depths of undercooling available in magmatic melts indicate that if phase equilibrium fields did apply, they would also yield pressures similar to those indicated above (see Figure 3, courtesy of P. Wiley). If a mafic melt with water content of 3 w/o at depth of 5 km were supercooled to 600° K, then the arrowed line indicates the direction in which the pressure line would be intersected. As these lines run nearly parallel, it is apparent that extremely high volatile pressures would eventually be obtained. Quench supersaturation explosions, i.e., the mechanism described above, is akin to "second boiling" as described by Morey in 1922 wherein high pressures may be generated by exsolving gases within a solidifying melt. Loper and McCartney (1988) refer to other work wherein pressures to 35 kbar were measured while cooling confined igneous melts, the limitations of the experimental equipment preventing the observation of higher pressures. These situations do not represent metastable, supercooled conditions and such pressures could not be obtained without the confinement. As will be discussed, Loper and McCartney's attempt to apply these results to the volcanic environment is incorrect.
ARGUMENTS
AGAINST
VOLCANISM
AS A SOURCE
OF SHOCK
PRESSURES
It has been suggested that the growth of the bulge on the north slope of Mt. St. Helerts was eventually responsible for the failure of the north slope and shock processes were not involved (Kanamori, personal communication, 1987). Rice (1985) has shown, however, that the north slope landslide of Mt. St. Helens did not have characteristics of a gravitational failure. The run and coverage were far too great and precursors were absent such as accelerating downward creep. For that matter, the significant pre-avalanche deformation that did occur, i.e., that of the bulge, was upward and outward and was of a de-accelerating nature (Voight et al., 1981). A slip plane slope stability analyses of Mt. St. Heleus just prior to the May eruption which includes the effect of the bulge, pore pressure estimations, etc. (Rice et al., 1988) strengthens the previous conclusion that the north slope could not have fallen by itself but required disruption from within (Rice, 1985). Kanamori (personal communication, 1987) has adhered to the original interpretation offered by the U.S. Geological Survey concerning the mechanism of the Mt. St. Helens eruption, i.e., the release of overburden due to the failure of the north slope ostensibly led to the flashing of the volatiles in the magma chamber (e.g., U.S.G.S. Prof. Paper 1250). The lateral exhaust through the side of the volcano was taken by the U.S. Geological Survey to be a directed blast in the horizontal. In analysing the near field seismic data Kanamori and Given (1982) indicated the arrivals of only Love and Rayleigh waves to be consistent with this interpretation. It should be recognized that landslides by themselves are effective Love wave generators (see Rice, 1985, for references) and that inversions and source characteristics determinations are not unique. Further, explosive sources are accompanied by a local shadow zone with respect to body waves and the lack of significant P arrivals at near field stations is consistent with an initiating explosion as was the situation at Mt. St. Helens. There is concurrence that the initiating earthquake preceeded the failure of the north slope by approximately I1 s which is consistent with the use here of a detonation point 4.5 km deep and shock propagation velocities of 0.5 km/s. Kanamori's own analyses show the earthquake to preceed the north slope failure (see, for instance text and fig. 16 in Kanamori et al., 1984). This point is dwelt upon here as Kanamori (personal communication, 1988) believes the landslide occurred before the quakes but this author is unable to establish how to arrive at this conclusion. In contrast, the U.S. Geological Survey considers the earthquake to have been responsible for the loss of the north slope (e.g., Voight et al., 1981). Two minutes after the initial quake - beneath all the rubble and overburden of what had beeffthe north slope - there was another magnitude 5 explosive - source earthquake and subsequent explosive venting. This contradicts the
~521 release of overburden as an explanation for volcanic explosions. The necessity of uncapping magma chambers to engender explosions leaves unexplained events in which no slide was involved such as Le~mington in New Guinea. The top of Leamington was uniformly blown off such that the resulting crater is completely symmetrical (P. Lippman, personal communication, 1985). Neither are slides involved in the explosions at Sakurajima (Ishihara, 1985) wherein explosion source quakes preceed by several seconds explosive eruption of the crater floor. These observations greatly strengthen the conjectures herein that an explosion in a magma chamber was the initiating event on 18 May 1980 at Mt. St. Helens. Further, the explosion on 18 May 1980 at Mt. St. Helens has been placed as deep as 7 km (Rutherford et at., 1985) which makes it even more difficult to appeal to the removal of north slope overburden as the initiator. Kerr (1987) reported in Science that the appearance of particles of shocked minerals as large as 100 m at the K/T boundary constituted an "insoluble problem for advocates of volcanic catastrophe" in explnlnlng the events at the end of the Cretaceous. Kerr states that the work of Lionel Wilson (e.g., Wilson & Huang, 1979) indicates grains of such size would be lofted no more than 500 km from the point of explosive volcanism. It is Kerr's conclusion that only a meteor impact could provide the worldwide dispersal that seems to attend the shocked minerals found at the K/T boundary and has been taken by others as proof of impact (e.g., Bohor et al., 1987). Kerr's conclusion and those of Wilson are incorrect. Bailey et al. (1984) report large concentrations of giant sized mineral particles in the present Arctic atmosphere. These concentrations appear to be uniformly distributed to 5 km in height. Some of these particles are reported to have the appearance of voleanic ash. Concentration does vary with size, i.e., the inverse cube of the particle diameter. From their data, these workers estimate the fall speed of a particle 100 m in diameter should be 0.05 m/s and much lower for plate llke aggregates. These fall speeds are considerably less than those of 0.25 m/s for equally large diameters as predicted by Wilson and Huang (1979). For that matter, quartz crystals of Asian origin of size around 300 m are known to be transported by wind from Asia to Hawaiian (Duce, personal communication, 1987), mineral particles in the 50 m range can be transported by wind distances in excess of 10.000 kin, and particles as large as 100 m have been transported from the Sahara to Miami, Florida (e.g., Betzer et al., 1986; Duce, 1986; Uematsu et at., 1985; Dauphin, 1983; Carder et al., 1986). It is to be noted that this material is of similar density, composition and even greater size than the shocked minerals from the K/T. It has recently been reported that explosions never develop pressures in excess of that provided by the tensile strength of the confining material or the hydrostatic head provided by overburden therefore volcanoes can never be a source of shocked minerals as confining pressures run to only several kilobars (de Silva & Sharpton, 1988). If explosions never developed pressures in excess of the confining strength, then open air blasts would never greatly exceed atmospheric pressure. Only a cursory examination of the blasting literature indicates that open air detonations of TNT and other high explosives can yield pressures at the detonation point on the order of a megabar (e.g., Cook, 1968). Not even confusion between detonation and deflagration attends the allegations of de Silva and Sharpton. Detonation driven reactions are carried forward at shock velocities in the reactant such that the reaction is over before there is any dynamic response of the reaction products. That is, pressure many orders of magnitude greater than that provided by confinement are released long before the confinement has a chance to respond. Gunpowder deflagrates, i.e., burns. Untamped gunpowder will simply flare. However, confined gunpowder allows a build up of pressure that greatly aecelerates the burning rate, carrying it to completion so rapidly that overburden and tensile strengths are considerably exceeded before the confinement fails. The reactions considered here to be responsible for fast rise time, high pressures in the volcanic environment are detonation driven; that is, driven by shock.
70 The observation that explosive discharge may be laden with phenocrysts has provided an objection to quench supersaturation explosions. Homogeneous nucleation of an undercooled melt driven by detonation kinetics is not expected to be charged with crystals. The speed of the shock driving the exsolution exceeds by many orders of magnitude crystal growth rates. Further, the existence of crystals indicates solidification which proceeded more as a heterogenous nucleation mechanism. In the latter case deep undercooling with attendent metastable conditions that are potentiaUy explosive would be difficult to obtain. This because of the available nucleation sites. The magma would off gas continuously in a much more subdued fashion as it cooled. The fact that magma chambers are zoned, however, provides an environment for phenocryst laden discharges from the upper reaches of the chamber as the result of explosive exsolution of supercooled melt nearer the bottom. This environment secures most of the crystal population in the upper portions of the chamber. There exists recent theoretical literature which would oppose such a population in spite of geologic evidence that it occurs. These views are discussed later. There are instances, however, wherein the phenocryst population is concentrated mostly at the top of the chamber. There is an approximately 20 meter thick mat of phenocrysts extending downward from the solidified surface of the Kil~uea Iki lava lake, with decreasing populations into a melt region approximately 20-30 m thick. Apparent deviations from this distribution such as the Shamsan caldera (e.g., Cox et al., 1979) wherein the crystal concentration appears to come from the bottom of the chamber will be dealt with elsewhere ~ i c e , in prep.) wherein it is shown that the increasing crystal content of the chamber exudate reflects its cooling history and not settling. The melt region of the lava lake is underlain by solid material thought to be the result of crystal settling. Although the lake is believed to initially have a depth of 100 m, apparently not until nearly two decades after emplacement were attempts made to sound the lake by several geophysical means. No consistent results were obtained (e.g., see Helz, 1987). It is not clear that the bottom of the present melt is not a surface that
APPROXIMATEVERTICAL DISTRIBUTION OF PHENOCRYST CONTENT FOR $HAMSAN CALDERA ROCKS {FROM COX ET AL, 1979), NOTE THE BOTTOM PORTION OF THE MAGMA CHAMBER WAS DEVOIDOF CRYSTALS
DEPTH BELOW SURFACEOF K1LAUEAIKI LAVA LAKE 1
......
1
ooy ij
> "o
..........
0D
i11
Figure 4 : The pbenocryst distribution in the above unit are concentrated near the top, suggesting the evolution of a magma chamber as shown in Fig. 6. Figure 5 : Phenocryst distribution is confined to the region above the melt at Kilauea Iki Lava lake (from Helz, 1987). This distribution is similar to that given in the previous Figure, Fig. 4, and sinailarly suggests the evolution of a magma chamber as shown in Fig. 6.
1
o
1.8 0 OL
CPX
/
E 5 0 0 /
I
0
31o
112
3.0
TOTALFELDSPAR
ORE
71 had been flooded. It is clear that the phenocryst distribution from the bottom of the melt to the surface of the lava take is in accord with models of magma chamber solidification from the top down which also yield banded structures (e.g., Rice, 1981, 1985). A qualitative depiction of the phenocryst distribution above the 40 m depth is given in Fig. 5. Note further that vesicle population correlates strongly with crystallization. Attention is now directed to Fig. 6 in which is depicted an idealized magma chamber whose chemical variation with depth again secures within it a multi-diffusive layering. Shown in Fig. 6 is the change in temperature (T), composition (C) and viscosity (V) with depth. The distribution of each of these entities (T, C and V) is as before (Fig. 2) a staircase function that increases with depth for T and C but decreases for it is only at the boundaries separating each layer that significant gradients exist. Shear entrainment at the separating boundaries does secure transport between layers as indicated by arrows crossing the boundaries: for instance, material from layer 2, i.e. C2, is engrained in layer 1 and vice versa. It is anticipated that the upper portion of the chamber is more acidic than the lower portion, hence the upper portions have lower melting points Tim than the lower portions. Material Cn has a higher melting point than C 1.
Figure 6 : A model convecting magma chamber depicTi ted above as suggested by MAXIMUM .':1. "° ., • :.," ". . . . . . % .;. ". e2entr;ined Into'GI , ~ Figs. 4 and 5 as well as VISCOSITY ¢~, • "manyl~anoer~sts C" "" ":" *'" 1"2 • ' . ~ m : ' . ." IN UPPER theoretical considerations. MOST LAYER The chamber is zoned. • , I . . ,.." [Tm2 ! C1 entrainedlntoC2 le 2 '. • : :. ! ! Lighter material is on the top and has composition C1 (e.g., is most silieic) which also fixes its melting point, T1. Take special note however of the dots representative of phenocrysts of material that has come up from depth which is of different ! C~'nn-1 entrained composition and h i g h e r melting point than In- 1 trained into Cn- 1 the material in which it LOWEST resides. As the chamber is VISCOSITY no phenocryst8 I CI~ cooling from the top, this IN LOWER ,Vn MOST layer also possesses the LAYER increasing lowest temperature T 1 and . is the most viscous material "Yl in the chamber, possessing viscosity ~t"Proceeding downward through the chamber, the magma becomes heavier and at the bottom has composltion C n (e.g.,most mafic) which implies material with the highest melting point in the chamber. This is also the warmest part of the chamber with temperature T n. It also possesses the lowest viscosity,V n. Being zoned, the chamber possesses both compositional and temperature variationwith depth which secures multi-dkffitsiveconvection, that is,the chamber supports a multi-layered system rather than a single layer system of convection. Physical properties are then distributed in a "staircase"fashion from top to bottom of the chamber as indicated above. As shown by the broad arrows, material from one layer can be entrained into another, and in fact be cascaded up or down through the layers of the chamber. Material moving upward into cooler,more viscous material willcool, crystallizeand if possessed of sufficientvolatilecontent, vesiculate.This due to its high melting point. Material moving downward from the top, however, is of lower melting point and ifithas crystallized,itwill have a proclivity to be resorbed. Moving up and down the staircase can lead to zoning and/or to reversed zoning. As the chamber cools, the staircasetemperature distributionmoves to the left.The distributionof melting point temperatures should not change significantly. Increasingci r~
Tmi
~
~ )lnCreraSlnl
72 Some C 1 is engrained into layer 2 (second from top) but as it enters a higher temperature environment it gets further from its liquidus. Crystals of C 1 engrained in layer 2 therefore have an increased likelyhood of being resorbed. For that matter some crystals of C 1 could cascade all the way down to layer n near the bottom where they would be completely resorbed as the highest temperatures in the magma chamber occur here and are above the melting point Tin1 of C 1. In a similar fashion, crystals of C2 may be engrained into layer 3, later 4, etc. Some C2 will also be engrained into layer 1 (at the top) where it will have a propensity to crystallize out. This because C2 has a higher melting point than C 1 and it enters a colder environment. Similarly for compositions of other layers, e.g. C4 engrained into layer 3, etc. It is possible for Cn material to cascade by entrainment all the way to the top of the chamber, crystallizing on the way up as it encounters colder and colder temperatures. There is evidence that there is a significant population of mafic crystals engrained in the more silicious material at the top of the chamber (e.g. Eichelberger, personal communication, 1981). Moving up and down the convection ladder will generate reaction rims, embayments, zoning, resorbtion in crystals. Most of the crystals reside in the upper, cooler reaches of the magma chambers. The lower regions of the chamber are too hot to support a crystal population. They would melt. The quench supersaturation explosion mechanism can take place in the crystal free region at the bottom of the magma chamber when these regions of the chamber have cooled sufficiently to reach the nucleation temperature. Note that the higher melting point of the material in the lower region of the chamber allows this material to become undercooled even though it is still the hottest part of the magma chamber. Mafic magma does not seem to occur at temperatures far removed from its liquidus which facilitates this undercooling of melt in the bottom of the magma chamber. A large temperature drop across the entire chamber itself is not anticipated because of the heat transfer efficacy of the convections. The upper portions of the chamber then, also being much further removed from their liquidus, will not be subject to undercooling until long after the material at the bottom of the chamber. Such evolution can be confirmed theoretically. The thickness of the exploding layer of magma is estimated to be on the order of some tens of meters (Rice, 1985). Although there is experimental variabilitywith regard to heterogeneous nucleation preventing the development of a supercooled melt (there is even some question as to whether homogeneous nucleation ever really exists), assuming the exploding layer to be phenocryst free in a magma chamber 1 km deep attributes to this layer only 10% of the discharge of the volcano. This assumes of course that material from this layer even reaches the surface. Fig. 7 indicates the situation from which it can be ascertained that phenocryst laden discharge is to be anticipated from a quench supersaturation explosion. Further, the propensity for ash to be lofted further than heavier phenocrysts offers the hazard of a skewed distribution of phenocryst content of the magma if the massive volumes of ash carried off by atmospheric processes are not taken into account. In any case, except close in to the lip of the volcano an explosive eruption through the compositional variation of the chamber will disrupt it to yield a mix as shown in Fig. 7. Obviously the outfaU may not accurately represent the distribution of material as it existed in the magma chamber. Such a distribution of crystals as indicated in Figs. 5, 6 and 7 runs cotmter to some recent work which has been proferred as supportive of segregation by crystal settling. It is surprising such effort is undertaken in view of the fact that one commonly sees advertised hydraulic or pneumatic transport systems carrying particles of coal, rock, minerals, limestone, etc. in sizes up to 8 cm as uniform suspensions in the working fluid up vertical hoists of height greater than a k m or over distances of 25 km. This in media orders of magnitude less viscous than magmas hence far more likely to facilitate settling out (e.g., Siemag, Inc.). Efforts that would support crystal settling still leave unexplained marginal border groups and heavier
73
material overlying light (e.g., as in the Jimberlana) unless one requires that somehow the magma chamber be rotated on its sides and roof occasionally to secure it. Further such efforts ignore a vast array of industrial literature regarding solidifying melts in which analogues of border groups and cryptic variation are
5
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commonplace as well as the occurrence of heavier material overlying light. The industrial phenomena however is due to convective fractionation with no crystal settling involved at all (see Rice, 1981, 1985 for refer-
ences). The effort of Weinstein et al. (1988) is representative of work attempting to support crystal settling as an important phenomena in magma chambers. Their theoretical study however, is restricted to laminar flow whereas magma chambers possess Rayleigh numbers readily of the order of 1014, about ten orders of magnitude higher than the transition to turbulent flow. The study then is restricted to melts about a meter thick hence have no application to magma chambers at all. This applies even to deep seated magma chambers. If the chamber is about a kilometer thick, a miniscule temperature of only one degree across the chamber will yield a Rayleigh number of 1012, well into the turbu-
DEGREE OF UNDERCOOLING
Figure 7 : After some time convective heat transfer drops the temperature of the magma chamber such that the lower layers become undercooled, (i.e., take on temperatures below their melting point), although the lower layers remain the warmest portion of the chamber. This sets up a potentially explosive situation in the bottom of the chamber. If the super cooled layer blows, it will yield a mix of material from all levels.
