RAPID SEDIMENT TRANSFER BY ‘ERRATIC' ICE STREAMS

Geologic evidence shows that ice streams are capable of transporting large volumes of sediment at high rates, and it is often assumed that transport of sediment occurs from deformation of a subglacial till layer. This assumption is, however, not consistent with observations from boreholes drilled to the bed of ice streams. The latter has revealed vertically limited extent of till deformation and material properties consistent with a Coulomb-plastic rheology. Although incorporation of Coulomb-plastic till yields better dynamic capability of ice sheet models, this particular basal parameterization can appear geologically problematic because deformation should collapse to a single shear plane when a yield stress criterion is exceeded, thereby limiting the magnitude of subglacial sediment transport. An important question is therefore: if ice-stream beds are as slippery as observations suggest, how is sediment transported in the system?

Here, we examine the interaction of a fast-flowing ice stream with a slippery bed by coupling a higher-order ice sheet model to a Coulomb-plastic subglacial processes model. The main difference compared to previous studies is that sediment transfer according to our model exclusively occurs from the development and fate of a debris-bearing basal ice layer. We run the model for 20,000 years and find that the inherent instability of ice streams cause erosion and sediment transport in asynchronous stages. Whereas erosion is high due to freeze-on of sediment when the ice stream stops, the rate of sediment transport is low due to low velocity. The latter is high when the ice stream is active because debris is transported in the basal ice layer. Erosion is on the other hand greatly reduced because fast sliding promotes basal melting.

We show that hydrologic forcing of the ice stream increases not only the flow speed of the active ice stream, but also the erosion rate of the subsequent ‘inactive’ period of stagnation. As a consequence of hydrologic forcing, the basal ice layer becomes thicker and richer in debris, and its formation is a very effective mechanism of sediment transfer. To verify the simulated development of basal ice, we use borehole camera imagery acquired from the basal zone of Kamb Ice Stream in Antarctica.