Description: Pine Island Glacier (PIG) in the Amundsen Sea sector of the West Antarctic Ice Sheet (WAIS) is losing mass and contributing to global sea-level rise at an accelerating rate. Although recent observations and modeling have identified the incursion of relatively warm Circumpolar Deep Water (CDW) beneath the PIG ice shelf (PIGIS) as the main driver of this ice-mass loss, the lack of precise bathymetry limits furthering our understanding of the ice–ocean interactions and improving the accuracy of modeling. Here we present updated bathymetry and sediment distribution beneath the PIGIS, modeled by the inversion of aerogravity data with constraints from active-source seismic data, observations from an autonomous underwater vehicle, and the regional gravity-anomaly field derived from satellite gravity observations. Modeled bathymetry shows a submarine ridge beneath the middle of PIGIS that rises ∼350 to 400 m above the surrounding sea floor, with a minimum water-column thickness of ∼200 m above it. This submarine ridge continues across the whole width of the 45-km wide ice shelf, with no deep troughs crossing it, confirming the general features of the previously predicted sub-ice-shelf ocean circulation. However, the relatively low resolution of the aerogravity data and limitations in our inversion method leave a possibility that there is an undetected, few-kilometers-wide or narrower trough that may alter the predicted sub-ice-shelf ocean circulation. Modeled sediment distribution indicates a sedimentary basin of up to ∼800 m thick near the current grounding zone of the main PIG trunk and extending farther inland, and a region seaward of the submarine ridge where sediments are thin or absent with exposed crystalline basement that extends seaward into Pine Island Bay. Therefore, the submarine ridge marks the transition from a thick sedimentary basin providing a smooth interface over which ice could flow easily by sliding or sediment deformation, to a region with no to little sediments and instead a rough interface over which ice flows mainly by deformation. We hypothesize that the post-Last Glacial Maximum retreat of PIG stabilized at this location because of the spatial transition in basal conditions. This in turn supports the hypothesis that the recent retreat of PIG was strongly forced, probably by changes in ocean circulation, rather than occurring because of ongoing response to the end of the ice age or other changes inland of or beneath PIG.

Description: The Amundsen Sea sector of West Antarctica Ice Sheet is losing mass at a rate that has more than doubled in the past four decades, and continues to increase. Pine Island Glacier (PIG), the second largest drainage basins in this sector, experienced the fastest grounding-line retreat and its ice-mass loss increased more rapidly than any others in the last two decades. The large mass imbalance of PIG is attributed to the increased sub-ice-shelf melting by the incursion of relatively warm Circumpolar Deep Water (CDW) beneath the PIG ice shelf (PIGIS), although the lack of precise bathymetry data have restricted thorough understanding of the ice-ocean interactions. Here we present updated bathymetry and sediment distribution beneath PIGIS, modeled by inversion of aerogravity data with constraints from active-source seismic and autonomous underwater vehicle data, and the regional gravity anomaly derived from satellite gravity observations. Modeled bathymetry shows that the submarine ridge beneath the middle of PIGIS appears to continue across the width of the ice shelf, with no major deep troughs crossing it, consistent with previously predicted sub-ice-shelf ocean circulation. However, the relatively low resolution of the aerogravity data and limitations in our inversion method leave a slight possibility that there is an undetected, few kilometer-scale narrow trough that may alter this predicted sub-ice-shelf ocean circulation. Modeled sediment distribution indicates that the submarine ridge marks the transition from a thick sedimentary basin (soft, smooth bed for ice flow) around the current grounding zone of the main PIG trunk to a region of thin-to-no sediment with some exposed crystalline basement (rough, resistant bed for ice flow) that extends seaward into Pine Island Bay. We hypothesize that this transition in basal conditions caused the post-Last Glacial Maximum retreat of PIG to stabilized near this geological boundary.