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phy (Kennett and Stott, 1991; Thomas, 1992) have strengthened the concept that the extinction was synchronous throughout the oceans. Nevertheless, this still requires confirmation with high-resolution studies at numerous sequences that can be accurately correlated by using carbon isotopic, paleomagnetic, and biostratigraphic data (Sinha and Stott, 1994).

Immediately following the extinction, the benthic foraminiferal assemblages were dominated by small, thin-walled specimens (Thomas, 1990). Benthic taxa that survived the extinction included Nuttallides truempyi, which became a dominant component in the Eocene, as well as Bulimina semicostata and other taxa making up what has been termed the Nuttallides truempyi assemblage. This new, relatively low-diversity assemblage includes about six forms that dominated Early to Middle Eocene benthic foraminiferal assemblages (Tjalsma and Lohmann, 1983; Miller et al., 1987). Faunal assemblages following the extinction are less cosmopolitan (Thomas, 1990).

Although the extinction event was abrupt, there is some evidence that the S. beccariformis assemblage became progressively restricted to shallower depths during the Paleocene (Tjalsma and Lohmann, 1983; Miller et al., 1987). Also, the relative abundances of certain forms in this assemblage decreased during the Late Paleocene and were replaced by forms more typical of the Nuttallides truempyi assemblage of latest Paleocene to Early Eocene age (Miller et al., 1987). These changes culminated at the mass extinction and suggest that some form of biological threshold was surpassed.

For several million years following the extinction there occurred a radiation of benthic foraminiferal taxa. These probably filled vacancies left by the latest Paleocene extinctions (Tjalsma and Lohmann, 1983; Miller et al., 1987). The postextinction assemblages included long-ranging forms such as Pullenia bulloides and Globocassidulina subglobosa (Thomas, 1990). The radiation caused a diversity increase that peaked during the early Middle Eocene. Nevertheless, the high diversity values of the Late Cretaceous and Early Paleocene were never attained again (Thomas, 1990).

In contrast to the benthic assemblages, oceanic planktonic microfossil assemblages underwent no mass extinction at the end of the Paleocene, but did exhibit distinct change in the species composition in the Antarctic. A general increase in diversity marks the Late Paleocene high- to middle-latitude assemblages of planktonic foraminifera, calcareous nannofossils, and dinoflagellates (Premoli-Silva and Boersma, 1984; Oberhänsli and Hsü, 1986; Stott and Kennett, 1990; Pospichal and Wise, 1990). This increase in diversity stemmed, in part, from the incursion of lower-latitude groups into the Southern Ocean. The diversity increase at the end of the Early Paleocene was superimposed on a longer-term increase that began during the Paleocene, following the K/T boundary extinctions (Corfield, 1987). The plankton diversity increase may have been caused by the increased surface water temperatures at high- to middle-latitude regions. This increase in surface water temperatures was particularly pronounced in the Antarctic during the latest Paleocene, as reflected by the relatively brief appearance of the subtropical-tropical morozovellid group and a peak in discoaster abundance (Pospichal and Wise, 1990; Stott and Kennett, 1990). The emigration of these warm-loving planktonic microfossils to the Antarctic was particularly pronounced during the mass extinction. In one Antarctic site 32% of the planktonic foraminiferal species appeared for the first time in the latest Paleocene, 27% underwent major abundance changes, and only 13% were eliminated from the assemblages (Lu and Keller, 1993). Most new entries were surface dwellers. Of those that were eliminated, most were deeper dwellers such as the subbotinids (Lu and Keller, 1993). Coeval low latitude planktonic assemblages underwent little change (Miller et al., 1987; Miller, 1991) presumably because of the relatively stable sea surface temperatures (Stott, 1992).

ASSOCIATION BETWEEN MASS EXTINCTION AND OCEANIC WARMING

In earlier work (Kennett and Stott, 1990; Stott et al., 1990) we discovered a dramatic negative oxygen and carbon isotopic excursion of brief duration (Figure 5.1) that coincided closely with the terminal Paleocene benthic foraminiferal extinction event in an Antarctic Paleogene sequence (ODP Site 690B) (Thomas 1989, 1990). This discovery stimulated a high-resolution study of the extinction event (Kennett and Stott, 1991). Results from that study demonstrated the intimate temporal relationship between the mass extinction and a large oxygen and carbon isotope excursion in both benthic and planktonic foraminifera. Planktonic values of d18O abruptly decreased by 1.0 to 1.5%o, and by about 2%o in the benthics; values of d13C also decreased by 4%o in surface-dwelling planktonic foraminifera, and -2%o in the deeper-dwelling planktonic and benthic forms. The planktonic foraminifer Acarinina praepentacamerata records the lowest oxygen and highest carbon isotope values within the excursion, consistent with an inferred near-surface habitat (Stott et al., 1990; Kennett and Stott, 1991). The species of Subbotina record higher d18O and lower d13C values, indicating a deep water planktonic habitat and/or a preference for cooler months of the year (Stott et al., 1990; Kennett and Stott, 1991). The highest d18O and lowest d13C values are exhibited by the benthic foraminifer Nuttalides truempyi, reflecting its habitat in relatively nutrient-rich high latitude deep water.



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