Pre-satellite retreat of Thwaites and Pine Island glaciers: Recent results from sediment cores

Thwaites Glacier (TG) is thinning and accelerating while sitting on a landward-dipping bed, with an ice shelf that is rapidly disintegrating and losing its ability to buttress ice flow from upstream, and is 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 already begun.  The Thwaites Offshore Research (THOR) group has just completed four field deployments aimed at understanding the recent history of TG and neighboring ice, including Pine Island Glacier (PIG).  Three cruises on the RVIB N.B. Palmer, combined with sub-ice-shelf sediment coring, provide a suite of new data along the TG and PIG margins.  Data include multibeam surveys, 3.5 kHz subbottom profiler, over 100 new sediment cores, and high-resolution seismic profiles.  Break-up of floating ice cover in front of TG in 2019 allowed surveying of previously unmapped seafloor.  Major calving of PIG in 2020 allowed marine surveying over the locations where sub-ice-shelf cores were collected in the past, allowing direct ties between ice-based and marine work.  As of this writing in January 2022, we are entering into the Amundsen Sea for our third marine field season. 

 

Sediment cores record the history of grounding-zone retreat and ice interaction with the ocean over timescales from decades to 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 contain laminated mud with sparse gravel and sand, suggesting deposition of meltwater deposits. Downcore ²¹⁰Pb measurements are used to create age models of the past ~100 years.  Combination of ages with facies models, including CT scans, reveals that the progressive detachment of Thwaites from pinning points began in the mid twentieth century, coincident with retreat of PIG (Smith et al., 2017) and with increasing advection of warm water onto the Amundsen shelf (Hillenbrand et al., 2017).  Conversely, Cranton Bay, to the northeast of PIG and separated from Pine Island Bay by a shallow sill, appears to be characterized by cold deep water and high productivity, allowing it to serve as an endmember different from the records obtained proximal to the large glacial outlets where CDW is impinging.

 

The satellite record of glacial retreat is inherently short.  Observations are accumulating about forcing mechanisms that can impact the stability of ice, such as increased CDW on the Amundsen Sea shelf.  However, without the time to observe the response of the ice, discovery of the forcing mechanisms is just half the story.  The other half of the story is completed by using the paleo record to see how ice has responded to drivers in the past.