lent regime. It should be kept in mind that the environs in which the chamber is immersed will be 15° C cooler at the top of the chamber than at the bottom simply due to the geothermal gradient. Particularly remarkable however, is the manner in which settling is theoretically secured in this work. The computer tracks the particles, tests to see if they are close to the bottom and if they are, the computer program, without appeal to physics, simply removes the particle from circulation and calls it settled. Harry Hess, as long ago as 1960, noted in his discussion of the Pzlisade sill that experimental evidence indicated strong vertical transport off the floor of a convecting body. In an attempt to experimentally verify such results, these authors fred support only when they cease heating the fluid. That is, settling of glass beads in syrup does occur but only when the convection comes to a stop. Otherwise the spheres remain entrained in the
74 fluid even though the relative density difference exceeds by an order of magnitude that to be found in magma chambers and even though the glass beads scale to "crystal" sizes of tens of centimeters. "Sticky bottom" is the term employed by these workers for the artifice of "logic gating" to settle particles. We have applied the same paradigm to "prove" that crystals accumulate on the "sticlq?' top of the chamber similar to crystal growth in industry which does occur in the more viscous chilled boundary layer at the top. These authors however indicate "the effects of boundary conditions are not important", hence dismiss the most important manifestation of convection itself, the boundary layer. The use by Weinstein et al. of the terms "homogeneous nucleation" to describe uniform crystal growth throughout the chamber and "heterogeneous nucleation" to describe growth confined at the top of the chamber indicates ignorance of solidification processes wherein these terms have quite different but such specific and singularly important meanings in the solidification literature (see for instance, Carter, 1979) that such confused use would adamantly be avoided. Further, the dismissal of the viscosity change of the upper boundary layer eliminates a powerful retention mechanism. Crystal retention correlates to increased viscosity of magmas as noted by other workers (Marsh & Maxey, 1985). The mechanism which glues crystals to the roof in the industrial setting is the crystal content of the melt adjacent to the roof. When this crystal content exceeds 65%, there is a sudden snap - through increase of viscosity by orders of magnitude (this has been noted also in the geological literature. See Shaw, 1965). Crystal content will taper off away from the wall but a "glue front" will advance down into the magma chamber with crystal content of 65%. It is interesting to note that it seems all orthocumulate structure has abut 65% crystal content imbedded in the matrix. Few lava flows contain 65% crystals. There is other work indicating results similar to Weinstein et al. that is also flawed. The treatment of this work is relegated to another paper (Rice, in prep.) to keep this one of reasonable size. It has been alleged by Shoemaker (personal communication, 1980) that explosive volcanism is a decompression process. If this were the case, first arrivals of P waves from such an event would be down. First arrivals from the Mt. St. Helens explosion were up, indicating to the contrary a compressive, i.e., an explosive event and not a venting event. Work on decompressive processes such as Wilson (1980) has been held up as an example of results calling into question high pressures inferred for magmatic processes from ballistic calculations (e.g., de Silva & Sharpton, 1988). Wilson cautions however that his inferences have no application to fuel-coolant interactions whose intensities are similar to the detonation processes discussed here. However, the inference of supersonic discharge from volcanic vents (Wilson et al., 1980) is at odds with flashing blow down experience in industry wherein choking limits the exit velocities to the speed of sound of the material of the flow (e.g., Please et al., 1986). Hydrofracking from a magma chamber to the surface of the earth may set up a blowdown situation such as occurs when a line of pressurized superheated water fails. Decompression occurs with a refraction wave passing back through the system wherein the attendant pressure drop causes massive flashing of the water with two phase flow exiting the break (e.g. Weisman, J., 1987). Although a diverging portion of the path may become supersonic, eventually the flow must enter the atmosphere and become choked. Exit flow with velocities greater than Mach number M > 1 occur only when PIAc/P2A2 > 2 using the ratio of specific heats of water. A c is the minimum area of the nozzle, P1 the pressure driving the flow. A 2 is the exit area, P2 the exit pressure. If the flow eventually dumps to the atmospheric then A2 is large enough to secure M < 1. Inclusion of slip between the components of the multiphase flow indicates gas velocities should be several times larger than magma velocities, an aspect neglected by Wilson et al. (1980). Further, there exist correlations that indicate the need for frictional corrections due to effects beyond viscous head loss: e.g. material colliding with walls, etc. These have forms Pt/Pv = 1 + R/K where R is the mass ratio of particles to gas, K an empirical constant, Pt is the total pressure drop, and Pv is the pressure drop of the vapor phase only as deduced from ordinary
75 viscous situations. These relations imply corrected pressures some tens of times larger but do not take in another important phenomena also neglected in the work of Wilson et al. (1980). Beyond the work of Shaw (1965) relating to magmas, it is well known elsewhere that when solid content gets beyond 50% of the volume of the flow, the effective viscosity of the flow begins to rise in a fashion even stronger than exponential. For instance, in slurries the effective viscosity can rise by a factor of 200 with a doubling of particulate content (e.g. Wohlbier, 1980). Inclusions of this effect in all assessments of magma chamber pressures from discharge rates, ballistic and otherwise, raises these assessments as high as a megabar, a pressure commonly associated with many commercial explosions. However, there is a fundamental difficulty in the mechanism of Wilson and coworkers that will not allow even the above problems to manifest themselves. Rapid degassing as the magma rises cannot take place as the diffusion coefficients of volatiles in melts are miniscule. For the same reason an oversaturated diver may not experience the bends until some time after returning to the surface, degassing due to loss of overburden head as magma rises is impeded by the time it takes to diffuse the gas from the melt. A volatile diffusion coefficient of 10_5 cm2/s in a melt 10 m across should take over a year to degas. In fact, the diffusion times of volatiles from magma is so great that it restricts the magma to boiling off over many, many days instead of acquiring within seconds the bouyancy of 70% volume gas content. The rise time of the magma will be further restricted as the viscosity increases greatly due to flash cooling as the magma boils (see Rice, 1985, for estimates). The mechanisms of Wilson and coworkers cannot occur. These latter difficulties also attend a model put forth by Loper and McCartney (1988) to generate volatile release and attendant pressure rise through magma mixing. Solidification by this process does not entail shock propagation speeds and must proceed by diffusional mechanisms alone which even when abetted by convection require at the minimum months to significantly degas. This excludes the similar dissipative cascade time scale necessary to break up large turbulent eddies into small. Driving gas out of a volume of solidified melt 1 micron across under near equilibrium conditions entails diffusion coefficients of 10_12 cm2/s which in turn implies a gas bleed-off time of many hours. For larger size melts, the diffusion time becomes monumental. Some workers (e.g. Wilson et al., 1980) contend degassing will not begin until depths of only 5 km have been reached and this initiates only slowly. In addition, the suggestion by Loper and McCartney that shock features may be formed by slow rise in pressure is incorrect and does not correspond to fact. Although shock features do not arise in minerals of relevance to K/T events until pressures exceed 20 kbars (Carter et al., 1989), shock features are never obtained unless there is a load rate sufficiently high to generate a shock front. Although a shock wave may have considerable length, shock fronts themselves (but not elastic precursors) rise within interatomic distances (e.g., Greene & Toennies, 1964). That is, they possess rise times of considerably less than a millisecond and not days as proposed by Loper and McCartney. There are other criticisms. While not applicable to detonation type reactions proposed here (in which the full pressure develops before the confinement can dynamically respond), De Silva and Sharpton (1988) have noted that the explosive mechanisms of Loper and McCartney are deficient. Although Loper and McCartney cite literature indicating laboratory confinement of pressures to 35 kbar developing in a solidifying magmatic melt, it is the confinement of the experimental apparatus that allows such pressures to develop in near equilibrium processes. The experimental apparatus did not have the strength to confine higher pressures. It is noted by De Silva and Sharpton that these relatively slow developing pressures exceed the overburden and tensile strength of the material confining magma chambers and cause it to fail before total ex.sotution of the volatiles. This failure opens the chamber to atmospheric conditions. Pressure then bleeds off with the confined volatiles never exceeding the local ambient pressure. Only slow bleed off of volatiles can take place
76 by the mechanism of Loper and McCartney leading to the gradual build-up of pressure until hydrofrak takes place to release the pressure. As pressure release exsolution is not expected to commence until the magma is some 5 km below the surface of the earth (e.g. Wilson et al., 1980) the mechanism of Loper and McCartney cannot be expected to be responsible for deep-seated crypto-explosions. Pressures attainable by the mechanism of Loper and McCartney are limited to that of the overburden: perhaps 2 kbar. The length of time it takes magma to diffuse off its volatile content has important ramifications, dictating that some sort of metastable conditions be set up conducive to instantaneous exsolution of volatiles. Huffman (personal communication) indicates that there isn't sufficient oxygen to appeal to chemical reaction. These situations dictate there can be only one mechanism by which explosive volcanism occurs: metastable undercooling, i.e. quench supersaturation.
CONCLUSIONS: SOURCE
OF
PERIODICITY K-T
IN
MANTLE
CONVECTION
THE
POSSIBLE
VOLCANISM
Several independent methods of demonstrating that volcanic processes are capable of producing shocked minerals have been reviewed here. It has also been shown here that a number of efforts to reach opposite conclusions, also independent of one another, are flawed and upon correction strengthen assessments that indicate shock may accompany exploding magma chambers. Some of these assessments made clear the importance of CO 2 in generating volcanically derived pressures sufficient to form shocked minerals. These results compliment the discovery of CO 2 inclusions along shock lameUae in minerals of the Vredefort and Sudbury (Medenbach et at., 1987; Fricke & Schreyer, 1987). Such inclusions do not seem to be reported in shocked minerals from underground nuclear testing which further supports suggestions that CO 2 is crucial to cryptoexplosions (e.g., Nicolaysen, 1985). Augmenting this view is the proposal that the Deccan Trap represented mantle CO 2 outgassing that led to a greenhouse effect which was deletorious to large body life forms (e.g., McLean, 1985). The Deccan Trap is now known to lie athwart the K/T boundary strengthening the suggestion that these massive flows may be a source of the K/T iridium (e.g., Vandamme et al., 1986). The extensive kimberlite pipe fields emplaced at the end of the Cretaceous could also provide an atmospheric CO 2 dump sufficient to overload oceans that at the time had little buffering capacity (Kurtz & Rice, 1988). It has been argued that although the marie flows of the Deccan Trap may be a source for the iridium, they could not be a source for the shocked minerals. There is, however, a common intimacy between marie flows and silicic flows, the latter which could be the source of explosive volcanism. This intimacy did exist with the emplacement of the Deccan Trap (e.g., Lightfoot et al., 1987; Setlma & Battiwala, 1977). For that matter, Sukheswala and Sethna (1977) note the appearance of patchy extinctions in phenocrysts in silicic outpourings associated with the Deccan Trap. Patchy extinctions are characteristic of mosalcism which is a diagnostic of extreme shock (see any reference to Carter herein). Although marie outpourings themselves are not to be an expected source of explosive volcanism, the converse is not necessarily true. Iridium can be expected from explosive volcanism. Note in Fig. 2 that the zoned magma chamber is mafic at the bottom and hence could also be a source of iridium. In this context, it is important to note that iridium has been found in volcanic ash layers preserved in Antarctic blue ice and at levels comparable to some K-T sites, e.g., 10 ppb (C. Koeberl, University of Vienna, personal communication, 1988). Although the Deccan Traps have attracted considerable attention with regard to the K-T debate and have been suggested as the manifestation of a single mantle plume that breaks away from the core - mantle boundary with a regularity of 30 my ~I.oper & McCartney, 1988), it has been noted (e.g., C. Officer,
77 personal communication, 1988) that not only were the Deccan Traps involved in the K/T events but there was also extensive explosive volcanism in the high latitudes of the Southern Hemisphere, e.g. Walvis Ridge, Kerguelan Plateau, Mand Rise, and Haiti. Although not well dated, both the diatreme fields in North America and Australia are of Late Cretaceous/Early Tertiary Age. There are kimberlite swarms of similar age in Africa as well as manifestations of volcanic ash throughout Europe, and Western North America. The source of the latter may have been obliterated with the San Juan volcanics. It is therefore incorrect to look just to the Deccan Traps as the volcanic cause of the K/T events as have Loper and McCartney (1988). The experiments which led Loper and McCartney (1988) to infer plume behaviour in the mantle leading to Deccan Trap type volcanism actually establish the opposite: i.e. the unlikelihood that plumes stream from the core-mantle interface through the mantle. Their experimental configuration of water below syrup overturns in a single blob without entrainment unless something akin to a silk membrane is placed between the two fluids. It cannot be expected then that the mantle is a prototype of this model unless a silk membrane exists between the core - mantle interface. The driving mechanism for this experiment are the interstices in the silk membrane which supplement the bouyancy head by an amount approximately 2 "y/r where y is the surface tension and r is the distance across the interstices. The plume will have greatest propensity to rise at the smallest grid. The Bond number for the mantle will be close to infinity and such mechanism is not expected there. These experiments fall in with a rash of others appearing in the earth science literature wherein no attempt has been made to assess if there is any figment of dynamical similitude with earth processes. No scaling is provided with this work which leaves one with no immediate sense if there is any applicability whatsoever and therefore ungraciously imposes a waste of time upon the reader. In addition, the experiment is far removed from the mantle at least in one manner. The experiment is not convecting. It has been shown elsewhere that attempted plume formation simply contributes to the mainstream flow and as in the ease of Loper and McCartney follow pre-existing routes (long established upwellings) and make no new ones for themselves (e.g. Bradley, 1986). The two dimensional theoretical analyses by Loper and Stacey (1983) suggesting plume formation is artificially constrained by boundary conditions convenient for analysis and could just as well be representative of an upwelling sheet. There are other difficulties. Theoretical attempts at modelling the core-mantle boundary layer indicate it most pronounced in the upwellings, in opposition to seismic inferences thast the opposite occurs (e.g. Lay, 1989). Further, plume mechanisms require cooling of the core, which is deemed to generate little energy but that of latent heat if ft is solidifying at a significant rate. It is accepted however, that most of the heat generated within the earth derives from the internal heating of the mantle and it is the heat source of the mantle that is responsible for mantle convection. The mantle then controls the thermal state of the core and until the mantle exhausts its internal energy sources the mantle dictates the thermal regime of the core and the heat flux from it if any (Tozer, 1989). This further implies that but for phase changes imposed by pressure (which must have been emplaced early in the history of the earth), no other phase changes can occur associated with heat flux wihtin the core until the mantle exhausts its own energy sources. This in turn means that there will be no transport of latent heat or otherwise from the core to the mantle to provide for plume formation a difficulty well recognized (e.g., Lay, 1989). Fully developed natural convection flow characterized by convection ceUs has also been known to display periodicity in transport for large enough Rayleigh numbers (e.g. Krishnamurti, 1970) and appeal need not be made to plumes of the nature of Loper and McCartney. Scaling to mantle conditions yields a range of periodicities depending on viscosity, depth and temperature differences (e.g., Rice, 1975, 1985). These periodicities are of geological time scales which include the magic number 30 million years. It is not surprising that boundary layer analyses yield time scales of similar order of magnitudes boundary layers are
78 the characterizing features of convection. Although perhaps not providing much geometric detail the treatment of boundary layers alone has yielded a great deal about expected transport rates and characteristic times of the systems. Although convection cells do possess periodicity, a combination of cells with different phases are more likely to yield a record that would appear episodic, generating a geologic signal perhaps closer to reality. There are two modes of convection in a layer of fluid: one in which the fluid rises in the center of a cell and sinks at the sides, the other in which fluid rises at the sides of the cell and sinks in the center. The latter can occur in internally heated fluids (e.g. Tritton, 1977). Appeal tO such type of convection cell in the mantle allows a surge in the upwelling along the perimeters of the cell which will yield manifestation of apparent plume activity all along the boundaries of the cell. This mode of convection will have a much larger geographic distribution than can be had by upwellings restricted to a central region. Appeal to this commonplace process in convection rather than the artifice of placing a silk membrane between the core and the mantle resolves the problem of the widespread distribution of K-T volcanics. Although Loper and McCartney note multiple plume formation in their experiments, these multiple sources converge on their way up to reach the surface as a single entity with no large surface distribution. Widespread jumps in sea floor spreading initiating at sea floor spreading ridges has been correlated with major rifting events, increased volcanism (both subareal and subacqueous), and sea level changes (e.g., see Campsie et al., 1984, and Hallam, 1984, for a literature review). Periodicities to these variations have been suggested (Vogt & Perry, 1981). The accompanying volcanism is thought to have brought about climatic changes by CO 2 injection into the atmosphere. It has been proposed that significant warming of ocean waters from episodic spreading ridge volcanism occurs even today with important consequences (e.g., El Nino) on ocean circulation and weather (Shaw & Moore, 1988). The points discussed here and the recent proposal that an Eocene CO 2 greenhouse event can be tied to enhanced or pulsed sea floor hydrothermal activity (Owen & Rea, 1985) gives indication that Shiva's abode may at last be discovered: the mantle. Footnote: It has been proposed that acid rains contributed to the K-T mass extinctions. However, volcanic CO 2 injection may be far more important than injection of acid into the atmosphere. Acid rains of pH 1.7 are common in the Los Angeles area (Roth et al., 1985) and non-anthropogenic rains of pH 3 occur in Alaska (Klinger, personal communication, 1988). Although it is estimated that the 1783 Lakagigar eruption dumped approximately a million tons of sulphuric acid into the atmosphere, the death of one quarter of the Icelandic population and three quarters of their livestock is attributed to dry flourine and dry sulfer products (Thorarinsson, 1969). Further, highly explosive volcanism apparently contains little acid (Stothers & Rampino, 1983).
