PLIOCENE SHIFT FROM POLYTHERMAL TO COLD-POLAR CONDITIONS OF THE ANTARCTIC ICE SHEET

When did Neogene Antarctic ice sheets switch from a ‘dynamic’ mode (polythermal) to a modern, cold, ‘stable’ (cold-polar) mode? Interpretations derived from landscape evolution, ash deposits and polar desert pavements in the Dry Valley region lead to the interpretation of persistent cold-polar climate from middle Miocene time. In contrast, interpretations based on glacigene strata and fossils of the Sirius Group in the Transantarctic Mts. and Pagodroma Group of the Prince Charles Mts. support the persistence of dynamic, polythermal ice sheets in East Antarctica until the late Pliocene. Marine sediments on the Antarctic continental shelf preserve a record of sea-ice history, as indicated by the presence/absence of a distinctive sea-ice diatom flora. The history of this flora bears on the timing of this climatic switch because the development of persistent sea-ice in the late Pliocene appears to have led to lowered regional marine and atmospheric temperatures, and provided a feedback mechanism for cooling of the Antarctic ice sheet. The modern sea-ice diatom flora is scarce to absent in available early and mid-Pliocene sediment records, yet this community evolved by the late Miocene. Expansion of the sea-ice belt around Antarctica during the late Pliocene-Pleistocene may have been a key climatic threshold leading into the modern cold, polar environment. Deposition in terrestrial and glacimarine environments during the late Miocene and Pliocene, as recorded in the Sirius and Pagodroma groups, suggest deposition by an ice sheet of different character than the present ice sheet, and a glacial regime that included significant erosion, transportation and deposition. In situ marine fossils within the Pagodroma Group indicate that the polythermal character of the East Antarctic ice sheet continued through the late Miocene and into the mid-Pliocene. The grounding zone of the paleo-Lambert Glacier/Amery Ice Shelf system fluctuated across a distance greater than 600 km during the Late Neogene, from a position on the continental shelf edge in Prydz Bay to the inner reaches of the Lambert/Amery Embayment. The appearance of the modern sea-ice diatom community as a dominant element in marine sediments on the Antarctic shelf may provide the best proxy evidence to date the shift to the present cold-polar glacial conditions.