Early Paleogene Benthic Foraminiferal Assemblages and Stable Isotopes in the Southern Ocean

ODP Leg 114 recovered sections at four sites east of the Falkland Plateau that cover a wide range of paleodepths and provide the opportunity to evaluate the response of benthic foraminifers to late Paleocene and Eocene Oceanographic changes.Early Paleogene paleodepth estimates were obtained by "backtracking" assuming simple thermal subsidence (Site 698, -900 m; Site 702, -2000 m; Site 700, -2400 m; and Site 699, -2800 m).These estimates agree with paleodepths determined by comparing our quantitative benthic foraminiferal assemblages to previously published assemblages associated with known paleodepths.Previous studies document that a major benthic foraminiferal crisis occurred in the latest Paleocene in the Atlantic, Caribbean, and Pacific; a similar faunal turnover occurred in the latest Paleocene throughout the Atlantic sector of the Southern Ocean.At the Leg 114 sites, Stensioina beccariiformis-dominated assemblages were replaced by Nuttαllides frwem/>y/-dominated assemblages just prior to the Paleocene/Eocene boundary.A preponderance of benthic foraminiferal taxa last appeared immediately prior to the Paleocene/Eocene boundary, as recognized at these high latitudes by the last appearance of the calcareous nannofossil Fαsciculithus spp.and the first appearance of the planktonic foraminifer Pseudohαstigerinα spp.Recovery and biostratigraphic control at the Leg 114 sites is insufficient to constrain precisely the timing of the extinction event, although studies of material from the Maud Rise (Weddell Sea) suggest that it occurred in the latest Paleocene.The benthic foraminiferal crisis may have been caused by deep-water warming, a drop in food supply, or changing deep-water source regions.Oxygen isotope data show that there is no clear correlation between δ 18 θ changes and extinctions.Similarly, most of the extinctions occurred well after the start of the drop in global δ 13 C values, which may, in part, reflect a decrease in productivity.Interbasinal carbon isotope comparisons suggest that the Southern Ocean was supplied "young" (high O 2 , low nutrient, and high δ 13 C) deep water in the latest Paleocene (approximately 60-58 Ma) and early Eocene (approximately 57-52 Ma).Oxygen isotope evidence indicates that the Southern Ocean was filled with cooler water than that in the Pacific beginning at approximately 60 Ma, supporting our contention that the deep-water source was antarctic.However, near the Paleocene/Eocene boundary (approximately 58-57 Ma), the supply of Southern Ocean "young" deep water was reduced or eliminated.We speculate that elimination of this inferred antarctic source between 58 and 57 Ma triggered the benthic foraminiferal turnover.