Circum-Antarctic warming events between 3.4 and 3.7 Ma recorded in sediments from Prydz Bay (ODP Leg 188) and the Antarctic Peninsula (ODP Leg 178)

Our study characterizes glacial and interglacial deposition on two Antarctic margins in order to discriminate between regional and continent-wide early to middle Pliocene warm intervals that caused sea-ice reduction and continental ice sheet retreat. We use a multi-proxy (i.e., sediment facies and grain size, siliceous microfossils, biogenic opal, geochemical composition and clay mineralogy) approach to examine sediments recovered in drill holes from the West Antarctic Peninsula and the East Antarctic Prydz Bay margins, focusing on the climatic record between 4 and 3.5 Ma. Warm conditions in both East and West Antarctica are recorded, which based on our age model correspond to periods of prolonged or extreme warmth correlated with isotopic stages Gi5, Gi1, MG11 and MG7. For the Gi5 interglacial our data corroborates the 60% Dictyocha percentage at 34.60 mbsf previously reported from Prydz Bay and interpreted to indicate a SSST of about 5.6 °C above present. Our higher-resolution sampling interval shows Dictyocha percentages up to 87.5%, suggesting even higher SSSTs above present levels. During MG11, which coincides with the section dated by the magnetic polarity reversal Gilbert-Gauss at 3.58 Ma, SSSTs were tentatively 2.5°-4° warmer than present, and reduced sea-ice cover in Prydz Bay and probably also west of the Antarctic Peninsula is indicated by increased primary productivity. In addition, a reduction of ice sheet size is suggested by the bioturbated and IRD-enriched facies that characterize these high-productivity intervals. Based in our age model and calculated sedimentation rates glacial-interglacial cyclicity between 4 and 3.5 Ma in the cores from Antarctic Peninsula and Prydz Bay Sites, result in frequencies consistent with obliquity and precession forcing. The prolonged early-middle Pliocene warm period was superimposed on a cooling trend recorded by the: 1) increase of the terrigenous sediment supply at all our sites starting between 3.7 and 3.6 Ma, and 2) decrease in SSSTs (from > 5.6 °C at 3.7 Ma to 4°-2.7 °C at 3.6 Ma, and 2.5 °C at 3.5 Ma.) indicated by the silicoflagellate W/C R from Site 1165. We postulate that, although the start of a cooling trend is recorded at about 3.7-3.6-Ma, relatively warm conditions prevailed until 3.5 Ma capable of maintained open marine conditions with reduced or no sea-ice and reduced ice sheet volume and extent. The information in this paper regarding the timing of continental-wide and regional warm events and the paleoenvironmental conditions that characterized them (i.e., SSST, extent of sea ice, and ice sheet size) are relevant to help constrain paleoclimate and ice sheet models for the early-middle Pliocene, a time period when the level of warming according to the Intergovernmental Panel on Climate Change 2007 report, is within range of the estimates of the Earth's global temperature increases for the 21st century. These data, when linked to modeling studies like those of Pollard and DeConto (2009) will further our understanding of how these ice sheets may respond to future warming of the southern high latitudes.