Gravel-rich layers from the Mid-Miocene Climate Transition near East Antarctica

A major step in the evolution of the East Antarctic ice sheet (EAIS) occurred ~14 million years ago during the Mid-Miocene Climate Transition (MMCT). This global climatic shift had a profound effect on Earth's climatic system, including a decrease in global temperatures and sea level, and an increase in global ice volume. Of particular interest during this transition is the growth of the EAIS; despite inferences on the total amount of ice growth on Antarctica as a whole, little is known of the temporal and spatial evolution of the East Antarctic ice sheet during this time. Here we present direct evidence of ice sheet dynamics during the MMCT from sediment in deep ocean cores close to the Antarctic continent. Between 14.2 and 13.8 Ma, coeval with the MMCT, unusually high concentrations of dropstones are found at IODP Site IODP Site U1356, Wilkes Land, and ODP Site 1165, Prydz Bay. Such concentrations of dropstones are found only in Neogene intervals within these cores, suggesting a particularly dynamic ice sheet during the MMCT. We interpret the IRD-rich layers to be caused by repeated, massive pulses of debris-laden icebergs. Taken together with evidence for global ice volume oscillations from seawater oxygen isotope records, our new records suggests that the MMCT featured repeated growth and collapse of the EAIS in at least two locations. At IODP Site U1356, we have obtained ⁴⁰Ar/³⁹Ar ages for all available ice-rafted hornblende grains (>150μm). Results suggest a consistent provenance from the nearby Mertz Shear Zone during at least six advance and retreat events of the EAIS within the Wilkes Subglacial Basin during the MMCT. While the ice margin probably sat along the extension of the Mertz Shear Zone at the western edge of the Wilkes Basin, variations in the ɛNd of the fine fraction of these sediments (<63μm) require significant changes in the overall provenance. The occurrence of radiogenic values during at least three times within the MMCT hints to supply of fine-grained material from the interior of the Wilkes Subglacial Basin and/or the Transantarctic Mountains. At ODP Site 1165, sediments deposited during the MMCT are comprised of fine-grained sediment (<63μm) granules (>1 mm), pebbles and a few cobbles (up to 7 cm) with virtually no sand fraction (63μm to 1mm). We have explored the provenance of the pebbles and cobbles; individual pebbles and cobbles were crushed and any available potassium bearing minerals (hornblende, biotite, muscovite) were analyzed for their ⁴⁰Ar/³⁹Ar ages. Based on these ages, the provenance of IRD at ODP Site 1165 shows a mixture of local (Prydz Bay) material and far-traveled (>1000 km) material derived from the Wilkes Land margin. These offshore marine sites provide a direct record of repeated collapse and re-growth of large parts of the East Antarctic ice sheet during the MMCT, and highlight the value of scientific offshore drilling for revealing the history of the Antarctic ice sheet.