| Summary: | Geological records from the Antarctic margin offer direct evidenceof environmental variability at high southern latitudes and provideinsight regarding ice sheet sensitivity to past climate change.The early to mid-Miocene (23–14 Mya) is a compelling interval tostudy as global temperatures and atmospheric CO2 concentrationswere similar to those projected for coming centuries. Importantly,this time interval includes the Miocene Climatic Optimum, a periodof global warmth during which average surface temperatures were3–4 °C higher than today. Miocene sediments in the ANDRILL-2A drillcore from the Western Ross Sea, Antarctica, indicate that the Antarcticice sheet (AIS) was highly variable through this key time interval.A multiproxy dataset derived from the core identifies four distinctenvironmental motifs based on changes in sedimentary facies, fossilassemblages, geochemistry, and paleotemperature. Four major disconformitiesin the drill core coincide with regional seismic discontinuitiesand reflect transient expansion of grounded ice across theRoss Sea. They correlate with major positive shifts in benthic oxygenisotope records and generally coincide with intervals when atmosphericCO2 concentrations were at or below preindustrial levels(~280 ppm). Five intervals reflect ice sheet minima and air temperatureswarm enough for substantial ice mass loss during episodes ofhigh (~500 ppm) atmospheric CO2. These new drill core data andassociated ice sheet modeling experiments indicate that polar climateand the AIS were highly sensitive to relatively small changes in atmosphericCO2 during the early to mid-Miocene.
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