Bed-type variability and till (dis)continuity beneath Thwaites Glacier, West Antarctica

Atsuhiro Muto,Sridhar Anandakrishnan,Byron R Parizek,Richard B Alley

doi:10.1017/aog.2019.32

Atsuhiro Muto, Sridhar Anandakrishnan + Show 2 more

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https://doi.org/10.1017/aog.2019.32

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Abstract

AbstractRecent seismic measurements from upper Thwaites Glacier indicate that the bed-type variability is closely related to the along-flow basal topography. In high-relief subglacial highlands, stoss sides of topographic highs have a relatively higher acoustic impedance (‘hard’ bed) with lower acoustic impedance (‘soft’ till) on lee sides. This pattern is similar to observations of many deglaciated terrains. Subglacial hydraulic-potential gradient and its divergence show a tendency for water to diverge over the stoss sides and converge into the lee sides. Convergence favors a thicker or more widespread water system, which can more efficiently decouple ice from the underlying till. Under such circumstances, till deformation does occur but, fluxes are relatively small. Till carried from the lee sides onto stoss sides of downstream bumps should couple to the ice more efficiently, increasing the ability for transport by till deformation. In turn, this suggests that steady-state till transport can be achieved if the stoss-side till layer is thin or discontinuous. In addition, the large basal shear stress generated in the highlands seems too high for a bed lubricated by a continuous although thin deforming till, suggesting till discontinuity, which would allow debris-laden ice to erode bedrock on stoss sides, supplying additional till for transport.

Highlights

Ice flow is dictated to a large extent by the nature of the bed
Prior modeling (e.g. Parizek and others, 2013; Koellner and others, 2019) has shown that the stability of the West Antarctic Ice Sheet may depend on the form of the basal-slip law, not just on the magnitude of the basal drag, with nearly linear, nearly plastic and mixed beds giving often significantly different thresholds for onset of retreat and for subsequent rates of retreat
To a greater extent than most other outlet glaciers such as Siple Coast ice streams (e.g. Anandakrishnan and others, 1998; Peters and others, 2006) and neighboring Pine Island Glacier (Jordan and others, 2010; Smith and others, 2013), Thwaites Glacier flows across the dominant topography rather than along it, giving strongly contrasting basal conditions along flow lines

Summary

Introduction

Ice flow is dictated to a large extent by the nature of the bed. Models exist for estimating the basal-slip law from the bed type. If a thin, deforming till layer was present on stoss sides, it could bury bumps at the sub-meter scale that may be most important in limiting basal sliding (Weertman, 1957) and prevent friction between bedrock and rocks carried in basal ice. In such a case, we would expect relatively low dynamic drag at the bed.

Results

Conclusion