Modeling englacial sediment entrainment using spectral methods

Abstract:

Records of ice-rafted debris are used as evidence of paleo-ice sheet dynamics, ocean circulation, and regional climate. On human time scales, sediment discharge from outlet glaciers impacts coastal infrastructure, fjord ecosystems, and cycles of glacial advance and retreat. Sediment is exported from the ice sheet through fluvial and englacial entrainment. Previous work has focused on sediment transport by basal melt or “bulldozing” in front of an advancing terminus, as well as sediment circulation throughout a fjord after iceberg calving, but models do not explicitly simulate the various mechanisms by which sediment becomes entrained in basal ice layers. Here, we develop a model to study the interactions between production, entrainment, and transport processes, with a focus on describing the sediment flux at the calving front of an outlet glacier. The ice dynamics model solves the depth-integrated stress balance from Goldberg (2011), using spectral collocation methods. These governing equations afford a closer approximation to full-Stokes models than the shallow-ice approximation, while scaling down the computational requirements to allow the model to run over time scales relevant to Quaternary ice-rafted debris deposits. We derive a grid mapping scheme to achieve spectral convergence even over complex geometries. Once the model has solved for ice velocity, thickness, and temperature fields, sediment is produced through abrasion and quarrying processes, then entrained englacially via vertical regelation, supercooling advection, refreezing, or glaciotectonic activity. We present results for sediment fluxes for a range of idealized glacier, climate, and bedrock configurations, and analyze the model’s sensitivity to changes in forcing parameters. Modeling the specific mechanisms of englacial sediment entrainment will inform estimates of sediment fluxes from outlet glacier systems, both in the geologic record and in the face of changing ice sheet and climate conditions.