APPENDIX
The difficulties in generating melt - coolant explosions in the magmatic context are briefly reviewed here (see Rice, 1985, for detail and references). The preponderance of industrial literature indicates explosive melt - coolant interactions to take place in less than a millisecond. If the explosion is due to heat transfer from the melt to the coolant, then intimate mixing must occur within this time frame that requires amongst things that the Weber number be large enough to break the melt up into droplets much less than a millimeter in diameter. This to secure a high enough surface to volume ratio to assure fast enough heat transfer to meet obvserved detonation time constraints. These observations negate speculation that lava
79 flows into the ocean will yield steam explosions of hydrogen bomb proportions as was feared might take place at Heimaey (Kanamori, 1986, personal communication). For that matter vast amounts of lava enter the sea from Hawaii without generating an explosive response, SCUBA divers filming red hot magma advancing across the sea floor under water. The constraints imposed by the industrial experience are not to be met by naturally occuring volumes of magma. For instance, pillow basatts are not of millimeter size and their occurrence argues against explosive melt-coolant interaction between magma and sea water. Otherwise they would have been blown to bits. There are further difficulties. Water above 80° C will not explode no matter how high the temperature of the melt. If the temperature of the melt is extremely high, there will be no explosion regardless the water temperature (e.g., Dutleforce et al., 1976). In addition, dry melts, i.e., melts sparged of volatiles never explode. The hosing down of white hot slag to granulate it is a case in point. This has led to the suggestion that the cause of these explosions is quench supersaturation (e.g., Rice, 1985), i.e., explosive offgassing during solidification. The quench supersaturation mechanism provides explanation for the observation that explosions will not occur for coolants too hot or melts too hot. In both cases it is likely that film boiling occurs which greatly inhibits the heat transfer from melt to coolant (by factors of 1000 or greater). This allows the melt to cool slowly in the coolant rather than be quenched which in turn allows volatiles to bleed off quietly. The above constraints will be even more rigid in the ground water context, the effective "viscosity" of earthen materials being considerably greater than that of water and impeding by as equally large factors the required rapid intimate mixing of magma and pore water in earthen material. This has relevance to a proposed explosion source for a peculiar quake off Tiro Shima which was thought to have been brought about by the flashing of water in sea floor sediments on contact with magma (e.g., Kanamori et al., 1986). The magma will pond between basement and sea floor sediments as its density may be anticipated to be somewhat larger than that of the overlying sediments. A necessary but not sufficient condition for the magma to penetrate the overlying sediments is the rough engineering rule of thumb given by (see Rice, 1981, for references)
v > \/2gha~/~ where V is the upward velocity of the magma and h the depth of the overlying material. As this estimate excludes yield strengths, virtual mass, viscosity,, etc., it is extremely conservative and for a sedimentary overburden of 3 km thickness, indicates a required penetration velocity considerably greater than 2.5 x 102 m/s. In any case, it is unlikely sediment or melt ~ break up into millimeter size particles within a millisecond as indicated to be required (see Rice, 1985) for an explosive interaction. For that matter, argillaceous sediments this thick as found in the Gulf Coast would have interlayer and pore water squeezed out of them at the basement boundary and in addition may have gone over to claystones. It would take perhaps an hour at 1100" K to cook out structural water at atmospheric pressure, this temperature needing to rise with increasing depth, i.e., pressure (R. Reynolds, Dept. of Geology, Dartmouth College, 1987, personal communication). It can be anticipated, however, that the influx of magma at the sedimentbasement boundary Hill hydrofracture the interface, allowing the magma to flood along the basementsediment boundary. This because the resisting horizontal stress 0 h = Ko c~ v where 0 v is the hydrostatic head will likely have K o very close to zero for sedimentary materials and even if K o = 1, the hydrofracturing in the vertical would be small in comparison to that in the horizontal (N. Y. Chang, Dept. of Civil Engineering, University of Colorado Denver, 1987, personal communication). Therefore the predominate source motion would be the lifting of overburden which is consistent with the predominance of the SV signal in comparison to P and SH motion reported for this earthquake. These analyses apparently
80 came from only LP data suggesting there was little SP component in the signal which in turn would be strong evidence that an explosion was not involved. If little SP component attended the signal, than a displacement with slow rise time is suggested. It is unusual for an earthquake of this magnitude (5.5) to generate tsunamis of the size reported for Tiro Shima unless it is a source with a rise time of some length, say 100s (e.g., Kanamori, 1972). It is likely that an overpressure above ambient of only several bars will see to separation of sediment from basement. If a dike is feeding the hydrofrac between basement and overlying sediment, an estimate of the time necessary to inject magma to a certain thickness can be had from A p = 13 (V212) (L/D) f where V is the average flow velocity, p is the melt density, L the crack length and D the hydraulic diameter is taken here to be half the thickness of the injected magma. The flow should stop when the injected sill is thick enough to overcome the overpressure above local hydrostatic conditions, i.e., 5-10 m thick. A large crack implies a small roughness ratio and iteration indicates a Reynolds number in the turbulent regime suggesting a friction factor f = 0.01. This will imply that a horizontal crack extending a kilometer either side of the dike could be filled to about 10 m thick in somewhat over a minute. This type of filling may also provide explanation for the peculiar directivity of the radiation pattern of this earthquake. Using E = Apdz to estimate the energy release, the above estimate of dz, i.e., 5-10 m and an overpressure above ambient of several bars yields an energy expenditure of ca. 1020 dyne cm if the effected area is some 6 - 7 km 2. It was assumed above that the sill extended in the horizontal 1 km either side of the crack, i.e., was 2 km across. Taking the flooded area to be about 3 km long provides the effected area used above. Using the relation log E s = 1.44 M + 12.24 for this magnitude 5.5 quake yields an energy release of 1.5 x 1020 ergs, similar to the first estimation above. Kanamori et al. (1986) indicate the source of the Tiro Shima quake to be a compensated linear doublet W I T H O U T a couple, hence the moment M o of this quake should be close to the above estimates of the energy release. Kanamori et el. (1986) report, however, a moment four orders of magnitude greater. The details of their estimation of moment are not spelled out but Kanamori and Anderson (1975) provide a graphical relation of M s vs M o. Kanamori et al.'s (1985) moment versus magnitude places one at the extreme left hand edge of the graphical relation provided by Kanamori and Anderson (1975), hence the curve cannot be followed out to lower moments. Further, the value of shear modulus p. assumed for this graphical relationship is 3 x 1011 dyne/era 2 whereas that of sea floor clays is about eight orders of magnitude less at a consolidation stress of ca. 370 kPa and at 5% shear strength. The deepest sediments will lie closer to the value used in the graph but if the following relationship (e.g., Pilant, 1979) is employed l o g M o = 1.5 M s + 11.8-1og(~] ~ / ~ ) we obtain a moment without couple closer to that of the above energy estimates, i.e. ca. 1020 dyne cm. This assumes a seismic efficiency "~ ca. 1 (since lifting, not sliding, is the form of displacement, little friction is assumed to be encountered in opening the crack). It also assumes ~ ca. 1 bar and p. ca. 105 dyne/era2. Hanks and Thatcher (1972) indicate a moment of this value to have a source dimension of a k m or so which supports the effected area assumed above as do the empirical formulations of Utsu and Seki (1955) and others. Little information is given regardhlg the characteristics of the tsunami associated with the Tiro Shima quake. Tide gauges don't often reflect the full height of a seismic sea wave. Apparently, the run up on the beach did not yield a bore of great height. A conservative estimate would be to pick the maximum stable height of the wave to be 1.5 m suggesting a wave height of 1 m just before landfall. Assuming constant depth
81
d, a rough guess as to the initial wave height might be had from H/K s Kf = H o where H = i m, K s is the shoaling factor, and Kf = fHo'Ks A x/d2 where f is the friction factor, H o' = the equivalent deep water wavelength, A x is normally the fetch but taken here to be the projected source dimension on the sea surface. Using values from Breitschneider (1969) suggests an initial wave height of ca. 2 m. If the directivity of the sediment displacement of 5-10 m is preserved, such an initial wave height is plausible. The point of this discussion is to emphasize that explosive volcanism is unique in its characteristics and that other mechanisms can be raised for seismic events that do not fall within these characteristics.
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VERTICAL WATER OF
ADVECTION FROM
RAPID
THE
MAIN
EXTINCTION
FROM
OXIC
PYCNOCLINE OR
RAPID
OR AS
ANOXIC A CAUSE
RADIATIONS
•• lJ
A contribution Proiect
to
GLOBAL BIO EVENTS
WILDE, Pat, QUINBY-HUNT, Mary S. & BERRY, Willim~ B. N. *)
Abstract: The vast majority of oceanic biomass fives in the surface wind-mixed layer (0-100 m) of the ocean,
trophicalty dependent on light or primary production based on photosynthesis. Waters from the main pyenocline (100-1000 m) or deeper naturally contain decay products from sinking organic matter as a function of the oxidation state of the waters. Such products, in proper concentrations, can inhibit photosynthetic growth or are toxic or debilitating to respirors. Usually, physical oceanographic processes of vertical circulation are slow or volumetrically small enough to permit "conditioning" or mixing of toxicants of the deep waters with surface waters so that the deleterious effects of deep water are neutralized or localized. However, rapid global to regional scale vertical advection of deep waters into the surface mixed layer could create an ecologic crisis for various marine groups through a combination of: (1) direct toxicity; (2) reduction or modification of nutrient and food supplies through inhibition of photosynthesis; 3) chronic debilitation caused by contact with such toxic waters; or (4) increased predation by more adaptive or less effected taxa. Such events are not necessarily universally deleterious as they could offer new opportunities for taxa ecologically restricted under prior conditions. During cool climates with oxic deep waters, a crisis may be caused by upwelling of metals concentrated with depth and resulting in reduced primary productivity, as well as metal toxic and/or chronic reactions in higher groups. During warm climates with anoxic to dysaerobic waters in the pycnocline, a crisis may result from contact with anoxic waters with a maximum effect on respirors and a minimal to enhanced effect on phytoplankton. Upwelling may come from three redox zones: I- oxic; II- nitric; and III- sulfatic. Each zone would be the source of waters of differing chemistries that could be advected into the photie zone. The effect on spedtfie taxa will be selective as a function of the depth and volume of source water, as different organisms have different tolerance limits or preadaptive capabilities. In the geologic record, significant upwelling events would be recorded initially as a general reduction in diversity, followed by mass extinctions in some groups and the possibility of rapid radiation in opportunistic groups. The ecologic requirements of both the extinct taxa and the newly enhanced taxa might be used to identify the type of any given major upwelling event.
I NTRODUCTION
Wilde and Berry (1984, 1986) discussed the oceanographic conditions which could contribute to rapid changes in marine communities and to apparent mass mortalities. They proposed physical oceanographic processes which bring chemically different waters from depth into the life range of fossil-producing organisms. Wilde and Berry (1984) concentrated on the physical mechanisms required to produce rapid overturn of deep water. Expanding on this theme, Wilde and Berry (1986) showed that chemical variability in the water colnmn with depth, produced by both oxic and anoxic decomposition of surface-produced organic matter, when introduced into surface waters, could trigger rapid changes in the marine biota. This paper discusses how the scale of vertical advection, both in depth and areal extent, influences the riving conditions of marine biotas ranging from essentially marked enhancement to rapid extinction.
*) Marine Sciences Group, University of California, Berkeley, CA, 94720, U.S.A.
86 GENERAL CHEMICAL CONDITIONS The ocean is generally stratified with respect to density (temperature and salinity) and light. Most organisms live in the photic zone. Organisms are exposed to and adapt to different chemical environments, depending on time, geographical location and where they live in the water column. In the modern ocean, phytoptankton and other organisms in the surface layers consume and sequester various metals and compounds keeping the surface waters low in nutrients (Figs 1 a-d) and dissolved metal ions (Vinogradov, 1953; Goldberg, 1957; Knauer & Martin, 1983). At depth, as organisms decay, nutrient species are released causing their concentrations to increase.
200
20
Retire,
40
400 =
=
Pacific O c e a n near H a w a i i a n Islands
600
60
Q 80O
80
1000
100
A
1200 0
120
-
10
20
Nitrate
30
40
50
0
(omollkg)
Concentration
1.0 Phosphate
2,0
3,0
Concentration
4.0
(pmot/kg)
200
200
400
400 £ o
Q
600
600
800
800
1000
1000 0
20
40 Silicate
00 Concentration
80 (IJmol/kg)
"~00
120
0 [ 0
50 I 1
100 I 2
150 pmol/kg I 3 mLIL
200 I 4
250 I 5
I 6
D.O, Concentration ( p m o l / k g )
Figure 1 : Distribution of the nutrients in the water column in the tropical-subtropical Atlantic and Pacific Oceans. A. Nitrate. B. Phosphate. C. Silicate. D. Dissolved 0 2. From Ouinby-Hunt and Wilde (1987) with permission of Marcel Dekker, copyright 1987. Data from the GEOSECS expedition.
87 The oxygen concentration in the oceanic water column varies significantly with depth (Fig. 1 d). At the surface the oxygen concentration is a function of the atmospheric concentration, temperature and salinity. Through the photic zone, oxygen is consumed or released by differewt organisms resulting in a subsurface mmfimum. Below the photic zone, as a result of the consumption of oxygen by oxidation of decaying organic material, the concentration declines dramatically. Below the pycnocline, the oxygen concentration in the modern ocean increases due to an injection of high latitude deep and bottom waters. Oxic waters are defined as having chemically active dissolved oxygen as the primary oxidant. If sufficient organic matter is present, all dissolved oxygen will be consumed and other agents will be used to oxidize organic matter. In the oceans, the thermodynamic sequence of available oxidants after free oxygen are the oxygenated nitrogen species followed by oxidized sulfur species, principally sulfate. Fig. 2 shows the areas of the modem ocean with dissolved oxygen below 0.4 mL/L. In the modem ocean the result of oxidation by oxygenated nitrogen species may be seen in the eastern tropical Pacific (Fig. 3).
.
