Bedforms of Thwaites Glacier, West Antarctica: Character and Origin

Bedforms of Thwaites Glacier, West Antarctica both record and affect ice flow, as shown by geophysical data and simple models. Thwaites Glacier flows across the tectonic fabric of the West Antarctic rift system with its bedrock highs and sedimentary basins. Swath radar and seismic surveys of the glacier bed have revealed soft-sediment flutes 100 m or more high extending 15 km or more across basins downglacier from bedrock highs. Flutes end at prominent hard-bedded moats on stoss sides of the next topographic highs. We use simple models to show that ice flow against topography increases pressure between ice and till upglacier along the bed over a distance that scales with the topography. In this basal zone of high pressure, ice-contact water would be excluded, thus increasing basal drag by increasing ice-till coupling and till flux, removing till to allow bedrock erosion that creates moats. Till carried across highlands would then be deposited in lee-side positions forming bedforms that prograde downglacier over time, and that remain soft on top through feedbacks that match till-deformational fluxes from well upglacier of the topography. The bedforms of the part of Thwaites surveyed here are prominent because ice flow has persisted over a long time on this geological setting, not because ice flow is anomalous. Bedform development likely has caused evolution of ice flow over time as till and lubricating water were redistributed, moats were eroded and bedforms grew. Plain Language Summary Thwaites Glacier, West Antarctica, is of great interest because ongoing retreat could lead to faster future changes that could notably increase sea-level rise. Wide-ranging studies are thus focused on gaining broad understanding of the glacier. Geophysical surveys discussed here show that the bed of the glacier has been sculpted by the flowing ice, eroding large "moats" upglacier and to the sides of bedrock obstacles, and depositing long tails of soft sediment downglacier of those obstacles. We use simple models to show that these features formed over time because of the interactions among ice, bedrock, and subglacial water and sediment, and that the behavior of the glacier must have changed as these features developed. This knowledge does not directly affect estimates of the potential for future sea-level rise, but does guide further studies to improve those estimates. This knowledge also may help understand the formation of glacially sculpted landscapes left in widespread regions by former ice sheets.