National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)


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Suggested Citation:"INTRODUCTION." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 94
Suggested Citation:"INTRODUCTION." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 95

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TERMINAL PALEOCENE MASS EXTINCTION IN THE DEEP SEA: ASSOCIATION WITH GLOBAL WARMING 94 5 Terminal Paleocene Mass Extinction in the Deep Sea: Association with Global Warming James P. Kennett University of California, Santa Barbara Lowell D. Stott University of Southern California ABSTRACT The end of the Paleocene Epoch was marked by an abrupt, worldwide extinction of deep-sea benthic organisms. At about 55 Ma, between 30 and 50% of the benthic foraminifers suddenly became extinct, in association with comparable ostracode extinctions. Extinctions of planktonic taxa were insignificant. This extinction event is considered the largest of the past 90 million years (m.y.) in the deep-sea. Although of major proportions, this biotic crisis was almost unknown before the past decade because it had little effect on shallow marine invertebrates and ocean plankton. This was a "bottom-up" extinction, compared with the "top-down" extinction that marked the Cretaceous/Tertiary boundary. High-resolution stratigraphic studies in deep-sea sediments indicate that the extinction occurred in less than 3000 yr. Foraminiferal oxygen and carbon stable isotope changes, in combination with a distinct change in benthic fauna, indicate an abrupt but temporary warming and oxygen depletion of deep waters related to a fundamental change in oceanic circulation. The available data point to an ocean temporarily dominated by warm saline deep water whose source was probably in the middle latitudes. Although the terminal Paleocene environmental changes were relatively brief, this transient global warming event affected both ocean and terrestrial spheres simultaneously and had a great influence on the course of global biotic evolution for the remainder of the Cenozoic. INTRODUCTION Much interest exists in the character and causes of major biotic extinctions in the geologic past. The stratigraphic record demonstrates that the biosphere, or parts of it, have experienced major disruptions in the geologic past, some involving mass extinctions. Mass extinctions involve a sudden, and short-lived, increase in extinction rates well above normal background levels, and can affect a great variety of biotic groups (Flessa, 1990). Numerous

