This paper reviews evidence for a significant ice-volume decrease in Antarctica during the Pliocene, as indicated by (1) recycled marine microfossils in glacial deposits of the Sirius Group and Pagodroma Tillite; (2) by numerous fossiliferous deposits along the Antarctic coastline, particularly that of Marine Plain in the Vestfold Hills; and (3) by paleobiogeographic distribution of diatoms in the Southern Ocean. This evidence indicates higher sea-level, warmer marine and terrestrial conditions, and retreat of the glacial margin during the Pliocene. Early to mid-Pliocene Antarctic paleogeography included (1) a greatly diminished, warmer and wetter ice sheet, compared to the present ice sheet; (2) an extensive austral vascular macroflora extending almost to the South Pole; (3) relatively warm seas (2°C to 6°C) extending into and across broad regions of East and West Antarctica; and (4) a weakly developed seasonal sea-ice belt, present only in isolated regions on the Antarctic continental shelf. The absence of sea-ice resulted in relatively mild winter conditions in Antarctica as oceanic heat warmed the continent and interior seas. This prevented the sharp depression of winter temperatures seen in Antarctica today.
Our initial interpretation of the Sirius Group diatom assemblages called for at least one, perhaps two, marine incursions into the Wilkes and Pensacola basins between 5.0 and 2.5 Ma. New evidence from the benthic oxygen isotope record and eustatic signature in the Wanganui Basin of New Zealand suggest a more dynamic history of ice retreat and advance during the Pliocene. Significantly warmer Pliocene climate may have rendered the East Antarctic Ice Sheet sensitive to the dynamics of orbital climate forcing. Repeated ice sheet growth and retreat would aid repeated isostatic depression of the Wilkes and Pensacola Basins to allow marine incursion evident by recycled microfossils in the Sirius Group.
The late Pliocene reglaciation of Antarctica was accompanied by the onset of the modern polar glacial regime and the development of extensive sea- ice. A portion of the isotopic shift at ~2.4 Ma, thought to represent the initiation of the Northern Hemisphere ice sheets, must also be attributed to the reglaciation of Antarctica. The identity of the Antarctic ice sheet as a potential source for this isotope shift was hidden by the assumption that the Antarctic ice sheet was a permanent and stable feature since the middle Miocene.