Glacial erosion beneath ice streams and ice‐stream tributaries: constraints on temporal and spatial distribution of erosion from numerical simulations of a West Antarctic ice stream

Abstract

Tulaczyk, S. 2003 (March): Glacial erosion beneath ice streams and ice-stream tributaries: constraints on temporal and spatial distribution of erosion from numerical simulations of a West Antarctic ice stream. Boreas, Vol. 32, Geologic evidence such as subglacial troughs and grounding zone wedges indicate that soft-bedded, West Antarctic ice streams are capable of eroding, transporting and depositing large volumes of debris at high rates ($100 m 3 yr À1 per meter width). In order to understand the dynamics of ice streams and the geologic effects of their activity, it is important to understand the physical mechanisms that control these high rates of sub-ice-stream sediment generation and transport. Here, we use a numerical model of Ice Stream C run over c. 8500 model years to quantify the effects of a recently proposed, till-ploughing mechanism of till formation and redistribution beneath ice streams Clark et al. in press). Our results show that this 'transport-limited' mechanism, in which till transport rates scale with ice velocity and erosion rates with spatial gradients of velocity, is consistent with existing constraints. For instance, our model results predict significantly higher ($0.6 mm yr À1 ) average erosion rates beneath ice-stream tributaries, which are underlain by deep subglacial troughs, than beneath ice-stream trunks ($0.2 mm yr À1 ), whose subglacial troughs have a significantly smaller relief. We would not obtain this satisfactory result if subglacial erosion was parametrized in the model using the more traditional approach of scaling erosion rates with ice velocity (what we call the 'production-limited' parametrization). Because of the requirement of ice continuity, the magnitude of ice velocity generally increases downstream in polar ice streams, and so do the production-limited erosion rates. Pending further investigations, we propose that geologic and geomorphic effects of soft-bedded ice streams should be quantified using some form of a 'transportlimited' parametrization of subglacial erosion rates, e.g. the till-ploughing mechanism.

Provenance of Des Moines lobe till records ice-stream catchment evolution during Laurentide deglaciation

“…Johnson & Hansel (1990) propose a changing location of subglacial debris entrainment or an unroofing of a rock sequence to explain the shift in lithologic character of the diamictons. Their description is consistent with the evolving-catchment-area model that we propose to explain similar unconformities and changes in till members of the New Ulm Formation of the Des Moines lobe and with current thinking about the evolution of West Antarctic ice streams (Joughin et al 2002;Bougamont & Tulaczyk 2003;Christoffersen et al 2010). …”

“…The process is especially effective in fine-grained tills like those described here because the pore water is difficult to extract and becomes supercooled and subsequently freezes to the glacier bed (Christoffersen et al 2010). Till becomes stiffer as water is withdrawn and the depth of the weakest zone increases, resulting in the plug flow (no deformation or mixing within the vertical section) of a layer of drained till above a shear zone located within lithologically similar material (Bougamont & Tulaczyk 2003). This is an efficient way to move till, results in progressive till consolidation and may even be a signature of palaeo-ice-stream stoppage (Bougamont & Tulaczyk 2003).…”

“…Till becomes stiffer as water is withdrawn and the depth of the weakest zone increases, resulting in the plug flow (no deformation or mixing within the vertical section) of a layer of drained till above a shear zone located within lithologically similar material (Bougamont & Tulaczyk 2003). This is an efficient way to move till, results in progressive till consolidation and may even be a signature of palaeo-ice-stream stoppage (Bougamont & Tulaczyk 2003). There would remain the potential to incorporate new material by drag-folding at horizons of contrasting strength or by freezing-on (e.g.…”

“…We know that modern ice streams are highly efficient at transporting and depositing till, and in fact their relatively high velocities coupled with low driving stresses are best explained by the presence of weak subglacial till with glacier motion resulting from a combination of bed deformation and sliding along a planar surface either within the till or at the ice-bed interface (Bougamont & Tulaczyk 2003). We see no evidence for our tills evolving significantly as they were transported because till sheets can be traced for several hundred kilometres using the textural and lithologic methods described.…”

Active lakes in Antarctica survive on a sedimentary substrate – Part 1: Theory

“…Christoffersen et al, 2014;Bougamont et al, 2014). These more sophisticated distributed/groundwater exchange modes show the most consistency with borehole (Engelhardt and Kamb, 1997;Christner et al, 2014) and seismic observations (Blankenship et al, 1987) of the basal environment. Channelized systems are those in which water flux is concentrated in one or more low-pressure conduits.…”