ALBERT

Description: The coastal bathymetry of Thwaites Glacier (TG) is poorly known yet nearshore sea-floor highs have the potential to act as pinning points for floating ice shelves, or to block warm water incursions to the grounding line. In contrast, deeper areas control warm water routing. Here, we present more than 2000 km2 of new multibeam echo-sounder data (MBES) acquired offshore TG during the first cruise of the International Thwaites Glacier Collaboration (ITGC) project on the RV/IB Nathaniel B. Palmer (NBP19-02) in February-March 2019. Beyond TG, the bathymetry is dominated by a 〉1200 m deep, structurally-controlled trough and discontinuous ridge, on which the Eastern Ice Shelf is pinned. The geometry and composition of the ridge varies spatially with some sea-floor highs having distinctive flat-topped morphologies produced as their tops were planed-off by erosion at the base of the seaward-moving Thwaites Ice Shelf. In addition, submarine landform evidence indicates at least some unconsolidated sediment cover on the highs, as well as in the troughs that separate them. Knowing that this offshore area of ridges and troughs is a former bed for TG, we also used a novel spectral approach and existing ice-flow theory to investigate bed roughness and basal drag over the newly-revealed offshore topography. We show that the sea-floor bathymetry is a good analogue for extant bed areas of TG and that ice-sheet retreat over the sea-floor troughs and ridges would have been affected by high basal drag similar to that acting in the grounding zone today. Comparisons of the new MBES data with existing regional compilations show that high-frequency (finer than 5 km) bathymetric variability beyond Antarctic ice shelves can only be resolved by observations such as MBES and that without these data calculations of the oceanic heat flux may be significantly underestimated. This work supports the findings of recent numerical ice-sheet and ocean modelling studies that recognise the need for accurate and high-resolution bathymetry to determine warm water routing to the grounding zone and, ultimately, for predicting glacier retreat behaviour.

Description: The geometry of the sea floor immediately beyond Antarctica's marine-terminating glaciers is a fundamental control on warm-water routing, but it also describes former topographic pinning points that have been important for ice-shelf buttressing. Unfortunately, this information is often lacking due to the inaccessibility of these areas for survey, leading to modelled or interpolated bathymetries being used as boundary conditions in numerical modelling simulations. At Thwaites Glacier (TG) this critical data gap was addressed in 2019 during the first cruise of the International Thwaites Glacier Collaboration (ITGC) project. We present more than 2000 km2 of new multibeam echo-sounder (MBES) data acquired in exceptional sea-ice conditions immediately offshore TG, and we update existing bathymetric compilations. The cross-sectional areas of sea-floor troughs are under-predicted by up to 40 % or are not resolved at all where MBES data are missing, suggesting that calculations of trough capacity, and thus oceanic heat flux, may be significantly underestimated. Spatial variations in the morphology of topographic highs, known to be former pinning points for the floating ice shelf of TG, indicate differences in bed composition that are supported by landform evidence. We discuss links to ice dynamics for an overriding ice mass including a potential positive feedback mechanism where erosion of soft erodible highs may lead to ice-shelf ungrounding even with little or no ice thinning. Analyses of bed roughnesses and basal drag contributions show that the sea-floor bathymetry in front of TG is an analogue for extant bed areas. Ice flow over the sea-floor troughs and ridges would have been affected by similarly high basal drag to that acting at the grounding zone today. We conclude that more can certainly be gleaned from these 3D bathymetric datasets regarding the likely spatial variability of bed roughness and bed composition types underneath TG. This work also addresses the requirements of recent numerical ice-sheet and ocean modelling studies that have recognised the need for accurate and high-resolution bathymetry to determine warm-water routing to the grounding zone and, ultimately, for predicting glacier retreat behaviour.