Oligocene Ice Sheets

A hiatus at the Early/Late Oligocene boundary marks a change to fluvial conditions, with grounded ice or possibly glaciomarine conditions. A distinct drop in sea level is noted at this time. Facies changes and diamictite beds in the Cape Roberts core are indicative of the periodic expansion of tidewater glaciers, typical of a temperate glacier regime with glaciers flowing from the young East Antarctic Ice Sheet across the continental margin.

Throughout the Oligocene, glaciation seems to have waxed and waned. A distinct record of glacial activity is recorded by glacial sediments in the Polonez Cove Formation on King George Island of mid-Oligocene age (Krakowiak Glacial Member). At that time, ice was grounded on a shallow marine shelf. More extensive ice sheets may have been present further south in the Weddell Sea region, from which clasts of rock from the Transantarctic Mountains may have been derived to be incorporated as exotic clasts in the Polonez Cove Formation. Sediments without a glacial signature that overlie these glacial deposits suggest a phase of climate warming and glacial retreat until the next glacial pulse in the Miocene.

Why did the climate cool during the Eocene and Oligocene, causing such a major change in Antarctic environments? The influence of palaeoceano-graphic changes is now considered less critical; instead, coupled climate-ice sheet modelling indicates that it was changing levels of atmospheric CO2 that controlled Antarctica's climate. Factors such as mountain uplift, vegetation changes and orbital forcing all played a part in cooling the polar climate, but only when CO2 levels fell to critical threshold levels (2.8 times present-day levels) did orbital forcing tip Antarctica into its icy glacial world.

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