PRODUCTION- AND TRANSPORT-LIMITED EROSION BENEATH ICE SHEETS: A COUPLED MODEL OF SUBGLACIAL EROSION AND SEDIMENT TRANSPORT

Many of the highly populated regions of the Northern Hemisphere were shaped by sub-ice sheet erosion and sedimentation. In addition, glacial landforms and sediments give insights into past physical conditions beneath ice sheets. Yet, subglacial geomorphic and sedimentary systems are much less understood than their fluvial and eolian counterparts. Most existing models of subglacial erosion are based on observational studies performed on mountain glaciers. These models typically assume that glacial erosion rates are (nearly)linearly proportional to ice velocity or to work done by flowing ice. The ‘classical' models of subglacial erosion assume that subglacial sediment production is independent of sediment transport and make no provision for negative erosion rates (i.e. deposition).

Here we present a mathematical model, which couples subglacial erosion, transport, and deposition. The model is an outgrowth of our laboratory simulations of rock erosion by shearing till. Results of these laboratory experiments indicate that glacial erosion rates should decrease by up to several orders of magnitude when subglacial till builds up beneath an ice base. The classical, production-limited model of subglacial erosion is valid when debris removal rates are greater or equal to the rate of debris production. This should be the case where ice overrides resistant bedrock and/or subglacial water drainage removes debris very efficiently. The other end-member situation is the ‘transport-limited’ erosion, which should predominate where ice overrides highly erodible bedrock or sediments and debris is stored in a subglacial till layer. Then, subglacial erosion rates are dictated by the horizontal divergence of till flux.

We conjecture that widespread subglacial till layers should occur in these parts of an ice sheet where ‘transport-limited’ erosion predominates. We use ice velocity data from West Antarctica to illustrate that there the transport-limited parametrization of subglacial erosion is more successful at explaining the regional topography than the classical, production-limited parametrization.

Our main conclusion is that realistic models of subglacial geomorphic and sedimentary processes should incorporate the close coupling between subglacial erosion and sediment transport.