Abstract

The fate of the West Antarctic Ice Sheet is the largest remaining uncertainty in predicting sea-level rise through the next century, and its most vulnerable and rapidly changing outlet is Thwaites Glacier. Because the seabed slope under the glacier is retrograde (downhill inland), ice discharge from Thwaites Glacier is potentially unstable to melting of the underside of its floating ice shelf and grounding line retreat, both of which can be enhanced by warm ocean water circulating underneath the ice shelf. Here we present the results of two missions underneath Thwaites ice shelf performed by the AUV ‘Ran’: The very first direct observations of ocean temperature, salinity, and oxygen underneath Thwaites ice shelf. Using the high precision environment payload suite, observations were obtained that indicate deep water (> 800 m) underneath the central part of the ice shelf is in connection with Pine Island Bay, a previously unknown westward branch of warm deep water entering the ice shelf cavity. Warm water also enters from the north in two troughs separated by a pinning point. Spatial gradients of salinity, temperature and oxygen recorded underneath the ice shelf indicate that this is an active region where several water masses meet and mixes. The observations identify the central buttressing point as a vulnerable region of change currently under attack by warm water inflow from all sides: a scenario that may lead to ungrounding and retreat more quickly than previously expected.