Characterizing Sediment Flux of Deforming Glacier Beds. Issue 4 (2nd April 2022)

• Record Type: Journal Article
• Authors: Hansen, D. D.; Zoet, L. K.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 4(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-04-02
• Subjects: subglacial mechanics -- sediment flux -- subglacial till -- basal slip -- glacier -- till deformation; Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2021JF006544
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: Deformation of subglacial sediment during basal slip shapes the beds of many fast‐flowing glaciers and ice streams. The resultant sediment flux impacts glacier dynamics and rates of subglacial erosion over a range of timescales, but its fundamental dependencies are not well understood. Using a cryogenic ring shear device, we conducted experiments to investigate the effects of both effective stress and slip speed on rates of till transport. Sediment fluxes were computed using digital image correlation from a photographic time series of the till bed. We find a near‐linear relationship between sediment flux and slip speed, but a non‐monotonic, double‐valued dependency of sediment flux on effective stress. Deformation primarily occurred in a thin shear band near the ice sole, the thickness of which could vary with both parameters. Coupling between ice and till increased at higher slip speeds and effective stresses and scaled the magnitude of the flow profile. Plain Language Summary: Rapid motion of many glaciers and ice streams occurs by ice sliding over soft sediment beds. This sediment is mobilized in response to stress exerted by the overriding ice, which influences rates of erosion and generates subglacial landforms over a range of timescales. However, the physical processes that control this sediment flux are poorly understood largely due to the difficulty of isolating the influence of specific variables in the field. We address this uncertainty with a custom ring shear device capable of simulating glacier slip in the laboratory at realistic pressures, temperatures, and ice speeds. We tested the influence of two key parameters on sediment deformation—the slip speed of ice and the pressure exerted by the ice on the bed—and computed the rates of sediment transport using an image processing technique called digital image correlation. We find a near‐linear relationship between sediment flux and slip speed, but a nonlinear relationship with ice pressure. Sediment deformation was mostly concentrated in a thin zone near the ice‐sediment interface. Coupling between ice and sediment increased at faster ice velocities and higher ice pressures, which scaled sediment transport rates. Key Points: Sediment flux in deforming till varies non‐monotonically with effective stress The relationship between sediment flux and the speed of basal ice is nearly linear Coupling between ice and till causes the grain velocity profile of the bed to scale with slip speed