Sediment Transport by Glaciers on Unconsolidated Beds: A Laboratory Study of Entrainment Mechanisms and Bed Lubrication

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9418671 Iverson Under certain conditions water-saturated sediment may, in effect, lubricate glacier beds. Such sediment, therefore, may have contributed to fast glacier flow and ice sheet instability during the Pleistocene. There is at present insufficient understanding of entrainment mechanisms to define the conditions under which subglacial sediment either persists as a lubricating layer, or is eroded. Similarly, the flow mechanism of glaciers on unconsolidated beds is not well known. Although flow is usually assumed to arise as a result of bed deformation, continuous measurements of bed deformation beneath some modern glaciers suggest that under sufficiently low effective pressures, the bed does not deform, and motion, instead, is focused at the glacier sole. To study both sediment entrainment and the coupling between glaciers and unconsolidated beds, a series of experiments is proposed with an existing apparatus that allows hypotheses to be rigorously tested under unusually well-controlled conditions. A sediment bed will be slid at a steady speed beneath a fixed block of temperate ice. One goal will be to study intrusion of ice into the sediment pore space by regelation. Regelation of ice through porous media has been studied successfully in previous experiments with the apparatus and may be the primary mechanism by which glaciers entrain basal debris. The principal uncertainty is the effect of sliding, which may impede intrusion. Intrusion rates well therefore be measured as a function of sliding speed. Post-experimental measurements of stable isotopes in debris-laden ice will be compared with isotopic data from modern glaciers. A second goal will be to develop an empirical sliding law for ice motion over sediment. In particular, the shear stress that ice exerts on the sediment bed will be measured as a function of effective pressure and sliding speed. The range of conditions under which sliding is favored over bed deformation, therefore, will be elucidated. Post-exper imental observations will also allow the relative importance of ploughing and sliding to be assessed under a range of conditions.