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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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