Heterogeneous subglacial sediments beneath Thwaites Glacier, West Antarctica, inferred from seismic data

Constraining the properties of subglacial materials is key to the accurate parameterisation of glacial slip in ice sheet models and is therefore important for accurate projections of sea level rise. The bed of Thwaites Glacier, West Antarctica, is particularly important because Thwaites is key to the stability of the West Antarctic Ice Sheet. We report the results of active-source seismic surveys undertaken during the 2023/24 Antarctic field season, immediately upstream of the subglacial ridge known as Ghost ridge. These include a 14.4 km profile of normal-incidence data and wide angle data over sections totalling ∼ 5 km. Estimation of basal acoustic impedance reveals that the bed in this region is composed of sediments, with regions of dilatant sediments and regions of stiffer sediments present. Amplitude-versus-angle (AVA) analysis of wide angle data supports this interpretation, with soft bed regions displaying high Poisson’s ratios (> 0.45). We image bedforms which are interpreted as sedimentary depositional bedforms, one of which we interpret as a crag and tail formation, and the other of which appears as a sedimentary tail emerging from rolling topography upstream. The tails of these features are probably composed of dilatant till. We apply Viscous Grain Shearing (VGS) theory to results obtained from AVA analysis to constrain the porosity and effective pressure of the subglacial sediments. VGS analysis suggests that the changes in reflectivity along our seismic profile may be primarily controlled by changes in effective pressure at the ice base rather than porosity.