III11~ e a wi~ maximum den~tT" of E. eximia ~ " ~ /Yea with maximum density of E. distinguenda
Figure 2 : Hachured regions show the distribution of modern marine waters with dissolved 0 2 < 0.4 mL/L (After Denser, 1975). Shaded areas show characteristic species endemic to oxygen-poor waters in the eastern tropical Pacific (After Brinton, 1980). Projection is Goode's Homosline equal-area, copyright, University of Chicago. (From Berry et al., 1987, with permission of Goological Society of Denmark,
copyright, 1987). In the nitric zone, nitrate and nitrite act as oxidizing agents producing nitrite, nitrogen and zmmonia. The concentration of nitrite in the nitric zone initially increases with depth, then decreases as nitrate is consumed, and subsequently, nitrite assumes the role as oxidant (Fig. 3). In the suffatic zone, sulfate is the oxidizing agent resulting in production of sulfides and ammonia. The sulfatic zone exists today in the Cariaco Trench, the Black Sea and in some tjords, such as Lake Nitinat. In the ancient oceans, particularly in warm, calm conditions, the extent of the nitric and sulfatic zones would have been greatly expanded. In the photie zone, various metals and compounds are taken out of sea water during primary production and released back into sea water below the photic zone by oxidation of sinking organic matter (Redfield, Ketehum & Richards, 1963). The distribution of such metals in the water column generally follow the concentrations of dissolved nutrients with depth, but as a function of their solubility (Fig. 3).
88 For oxic conditions seen in the modem ocean, Quinby-Hunt and Wilde (1987) reported that the concentrations of Cr, Ni, Zn, Ge, Se, Sr, Cd, I, and Ba correlate quantitatively with the concentrations of the nutrients N, P, or Si. In addition, Be, Mn, Fe, Co, Cu, Ga, As, Pd, rare earth elements, Hg, Rn, and Ra also are partially nutrient-related, but no relatively simple expression has been developed relating these elements with nutrients. As the concentration of the nutrient-related elements generally increases with depth in oxic waters, their effect on organisms living in the photic zone as a function of concentration can be related to the depth of vertical advection or upwelling. The concentrations of some of these elements, notably Fe and Mn, also controlled by scavenging and redox considerations, tend to decrease below the mixed layer, although the concentration may increase to some degree in the oxygen-minimum zone. Under anoxic conditions, the distribution of nutrient-related elements may differ from that under oxic conditions. If the waters are from the nitric zone (Wilde, 1987), such as the denitrification zone of the Eastern Tropical Pacific and the Northern Arabian Sea of the Indian Ocean, then the concentrations of many dements may be expected to increase. Notably, the concentrations of Fe and Mn might be expected to increase because neither the O =, or S =, which tend to precipitate these elements are present. Other elements whose concentration in the oxic and sulfatic zones is solubility controlled will also increase. If anoxia has reached the stage of sulfate reduction, those elements in sea water which form sulfides will form mineral complexes and be removed from the water column. Thus, Cu, Cd, and Zn are depleted below the redoxocline in the Black Sea (Brewer & Spencer, 1974) and the Cariaco Trench (Jacobs et al., 1987) (Figs. 4 b, 4 c). In anoxic waters, metals that form sulfides are depleted. In the sulfatic zone, Fe is more depleted than in the nitric zone, but appears in significantly higher concentrations than in the oxic zone (Fig. 4 a) due to greater solubility of the iron sulfide vs ferric hydroxide. Manganese occurs in greater concentrations in the nitric and sulfide zones than in the oxic zone (Fig. 4 d). Oxygen Minimum Zone Off Peru Micromoles/Liter 02
0
250
NO~ 0 5 0 '~a.,~&.;---~,
500 10 ,
,
,
Au~ 2 0 0
200
4oo
,oo
600
i o2 i ;
800 0
, 20
, NO 3
~ 40
.o3 ~. , ~ T 60
600
oo,,L .-.M.~ ~ 320
a
800 280
240
200
160
120
80
40
0
Distance From Shore (kilometers)
Figure 3 : Vertical distribution of nitrate, nitrite and dissolved 0 2 .in the eastern tropical Pacific off Peru (After Anderson et al., 1982). From Berry et al., 1987, with pernnsslon of Geological Society of Denmark, copyright, 1987.
Figure 4 (opposite side) : Comparison of the vertical distribution of trace metals in the oceanic water column in the modem oxic ocean and the Cariaco Trench. A. Iron (N. Pacific: Gordon et al., 1982). B. Copper (N. Pacific: Bruland, 1980). C. Cadmium (N. Pacific: Bruland, 1980). D. Manganese (N. Pacific: Martin & Knauer, 1984). All data for the Cariaco Trench are from Jacobs et al.. 1987.
89
Cu conc. (nM/kgJ
Fe conc. {nM/kgJ
i
t
l
l
t
r
(
l
l
l
l
o o o o o o l t l
l
/ f Cariaco Trench
Cariaco Trench
N.Pacific
o o
N.Pacific
A Mn conc. (nM~
Cd conc,(pM/kg) (Thousands)
o
~
~
~
~
g
~
~
~
~
-
=
I
Cariaco Trench
N.Pacific
N.Pacific
o
o
o Cariaco T r e n c h
C Figure 4 (see opposite side for explanation)
D
90 PHYSICAL
PROCESSES
OF V E R T I C A L
ADVECTION
Wilde and Berry (1986, p. 81) discussed three major geographical scales of upwelling produced by several physical processes: I. Planetary (Oceanic); II. Regional; and Iit. Local. Upwelling on a oceanic scale may be the result of divergence of water by Ekman transport produced by change in direction of zonal surface winds at boundaries of major climatic belts. The effective maximum depth for upweUing in this case is that of mixed layer (ca. i00 meters); the vertical rise would be 10 to 80 meters/month. (Apel, 1987, p. 270, 10-6 m/see = 30 m/yr, equatorial). Divergence due to Ekman transport occurs at the equator and the temperate/polar boundary (60°N and 60"S). Vertical advection on an oceanic scale also may occur as a result of displacement by continual renewal of water masses at source. In this case, the vertical rise is of the order of milllmeters/day. Thus, one mixing cycle occurs about every 1000 years (3 m/yr: Apel, ].987). Overturn of deep water to the surface could also occur on an oceanic scale. The vertical rise probably would be rapid, greater even than for Ekman transport. Horizontal movement is from high to lower latitudes. Regional (].00 to 1000 km) upweUing may be due to seasonal Ekman transport: for example, due to atmospheric high pressure off western coasts. Regional upweUing also may be due to off-shore advection, that is, entrainment by major oceanic currents moving off-shore or by vertical Coriolis deflection and surfacing of Equatorial undercurrents as eastward flowing undercurrents rise. Wilde and Berry (1986, p. 8].-82) listed some of the physical processes that can produce local ( < 100 kin) upweUing. Obstruction of horizontal current flow by banks or seamounts produce vertical advection in Taylor columns along the leading edge of the obstruction. Closed cyclonic eddy circulation, that is, oceanic "weather" spin-off, and migration of cold-core eddies from Rossby waves in major currents may cause vertical advection of deep waters causing local upwelling. Bernoulli uplift, by flow through constricted straits; breaking internal waves or internal surf also cause local upwelling.
BIOLOGICAL
IMPLICATIONS
DEPTH AND REDOX
OF T H E V A R I A T I O N
OF CONCENTRATION
WITH
CONDITIONS
The effects of rapid global to regional vertical advection of deep waters into the mixed layer could cause ecologic crisis due to: (1) direct toxicity or enrichment; (2) reduction/modification of nutrient or food supply;, (3) chronic debilitation due to contact with deep waters; and (4) increased predation by taxa adapted to the new water mass chemistry. The effect of a dissolved chemical on a biotic community is a function of the concentration of that chemical, the temperature and pH of the water, and other constituents present in the water column. A particular compound may be inert (no effect), limiting (required at a certain minimum concentration), inhibiting (also referred to as sublethal or chronic, that is debilitating for short exposures or with lifeshortening consequences), or toxic (lethal at certain concentrations). Elemental concentrations do not always indicate whether the above effects will occur. Chelating agents, antagonistic or synergistic elements (antagonistic elements reduce the effect of an element; synergistic elements enhance the effect), redox state, pH or temperature modify the actual chemical configuration or activity experienced by the organism (Bewers & Yeats, 1977; Anderson & Morel, 1978, 1982; Thomas et al., 1980). Thus, the activity of the chemical species rather than the absolute concentration is the important consideration (Jackson & Morgan, 1978). Fe, Mn, and Zn are limiting for a number of organisms. Diatoms, eoccolitt~, dinoflagellates, and
91
cyanobacteria are Fe-limited in neritie waters for activities less than 10-7 M and in pelagic ocean waters at activities less than 10-9 M (Brand et al., 1983). The concentration of dissolved Fe in the open North Pacific Ocean is much less than 10-9 (Gordon et al., 1982). The same groups of organisms are Mn-limited at activities less than 10-10 M in neritie waters. In the oceanic pelagic realm, all but coccolitlas are limited at less than 10-10 M. In the open ocean, dissolved Mn is about 0.2-4 x 10-9 in the mixed layer (Landing & Bruland, 1980; Martin & Knauer, 1984). Concentrations of both Mn and Fe are sufficiently low in the open ocean that they may be controlling for some protic species (Martin & Gordon, 1988; Foster & Morel, 1982). In neritic waters, Brand et al. (1983) reported that certain neritic diatoms are Zn-limited at activities less than 10-11"5 M; but in the pelagic oceans, other phytoplankton (including diatoms, coccoliths, dinoflagellates and cyanobacteria) are not limited even at activities as low as 10-13 M (Brand et al., 1983). Although metals such as Zn and Cu are limiting at low activities, they may be toxic at high activities (Sunda & Guillard, 1976). For example, the deleterious effects of copper on the diatom, Thalassiosirapseudonana, are chronic above activities of 3 x 10"11 M, and toxic above 5 x 10"9 M. The concentrations of Cu in the upper open ocean are of the order of 10-9; therefore organisms with sensitivities similar to T. pseudonana exist in conditions where Cu concentrations, even if strongly chelated, are inhibiting to growth. Not all of the nutrient-related elements have either a plastic or catalytic role (senso Dietrich, 1963, p. 246) in the biology of marine organisms, particularly those in higher trophic levels. Some elements or compounds are sequestered passively in the lipids of living organisms (for example, the highly toxic methyl mercury, Boney, 1975, p. 103). For lower trophic levels, such concentrations may be harmless. But by concentration in successive trophic levels, certain dements or compounds may reach chronic or toxic levels detrimental to organisms ha higher trophic levels. Provasoli (1963) found such a situation for the toxicity of dinoflagellates to higher trophic level during Red Tide upwellings off the southwest coast of Africa. Morel (1986) has noted that for algae, which in the ocean are the major primary producers, Cd, Pb, and Hg (all non-essential elements) are toxic at low concentrations. Accordingly, a crisis could be produced by upwelling of enriched metals or other compounds from deeper water into the surface layers. UpweUed toxic waters that inhibit photosynthesis could cause a crisis in groups with short food chains by reduction of primary production. During warm climates and potentially anoxic waters in the pycnoctine below the mixed layer, the crisis could be due to low levels of oxygen and its related chemistry so that more active organisms would be affected more than sessile or low-oxygen tolerant organisms. Conversely, upwelling of nutrient species including those trace metals whose concentrations are limiting, could result in enhanced living conditions for some species, resulting in a bloom. In either case, the species composition may change (Sunda et al., 1981). Sunda et al. (1981) have shown that in water from off the North Carolina coast in which the diatom Nitzschia and the green flagellate Chaetoceros where common elements of pelagic communities, Cu additions caused a shift in species dominance from the diatoms to the flagellate.
POTENTIAL
I M P A C T S OF U P W E L L I N G E V E N T S
The effects of upwelling on biotas depend on the depth of upwelling which will in turn control the physieo-chemical conditions of the upwelled water. For modern open ocean conditions with well-ventilated deep waters, generally oxic conditions occur regardless of the depth of upweUing. In restricted locations where a major denitrilication zone occurs, such as the Eastern Tropical Pacific (see discussion in Berry et al., 1987), waters representative of the nitric zone might be vertically advected during enhanced upwelling.
92 In basins such as the Cariaco Trench, the Black Sea, or tjords such as Lake Nitinat, upwelling of sulfatic waters could occur. Brongersma-Sanders (1957) and Richards (1965) have given various incidences of mass mortalities due to the introduction of sulfides into the water col~lrnn: For the Phanerozoic, Wilde (1987) proposed that three redox zones: oxic; (2) nitric, i.e. anoxie, with no sulfide production; and sulfatic, i.e. anoxic, with sulfate-reduction, existed in the open ocean on a regional to planetary scale. This zonation assumes a wind-mixed oxygenated surface layer of 50 to 100 meters below the surface. Variations in redox occurred in the underlying pycnoeline as a function of the efficacy of deep ventilation (Wilde & Berry, 1982), which, presumably, was related to climate. Upwelling of increasing depth would result in penetration into the superimposed oceanic redox zones. Thus, for an ocean with a discrete anoxic layer below the surface mixed layer, such as proposed by Wilde (1987; Wilde & Berry, 1982), there is a potential for u p w e 11 i n g f r o m a 11 t h r e e z o n e s at the same location, depending on the depth of upwelling. Upwelling, in order to influence extinction or killing events, must be sufficiently rapid that chemical eqttilibrium is not attained during the rise.
Upwelling from the Oxic Zone
With duration of upwelling and/or increasing depth of the source of upwelled water, more decay products would be brought into the photie zone as oxidized metals. Therefore, with increasing depth, increased quantities of the dissolved micronutrients, N (as nitrate), P, and Si wilt be brought to the surface layer. Increased levels of Cd, Cu, Zn, Co, Ni, Se, Cr, Ba, Ge, As, Pal, Te, I, REE and organic decay products are also expected. Lower concentrations of dissolved oxygen would be brought to the surface from the oxygen minimum zone. Fe and Mn concentrations generally decline with depth, although there can be increased concentrations in the oxygen-minimmn zone. Thus, Cu and Cd algal toxicity would increase. However, the availability of nitrate, phosphate and silicate would increase. The upwelling of Mn-poor waters, such as those of the Sargasso Sea, may result in acute Mn deficiency (Sunda & Huntsman, 1983). The conjunction of waters low in Mn and higher in Cu could result in enhanced toxicity as Cu is antagonistic to Mn-usage in some diatoms (Sunda & Huntsman, 1983). A similar relationship exists between Cd + 2 and Fe + 3: Cd toxicity can be reversed if there is sufficient Fe + 3 present (Foster & Morel, 1982). Upwelling of deep oxic waters depleted in Fe + 3 or Mn, but with high levels of Cd + 2 or Cu could have greater to,deity for some species than would nitric zone waters that would have dramatically increased levels of Fe and Mn. Due to the influx into the photic zone of unchelated metal ions, such as Cu or Cd (Barber et al., 1971; Terry & Caperon, 1982) and low levels of 02, photosynthesis would initially be suppressed even though additional nutrients also would be entrained. If the depth of upwelling is great enough, suppression might continue to drastically reduce primary productivity. This would be particularly critical in middle to high latitude locations if the upwelling occurs during the normal time of plankton growth when seasonal light levels also are critical. With the suppression of primary productivity, effects would be felt in the higher trophic levels in the water column and eventually among the benthic seston feeders. Under most situations involving upwelling of oxic waters, a phytoplankton bloom (due to increased levels of nutrients), and an increase in zooplankton grazers would be expected to follow the initial suppression. These upwelling conditions could be advantageous to benthos above the oxygen minimum zone. If the oxygen demand of the bloom is excessive, eutrophication may occur causing death of respirors and benthos if anoxia reaches the bottom. Blooms of certain dinoflagellates may be inherently toxic to predators.
93 Upwelling from the Nitric Zone
In the nitric zone (where nitrate is the oxidant), concentrations of micronutrients (P as phosphate, Si as silicate and N as nitrite and ammonia) increase with increasing depth. Certain trace met',d concentrations, Fe, M.n, Cd, Cu, increase with depth because of the lack of precipitating anions such as sulfide. The nitric zone occurs at relatively shallow depths at the top of the pycnocline, about 100 m. The increased concentrations of micronutrients, as well as reduced nitrogen compounds 0~ppley et al., 1969) and the limiting trace metals, Fe and Mn, would increase productivity and may suppress Cu or Cd toxicity. In the nitric zone, there would be reduced nektonic activity, and little or no benthic megafauna. The oxygen-poor waters would inhibit respiring organisms, especially nekton. Such conditions would give advantage to specialized low-oxygen tolerant types, such as the copepods Euphausia distinguenda and E. eximia (Brinton, 1980), which occur in the eastern tropical Pacific off Peru (Fig. 2). The sequence of biological events as a consequence of upwelling from the nitric zone initially would be enhanced primary productivity, but rapid reduction in active grazers and nekton. Increased productivity would secondarily add to the oxygen demand in the water column and cause a reduction in benthic populations, even with additional seston food supply.
