Millennial-scale variations of an East Antarctic outlet glacier during the Last Glaciation

Antarctica's ice sheet response to ongoing climate change is the largest uncertainty in future sea-level projections. Recent warming has forced potentially irreversible retreat of the West Antarctic Ice Sheet. However, the vulnerability of marine-based portions of the East Antarctic Ice Sheet (EAIS), which contain 19 meters of sea-level-equivalent, remains uncertain due to a lack of data and long-held assumptions about EAIS stability. This uncertainty is concerning because numerical ice sheet models indicate that Antarctica's ice shelves and marine-terminating outlet glaciers respond rapidly to ocean perturbations (e.g., increased ocean heat flux, sea level rise), resulting in rapid regional ice mass loss. Ice-proximal marine geologic records are useful for informing and refining model outputs, yet few well-dated records of past East Antarctic marine-based outlet glacier response exist. Here we show that East Antarctica's largest marine-based outlet glacier system, the Lambert Glacier-Amery Ice Shelf system, experienced at least three millennial-scale advance and retreat cycles during the last glaciation. Because this system had retreated from the shelf edge before the Last Glacial Maximum, a late Quaternary ice-proximal sedimentary sequence is preserved in Prydz Channel. We have accurately dated these sediments using a newly established radiocarbon dating method that separates autochthonous marine from glacially reworked organic carbon. Three diatom-rich sedimentary units with elevated radiogenic beryllium-10 (10Be) concentrations reveal three open water intervals in inner Prydz Bay between 40 and 20 ka. Our sedimentologic and geochemical evidence indicates that the Lambert Glacier-Amery Ice shelf system had retreated landward of our study site coincident with millennial-scale atmospheric warm events and reduced Southern Ocean sea ice extent. The observed sensitivity of East Antarctica's largest outlet glacier system to climate forcing during the last glaciation has important implications for past and future EAIS stability and global sea levels, particularly during warm intervals with high atmospheric CO2 concentrations.