Latitudinal Sea Surface Temperature Gradient of the Oligocene Southern Ocean

During the Oligocene, surprisingly warm sea surface temperatures (SST) prevailed in the vicinity of the Antarctic Ice Sheet. However, this warmth has not been put into context with the oceanographic conditions of the rest of the Southern Ocean. We improve the understanding of the Southern Ocean dynamics by reconstructing the latitudinal SST gradient as well as the variability and position of ocean frontal systems of the Southern Ocean from the late Eocene until early Miocene (3520 Ma) using new TEX86-based SSTs south of Australia (ODP Site 1168). We integrate our data into existing temperature reconstructions from other marine sediment cores around the Tasmanian Gateway (TG) and ultimately compare these to fully coupled climate, and high-resolution ocean-only model simulations. The novel TEX86 record at ODP Site 1168, south of Australia, indicates SSTs of 2029C (BAYSPAR calibration), with a relatively flat long-term trend, but with marked short-term variability. The SST compilation reveals an increasing latitudinal temperature gradient around the TG from 6C prior to 26 Ma to >10C by 23 Ma, particularly due to profound ice-proximal cooling. Climate models replicate the increasing temperature gradient in the late Oligocene. The increasing SST gradient from 26 Ma onwards is consistent with timing of Drake Passage (DP) opening, derived from kinematic reconstructions, and with ocean model evidence of ice-proximal cooling once the second Southern Ocean gateway deepens. Amplitudes of glacial interglacial SST variability also increase after 26 Ma, with a weaker latitudinal SST gradient during interglacials. This suggests strong latitudinal shifts in ocean frontal systems and the expansion of ice sheets strongly affecting the ice-proximal records during glacials. The Oligocene opening of DP and widening of the TG with northward migration of Australia, allowed for an increased water transport south of Australia and for the ocean flow through the gateways to align with the westerly winds. We propose that the increasing Southern Ocean latitudinal SST gradient strengthened the influence of westerly winds on ocean flow, which in turn drove an intensification of the Antarctic Circumpolar Current and strengthening of frontal systems in the late Oligocene, intensifying the thermal isolation of Antarctica around 26 Ma.