A ploughing model for the origin of weak tills beneath ice streams: a qualitative treatment

Abstract

Glaciological studies of West Antarctic ice streams have shown that weak sub-ice-stream tills provide the basal lubrication that makes fast ice streaming possible under low driving stresses. Given the significant current interest in time-dependent ice stream behavior, there is a clear need for a conceptual model of weak sub-ice-stream tills that treats in a simple, but physically correct, way the coupling between evolution of till properties and ice stream dynamics. As a possible alternative to the previous, viscous-bed model, we propose a ploughing model that is consistent with the experimentally determined Coulomb-plastic rheology of sub-ice-stream till. In the ploughing model, the till is a several-meters-thick layer of sedimentary material that is disturbed and transported by ploughing that occurs during sliding of a bumpy ice base. The thickness of the till layer is determined in the ploughing model by the amplitude of the largest roughness elements (“ice keels”). There is no direct proof for the existence of ice bumps and ice keels beneath the modern West Antarctic ice streams but bedforms (e.g. megalineations and bundle structures) left behind by Pleistocene ice streams strongly support our assumption that an ice stream base is irregular. Generation of new till material occurs when ice keels protrude through the existing till layer and erode the top of the sub-till preglacial sediments. Based on a single tethered stake measurement of Engelhardt and Kamb (J. Glaciol. 44 (1998) 223) made at the UpB camp, Ice Stream B, West Antarctica (Fig. 1), we estimate that the till flux due to sliding with ploughing is there <88 m 3 yr −1 per meter width. To balance the estimated till flux in the UpB area, substrata erosion by ice keels would have to take place at a high, but not unreasonable, non-dimensional rate of <1.7×10 −4 (assuming 1% contact area). In the case of the West Antarctic ice streams, erosion of sub-till materials by ice keels may be particularly fast and unimpeded because these ice streams are overriding unlithified preglacial (Tertiary) sediments. The most significant implication of the proposed ploughing model is that it permits treating basal resistance to ice motion as being velocity independent (plastic till rheology) while allowing subglacial transport of till as in the viscous-bed model. Models of ice streams with a plastic bed exhibit a greater potential for unstable behavior than models of ice streams with viscous beds.