THWAITES GLACIER, ANTARCTICA, OFFSHORE RESEARCH: INITIAL SEDIMENTARY RECORDS FROM 2019 AND 2020 CRUISES (Invited Presentation)

Thwaites Glacier (TG), which currently contributes ~4% to global sea-level rise, is thinning and accelerating while sitting on a landward-dipping bed, without a buttressing ice shelf, and in deep water that allows warm Circumpolar Deep Water (CDW) to reach its grounding zone. Significant retreat of TG would trigger loss of ice across the region. In recent decades, the mass balance of TG has become increasingly negative, suggesting that unstable retreat may have begun. The Thwaites Offshore Research (THOR) group has just completed three separate field deployments aimed at understanding the recent history of TG. Two cruises on the RVIB N.B. Palmer, combined with sub-ice-shelf sediment coring, provide a suite of new data along the Thwaites and Pine Island glacier margins. Break-up of floating ice cover in front of TG in 2019 allowed surveying of previously unmapped seafloor. Major calving of Pine Island Glacier in 2020 allowed marine surveying at essentially the same location as past ice-shelf coring, allowing direct ties between land-based and marine work. Data include multibeam surveys in front of the ice, 3.5 kHz subbottom profiler data, over 100 new sediment cores, and high-resolution seismic profiles. Bathymetric data show deep troughs that allow CDW to reach the glacier front. Images of seabed landforms provide information about past ice flow and pinning points.

Sediment cores record the history of grounding zone retreat and its interaction with the ocean over timescales from the last decades, extending the instrumental record, through several thousand years. Proxies used to reconstruct ice and ocean histories include sedimentary facies analysis, diatom and foraminiferal assemblage data, and geochemical analyses. Sedimentological analyses show a diverse array of lithofacies attributed to different environmental conditions. Many cores across the region only contain laminated mud with sparse gravel and sand, suggesting deposition of meltwater plumites at high-accumulation rates. Cores collected next to the 2019 ice margin transition from gravelly sandy mud near the base to mud at the surface, which likely results from ice-shelf retreat. Sediment accumulation rates, obtained through ²¹⁰Pb geochronology, vary spatially and temporally, sometimes revealing discrete depositional events.