Significant Role of the East Antarctic Ice Sheet in Miocene Global Climate Change

Abstract:

The Miocene was a period of climatic extremes, with indirect records suggesting major retreat and advance of Antarctica's ice sheets. Climatic warmth of the Early Miocene and Miocene Climatic Optimum (MCO; ~17-15 Ma) was followed by cooler conditions and growth of the East Antarctic ice sheet (EAIS) during the middle Miocene Climate Transition (MMCT; ~14-15 Ma). This cooling occured with decreasing CO₂ levels and deepening of the CCD. Weathering on Antarctica associated with ice sheet variability likely played a role in global climate, but the temporal and spatial evolution of the EAIS is not constrained well enough to characterize this relationship. We use Pb and Nd isotopes of authigenic Fe-Mn oxide coatings, which preserve bottom water compositions, and associated detrital bulk deep sea sediments, to provide new insights into continental weathering and ice sheet evolution during the Miocene. Comparison of Pb IC’s of coupled detrital and authigenic phases are used to reconstruct chemical weathering patterns. In contrast, Nd IC’s provide insights into bedrock erosion and ocean circulation patterns. Study sites are located on Maud Rise (ODP Sites 689 and 690) in the Atlantic sector, and on Kerguelen Plateau (ODP Site 744) off of Prydz Bay.

Results reveal increased offsets between seawater and detrital Pb IC’s during the Early Miocene and MCO, and decreased values and offsets during the MMCT and Late Miocene for all three sites, a pattern that is most pronounced at Maud Rise. There is a long-term trend to more unradiogenic Nd IC and more radiogenic Pb IC values of detrital sediments during the MMCT and Late Miocene. Comparison with modern day coretop data suggest this pattern indicates increased supply of local continental material. Our data suggest enhanced chemical weathering accompanied reduced ice volume and high atmospheric CO₂ during the Early Miocene and MCO, likely contributing to the drawdown of CO₂ that lead to the MMCT, as well as deepening of the CCD. Growth of the EAIS during the MMCT and the Late Miocene resulted in increased input of continental material into the Southern Ocean and reduced chemical weathering. Similarities in the records from these two locations demonstrate these changes were continental in scale, indicating a major role for the EAIS in global climate change during the Miocene.