Upwelling from the Sulfatic Zone
In the sulfatic zone, micronutrient concentrations, N (as ammonia), phosphate, and silicate, increase with depth due to continuing decay of organic matter. Concentrations of Fe would be greater with depth than in oxic ocean waters, though probably less than in nitric zone waters (Fig. 4 a). Mn concentrations are greater in the sulfatic zone than in the oxic zone, and similar to those in the nitric zone, based on observations from the Cariaco Trench (Fig. 4 d). The concentrations of Cu, Cd, Zn, and any cations that form sulfides will decline due to sulfide precipitation (Figs. 4 b, 4 c). Ni is unchanged with depth. Sulfatic zone waters are oxygen-depleted, and with depth have increasingly higher concentrations of toxic hydrogen sulfide. During upwelling of sulfatic zone water, primary productivity will increase due to increased levels of ammonia, particularly for those phytoplankton that are Fe- or Mn-limited. On the other hand, primary productivity may decrease among other phytoplankton due to low levels of trace nutrients, such as Zn and Cu. For animals, acute hydrogen sulfide toxicity will cause losses of nekton and benthic megafauna with an excellent chance of preservation of large organisms in anoxie bottoms. Thus, upwelling from suIfatic waters would be characterized by enhanced primary productivity and elimination of nekton and respiring benthos.
P O T E N T I A L E X A M P L E S IN G E O L O G I C RECORD
Evidence of extinctions or killing-events in the geological record would appear as reductions in diversity, mass extinction of some taxa, followed by rapid radiations among surviving and newly adapted organisms if conditions remained stable. Survivors could return if preexisting conditions were reestablished. Evidence for mass mortality/radiation events due to vertical advection from the oxie, nitric or sulfatic zones may be found in the rock record and are discussed below. Possible examples of vertical advection from the oxic zone are found in sequences from the end of the Ordovidan and from the end of Triassic. A mass mortality/radiation event in the late Weuloek could be due to upwelling from the lower nitric zone. The extinctions evidenced in the early Jurassic Toareian shales may be the result of upweUing from the sulfatic zone.
94 Advection from the Oxie Zone
At the end of the Ordovician, graptolites suffered mass mortality at various tropical localities, including Dob's Lima, Scotland, Anhul, China, and Mirny Creek, USSR (Berry et al., this volume). The latest Ordovieian graptolites were restricted to limited areas in the tropics of the time. Each incidence of near extinction was a cousequence of a local event. At each site, the environment changed from one inhabited by graptolites to one in which graptolltes were virtually excluded. That change involved introduction of oxic waters into the oxygen-poor waters inhabited by graptolites resulting in graptolite mass mortality. Such oxygenation could be the result of vertical advection of pyenoclinal waters vent~ated during the glaciation that occurred at the end of the Ordovidan. Advection of colder, more oxygenated, and more metalliferous water may have affected the graptolites themselves or their food supply. In the late Triassic, House (1985) has shown a massive, world-wide decline of ammonites in the interval of the latest Norian through Rhaetian, the latest part of the Triassic. By the close of the Triassic, ammonites had become virtually extinct. Triassic ammonites lived primarily on the margins of the tropical shelves of the time. They seemingly lived primarily in low oxic (dysaerobic) waters under similar environmental conditions as those inhabited by graptolites at an earlier time. During the late Triassic, major marine transgressions began across many of the low-lying continental areas (Sellwood, 1978). Anderton et al. (1979) described a major transgression in Britain that began in the south and spread initially over lagoons. Volcanism occurred in southern Europe, and rocks from the Aquitaine and New England suggest that an initial phase of North Atlantic rifting began in the late Triassic (Anderton et al., 1979). Accordingly, the new ocean became connected to the world ocean with better circulation. As the Atlantic opened, the rise in sea level presumably was accompanied by advection of more oxic waters into ammonite habitats. Introduction of these waters resulted in ammonite extinctions that took place over a several million year interval. The length of time of the extinctions suggests that the demise of the ammonites may have resulted from a long-term chronic condition. Throughout the Jurassic, ammonite diversity increased, notably in species better adapted to oxygenated waters. Apparently, as a result of the opening of the Atlantic, new patterns of vertical advection supported the radiation of these species.
Advection from the Nitric Zone
In the late Wenlock, mass mortalities of tropical graptolites (Rickards et al., 1977) occurred concurrently with development of massive carbonate reefs after a long interval of limited reef growth (Berry & Boucot, 1970; Ziegler et al., 1974), as did the development of land plants with vascular tissue (Cooksonia) (Edwards & Feehan, 1980). Both marine events were of relatively short duration. The marine events took place in water oceanward from the shallow subtidal zone. The oldest vascular tissue was found in early Ludlow strata in Wales, which was tropical in the Silurian. These events can be explained by expansion of the nitric zone toward the surface. UpweUed waters from the deep nitric zone would contain no oxygen and would be elevated in ammonia (see above). Vertical advection of anoxic waters could have caused mass mortalities of graptolites via a number of mechanisms. Mortalities might be caused by lack of sufficient oxygen to support either graptotites or their food. If graptolites were forced into upper levels of the mixed layer, they may have been exposed to greater predation. By the time ammonia was advected into shallow waters, it would have been diluted sufficiently to be a nutrient rather than a toxicant. This diluted ammonia would have increased productivity, thereby providing increased food supply for the reef-building corals.
95 Increased levels of ammonia could also increase the alkalinity sufficiently to favor carbonate precipitation necessary for coral reef building. Nutrient levels of ammonia in tidal waters may have provided the necessary fertilization for the development of vascular land plants in the nearshore environment. As upwelling from this zone ceased, the graptolites reradiated, and coral reefs receded.
Advection from the Sulfatie Zone
Hallam (1967, 1975, 1977, 1981), Jenkyns (1985), Wilde et al. (1986), and Riegel et al. (1986) discussed the Lower Jurassic Toarcian extinctions in terms of physical conditions relating oceanic redox conditions with transgression and regression. The extinctions can be explained in terms of an upweUing of sulfidic waters. Toarcian rocks in Great Britain and Europe are organic-rich shales with a non-bituminous argillaceous facies above and below. The fauna is characterized by nektonic ammonites, belemnites and fish scales; and mierofossils: ostracods, foraminifera, and radiolarians (Kauffman, 1978, 1981). The benthic fauna are of limited diversity with large populations. Massive mortalities, consistent with upwelling from the sulfidic zone, are observed in Yorkshire at the base of the Jet Rock (Hallam, 1967; Jenkyns, 1985) and in Germany, in the Posidonia Shale (Riegel et al., 1986). In the Posidonia shale, pyritic concretions, indicative of the presence of sulfidic waters, are common in the lower bifrons zone (Riegel et al., 1986).
CONCLUSIONS Vertical advection or upwelling is a mechanism that could cause selective extinctions or killing events. By this mechanism, step-wise or gradual extinctions are readily explained as are survivals. The impact of upwelling is a function of depth, temperature and the chemical properties of the source water. The redox properties, that is whether the water is ode (oxygen as oxidant), nitric (oxygenated-nitrogen species as oxidant), or sulfatic (sulfate as oxidant) control the effect of such waters on the biota. The chelating tendencies of the water, organismal tolerances, and the presence or absence of antagonistic or synergistic elements may determine whether an organism thrives, exists or dies. Impacts equally depend on the taxa present, their tolerance limits and preadaptive capabilities. The results of massive upwelling events would be characterized in the geologic record by decreased diversity of species, and, possibly, certain areas depanperate of species followed by radiations.
ACKNOWLEDGEMENTS
The authors wish to thank Profs. A. Boucot, W. Holser, E. Kauffman and O. Walliser for useful discussions and encouragements at Boulder. M. Krup drafted the figures and coordinated the preparation of the manuscript with her usual efficiency. This is contribution number MSG-88-003 of the Marine Sciences Group, University of California, Berkeley.
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AN A S T R O N O M I C A L
EXPLANATION
LOUS CONCENTRATIONS DURING CATASTROPHIC
OF IRIDIUM EXTINCTIONS
OF
ANOMAELEMENT
•? U
A to
contrlbutlon Project
GLOBAL BIO EVENTS
YANG, Zhengzong *)
Abstract: In this paper a new possible explanation of iridium anomalies in the extinction boundaries is proposed. I found that the impact of an object is not necessary to be in the theory when we try to explain the iridium anomaly.
INTRODUCTION
Kepler's law states the solar system revolves around its galactic center and therefore must be affected by many astronomical factors. These effects can bring about many anomalous events in the solar system, which can, in turn, affect the Earth. There are many astronomical factors that can ~ffect the Earth directly in the solar system. These combined effects may cause many occurrences of rare events on Earth. Rare events are also produced by the Earth itself. Many catastrophic events of varying magnitude have occurred since the Earth was formed. When these events occur, they may reflect certain galactic laws. In this paper, the anomalous concentration of Ir in the K-T boundary clay is discussed. There are many causes of catastrophic events. Many astronomical phenomena, for example the impact of an asteroid or a comet showers, cosmic dust storms, etc. can result in a series of rare events including anomalous concentrations of Ir, mass extinction, and glacial ages. We have no definite explanation for the anomalous concentration of Ir at the K-T boundary. Several models have been proposed to explain this phenomenon, such as the impact of an asteroid or a comet, the explosion of a supernova, etc. Here we propose a model for the occurrence of a large meteoric storm at the end of the Cretaceous.
A
POSSIBLE ASTRONOMICAL EXPLANATION OF I R I D I U M ANOMALIES
What is the cause of iridium anomalies at the extinction boundaries? What was its influence throughout geological history? Many workers have tried to interpret the iridium anomaly found at the K-T boundary. Some think it was generated by a reaction inside the Earth (e.g. volcanism). But this hypothesis has difficulty in explaining some of the facts (e.g. trace element composition, microsphere, multiple shock lamellae). Some workers have ascribed it to an extraterrestrial cause. There are four major extraterrestrial hypotheses that have been proposed: the explosion of a supernova (Napier & Clube, 1979), the impact of an asteroid (Alvarez et al., 1980), the impact of a comet (Urey, 1973; Hsu, 1980), and periodic comet showers triggered by close passage of solar companion star or field star. Other hypotheses exist (Gartner & Keany, 1978; McLean, 1978; Hut et al., 1987; Rampino & Volk, 1988). Alvarez et al. (1980) proposed that an asteroid impacted on the Earth, causing the iridium anomaly. However this hypothesis has a problem: An asteroid is a solid mass. When it impacts on the Earth, the kinetic energy transferred creates a huge *)
Beijing Astronomical Observatory, Academia Sinlca, Beijing, China
100
meteorite crater and impact debris. This hypothesis fails to explain why the density of iridium in some areas, or craters is less than the normal predicted density of Ir. In 1973, Urey proposed the cometary hypothesis. Hsu (1980) elaborated on it further. This hypothesis has some advantages. But it has similar difficulties as the Alvarez et al. (1980) hypothesis, the main characteristic of the two hypothesis is the impact. In this paper a new hypothesis is proposed: Anomalies of iridium at the K-T and other extinction boundaries were the result of large meteorite storms. The process is described as follows. A meteoric shower (not a comet shower) is composed of many meteors falling into the atmosphere in a short period of time. A large meteorite storm is judged by the scale of the meteoric stream. There are several sources which may produce a large meteoric storm: a) A comet may collapse in on itself after revolving round the sun for about 106 years. The collapsed comet turns to many meteoroids. Newly formed meteoroids distribute nonuniformly into Earth's orbit. The large meteoroid group is more nonuniform in distribution. The meteoroids scatter because of radiations. A large meteoric shower would occur if the Earth ran into a large meteoroid group, b) Owing to the perturbations of stars in the Galaxy and planets in the solar system, a comet which is normally far from the sun could enter the Earth's gravitational field. The Earth's tidal action and radiative heating of the sun could force the comet to break apart if it is made of brittle matter. The collapsed comet becomes many meteoroids. If the Earth attracts these meteoroids into the atmosphere in a short period of time, a large meteorite storm will occur. The influence of a large meteoric storm on the Earth can be estimated by a simple calculation. The mass of a large meteorite storm is nearly the mass of a comet, about 1013 tons. In normal situations the total mass of the fallen meteors is about 2 x 105 ton each year. When a large meteoric storm occurs the total mass is 108 times higher than normal. The total volume of the meteors in a large meteorite storm is: M
V -- - ~ = 1019em 3
(1)
where M is the total mass, which is about 1019g, and D is the density of the meteoric stream which hasn't scattered; this is the same as the density of the cometary head, about I g/cm3. Thus, after the occurrence of a large meteoric storm, the thickness of the meteoric dust deposited on the Earth's surface is: h= ~
V
= 1.96era
(2)
where R is the radius of the Earth. This estimate of the thickness is nearly at the upper limit (h varies with M). The fallen dust of meteoric debris would be very thick, and sufficient to be one of the causes of iridium anomalies. After a large meteorite storm appears, the time of suspended meteor dust in the atmosphere can be calculated. According to the Newton's law, we have dv m - ~ = m g - 67r~rv
(3)
where ~ is the viscous coefficient of the atmosphere (we take it as 180 x t0-6p), r is the radius of the dust (the dust is spheric in the model), and m is the mass of the dust. The equation can be rewritten as dv ~ + c v =a
(4)
101
where
c = 6~r~?r/m.
We assume that v = v0 at h0, the equation is solved v=
~ ( e ° ~ + v ° - - £c - 1)e - ° t g
(5)
ho = tt + ~(~0 + ~)(I- e -°')
(6)
and
C
Once we knov¢ c, we can fred the settling
180 x lO.Sp (atmosphere)
1.5x 10"2p (sea)
0,01
2.7
225
0.001
270
225 x 102
0.0001
27000
225 x 106
time t of the dust. Putting the corresponding data into the above equations, we can obtain the constant c and settling time t.
200 km (atmosphere)
100 km (atmosphere)
1000 m (sea)
0.01
0.6
0.3
0.3
0.001
64
32
26,5
Table 2 : particles.
17.5 years
culates c. In Table 2, the settling times are calculated. During the settling time the
Table 1 : The value of constant c.
0.000t
Table 1 gives reasonable 3] and r and cal-
8.75 years
7,3 years
dust is floating in the atmosphere, and may block radiation from the sun. This effect can be calculated as follows. The decrease of the radiation flux in a layer dl is proportional to the number of dust particles and size of the particle. So we have:
The settling time of different radius of dust
dI I
N=r2dl
(7)
where I is radiation flux and l is penetration length, which is h 0 in our case. Solving this equation we derive: I = Ioe - m ~
(8)
= e-x = T
where T is the fraction of the solar radiation penetrating the dust. N I is the number of dust particles per unit area on the Earth's surface. If we take the mass of the comet is 1015g and r is 0.001 cm, we have T = 1. If we take the mass of comet is 1018g, we have T ~ 0. Here we can see that if a meteoric storm caused by a collapsed comet is small (M = 1015g) and has only slight influence; but in a large storm caused by rather big meteoric (M = 1018g), the situation will be very serious and mass extinction may occur.
THE
EFFECT
OF A LARGE
METEORIC
STORM
ON THE
EARTH
When a large meteoric storm appears in the atmosphere, one possible effect on the Earth would be to change the temperature. The temperature would increase in a short time and then decrease. The low temperature phase would remain for a long time. The explanation is as follows. When a meteor falls into the atmosphere, it is burning due to friction. Finally, it may totally become dust before impact. The dust remains suspended in the atmosphere. In this process the potential energy of the meteor turns into kinetic energy and then turns into heat. The heat increases the temperature of the
102
I
Cornet (in the spacesphere)
I
Meteoroids (in the spacesphere)
l1
t
''' A laq~ n ~ t ~ storm (inthe spacesphete) i
XCN + 0 2 . . . . . • + CO2 C02 in the meteoroids (in the atorosphere)
I RichIr in the dust (in the lithoshphe~e) The anomalyof Ir coverslargearea aroundthe world
Reductionof radiativeenergy I I from the sun (in the atomsphere)
T
1
"'
....,,
t
I
Lessdeathsof marine t t I Lessdeaths I Jbiocommunilies J I°!f'~n"=~l
I
balanceof CO2 (in the atomspehre andwatersphere)
Damageof (ntPh°~¢:Y~o~hpe~ei']Sre)I~eari~r~2:here,,I
!
t
Increaseof the temperature J (in the atomspher)
I
Damageof CCD (in the hy~oc~phere) I
t
I~°fterrestlia'
I
t
T
I Deathsof calcurcous marinefauna 1 Figure 1 : The Extinctional Process by a Large Meteoric Storm.
atmosphere because of conduction and circulation of the air. With the diffusion of the dust into the atmosphere, the atmosphere becomes densely saturated. According to my calculations, the intensity of light would be nearly zero on the ground. If the surface of the Earth cannot receive radiative energy from the sun, very low temperatures will result. The affect of this scenario on the biosphere is obvious. The dust prevents the plants from receiving adequate sunlight, the plant's photosynthesizing process weakens, and plants broadly die out. The rapid decimation of the plants destroys the base of the food chain, and as a result many kinds of animals die out. Depending on the nature in the atmosphere the meteoric dust may be distributed uniformly, causing a global effect for darkness, fallout debris and iridium anomalies. After a large meteoric storm occurs, changes in hydrosphere are the same as predicted for a comet falling into the sea in that meteoric dust will eventually fall into the hydrosphere, contaming the water down to the bottom of the sea. When a comet collapses, its chemical composition does not essentially change. After many meteors fall into the atmosphere, its chemical properties should also not change. The process of extinction by a large meteorite is described in Fig. 1.
