Reconstructing glacial and environmental change on the South Orkney Plateau, sub-Antarctica

Glaciers and ice shelves in the Antarctic Peninsula (AP) region, including neighbouring sub-Antarctic islands, have undergone sustained retreat over the past two decades resulting in enhanced global sea-level rise. These changes have been linked with rapid regional increases in atmospheric and ocean temperatures far greater than the global mean. To understand the mechanisms of glacier retreat and help predict the future response of Antarctic ice sheets, it is crucial that longer term ice-sheet history is investigated. The glacial histories of sub-Antarctic islands are particularly poorly known despite their potential to provide important insights into the sensitivity of sub-Polar ice caps to atmospheric and ocean forcing. This study uses new marine geophysical and geological data to reconstruct the style, timing and rates of past glacial and environmental change(s) on the South Orkney Plateau (SOP), at the northern tip of the AP. A comprehensive compilation of available bathymetric data reveals several large cross-shelf troughs emanating from the South Orkney Islands. These extend to the shelf edge in the north, whilst a large glacial depocentre (a grounding zone complex) on the southern shelf indicates that grounded ice previously extended to the mid shelf in the south. A seismic line acquired through the complex reveals that shelf stratigraphy was formed by repeated glacial advances which span the Plio-Pleistocene but could extend back to Miocene times. Former ice-cap dynamics during the most recent glaciation have been reconstructed from high-resolution multibeam data collected within and between cross-shelf troughs. Lineations within troughs indicate streaming ice, whilst their absence in inter-trough areas implies slower moving ice. Channels and grooves on the northern shelf suggest meltwater played a key role in former ice-cap dynamics. Detailed analyses of sediment cores combined with geophysical observations show that the grounding line retreated from the outer shelf prior to 16690 cal. yrs B.P., with mid and inner shelf deglaciation between 14600-12950 cal. yrs B.P. Deglacial moraines are absent from the northern shelf, although a suite of moraine ridges and grounding zone wedges from intra- and inner-trough regions on the southern shelf suggests a complex pattern of grounding line retreat. Initial deglaciation of the SOP was probably driven by postglacial sea-level rise, whilst a potential readvance of the ice cap during the Late Glacial coincides with the Antarctic Cold Reversal. Ice cap history is supplemented with a multi-proxy analysis of a long piston core recovered from an inner shelf basin. 18Odiatom data provide a detailed record of glacier discharge and melting through the Holocene and highlight the importance of tropical teleconnections in controlling the climate of the SOP and wider Antarctic environment. Crucially, the record shows a rapid and sustained increase in glacial melt since 1450 A.D., almost 500 years before the onset of anthropogenic warming, suggesting environmental response to recent forcing is more complex than previously thought.