Spatial distribution of englacial layer slope as a constraint on ice sheet basal conditions

Englacial layers are a ubiquitous indicator of internal deformation within ice sheets, as well as a common finding in radio echo sounding data. In spite of this, placing direct constraints on present or past ice flow through englacial layers remains, to date, a challenging task. Our work leverages recent advances in the processing of airborne radar sounding data along with modelling work to address this challenge. Here we present an application to the case of an abrupt change in basal friction due to a transition from frozen to temperate basal conditions, which we seek to detect from radar sounding data through its signature in englacial layer geometry. We first formulate a first-principle model for ice flow across an abrupt change in basal friction, and use it to show that this setting produces quantifiable, large, anomalies in layer slope. Then we exploit a recently developed layer-optimized, unfocussed SAR processing technique that automatically estimates layer slopes with high accuracy to look for this signature in the onset region of Institute Ice Stream (West Antarctica). We find that observed slopes are incompatible with an abrupt sliding initiation. Our results instead provide evidence for the existence of a slow (in space) transition from fully frozen to temperate beds, as consistent, for instance with a spatially extended region of subtemperate sliding. We conclude by discussing implications of this finding with respect to the present and past history of Institute Ice Stream.