CONCLUSION The difference in the present hypothesis and previous ones is in astronomy. According to our hypothesis the iridium anomaly at extinction boundaries is explained by a large meteoric storm. This hypothesis is supported by astronomical calculations, which may explain why we have failed to definitely fred the impact crater causing the K-T anomaly iridium, although the Manson structure is a strong candidate.
103
We agree that impact events do occur in the Earth, which can be confirmed by comparing the Earth's surface with other planets, and actual craters on Earth. However, these impact events may not be the sources of the anomalous iridium. In my theory an impact is not necessary to explain anomalous iridium levels. The Ultimate effects of a meteorite storm on Earth geology and paleontology in my theory are similar to those of other theories. The current theory is a reasonable alternative to other hypotheses.
REFERENCES
Alvarez, L. W. et al. (1980): Extraterrestrial Cause for the Cretaceous-Tertiary Extinction. - Science, 208, 1095-1108. Davis, M. et al. (1983): Berkeley Laboratory, preprint LBL-17298. Gartner, S. & Keany, J. (1978): The terminal Cretaceous event. A geologic problem with an oceanographic solution. - Geology, 6, 708-712. Hsii, K. J. (1980): Terrestrial catastrophes caused by cometary impact at the end of Cretaceous. - Nature, 285, 201-203. Hut, P. et al. (1987): Comet showers as a cause of mass extinction. - Nature, 329, 118-126. McLean, D. M. (1978): Land Floras: The Major Late Phanerozoie Atmospheric Carbon Dioxide/Oxygen Control. - Science, 200, 1060-1062. Napier, W. N. & Clube, S. V. M. (1979): A theory of terrestrial catastrophism. - Nature, 282, 455-459. Rampino, M. R. & Volk, T. (1988): Mass extinctions, atmospheric sulphur and climatic warming at the Kfr boundary. - Nature, 332, 63-65. Urey, H. C. (1973): Cometary Collisions and Geological Periods. - Nature, 242, 32-33. Whitmire, D. P. & Jackson, A. A. (1984): Are periodic mass extinctions driven by a distant solar companion? - Nature, 308, 713-715.
EVOLUTIONARY CRISIS WITHIN ClAN ACROTRETID INARTICULATE OF POLAND
A contribution
THE ORDOVIBRACHIOPODS
to
U
Project
GLOBAL BIO EVENTS
BIERNAT, Gertrude *) & BEDNARCZYK,Wieslaw **)
Abstract: Acrotretidae, a group of short time range, dynamically evolved during the Cambro-Ordovician
times and became extinct by the end of the Ordovician except for three genera of the Scaphelasma line going up to the Silurian and of the Torynelasma line going on to the Lower Devonian. In Poland, acrotretids are very diversified in Ordovician, especially in the Lower Ordovieian/Upper Tremadocian-Llanvirnian), similarly as e.g. in Balto-Scandia (Sweden, Estonia, Leningrad environs). The main factors responsible for their mass extinction are discussed: they comprise, among others, changes of sea level, changes of climate in connection with glaci-enstatic events and fluctuations in the rate of the sea-floor spreading.
INTRODUCTION
Acrotretids within the Ordovician shelly faunas of Poland constitute a distinctive and largely diversified fossil group. Available records reveal that these minute inarticulate brachiopods have developed a wide range of variation in morphology, sometimes of bizarre appearance. Only some genera appear to be of rather wide geographic distribution but all are, as a rule, of limited stratigraphic range. These lithofacies dependent animals appear to have been susceptible to the environmental changes caused by various factors, including, among others, the Late Ordovician cooling. The presented brief review of the aerotretid taxa throughout the Ordovician of Poland illustrates sufficiently well their very discontinuous ranges and points out their stepwise and almost complete extinction by the end of the Ordovician.
GENERAL
R E M A R K S ON T H E O R D O V I C I A N
IN P O L A N D
In Poland, three basic Ordovician sedimentary regions are distinguished (Fig. 1). These are as follows: 1. Pre-Vendian East-European Craton comprising northern and eastern Poland (Bednarczyk, 1979, 1986; Podhalanska, 1980); 2. Kielce region, southern part of the Holy Cross Mountains (Bednarczyk, 1964; Bednarczyk & Biernat,
19'78); 3. Koszalin-Chojnice to the Lysog6ry region skirting the south-western margins of the Craton and lying on the protracted Oslo-Seanian belt (Erdtmann, 1965; Bednarczyk, 1974; Jaanusson, 1979). All the sedimentary processes in the above mentioned regions and those influencing the development of the fauna associations appear to be comparable to those of Scandinavia. Main events connected with the transgressive-regressive phenomena which are recorded from north-western and central Poland appear to coincide with those occurring in global scale (i.e. Erdtmaun, 1986; LindstrSm & Vortisch, 1983; Walliser,
1984). The Acrotretidae which are considered here derive from the two first of the three sedimentary regions *) **)
Zaklad Paleobiologii PAN, A1. Zwirki i Wigury 93, PL 02-089 Warszaw, Poland Institute of Geological Sciences PAN, A1. Zwirki i Wigury 93, PL 02-089 Warszaw, Poland
106
mentioned above, starting there with the Tremadocian deposits. Only very few informations come from the third region where the Ordovician deposits begin with Ltanvirnian-Llandeilian silts with graptolites (Bednarczyk, 1974).
~'~cs e ~ ~
.~
w~
~
~ e r l b a l t i c Depression
~
T ~
~Q
--
~. "~ e.~ \ •.-...~ "o. \
-~..~ .\
~
.. t
O
%. ~ xi~.~"~,~=z:~ "~,, \ - T"TT/.J" "><.W ~ ' ,l
%_ \
\
{.,.,~._x,,v. •
~
"o%.k~
200kin X
tO
.
-.~__.=~/
/~__C_a~__athian~~
.~Carpathians
f
~ ' ~ / "~ ~ . J - ~ " ~ - , . ~ \ ~d" "~-~ ~]1
~ 2
SEDIMENTARY
~
~Z~-Z~4 Z55 REVIEW
WITH
/6 REGARD
TO
Figure 1 :
Lithofacies distri-
bution i~ the Early ~ e x ~ i ~ time on the ground of structural t = area without deposits; 2 = carbonate lithofacies; 3 = glaueonite lithofacies; 4 = muddyclay lithofacies; 5 = occurrence of acrotretids; 6 = boundaries of structural units.
ACROTRETIDS
Six main stages in the development of sediments which affect different groups of fauna, including aerotretids, can be recognized in the Ordovician of Poland. These stages, to a great extent, appear to be connected with transgressive-regressiveevents (Fig. 2). 1. Uppermost Cambrian - Lowermost Tremadocian regression coinciding with the Acerocare Regressive Event of Erdtmann (1984) or Lange Ranch Eustatic Event of Miller (1984). Acrotretids are rare and are known to occur mostly in dark silts. Unfortunately, they are still poorly known; the recent record from the LysogGry region have not been studied yet. 2. Upper Tremadocian transgressive phase, preceded by a regression connected with Peltocare Regressive Event (Erdtmann, 1986), or Black Mountain Regressive Event (Miller, 1984). This phase is characterized by muddy - silty - glauconitie or marly - glauconitic sediments or siliceous rocks (numerous specimens being immersed in silica). The sediments contain a rich and diversified fauna. Among them, the abundantly represented aerotretids are of relatively great variety in taxonomic respect. In addition, they also abound in specimens. Distinctive trait of this fossil group is, that many genera are monospecific and these being of a rather very short stratigraphie range. 3. Tremadoeian-Arenigian transgressive-regressive phase with a carbonate and/or sandy carbonate (with glauconite) sedimentation. There occur many stratigraphic gaps, usually local, sometimes very short and, in some cases, sedimentary stagnation is stated and judged to be connected with a deep erosion. This phase is connected with the Ceratopyge Regressive Event (Erdtmann, 1984).
JERRESTADIAN
STAGE CLASSlFlCATlON NOT SAFELY
ESTABLISHED
UHAKUAN L A S N M G IAN
KUNDAN
UPPER
Figure 2 : Lithofacies and discontinuities in the Ordovician of Poland in comparison to the Eustatic Events. 1 = sandy lithofacies; 2 = clay lithofacies; 3 = carbonate lithofacies; 4 = benthonite; 5 = hiatus; 6 = fault; 7 = discontinuitr, 8 = events affecting development of acrotretids. ARE = Acerocare Regressive Event; PRE = Peltocare Regressive Event; CRE = Ceratopyge Regressive Event; VRE = Valhd Regressive Event (after Erdtmam 1986, slightly modified by present authors).
108
Benthic fauna is still richly developing. As to the acrotretids, they are, as a rule, well represented, and many of the genera are also monospecific. Some of the species can be treated as index fossils for the Lowermost Axenlgian, i.e. Emytreta minor Biernat, Pomeraniotreta biernati Bednarczyk (Biernat, 1973; Bednarczyk, 1986). 4. Regressive phase at the end of the Lower Llanvimian is characterized by oolithic - ferrugineous chamosite deposits due to local deep submarine erosion. The washed-out Arenigian and Llanvirnian conodonts were redeposited in Upper Llanvirnian sediments (Eoplacognathus robustus Zone). Aerotretids are poor in terms of taxa and specimens. 5. Phase of radical changes in the character of sedimentation. In the Caradocian, limy-carbonate facies disappeared in the majority of the Craton regions and were replaced by marly and dolomitic lithofacies with some inclusions of benthonites. Fauna, in general, is extremely poor and aerotretids are scarcely represented in respect to both taxa and specimens. The characteristic event in the Caradocian of Poland is the greatest extent of transgression and therefore a sea of relatively great depth. Therefore, the silty-marly lithofacies with graptolites has the comparatively widest horizontal extent. Probably, (enstatic) changes of the sea level have not been very distinct as compared to those mentioned by Fortey (1984) and Barnes (1986). 6. Phase of regression connected with the Upper Ashgillian glacial event of Barnes (1986). Similar to the preceding phase, acrotretids are rare in terms of taxa and specimens. In the authors' opinion, the above mentioned transgressive-regressive phases in the Ordovician of Poland are a reflection of eustatic sea level changes and of considerable climatic alterations caused by the subsequent intensifying glacial periods. Only during the Caradocian tectonic events might have influenced e.g. the initiation of volcanism which seems to be proved by numerous bentonite interbeds in the Holy Cross Mts. region and in the Koszalin-Chojnice area as well (i.e. Bednarczyk, 1971; Podhalanska, 1980; Przybylowicz, 1980; Tomczyk, 1971). The distributive pattern of the inarticulate braehiopods in the Cambrian-Ordovician profiles of Poland supports the generally accepted view of a strong dependence of these animals on lithofacies.
SOME
COMMENTS
ON ACROTKETIDS
As mentioned above acrotretids are poorly represented in the Cambrian deposits of north-western and central Poland. They comprise, up to now, only three genera of which two, namely Acrothyra and Linnarsonia are monospecific and the third one, Acrotreta is represented by three species (Tab. 1, 2), i.e. Middle Cambrian Acrotreta socialis Seebach (Bednarczyk, 1972), Upper CambrianAcrotreta multa Orlowsld and A. upplandica Wiman (Orlowski, 1968). Particular species usually occur in a number of separate brachial and/or pedlele valves and these are, occasionally, aecumnlated in some sort of banks. Generally speaking the Cambrian acrotretids were connected with the offshore shelf area being associated with the agnestid and ptyehopariid trilobites. Associations of the Ordoviclan acrotretids, contrary to the Cambrian ones, are much more diversified and even more rich in specimens (Fig. 3). As a matter of fact, the richest record comes from Tremadocian/Arenigian strata. In that interval, evolution of this animal group had attained its apex. Some genera like Pomeraniotreta (P. biemati) are monospecifie but found in hundreds of specimens of brachial and pedlele valves. Closed shells are sporadic. The available collection includes many complete shells with both valves closed but belonging to the genus Ephippelasma.
10g
"l SI
LU
RIAN
AshgilI-
C&r~do¢i;tn Ll&nde
i I -
iart
Llanvlrn
-
Jan
Ar
e nl
g-
iart Tremadocian U P P E R C A M B R I A N
a
b
Figure 3 : Diversity of acrotretid genera (a) and species (b) progressively decreasing during the Ordovician.
All of these acrotretid brachiopods, probably, occupied the inshore shelf area where, however, the bathymetric oscillations still existed. These, in all probability, epibiotic animals, could be attached by a pedicle to living ? algae, the latter probably flourished on bottom of the shallow, photic and sufficiently well aerated basin. Some thick walled forms, i.e. Conotreta or Spondylotreta could occur in environments of sandy lithofacies. Also in such a kind of environments they were rather numerous even in species (about ? eight species) and individuals. All together, eleven genera and eighteen species of acrotretids are known from the Upper Tremadocian to Lower Arenigian time (Tab. 1, 2).
Ordovician
Camb r i an Taxa Lower
Middle
Upper
T
A
Lv
L
A
C
Aorotreta Kutorga Ditreta Biernat Paratreta Biernat Eurytreta Rowell Semitreta Biernat Spon~lotreta Cooper Myotreta Gorjanski Conotreta Walcott Linnarssonia Walcott Acrothyra Matthew Seaphelasma Cooper Torynelasma Cooper Pomeraniotreta Bednarczyk ~phippelasma Cooper Hisingerella Hen i ngsmoen
_-
Table 1 : Distribution of acrotretids (genera) across the Cambrian/Ordovician sections in Poland with references to their occurrence in the profiles of Europe and other continents.
110
Cambrian
Ordovlcian
Taxa
L Acrotreta gemmula Matthew Aorotreta multa
M
U
T
A
Lv
L
C
A
m
Or~ow.ski
Aorotreta 8ooialis Seebach Acrotreta uplandida WIm~n ~itreta dividua
Biernat
Conotreta oalva montana Bednarczyk Conotreta ozarnockii Bednarczyk
Conotreta kozielensis Bednarczyk Conotreta koz~owskii
Bednarczyk
Conotreta mica Goryansky Conotreta parva Bednarcz.yk
aff, ' af~
Conotreta pSana Cooper Conotreta polonica Bednarczyk Conotreta samsonow~ozi Bednarczyk Conotreta staszioi Bednarczyk
Paratreta similis Biernat Paratreta sp. Eurytreta intermedia Bi e rnat Eurytreta minor Blernat Eurytreta sp. Semitreta major Biernat Spondylotreta dissimilis Biernat b~pondylotreta major Bednarczyk& Biernat Myotreta craasa Gorjansky
N
.
u
Myotreta estoniana Gorjanaky Myotreta gorjanBky Bednarczyk Linnarssonia ? sp. sp, Acrothyra sp.
m
ct
Scaphelasma septatum Cooper Bcaphelasma aubquadratum Biernat Scaphe lasma bukowkenne Bedna rczyk & B Ie rnat Torynelasma rarum Blernat Torynelasma rossic~n Gorjansky Pomeraniotreta biernatl Bednarczyk
B
~ohippelasma minutum Cooper Ephippelasma spinoa~n Biernat Ephippe lasma latior B ie rn a t Ephippelasma sp. sp. Hisingerella nitene( Hen Ingsmoen) Table 2 : Distribution of acrotretids (species) across the Cambrian/Ordovician sections in Poland with reference to their occurrence in the profiles of Europe and other continents.
