Response of the East Antarctic Ice Sheet to Middle Miocene Global Change

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Abstract Marchant OPP-9811877This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports research to determine the response of the East Antarctic Ice Sheet to global climate change. One of the largest global climate shifts occurred between about 15.6 and 12.5 million years ago (Middle Miocene time). During this time, dramatic global cooling and reorganization of ocean circulation patterns were recorded as a shift in the isotopic composition of oxygen in the oceans. This dramatic and permanent shift set the stage for modern oceanic and atmospheric circulation and ushered in the bipolar ice ages that have dominated climate records for the last 2.5 million years. How did Antarctica respond to the great climate shift at about 14 million years ago? Did growth of the Antarctic Ice Sheet in fact initiate this shift? If so, how will future fluctuations in the volume of East Antarctic ice influence atmospheric and oceanic circulation?The recent (and unexpected) breakthrough in Antarctic geology that now allows us to address the fundamental problem of middle Miocene global climate change and ice sheet evolution is the discovery of Miocene-age volcanic ashes that are interbedded with surficial sediments in southern Victoria Land. For the first time, it is now possible to generate precise climate and glaciological reconstructions from direct examination of unambiguous, Miocene-age terrestrial deposits on the Antarctic continent. As the only place in Antarctica where pristine Miocene-age unconsolidated deposits are preserved at the ground surface, southern Victoria Land is unrivaled as a storehouse of Miocene glacial-geologic data. Key questions that now can be addressed include; What contributing factors on Antarctica led to abrupt global cooling at about 14 million years ago? Does the middle Miocene shift in the isotopic composition of the oceans signify a major expansion of East Antarctic ice? Or, does the isotopic shift instead reflect a change in ocean temperature or circulation? A related question is when did hyper-arid, cold polar-desert conditions (signifying the development of a polar East Antarctic ice sheet) first evolve in Antarctica?Precise chronological control will be based on 50 laser-fusion isotopic analyses of in-situ volcanic ashes and 20 cosmogenic exposure-age analyses of ancient deposits. A coeval record of Miocene paleoclimate will come from textural changes in alpine drifts, the areal distribution of ice-marginal lakes, the abundance of dated patterned ground and ventifact pavements, and the geochemistry of buried soils and volcanic-ash deposits.