Sea ice and ocean circulation changes during the last 140 kyr, offshore Adélie Land, East Antarctic continental margin, with special emphasis on last two deglaciations

Fuelled by anthropogenic warming, the duration of sea ice and ocean circulation near the East Antarctic continent at present-day, is changing and contributing to further ice sheet melt. Studying the functioning of these parameters, atmosphere, ocean, and sea ice, during past climate transitions (such as warming stages, i.e., deglaciations, and cooling stages, i.e., glaciations) may give us more understanding to the stability of the East Antarctic ice sheet in the future. Fossil diatom records found in deep ocean sediment cores can provide indications of past sea ice and surface temperature variability and can therefore deepen our understanding of future outcomes of present-day anthropogenic warming, including ocean circulation changes. However, very few sea ice and sea surface temperature reconstructions exist from past warming stages (deglacials) near the Antarctic continent. This is partly due to the fact fossil records are lacking from sediment archives retrieved from the Antarctic continental margin, possibly as a result of extended ice sheets and prolonged periods or permanent sea ice cover. In this study we analysed diatom assemblages (based on relative abundances of identified species and statistical analysis), the Eucampia index, Thalassiothrix antarctica, ice rafted debris (IRD), and the geochemical productivity indicators (biogenic silica and XRF derived Si/Al and Ba/Ti) in core TAN1302-44, collected from the slope offshore Adélie Land. The results show a pattern of glacial (cool periods) to interglacial (warm periods) sedimentary facies changes and include last two deglacials and the last glaciation stage. Two diatom assemblages coincide well with changes in glacial (low IRD, low productivity) to interglacial (high IRD, high productivity) facies. The persisting presence of Thalassiosira lentiginosa, representative of Assemblage 1, suggests the last glacial (MIS 4-2) is characterised by an open ocean environment with respect to sea ice. However, due to the increasing presence of Fragilariopsis obliquecostata, representing Assemblage 2, we conclude the last glacial also comprised a gradual build-up of sea ice, reaching a maximum duration at the end of MIS 2, before rapidly vanishing. Following the decrease in sea ice, based on the increased of Thalassiothrix antarctica within the deglaciation facies, we conclude Circumpolar Deep Water (CDW) influx increased over the slope. This observation occurs in both deglacials, one leading to MIS 5e, and other to Holocene interglacial. Finally, based on IRD rich interglacial facies, we conclude the CDW increase occurred prior to regional ice sheet retreat, leading into the Holocene. Together, these datasets suggest major sea ice, and oceanographic changes occurred prior to the last major ice sheet retreat, suggesting a progression of events may influence the demise of the East Antarctic ice sheet in the future.