ONSET AND DEVELOPMENT OF THE ANTARCTIC CIRCUMPOLAR CURRENT

A major question in paleoclimate research concerns the possible causal connection between tectonic events and climate. The opening of Drake Passage between South America and the Antarctic Peninsula made it possible for the Antarctic Circumpolar Current (ACC) to circle the globe, and has been argued to have strongly influenced global climate and the glaciation of Antarctica. The timing of this event, however, is not well constrained: it occurred between the late Eocene and the late Miocene. A deep-water gateway between Australia and Antarctica opened at around the Eocene-Oligocene boundary, the time of major though gradual restructuring of deep-sea benthic foraminiferal faunas, onset of Antarctic continental glaciation, and changes in Southern Ocean water mass structure in the SE Indian/SW Pacific sector interpreted as marking onset of the ACC. Glaciation on Antarctica, however, might have been triggered by a decline in concentration of atmospheric greenhouse gases, and the timing of ACC onset, development and its likely effects on circulation and climate must be re-assessed. There is considerable evidence that a deep-water connection between South America and Antarctica did not originate until after the initiation of Antarctic glaciation in the latest Eocene, because neritic, low-salinity benthic foraminiferal assemblages in uppermost Eocene sediments at ODP Site 696 (South Orkney Microcontinent) show very close similarity to assemblages in Tierra del Fuego. The precise timing of the creation of a complete deep-water circum-Antarctic pathway is thus unknown. Key questions are: (1) what was the Southern Ocean circulation in the Oligocene and early Miocene? (2) did this circulation contribute to Antarctic glaciation or were the circulation changes the effect of Antarctic glaciation rather than its cause? (3) when was a complete deep-water circumpolar path produced? (4) when did the ACC develop as we know it today and what was the effect of that development on global climate and biosphere?