111
From the Upper Arenigian sediments in Poland only seven species belonging to six genera are noted. They include thin-walled and much less conical specimens of Eurytreta and/or Paratreta (Biernat, 1973). Such a situation probably has continued into the Lower Llanvirnian. From that interval six genera and nine species have been recorded. More serious changes in sedimentation (ferrugineous-oolitic facies appearing) have caused in a way the continued decrease in acrotretid population in the Upper Llanvirnian of Poland. The next crisis in aerotretid evolution is to be noted in the Ltandeilian and Caradocian (only four genera and six species are known). Usually, the specimens occur in marly and limy-dolomitic sediments which presumably were laid down in the outer part of the shelf and partly on its slope. Only three genera (Acrothyra, Scaphelasma, Hisingerella), all monospecific are known up to now from the Ashgillian, occuring in very poor associations of fauna presumably at the slope of the outer shelf boundary. This brief review of distribution pattern of the acrotretid brachiopods appears to fit, to some extent, to the model of faunistic differentiation during the Ordovician as given by Sepkoski and Sheehan (1983). In our opinion, the changes of acrotretid associations, specific differentiation, and increase and decrease of their evolution as suggested by the above mentioned authors can be confirmed in a way by our observations. It seems obvious, that the relatively greatest diversification and individual development of nnlmals, aerotretids included, were connected with necessity of adaptation and co-adaptation to the changing environmental conditions. There were, probably, frequent changes owing to the eustatic movements and particularly perceptible in the inshore shelf area. There could occur also a local defaunation and recoloniTation. Such a picture could be characteristic for the marginal area of the East European Craton including southern part of the Holy Cross Mountains at the Tremadocian-Arenigian boundary. The progressing Ordovician transgression, also some bathymetrie stabilization throughout the Middle and the beginning of the Upper Ordovician could not favour the development of acrotretids. To these factors should also be added the climatic changes with a particularly intensified cooling in the Upper Ordovician.
CONCLUSIONS 1. Acrotretids, well established within the Lower Cambrian benthic fauna quickly evolved throughout the Cambrian and Ordovician. In Poland, they are known to be well diversified particularly in the Lower Ordovician, the time span comprising Upper Tremadocian to the Lower Ltanvirnian. Up to now, about twenty genera are cited from that time. 2. These animals displaying phosphate shells resistant to dissolution are preserved in a rather wide range of rock types i.e. silty, muddy, marly shales, marly limestones, but particularly diverse and numerous they are in carbonates. Some forms have developed few internal structures of very variable appearance, e.g. the dorsal median septum of Ephippelasma or of Myotreta. This is an inadaptative character, particularly in the changing environments and the acrotretids displaying it disappeared very quickly. Only those forms with a simple septnm, e.g. Conotreta, have had some chance to survive from Tremadocian even to AshgiUian. To mention, members of the Torynelasma and Scaphelasma lines are known to pass into the Silurian (scaphelasmatids) and even into the Devonian (torynelasmatids) (Fig. 4). Their shells are conical to a varying degree and the dorsal septum is simple or even much reduced (Biernat, 1984). 3. Very characteristic in acrotretids is a pitted firstformed larval shell, planktonic, of very small size. This
112
I
Artiotreta (S) ~ s 0 ~ Acrotretella
/
Scaphelasma . ( A - C ) ca30x ~1 I I
Torynelasma
a?
(A- Lv)
ca40x
Figure 4 : Scheme of developmental trends suggested for scaphelasmatids (a) and torynelsmatids (b); Note, the Silurian Artiotreta is not yet found in Poland and Acrotretella being recently found but from the Llanvirnian-Llandilian deposits of Poland (not yet described). T - Tremadocian, A - Arenigian, L v Llanvirnian, L - LlandeiUian, C - Caradocian
Pomeraniotreta
(T-A)
(Lv-L) caSOx
ea75x
bl stage is stronglymarked on adt~t shelland separated from the adult stage by the development of a relatively thick growth line. These pits could have been helpful in achieving buoyancy of the larvae (Bicrnat & Williams, 1970; Krause & Rowell, 1975; Bitter & Ludvigsen, 1979; Nazarov & Popov, 1980). To note, these larvae could have been drasticallydecimated, among others due to cooling (glacitectonicevent); the more so, because proctrated larvalstages have been propitiousfor a wide geographical distribution. These are a few complementary factors considered as influencing the extinction of acrotretids by the end of Ordovician. Unfortunately there is still a shortage of information on these animals which makes difficult a full explanation of their almost complete extinction. Anyway, it seems that a few number of factors complementing each other give rise to a stepway mass extinction of this group.
113
REFERENCES
Barnes, C. R. (1986): The faunal extinction event near the Ordovician-Silurian boundary: a climatically induced crisis. - Lecture Notes in Earth Sciences, 8, 121-126. Bednarczyk, W. (1964): The stratigraphy and fauna of the Tremadocian and Arenigian (01andian) in the Kietce region of the Holy Cross Mountains Middle Poland. - Biuletyn Geologiczny Uniw. Warsz., 4, 3-88 [in Polish], 188-197 [summary in English], 206-216 [summary in Russian]. Bednarczyk, W. (1968): The Ordovician, in the region of Ketrzyn - NE Poland. - Aeta Geologica Polonica, 18 (4), 70%749 [in Polish, English summary). Bednarezyk, W. (1971): Stratigraphy and palaeogeography of the Ordovician in the Holy Cross Mts. - Acta Geologica Polonica, 21 (4), 573-616. Bednarezyk, W. (1972): The Precambrian and Cambrian of the Leba Elevation (NW Pland). - Acta Geologica Polonica 22 (4), 685-710 [in Polish, English summary]. Bexinarczyk, W. (1974): The Ordovician in the Koszalin - Chojnice region Western Pomerania. - Acta Geologica Polonica, 24 (4), 581-600. Bednarczyk, W. (1979): Upper Cambrian to Lower Ordovician conodonts of the Leba Elevation, NW Poland, and their stratigraphic significance. - Acta Geologica Polonica, 29 (4), 409-442. Bednarczyk, W. (1984): Biostratigraphy of the Cambrian deposits in the Leba area. - Acta Geotogica Polonica, 34 (1-2), 95-110. Bednarczyk, W. (1986): Inarticulate brachiopods from the Lower Ordovician in northern Poland. - Annales Soeietatis Geologorum Poloniae, 56, 409-418. Bednarczyk, W. & Biernat, G. (1978): Inarticulate brachiopods from the Lower Ordovician of the Holy Cross Mountains, Poland. - Acta Paleontologica Polonica, 23, 293-316. Biernat, G. (1973): Ordovician inarticulate brachiopods from Poland and Estonia. - Palaeontologica Polonica, 28, 1-116. Biernat, G. (1984): Silurian inarticulate brachiopods from Poland. - Aeta Paleontologica Polonica, 29, 91103. Biernat, G. & WiUiam.% A. (1970): Ultrastructure of the protegulum of some acrotretid brachiopods. Palaeontology, 13, 491-502. Bitter, P. H. & Ludvigsen, R. (1979): Formation and function of protegular pitting in some North American acrotretid brachiopods. - Ibidem, 22 (3), 705-720. Erdtmann, B.-D. (1965): Eine sp/it-tremadocische Graptolithenfauna yon Toyen in Oslo. - Norsk Geologisk Tidsskrift, 45, 97-112. Erdtmann, B.-D. (1984): Outline ecostratigraphic analysis of the Ordovician graptolite zones in Scandinavia in relation to the paleogeographic disposition of the Iapethus. - Geologica et Palaeontologica, 18, 915. Erdtmann, B.-D. (1986): Early Ordovician eustatie cycles and their bearing on punctuations in early nematophorid/planctic graptolite evolution. - Lecture Notes in Earth Sciences, 8, 139-152. Fortey, R. A. (1984): Global earlier Ordovician transgressions and regressions and their biological implications. - In: Bruton, D. L. (ed.): Aspects of the Ordovician System. - Palaeont. Contr. Univ. Oslo, 295, 37-50. Jaannsson, V. (1979): Ordovician. - In: Robinson, R. A. & Teiehert, C. (eds.): Treatise in Invertebrate Paleontology. A. Introduction. - Geol. Soc. America, A 136-166; Univ. Kansas Press. Kranse, F. F. & Rowell, A. J. (1975): Distribution and systematics of the inarticulate brachiopods of the Ordovician carbonate mud mound of Mejlde john Peak, Nevada. - Paleont. Contr. Univ. Kansas, 61, 1-74. Lindstrrm, M. & Vortisch, W. (1983): Indications of upwelling in the Lower Ordovician of Scandinavia. In: Thiede, J. & Suess, E. (eds.): Coastal UpweUing. B. - Plenum Publishing Co., 535-551. Miller, J. F. (1984): Cambrian and earliest Ordocician conodont evolution, biofacies, and provincialism. - In: Clark, D. L. (ed.): Conodont Biofacies, and Provincialism. - Geol. Soc. America Spec. Paper, 196, 4368. Modlinski, Z. (1982): The development of Ordovician lithofacies and palaeotectonics in the area of the Precambrian platform in Poland. - Prate Instytutu Geologicznego CII, 5-66 [in Polish and English and Russian summary]. Nazarov, B. B. & Popov, L. E. (1980): Stratigraphy and fauna of Ordovician siliceous-carbonate deposits of
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Kazakhstan/radiolarians and inarticulate brachiopods. - Transactions of Geol. Inst. of Acad. Sci. USSR, 331, 3-189 [in Russian]. Orlowski, S. (1968): The Cambrian stratigraphy in the Holy Cross Mts Poland. - XXIII Intern. Geol., 9, 127131. Podhalanska, T. (1980): Stratigraphy and facial development of Middle and Upper Ordovician deposits in the Leba Elevation NW Poland. - Acta Geologica Polonica, 30 (4), 328-390. Przybylowicz, T. (1980): Petrography of the Ordovician pyroclastic sediments in the Leba Elevation area. Archiwum Mineralogiczne, XXXVI (1), 73-84 [in Polish, English summary]. Sepkoski, J.J., Jr. & Sheehan, P. M. (1983): Diversification, faunal change, and community replacement during the Ordovician radiations. - In: Tevesz, M. J. S. & McCall, P. L. (eds.): Biotic interactions in Recent and fossil benthic communities. - Plenum Press, 673-717. Tomczyk, H. (1971): The Arenigian transgression in Poland and its relation to earlier formations. Memoires du B.R.G.M., 73, 423-430. Walliser, O. H. (1984): Global events and evolution. - Proc. 27th Internat. Geol. Congr., Palaeont., 2, 183192.
LATE
ORDOVICIAN
GRAPTOLITE
ITY AND SUBSEQUENT RE-RADIATION
MASS
EARLY
MORTAL-
SILURIAN
A contribution to Pro~ect
N•
GLOBAL
U
BIO
-
EVENTS
BERRY, William B. N., WILDE, Pat & QUINBY-HUNr, Mary S.*)
Abstract: Late Ordovician graptolite mass mortality has been examined closely in only a few continuous stratal succession. These sections include those at Dob's Liun, Scotland; Anhni, China; and Mirny Creek, USSR. Correlations among these areas are not precise; however, many characteristic Late Ordovician graptolites appear to persist longer in the Anhui area than at either Dob's Linn or Mirny Creek. Graptolites disappear from the stratigraphic record more sharply at Dob's Lima than at the other localities. Lithologic and chemical aspects of the strata at each locality suggest that a significant environmental change occurred at the stratigraphic position at which the graptolites disappeared. That change appears to reflect both a diminution and a deterioration in environmental conditions under which the graptolites previously had flourished. The optimal conditions for graptolites appear to have been in low oxygen but bacteria-rich waters similar to those found in the modern Eastern Tropical Pacific. Areal reduction in those environments as well as reduction in food resources preferred by graptolites resulted in diminished graptolite populations. These reductions took place gradually in many parts of the world, generally commencing in high latitudes. The final Late Ordovician mortality, in the Tropics, may have been the result of introduction of toxins to graptolites or their food supply by local overturn and increased vertical advection from moderate depths. The timing of such events in the Tropics is non-synchronous, suggesting local environmental influences. Geochemical and lithologic evidence links the Late Ordovician graptolite mass extinction to progressive latitudinal habitat destruction commensurate with the final pulse of the Late Ordovieian glaciation. Re-radiation of the surviving taxa in the Early Silurian followed deglaciation and redevelopment of marine environments preferred by graptolites.
I NTRODUCTION
A mass mortality occurred among graptolites in the Late Ordovician (Berry & Boucot, 1973; Berry, 1979). Koren et al. (1979, p. 131) discussed this mass mortality, or near-extinction, noting that the characteristic Ashgill (Late Ordovician) graptolltes disappear completely at this time. This extinction event corresponds with mass extinction in other groups documented by Ranp and Sepkoski (1984). Details of this major bioevent among the graptolites have been examined closely in only a few areas with continuous marine sections, primarily because major changes in the sedimentary regime occur in many areas of the world about the time of the mass mortality. Berry and Boucot (1973), Sheehan (1973) and Brenchley and Newall (1984) reviewed the influence of glacioeustatic sea level lowering on marine sedimentary environments during the Late Ordovieian glaciation. Many areas that had been sites of marine deposition prior to continental glaciation in the south polar area became sites of erosion or non-marine deposition when sea level dropped. Even continental slope and deeper marine stratigraphic successions of Late Ordovician age, display features suggestive of change in some aspect of depositional environments. Graptolite-bearing stratal sequences that include an unbroken depositional record from strata prior to, during, and after the mass mortality include only three that have been studied closely: the OrdovicianSilurian stratotype at Dob's Linn, southern Scotland (Williams, 1983, 1986); Jingxian, south Anhui, China *) Marine Sciences Group, University of California, Berkeley, CA 94720, U.S.A.
116
1977; Koren et al., 1979; Koren et al., 1983). Williams (1983, 1986), Berry (1987), and Berry et al. (1987) described the Dob's Lima mass mortality stratal interval. The pre-mass mortality faunas there occur in black shales and mudstones. An interval of non-fossiliferous, grey to green-grey mudstones stratigraphically overlies the black shales bearing the faunas prior to mass mortality (Williams, 1983, 1986; Berry, 1987). Berry et al. (1987) pointed out that a change in oxidation state, from anoxic to oxic, took place in the Dob's Lima area at the lithologic change from the pre-mass mortality black shale to the non-fossiliferous grey mudstones. That change in depositional environment is reflected not only by change in color of the strata, but also in their geochemistry (Berry et al., 1987). Li et al. (1984, p. 292) described a 0.25 m. layer superjacent to that bearing the pre-mass mortality graptolites as "yellow-brown or yellowish brown thinbedded mudstone, carrying nautiloids and spongy spicules." The subjacent graptolite-bearing layers are "black carbonaceous mudstone, rich in graptolites" (Li et al., 1984, p. 292). As at Dob's Linn, the color and lithologic change reflects a significant environmental change. That change appears to include change in oxidation state of the depositional environment from anoxic to oxic. At Mirny Creek in northeastern USSR, the pre-mass mortality layers are dark, platy limestones bearing graptolites, and the post-mass mortality strata are siltstones (Koren et al., 1979, Fig. 3). Again, the change in sedimentary regime seems to include a change in oxidation state in the depositional environment.
THE TIME (ZONAL)
FRAMEWORK
Koren et al. (1979) and Wang (1987) reviewed graptolite zonal sequences across the Ordovician-Silurian boundary interval. Late Ordovician zones recognized in Kolyma (Siberia), USSR, Kazakhstan (Koren et al., 1979), South China (Mu, 1984), and Dob's Lima, Scotland (Williams, 1983, 1986) differ because varied taxa and differing combinations of taxa occur in each of these areas (Wang, 1987). Nonetheless, enough similarities exist among the stratigraphic sequences bearing Late Ordovician-Early Silurian graptolite faunas to use the following zonal scheme for discussion of timing of the Late Ordovician graptolite mass mortality.
G RAPTOLITE
SILURIAN ORDOVICIAN
ZONES
Parakidograptus acuminatus Glyptograptus persculptus Clirnacograptus extraordinarius Paraorthograptus sp.