TERMINAL PALEOCENE MASS EXTINCTION IN THE DEEP SEA: ASSOCIATION WITH GLOBAL WARMING 95 theories have been proposed to explain mass extinctions (for reviews, see Stanley, 1984, 1987; Hallam, 1989). All involve a response by the biosphere to radical changes in the environment on regional or global scales. However, a persistent problem requiring resolution is how to explain contemporaneous extinctions over broad areas of the Earth's surface and over a wide range of habitats. Why did certain groups of taxa, particularly those that were abundant over large areas of the Earth, suddenly cease to exist? What factors control the timing of mass extinctions and the rate of biotic turnover? Theories advanced to explain mass extinction are of two general categories. The first, and perhaps most popular, has invoked extraterrestrial causes, particularly massive global environmental change resulting from bolide impacts on Earth (Alvarez et al., 1980). The second involves intrinsic changes exclusively within the Earth's environment (Stanley, 1984). General popularity for an extraterrestrial cause of mass extinction stems in part from the fact that it seems to provide a mechanism for sufficiently large and rapid changes in the global environment. In contrast, it appears more difficult to explain how the Earth's environment might have changed intrinsically on a magnitude necessary to cause mass extinction. Have intrinsic changes in the global environment ever been large and rapid enough to cause biotic crises of this scale? Stanley (1984) argued that most mass extinctions have resulted from climatic change, particularly cooling. Debate continues, however, about the relative merits of extrinsic and intrinsic causes. The purpose of this essay is to briefly summarize evidence for the possible cause of a mass extinction of deep-sea biota near the end of the Paleocene about 55 million years ago. Existing data suggest that the extinction event resulted from large, rapid changes within the Earth's environmental system without extraterrestrial forcing. Evidence for mass extinction comes exclusively from the stratigraphic record, and the quality of the stratigraphic data, including their resolution, is usually the key to better understanding of causes. Critical information includes the rate of extinction; which sectors of the biosphere were involved; description of the taxa that did or did not become extinct; the sequence of extinctions in taxa; and relationships of the extinction event to a wide variety of paleoenvironmental proxies. Relatively few high-quality sediment sequences are available that are sufficiently fossiliferous and were deposited continuously at high enough rates of sedimentation to provide the required resolution. Numerous stratigraphic sections contain hiatuses of various duration contemporaneous with major extinctions events. Such disruption in the stratigraphic record probably resulted from sediment erosion related to changes in oceanic circulation and/or sea-level change at times of major global environmental change. During the past two decades, the Deep-Sea Drilling Project (DSDP) and its successor, the Ocean Drilling Program (ODP), have provided fossil-rich ocean sediment sequences covering vast, otherwise inaccessible, areas of the Earth's surface, including the high latitudes. The problem of mass extinction has thus been assisted by the availability of a broader array of sequences that record such events and of critical new information about changes in the deep-sea environments and biota. The ocean ecosystem deeper than the continental shelf is vast, forming more than 90% by volume of the Earth's habitable environments (Childress, 1983). Questions about mass extinctions require information about any response or role played by the deep-sea habitat. The Paleocene-Eocene transition has long been differentiated by stratigraphers based on significant biotic changes at the end of the Paleocene. However, knowledge of stratigraphic relationships between marine and terrestrial records has been hampered by poor chronostratigraphic control. The primary problem has been the discontinuous nature and poor biostratigraphic control of the classic Late Paleocene-Early Eocene European stratotype sections that form the foundation of global stratigraphic correlations. Terrestrial to marine correlations have been improved with the application of carbon isotope stratigraphy. During the Paleocene and Early Eocene there were large, systematic patterns of δ13C variability in the ocean that have been correlated globally (Shackleton and Hall, 1984; Stott etal., 1990; Pak and Miller, 1992; Stott, 1992; Zachos et al., 1993a,b). These carbon isotope (δ13C) variations reflect changes in the 12C/13C ∑CO2 in the ocean. Because the ocean and atmospheric reservoirs of CO2 tend to maintain approximate isotopic equilibrium, variations in the ocean's δ13C composition will be transmitted to the terrestrial reservoirs of carbon via the atmosphere. Soil carbonates, freshwater fossils, and terrestrial biomass, therefore, also exhibit the large carbon isotopic variations of the marine fossil record. The absolute isotopic values differ among these various terrestrial and marine carbon reservoirs due to systematic differences in the fractionation of 12C and 13C. However, these fractionation patterns are known and can be used to predict isotopic stratigraphies in terrestrial sections. Hence, the large-scale patterns of δ13C variability recorded in the marine sections across the Paleocene-Eocene boundary are now being discovered in terrestrial sections (Koch et al., 1992; Sinha and Stott, 1994). With this new global stratigraphy it has become apparent that accelerated evolution in terrestrial mammals during the Late Paleocene coincided with the extinction and environmental changes recorded in the deep sea (Rea et al., 1990; Koch et al., 1992). Although this chapter is concerned only with the marine record of extinction, the

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What can we expect as global change progresses? Will there be thresholds that trigger sudden shifts in environmental conditions—or that cause catastrophic destruction of life?

Effects of Past Global Change on Life explores what earth scientists are learning about the impact of large-scale environmental changes on ancient life—and how these findings may help us resolve today's environmental controversies.

Leading authorities discuss historical climate trends and what can be learned from the mass extinctions and other critical periods about the rise and fall of plant and animal species in response to global change. The volume develops a picture of how environmental change has closed some evolutionary doors while opening others—including profound effects on the early members of the human family.

An expert panel offers specific recommendations on expanding research and improving investigative tools—and targets historical periods and geological and biological patterns with the most promise of shedding light on future developments.

This readable and informative book will be of special interest to professionals in the earth sciences and the environmental community as well as concerned policymakers.

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