Sediment Flux of Deformable Glacier Beds

Abstract:

Advection and deposition of sediment mobilized during glacier slip shapes the beds of fast-flowing ice streams, reroutes subglacial hydrologic systems, changes the bed slope, and constructs bedforms that impede flow. As such, subglacial sediment flux can significantly impact glacier dynamics and accurate estimates of this parameter are necessary to achieve reliable predictions of ice sheet stability. Subglacial sediment transport is facilitated in part by the deformation of “soft” glacier beds (i.e. unconsolidated, water-saturated granular materials) in response to overriding ice. However, the mechanisms that control vertical distribution of strain in the sediment are not well understood. To address this deficiency, we conducted a suite of experiments using a novel ring shear device designed to systematically assess the influence of two likely controlling factors, overburden and sliding velocity, on subglacial sediment deformation. We simulated glacier sliding using by sliding rings of temperate ice over a fine-grained, sandy, diamicton. A range of realistic normal stresses (~25 kPa to ~215 kPa) and sliding velocities (~50 m/yr to ~300 m/yr) were implemented, and strain distribution in the sediment was recorded at each stage. Deformation was quantified by photographing the till bed through clear acrylic walls at regular intervals in time and using digital image correlation to estimate the displacement that occurred between image captures. At all stages of the experiment, sediment deformation was concentrated in a thin band (O(10^-2 m)) near the ice-bed contact. Preliminary results indicate sediment flux increases with sliding velocity. However, the relationship between overburden and depth of deformation is more varied, with minimal deformation occurring at high stresses. These data provide unique insight into the dynamics and sediment flux inferred for both modern glacial systems and the glacial geologic record.