NSFPLR-NERC: Geological history constraints on the magnitude of grounding line retreat in the Thwaites Glacier system

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We propose to obtain geological evidence from the Thwaites-Pine Island glacier system that will show whether glaciers were less extensive than they are at present, and, if so, when. Our goals are to: (i) determine whether previous grounding-line retreat-advance cycles, as suggested by existing geological evidence, occurred in the late Holocene, and (ii) establish under what climate and sea-level boundary conditions they took place. Determining the conditions under which the Thwaites and Pine Island Glacier grounding lines have retreated and re-advanced in the past is critically relevant to determining whether or not present-day grounding-line retreat is irreversible.We will utilize two approaches to achieve these goals. To obtain geological evidence for past grounding-line retreat episodes, we will apply cosmogenic-nuclide exposure-dating of subglacial bedrock. Using drill systems recently developed in the USA for subglacial bedrock recovery and proven in the 2016-17 Antarctic field season, we will obtain subglacial bedrock from sites where ice thickness is dynamically linked to grounding line position in the Thwaites system. The observation of significant cosmogenic-nuclide concentrations in these samples would provide direct, unambiguous evidence for past episodes of thinning linked to grounding line retreat as well as constraints on their timing and duration. To reconstruct Holocene relative sea level, we will map and date raised marine and shoreline deposits throughout Pine Island Bay.