Latest Cretaceous to Cenozoic Climate and Oceanographic Developments in the Weddell Sea, Antarctica: an Ocean-Drilling Perspective

This is a summary of principal findings made by ODP Leg 113 investigators concerning the latest Cretaceous-Cenozoic climatic, cryospheric, and oceanographic history, and biogeographic developments of the Weddell Sea region, Ant­ arctica. During Leg 113, 22 holes were drilled at 9 sites that sampled 4 contrasting environments: open-ocean pelagic sedimentation on Maud Rise (Sites 689 and 690), hemipelagic and terrigenous sediments on the East Antarctic Conti­ nental Margin (Sites 691-693), a turbiditic sequence in the deep Weddell Basin (Site 694), and hemipelagic and biogenic sediments on the South Orkney microcontinent (Sites 695-697). A wide range of sedimentologic, biotic, and isotopic evidence obtained in Leg 113 material indicates that sequential cooling and cryospheric development of Antarctica and the surrounding oceans during the Cenozoic profoundly af­ fected the ocean/atmosphere circulation, sediments, and biota. Important cooling steps occurred during the latest Cre­ taceous, the middle Eocene, near the Eocene/Oligocene boundary, in the middle Oligocene, the middle Miocene, the early late Miocene, the latest Miocene, and the late Pliocene. Distinct but temporary warming trends occurred during the late Paleocene and the latest Oligocene to early Miocene. Principal findings include the following: 1. Late Cretaceous climatic conditions were temperate to cool subtropical in the Weddell Sea sector of the Antarctic with no evidence of cryospheric development. 2. A significant warming episode began about 500 k.y. prior to the mass extinction event at the Cretaceous/Tertiary boundary. This was followed by a marked cooling and a positive carbon isotopic shift that immediately preceded (~ 200 k.y.) the boundary. 3. The Paleocene section at Site 690 contains fine-grained sediments, possibly of eolian origin and derived from East Antarctica; the presence of this material suggests that climate was warm, semiarid, and continental at that time. 4. A warming trend occurred during the late Paleocene, leading to the climax of Cenozoic warmth during the early to early middle Eocene (~ 58 to 45 Ma). Conspicuous paleoenvironmental and biotic changes mark the Paleocene/Eo­ cene boundary, including a brief climatic spike when surface water temperatures in the Weddell Sea increased to as much as 20°C. 5. The early Paleogene high-latitude ocean seems to have been marked by high seasonal variation in sea-surface temperatures and a well-developed seasonal thermocline. With cooling of the high latitudes during the middle to late Paleogene, this seasonal variation decreased, as did vertical water mass stratification. 6. Antarctica may not have been the primary source of deep waters during at least part of the Eocene. Oxygen iso­ topic evidence indicates the presence of warm saline deep water in the Weddell Sea at times during the Eocene. The warmer deep waters are inferred to have been produced at lower latitudes. 7. At the beginning of the Oligocene, cold deep water began to form in the Antarctic and upwardly displaced warmer waters that were probably still being formed at lower latitudes. 8. On Maud Rise, a siliceous biogenic facies began to replace a carbonate facies with initial siliceous sedimentation in the latest Eocene-earliest Oligocene, leading to an almost completely siliceous biofacies by the late Miocene (~9 Ma). Near the beginning of the Neogene, there was a pronounced increase in the siliceous relative to the calcareous bio­ facies on Maud Rise. This coincides with the development of the siliceous biofacies observed in other sectors of Antarc­ tica and probably reflects the development of a strong modern upwelling regime in the Antarctic that was also typical of the late Neogene. 9. Diverse calcareous planktonic assemblages reflect the relative warmth of surficial waters off Antarctica from the Late Cretaceous through the Eocene. Near the northern Antarctic Peninsula, a diverse middle to late Eocene palynoflora indicates the presence of forest dominated by Nothofagus with an undergrowth containing ferns. During the late Eocene, diversity began to decrease in planktonic assemblages as the Southern Ocean cooled; diversity sharply dropped near the Eocene/Oligocene boundary. The ultimate replacement of calcareous by siliceous assemblages by the late Miocene was in response to continued cooling and upwelling conditions throughout the circum-Antarctic. 10. The Leg 113 evidence suggests that cryospheric development began early in the Oligocene (36 Ma) at least in this sector of the Antarctic. Oligocene sequences drilled during Leg 113 on the Antarctic continental margin (Sites 692, 693, and 696) do not provide the required resolution to decipher details about cryospheric development during the Oligo­ cene, especially with respect to determining if an ice-sheet accumulated on the continent at this time. We found no evi­ dence for major ice accumulation during the Oligocene. The oxygen isotopic record from Site 689 contains evidence of some ice growth on East Antarctica (isotopic values >2.5%o) during the early to early late Oligocene. This is further supported by the presence of probable ice-rafted detritus at Site 693 during the late early Oligocene, the earliest detected during Leg 113. Comparison with evidence from other sectors suggests possible diachronism of ice accumulation around the margin of East Antarctica. 1 Barker, P. E , Kennett, J. P., et al., 1990. Proc. ODP, Sci. Results, 113: College Station, TX (Ocean Drilling Program). 2 Marine Sciences Institute and Department of Geological Sciences, University of California at Santa Barbara, Santa Barbara, CA 93106. 3 British Antarctic Survey, Madingley Road, Cambridge CB3 OET, United Kingdom.