Revealing the former bed of Thwaites Glacier using sea-floor bathymetry

Abstract

Abstract. The geometry of the sea floor beyond Thwaites Glacier (TG) is a major control on the routing of warm ocean waters towards the ice stream’s grounding zone, which has led to increased mass loss through sub-ice-shelf melting and resulting accelerated ice flow. Nearshore topographic highs act as pinning points for the Thwaites Ice Shelf and potentially provide barriers to warm water incursions. To date, few vessels have been able to access this area due to persistent sea-ice and iceberg cover. This critical data gap was addressed in 2019 during the first cruise of the International Thwaites Glacier Collaboration (ITGC) project, with more than 2000 km2 of new multibeam echo-sounder data (MBES) were acquired offshore TG. Here, these data along with legacy MBES datasets are compiled to produce a set of standalone bathymetric grids for the inner Amundsen Sea shelf beyond both Pine Island and Thwaites glaciers. At 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 parts having distinctive flat-topped morphologies produced as their tops were planed-off by erosion at the base of the seaward-moving Thwaites Ice Shelf, suggesting a positive feedback mechanism for ice-shelf ungrounding. Knowing that this offshore area is a former bed for TG, we applied a novel spectral approach to investigate bed roughness and find that derived power spectra can be approximated using an inverse-square law, a result that is consistent with spectra for bed profiles from the modern TG. Using existing ice-flow theory, we also make a first assessment of the form drag (basal drag contribution) for ice flow over this topography. Ice flowing over the sea-floor troughs and ridges would have been affected by similarly high basal drag to that acting in the grounding zone today. We show that the sea-floor bathymetry is an analogue for extant bed areas of TG and that more can be gleaned from these 3D bathymetric datasets regarding the likely spatial variability of bed roughness and bed composition types underneath TG. Comparisons with existing regional bathymetric compilations for the area 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 capacity of bathymetric troughs, and thus oceanic heat flux, may be significantly underestimated. This work meets 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.

Pathways and modification of warm water flowing beneath Thwaites Ice Shelf, West Antarctica

“…1 and see Methods). These troughs are notably deeper (by 100 to 300 m) than predicted by gravity models (8,21). Temperature, salinity, oxygen content, and current velocity were measured in these troughs by ship-borne instrumentation and an autonomous underwater vehicle (AUV).…”

Rapid retreat of Thwaites Glacier in the pre-satellite era

“…Fast retreat from flat-topped ridges may be a process that is already occurring in several places at Thwaites Glacier today 9 . In previous work, the process of sediment removal from offshore sea-bed highs was suggested to have made the Thwaites Ice Shelf more sensitive to thinning in the past by lowering sea-bed topography 18 . Our results show that, in addition to increasing bathymetric depth, the process of flattening sea-bed highs during grounding may have also increased the likelihood of rapid retreat at Thwaites Glacier by promoting the development of ice plains at which rapid inland tidal migration of the grounding line occurred.…”

“…One place where information on patterns and processes of retreat might be studied is in former ice-sheet grounding zones. Offshore of Thwaites Glacier, recent sonar mapping documented numerous sea-bed promontories where the ice was previously grounded 18 . Spectral analyses of this former bed topography indicate that these regions are geologically analogous to the modern grounding zone 18 , making them ideal sites to target the processes and behaviour of Thwaites in the recent past.…”

Twenty-first century sea-level rise could exceed IPCC projections for strong-warming futures

“…Changes in prevailing winds and currents may draw up these warm waters over the continental shelf edge, where they can then drain downward along deep troughs carved by formerly more extensive ice streams to reach the modern ice edge. [95][96][97] Many of these deep troughs have not been mapped satisfactorily, especially in East Antarctica. 62 As these natural conduits that allow warm waters to reach the ice sheet are missing in models, the impact of ocean-driven forcing on ice sheets is likely to be systematically underestimated.…”