Changes in Southern Ocean circulation in late Oligocene to early Miocene time

Benthic stable isotopic measurements from four ODP Leg 189 sites, in and near the Tasmanian Gateway, link existing southern high latitude records from the Oligocene. Bulk sediment isotopic analyses at Sites 1170-1172 present the first moderate resolution, high latitude “planktic” late Oligocene to early Miocene (-28-21 Ma) data. Stable isotopes and records of carbonate content suggest similar surface hydrography for southern Sites 1170-1171, and strongly reduced offsets at Site 1172 to the north, following-22.8 Ma. Benthic δ18O values at these sites show low variability prior to earliest Miocene cooling. Greater offsets of 1170-1172 benthic δ18O from southern Kerguelen Plateau values, continuing until 25.4 Ma, cast doubt on hypotheses of earlier development of circum-Antarctic circulation and therefore deep opening of the Drake Passage. The data also suggest that Southern Hemisphere temperatures in this area were relatively warm throughout the late Oligocene, consistent with published Mg/Ca temperature reconstructions for bottom waters. The global ice-volume signal of the early Oligocene (Oi-1) records a brief event which did not lead to a permanent ice sheet, but high δ18O values at Maud Rise (Sites 689-690) continuing into the late Oligocene suggest an increase in Antarctic Bottom Water production in the Weddell Sea. Near complete similarity of surface hydrographic conditions at Sites 1170-1172 following-22.8 Ma suggests this as a youngest date for full development of the Antarctic Circumpolar Current.