Frictional Origin of Slip Events of the Whillans Ice Stream, Antarctica. Issue 11 (7th June 2021) - Guerin, Gauthier
Abstract: Ice sheet evolution depends on subglacial conditions, with the ice‐bed interface's strength exerting an outsized role on the ice dynamics. Along fast‐flowing glaciers, this strength is often controlled by the deformation of subglacial till, making quantification of spatial variations of till strength essential for understanding ice‐sheet contribution to sea‐level. This task remains challenging due to a lack of in situ observations. We analyze continuous seismic data from the Whillans Ice Plain (WIP), West Antarctica, to uncover spatio‐temporal patterns in subglacial conditions. We exploit tidally modulated stick‐slip events as a natural source of sliding variability. We observe a significant reduction of the till seismic wave‐speed between the WIP sticky‐spots. These observations are consistent with a poroelastic model where the bed experiences relative porosity and effective pressure increases of >11% during stick‐slips. We conclude that dilatant strengthening appears to be an essential mechanism in stabilizing the rapid motion of fast‐flowing ice streams. Plain Language Summary: The rate of ice‐mass loss from the Antarctic ice‐sheet, and hence sea‐level rise, is governed by the rate of ice flow. The frictional force of the glacier bed plays a large role in ice flow speed. For fast glaciers, this force is often controlled by the deformation of subglacial sediments. Therefore, understanding their mechanical properties is crucial to understanding glaciers' contribution to sea‐level rise. This task remains challenging due to the lack of direct, in‐situ observations. This paper analyzes seismic data recorded on the Whillans Ice Plain (WIP) in West Antarctica to reveal spatio‐temporal phenomena in subglacial conditions. We exploit glacier stick‐slip events as natural sources of slip variability. We observe a significant reduction in seismic wave‐speed within the till between the WIP's sticky‐spots. On the contrary, the sticky‐spots themselves show no change. These observations are consistent with a poroelastic model. In between the sticky‐spots, the bed undergoes an increase in effective pressure and porosity due to the sediments' plowing during the slip. These till variations slow down the glacier slip and prevent it from accelerating catastrophically. This phenomenon of strengthening by dilation of bed sediments appears to be an essential mechanism for stabilizing the rapid movement of fast‐flowing ice‐streams. Key Points: The frictional properties of the Whillans Ice Stream bed are spatially heterogeneous Seismic velocities are sensitive to small changes in till poroelastic properties Dilatant strengthening of till stabilizes the glacier during stick‐slip cycles