Reconstructing Antarctic winter sea-ice extent during Marine Isotope Stage 5e

Environmental conditions during Marine Isotope Stage (MIS) 5e (130-116 ka) represent an important ‘process analogue’ for understanding the climatic responses to present and future anthropogenic warming. Our understanding of the response of Antarctic sea ice to global warming is particularly limited due to the short length of the observational record. Therefore, reconstructing Antarctic winter sea-ice extent (WSIE) and Southern Ocean sea-surface temperatures (SSTs) during MIS 5e provides insights into the temporal and spatial patterns of sea ice and SST change under a warmer than present climate. This thesis presents new MIS 5e proxy records from a set of marine sediment cores located south of the Antarctic Polar Front, between 55 and 70 o S, alongside an analysis of published Southern Ocean records. The MIS 5e diatom species assemblages in these new cores are used to reconstruct and investigate changes in WSIE and SSTs between the three Southern Ocean sectors. The new proxy reconstructions of MIS 5e WSIE and SSTs are also compared to state-of-the-art climate model outputs. There was high variability in both WSIE and SSTs in the Atlantic and Indian sectors during MIS 5e, compared to the largely stable conditions found in the Pacific sector. All three Southern Ocean sectors reached their minimum WSIE during MIS 5e concurrently, within chronological uncertainties, both with each other and with the peak atmospheric temperature in the EPICA Dome C ice core record. The high variability in the Atlantic sector records is attributed to high glacial meltwater flux into the Weddell Sea whereas the high variability in the Indian sector likely resulted from large latitudinal migrations of the Antarctic Circumpolar Current occurring on a millennial�scale. The different variability and sensitivity of Antarctic WSIE and SSTs between the three Southern Ocean sectors may have significant implications for the Southern Hemisphere climate system under future warming. The latest climate models are currently unable to recreate the MIS 5e WSIE and SSTs seen in these new proxy records, with colder SSTs and a greater WSIE in the models than the proxy records. Inclusion of Northern Hemisphere Heinrich 11 meltwater forcing into model runs reduces the discrepancy with proxy values, but longer duration (~3-4 ka) model runs, with Heinrich 11 meltwater forcing included, are required to fully test whether state-of-the-art climate models can match Southern Ocean MIS 5e WSIE and SSTs reconstructed from proxy records