Development of Cenozoic deep-sea benthic foraminiferal faunas in Antarctic waters

Upper abyssal to lower bathyal benthic foraminifera from Ocean Drilling Program Sites 689 (present water depth 2080m) and 690 (present water depth 29I4m) on Maud Rise (Antarctica) recorded changes in deep-water characteristics at high southern latitudes during the cenozoic. The benthic foraminiferal faunas show only minor differences as a result of the difference in water depths between the sites, and changes in faunal composition were coeval. These changes occurred at the early/late paleocene boundary (+61.6Ma), in the latest Paleocene (+57.5Ma), in the middle early Eocene (+55.0Ma), in the middle middle Eocene (+46.0Ma), in the earliest Oligocene (+36.5Ma) and in the early middle Miocene (+14.5 Ma). The faunal change at the end of the Paleocene was the most important and has been recognized world-wide. On Maud Rise, the diversity decreased by 50% and many common species became extinct over a period of less than 20000 years. Diversity increased again during the early Eocene, and reached the same values as in the Paleocene by the middle Eocene. In the middle Eocene the diversity started to decrease, and continued to decrease until the middle Miocene. From the beginning of the middle Miocene until today biosiliceous oozes accumulated and calcareous benthic foraminifera were generally absent, with the exception of part of the late Mrocene (*8.5-7.5Ma) and the Quaternary. Changes in composition of the benthic foraminiferal faunas over a wide depth range (upper abyssal-lower bathyal) probably indicate periods of major changes in the formational processes of the deep waters in the oceans. The earliest Eocene faunas, living just after the major extinction at the end of the Paleocene, are characterized by low diversity and high relative abundance of small species that probably migrated downslope into the deep waters. These faunas, and to a lesser degree those in the early middle Eocene, are characterized by high relative at'undance of biserial and triserial species. In contrast, older and younger faunas have high relative abundances of spiral species. This suggests that bottom waters on Maud Rise were poor in dissolved oxygen in the latest Paleocene through early middle Eocene, and that the major extinction of benthic foraminifera at the end of the Paleocene might have resulted from a decrease in availability of dissolved oxygen as a result of warming of the deep waters. Warming might have been caused by a change in sources of deep waters, possibly as a result of plate-tectonic activity. The overall decrease in diversity from middle Eocene through Miocene probably reflects continual cooling of the deep waters. Benthic foraminiferal faunas thus indicate that Cenozoic changes in the deep oceanic waters at high latitudes did not consist of gradual progression from Cretaceous circulation to the present-day pattems of formation of deep water: the benthic faunal changes occurred in discrete steps. Benthic faunal composition indicates that deep water most probably formed at high latitudes during the Maastrichtian-early Paleocene, and from the middle Eocene to Recent, with episodes of deep water formation at low latitudes (warm, salty deep water) during the latest Paleocene and early Eocene. Goals of the research The Earth's climate and the temperaturestructure of the oceans have changed considerably during the Cenozoic, from a warm climate with equable temperatures in the Cretaceous to the present-day situation with extensive polar ice caps and strong latitudinal and vertical temperature gradients in the oceans (e.9. Douglas & Savin 1975; Savin 19771' Barron 1985). The world may not have been completely ice-free in the Cretaceous (Frakes & Francis 1988), but ice-caps were certainly either absent or much smaller than they presently are (e.9., Frakes 1979; Schnitker 1980; Berger et al. l98l) . The changes in climate were largest at high latitudes: cooling was of little importance in the trooics. but it was considerable in Antarctica. Beech-type (Nothofagus) forests were growing on the Antarctic Peninsula during the Eocene, where vegetation is much more limited today (e.g., Frakes 1979; Mohr in Barker et al. 1988; From Crame, J. A. (ed.), 1989, Origins and Evolution of the Antarctic Biota, Geological Society Special Publication No. 47, pp.283-296.