A thin predominantly cold-based Late Miocene East Antarctic ice sheet inferred from glaciovolcanic sequences in northern Victoria Land, Antarctica

Glaciovolcanic sequences are widespread in northern Victoria Land and their interpretation enables estimates of past ice thicknesses and other defining parameters of the coeval ice sheet to be made with considerable certainty. The terrestrial sequences are mainly Late Miocene in age (c. 12-5 Ma) and conform to five main types: aa lava-fed deltas, volcanic sheet-like sequences, a glaciolacustrine sequence, scoria cones and tuff cones. Apart from the environmentally undiagnostic pyroclastic cones, the volcanic sequences were erupted in association with a glacial cover that was typically, and possibly never much more than, a few hundred metres thick (<300 m). Despite gaps in the record, no evidence was observed for prolonged subaerial (i.e. fully ice-free) eruptive conditions, suggesting either that ice-free conditions did not occur or else were of such limited areal extent or short duration that they left no record. These observations are consistent with the presence of a thin persistent Late Miocene ice dome or icefield draping the pre-Miocene topography in northern Victoria Land for the period, although it may have been confluent with the greater East Antarctic Ice Sheet similar to conditions present today. The presence of glacially emplaced sequences right down to present datum suggests that the Transantarctic Mountains in the region were already uplifted to their current elevation prior to the volcanism. The high terrain would have helped the ice sheet to establish and persist. The glacial thermal regime varied from wet-based and dynamic, to cold-based (frozen to its bed) and presumably relatively stable. A major transition from wet-based ice to cold-based ice might have occurred between 9.7 and 7.5 Ma consistent with published interpretations of the regional landscape evolution. Alternatively, the glacial thermal regime was dominantly polar since the earliest eruptions but with several temporary changes to a wet-based (sub-polar or temperate) more dynamic regime.