Paraorthograptus pacificus zone faunas of Koren et al. (1979) are approximately equivalent to the Dicellograptus anceps zone of Williams (1983, 1986) at Dob's Linn. The Paraorthograptus zone in the above scheme includes the Dob's Lima D. anceps and most, if not all, of the subjacent Dicellograptus complanatus zone. It is about equivalent to the Paraorthograptus pacificus and subjacent Climacograptus longispinus zone Siberia of Koren et al. (1979). Using the suggested zonal scheme, the mass mortality took place at the C. extraordinarius - G. persculptus zone boundary in Jingxian, south Anhui, China (Li et al., 1984). Koren (1988) and Koren et al. (1979) noted that the mass mortality in the USSR occurs at the Paraorthograptus-C. extraordinarius zone boundary there. Koren (1988) said that the mass extinction, which she termed the "pacificus event", coincided with "maximum glaciation in the Late Ashgill". Koren's (1988) review of the Late Ordovician graptolite mass mortality (the pacificus event) implies that it was time synchronous. The data from south Anhui suggest that
117
it occurred later there than in Siberia and Dob's Lima. Because a significant non-fossiliferous interval is superjacent to the mass mortality at Dob's Linn, and the faunas there are somewhat different forms than approximately coeval faunas in Siberia, the mass mortality at Dob's Lima may not have been coeval with that in Siberia. Comparison of stratigraphic occurrences of graptolites in the Dob's Lima sections with those in Siberia suggest the possibility that the mass extinction occurred slightly earlier at Dob's Linn. Koren (1988) and Rickards (1988) reviewed C. extraordinarius, G. persculptus, and P. acuminatus zone faunas. Aside from the relatively diverse C. extraordinarius zone fauna in south Anhui, the fauna of that zone is typified by a few glyptograptids and three small climacograptids. These climacograptids co-occur joined in the G. persculptus zone with the first of a new style of graptolite colonial organization, the monograptid or tmiserial seandent. Parakidograptus acuminatus zone faunas are characterized by radiation among graptolites with this new colonial organization as well as by radiation among biserial graptolites in which the proximal end is drawn out to an elongate, thorn-like structure. The base of the P. acuminatus zone is the base of the Silurian by international agreement. As detailed stratigraphic observations at Dob's Linn (Williams, 1983, 1986; Berry, 1987; Berry et al., 1987), south Anhui (Li, 1984; Li et al., 1984) and northeastern USSR (Koren et al., 1983) have shown, the "pacificus event" mass mortality took place at a stratigraphic position at which a significant change in lithologic aspect of the strata is seen. In all three stratal sequences in which the mass mortality has been examined closely, this includes a change from dark gray to black graptolite-bearing mud rock to a nonfossiliferous, light colored rock. The available evidence indicates that a change in environment coincided with the biological mass mortality.
GRAPTOLITE ENVIRONMENTS
Inasmuch as the "pacificus event" (Late Ordovician graptolite mass mortality) occurs at a significant change in environment, relevant questions are: (1) In what environment or environments did graptolites live? (2) What changes in oceanographic conditions could have led to lethal changes in the environments in which graptofites lived? 1. Modern Analogs of Graptolitic Environments
Berry and Wilde (1978); Wilde and Berry (1982, 1984, 1986) and Wilde (1987) applied basic concepts of physical and chemical oceanography to ascertain those oceanic conditions that would lead to development of widespread, dark, commonly organlc-rich mudstones and shales that typify the graptolite biofacies. Thus, modern analogs must have both biofacies and lithofacies similarities. Modern analogs of anoxic and near anoxic oceanic environments are those in which black muds similar to those of the graptolite facies are limited as a consequence of the well-oxygenated aspect of much of the deep ocean and the generally cool modern climate. Modern oceanic environments analogs to graptolite facies occur only in warm tropical waters. As Berry et al. (1987) discussed, waters in two large, near-parallel lobes flanking the equator off the Central American and South American coast, the northern Arabian Sea in the Indian Ocean, and off the West African coast meet many of the requirements for a graptolite habitat (Fig. 1). In these areas, the oxygen content declines rapidly to a minimum or undetected value as shallow as 50 meters. This depth is characterized by a nitrate maxima below which nitrite increases (Anderson, 1982; Anderson et al., 1982). The physical characteristics of the Eastern Tropical Pacific waters have been described by Wooster and Cromwell (t958), Wyrtki (i967) and Brinton (1980).
118
ao.E
\ / J ~/, ~-LJ--~~
IIIIIArea with maximum . . .density . . . . of E / ~ X m i d / : ? " f"//~ Area with maximum density of E, distinguenda
#
Figure 1 : Modern Areas of Low Oxygen Waters. Hachured regions show the distribution of modern marine waters with dissolved O < 0.4 mL/L (after Denser, 1975). Stripped areas show characteristic species endemic to oxygen-poor waters in the Eastern Tropical Pacific (after Brinton, 1980). Projection is Goode's Homosline equal-area, copyright, University of Chicago (from Berry et al., 1987, with permission of Geological Society of Denmark, copyright, 1987). Brinton (1980) described euphansiid faunas related to the oxygen-poor waters of the Eastern Tropical Pacific and Ebeling (1967) pointed out that a distinctive fish fauna inhabits the same waters. Mul!ins et al. (1985, p. 491) noted from their study of low oxygen waters off the central California that "the edges of the boundaries of the oxygen minimum zone are 'hot spots' of increased biogeochemlcal activity." Holligan et al. (1984) cited dinoflagellate blooms in similar waters off both the Peru-Chile and west African coasts. The reasons for the enhanced biological activity at the margins of the oxygen minimum zone in the Eastern Tropical Pacific were analyzed by Anderson et al. (1982, p. 1133) in terms of the various pathways that nitrogen is available as a chemical nutrient by bacterial activity. The interaction between denitrifying and nitrifying bacteria augment the amount of biologically useful nitrogen for phytoplankton as well as increasing the biomass of bacteria available as a food source. MuUins et al. (1985, p. 494) concluded that the edges of oxygen minimum zone water were preferred sites for enhanced biological activity for both autoand heterotrophes "became of greater nutrient concentrations plus larger food supplies in the form of bacteria." Longhurst (1967) and Brinton (1980) reviewed zooplankton distributions in and near the Eastern Tropical Pacific oxygen minimum zone. Brinton (1980) cited euphausfids endemic to these waters that spend the daylight hours in waters in which the oxygen content is less than 0.4 ml/1. During the night, these zooplankton migrate vertically upward as much as 300 meters to oxygenated waters to alleviate the oxygen debt incurred during their daylight stay in oxygen-poor waters. Other euphausiids spend most of their lives in waters on the margins of the oxygen minimum zone (Brinton, 1980). Longhurst (1967) studied copepods and euphansfids lixing on the northern margins of the Eastern Tropical Pacific lobes, noting that certain species also migrate daily into the oxygen minimum zone from oxygen-rich waters above. Zooplankton distributions in and near the modern Eastern Tropical Pacific band of low oxygen waters thus include faunas that: (1)live within oxygen deficient waters most of the time; (2) live on the oxygen-rich side of the boundary; and (3) migrate daily into the oxygen-poor waters. Faunas living on the maron~ of the zone apparently utilize the food resources found in the denitrificadon zone (HoUigan et at., 1984; Mullln.~ et al., 1985).
119
The zooplankton distributions near the modern Eastern Tropical Pacific oxygen-poor waters provide clues to environments inhabited in the Paleozoic by graptolites if they occupied a niche similar to euphausiids of today. Graptolites are most abundant in shales formed on the margins of Early Paleozoic shallow shelf seas on which calcium carbonate accumulated (Berry, 1979). These shallow shelf seas were on cratons or continental blocks that were in the tropics of those times (Skevington, 1976; Berry, 1979). Less richly fossiliferons graptolitie successions occur in temperate to sub-polar oceans of the Ordovician and Silurian. At least some of these Early Ordovician faunal successions developed in areas of oceanic upwelling, which permitted high organic productivity of phytoplankton in the surface and near-surface waters (Wilde et al., 1990). High organic productivity would have resulted in consumption of oxygen below the photie zone as organic matter sank in the water cob!ran High rates of oxygen consumption would lead to oxygen depletion and expansion of the denitrification zone into the mixed layer from the pycnocline, as is seen today in the Eastern Tropical Pacific. Thus, analysis of processes related to nutrient availability in modern denitrified waters suggests that graptolites occupying a similar niche also would have been linked trophically to oxygen-poor, denitrification zone waters. During the span of the graptolites in the Early Paleozoic, such relatively shallow anoxic and near-anoxic waters were more extensive than seen in the more ventilated ocean in the post-Devonian. Accordingly, Wilde et al. (1990) proposed that graptolitic facies faunas were both anoxitrophic and anoxitropic, being closely dependent upon oxygen deficient to anoxic waters. Modern ETP Model "Peru"
Micromoles/Kilogram 0
t
~" 250
100
200
09
300
A
,,
¢-
a Isog~,,ed Zo.e~No ' ~
PhoticZone
_1 A b u n d a n t _
Ui
-'G=
__,
t /
100
200
f
I
I
I
*'. [
/
Figure 2 : Proposed changes in graptolite habitats from (A) early Late Ordovician to (B) Late Ordovician glacioeustatic lowstand of sea level. Chemical profile on right based on conditions in modern Eastern Tropical Pacific (ETP) using model of Wilde (1987). From Berry, Wilde and Quinby-Hunt (1987) with permission of the Geological Society of Denmark, copyright, 1987.
120
2. Modifications in the Paleozoic Environment
Berry and Wilde (1978), Wilde and Berry (1982) on lithofacies arguments, and Wright et al. (1986) on geochemical grounds suggested that the Paleozoic oceans through the Devonian contained a significant volume of anoxic waters. This implies the oceans were oxygen deficient to anoxic in the pycnocline and lower surface, wind-mixed layer. Due to the increased solubility of oxygen in sea water with decreased temperature (Weiss, 1970) oceanic anoxia would decrease during intervals of globally cool to cold climates. Thus, during the Late Ordovician glacial interval (the "pacificus event") and the late Early Devonian cool interval (the terminal extinction of graptolites), climatic and oceanographic conditions favored constriction of anoxie waters to the pycnocline and general oxygenation of the oceans. Late Ordoviclan glaciation resulted in glacioenstatic sea level lowering (Berry & Boucot, 1973). Brenchley and Newall (1984) suggested that sea level may have lowered by nearly 100 meters. They describe a number of paleogeographic changes that took place as a consequence of such marked sea level drop. These changes included: 1) exposure of former areas of shallow shelf sea deposition with development of widespread karst topography on former marine carbonate depositional facies; 2) deep channeling of former shelf siliclastic environments; 3) "deep erosion at shelf margins"; 4) formation of "mass flow deposits and fans at the base of slopes"; and 5) extensive sediment slumping along the continental slopes as a consequence of high rates of sedimentation. Brenchley and Newall (1984) recognized at least one major oscillation in sea level during the Late Ordovician glaciation. Berry and Boucot (1973) and Sheehan (1973) summarized the evidence for mass mortality among planktonic and benthic organisms in the Late Ordovician. Brenchley and NewaU (1984) noted that the extinctions took place in at least two steps. The first phase is seen among the planktonic faunas and took place in the earliest part of the glaciation as a result of lower sea surface temperatures. The second step occurred among benthic faunas as sea level lowering reduced their habitable riving space. As sea level lowered in tropical seas, the top of oxygen-poor denitrified waters inhabited by graptolites dropped as well. Berry et al. (1987) suggested that the top of this zone was lowered to a position about that of the middle to lower continental rise from a high stand position at the upper slope or shelf edge. As sea level and the denitrified waters lowered during expansion of continental ice, deep ocean circulation and ventilation became more vigorous. Extensive long-term development of a polar ice cap would have generated large volumes of oxygen-rich oceanic bottom waters expanding the deep waters and compressing the pycnocline (Wilde & Berry, 1982). Oxygen would diffuse upward into the anoxie and oxygen-poor waters of the pycnocline, reducing anoxicity. Cooler surface waters with increased oxygen solubility coupled with reduced productivity as the result of higher turbidity would permit downward ventilation, lowering the top of the denitrification zone well below the photic zone. Accordingly, phytoplankton could no longer take advantage of increased nutrient availability at the oxic-denitrificationzone boundary. Thus, during increased glaciation, the preferred environment of graptolites would have been reduced or eliminated in high latitude to temperate areas and restricted to the tropics, as in the modern ocean. AS the graptolite environment became limited, most of the graptolites adapted to life close to denitrified waters disappeared. The survivors would have been those in the tropics that could live in more oxic waters and those that could survive crowded conditions in the diminkhed area of denitrified waters. The stratigraphie evidence indicates that the former group were climacograptids of the C. rniserabilis and C. normalis type. The graptolites surviving in relatively crowded conditions were diverse glyptograptids and diplograptids that were very small. Small colonies doubtless had relatively minimal nutritional needs and could survive in environments in which food supplies were limited.
121
Skevington (1976, p. 196) pointed out that "the lowering of surface water temperatures in mid and high latitudes, consequent upon the Late Ordovieian glacial phase, appears to have imposed a restriction on the overall distribution of graptolite faunas." He also noted that "Late Ordovician graptolites do not occur in North Africa," and that early Late Ordovician graptolites are rare in South America, both regions then being in high southern latitudes. Skevington (1976, p. I96) stated that graptolite faunas progressively withdrew from the south polar areas toward the tropics during the latter part of the Ordovician. By the Late Ordovician, Skevington (1976) noted that graptolites were virtually restricted to the tropics. Thus oceanic conditions also must have changed during the latter part of the Ordovician to limit most graptolites to tropical habitats. The stratigraphic record suggests that, by the latest Ordovician, graptolites were limited to environments in Anhui, China; modern northeast Siberia; and Dob's Lima, Scotland, which was located on the margin of northeastern North America in the Late Ordovician tropics. The final mass mortality in these areas may have taken place as a consequence of an overturn along the equator as described by Wilde and Berry (1984). Alternately, upwelling of toxins from a modest depth as described by Wilde and others (This volume) could have provided the final blow. The relatively narrow stratigraphic interval in south China in which no graptolites occur, and the relative richness of the Clirnacograptus extraordinarius zone faunas in China suggests that the events leading to the ultimate mass mortality of the "pacificus event" took place later in China than in either Scotland or Siberia. Indeed, based on the richness of the graptolite faunas in the C. extraordinarius zone, as noted earlier, thepacificus event took place at Dob's Lima slightly earlier than it did in Siberia. In both of these areas, thepacificus bioevent was at the Paraorthograptus sp. - Climacograptus extraordinarius zone boundary, as that boundary is recognized in each area. The pacificus event occurred at the next younger zone boundary, or that between the C. extraordinarius and Glyptograptus persculptus zones in south China.
POST-PACIFICUS
EVENT
GRAPTOLITES
Rickards (1988) and Koren (1988) both noted that the post-pacificus event graptolites were small colonies. In addition, two new colonial organizations appeared in the G. persculptus zone, each involving reduction in the number of zooids (at least in the proximal part of those biserial scandent forms with an elongate, thorn-like proximal end). The monograptids appeared first in the English Lake District (Rickards & Hutt, 1970). They developed there in an oceanographic setting of relatively cool waters but with seasonal upwelling (Wilde et al., 1990). Rickards (1988) noted that this new colonial type spread to other areas in a short time. Koren (1988, p. 48) said that these new developments among graptolites took place "as a response to deglaciation and the onset of widespread black shale sedimentation." Post-pacificus event radiation was closely linked to re-establishment of the oxygen-poor, denitrification zone waters on the shelf under which richly fossiliferous, graptolite-bearing shales accumulated.
S UMMARY
The stratigraphie record of the Late Ordovieian graptolites indicates that they almost became extinct as a result of mass mortality during the pacificus event (Koren, 1988). This event occurred during a stratigraphic interval characterized by evidence of global cold climates. The mass mortality appears to have been diachronons, initiating in high latitudes and moving equatorward in time, which is consistent with developing glaciation. The relationship to the Late Ordovician glaciation suggests the cause was
122
equatorward ventilation of anode and denitrified waters, which provided the major source of food for graptolites. This ventilation was caused by increased oxygen solubility and oceanic circulation related to colder glacial climates. In the tropics, only three localities, south China, Siberia, and Scotland contained taxa that survived into the Silurian. Even at these areas, the event apparently occurred latest in south China, where it is found one zone younger that in Siberia and Scotland. The change in sub-polar and temperate areas appears linked to reduction of the graptolites' preferred habitat near nutrient-rich but oxygen-poor to denitrified waters. With such habitats restricted during maximum glaciation to tropical waters, extinctions at south China and perhaps in Siberia may be linked to increased circulation producing local upwelling of toxins (Wilde et al., this volume).
ACKNOWLEDGEMENTS
The authors thank Prof. Dr. O. H. Walliser and Prof. E. G. Kauffman for their continued support of our research on Bio-events. In particular, Dr. T. Koren graciously provided us with insights on the Siberian section at the Ordovieian Conference in Newfoundland. M. Krup did her usual outstanding job in preparing the figures and designing the manuscript. This is contribution MSG-88-006 of the Marine Sciences Group of the University of California, Berkeley.
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