Abstract. Antarctic continental ice masses fluctuated considerably in size during the elevated atmospheric CO2 concentrations (~ 600–800 ppm) of the Oligocene “coolhouse”. To evaluate the role of ocean conditions to the Oligocene ice sheet variability requires understanding of past ocean conditions around the ice sheet. While warm ocean conditions have been reconstructed for the Oligocene Wilkes Land region, questions arise on the geographical extent of that warmth. Currently, we lack data on surface ocean conditions from circum-Antarctic locations, and ice-proximal to ice-distal temperature gradients are poorly documented. In this study, we reconstruct past surface ocean conditions from glaciomarine sediments recovered from the Deep Sea Drilling Project (DSDP) Site 274, offshore the Ross Sea continental margin. This site offshore Cape Adare is ideally located to characterise the Oligocene regional surface ocean conditions, as it is situated between the colder, ice-proximal Ross Sea continental shelf, and the warm-temperate Wilkes Land Margin in the Oligocene. We improve the existing age model of DSDP Site 274 using integrated bio- and magnetostratigraphy. Subsequently, we analyse dinoflagellate cyst assemblages and lipid biomarkers (TEX86) to reconstruct surface paleoceanographic conditions during the Oligocene (33.7–25.4 Ma). Both TEX86-based sea surface temperature (SST) and microplankton results show temperate (10–17 °C ± 5.2 °C) surface ocean conditions at Site 274 throughout the Oligocene. Increasingly similar oceanographic conditions between offshore Wilkes Land margin and Cape Adare developed towards the late Oligocene (26.5–25.4 Ma), likely in consequence of the widening of the Tasmanian Gateway, which resulted in more interconnected ocean basins and frontal systems. To maintain marine terminations of terrestrial ice sheets in a proto-Ross Sea with as warm offshore SST as our data suggests, requires a strong ice flux fed by intensive precipitation during colder orbital states in the Antarctic hinterland, but with extensive surface melt of terrestrial ice during warmer orbital states.
Temperate Oligocene surface ocean conditions offshore Cape Adare, Ross Sea, Antarctica
