THE ENTRAINMENT OF SUBGLACIAL MATERIAL VIA GLACIOHYDRAULIC SUPERCOOLING

Beneath many glaciers and ice sheets, hydrology modulates basal processes, including sliding, erosion, and sediment transport. Recently, the role of glaciohydraulic supercooling was exposed as an important process in the overdeepening near the toe of Matanuska Glacier, Alaska. Water that rises rapidly along the subglacial slope has the ability to translate its latent heat from an area of higher pressure to an area of lower pressure without raising its internal energy above the ambient pressure--dependent melting point. Thus, the water supercools. Because it is moving relatively rapidly, the flow also entrains local sediments. As the undercooled water crystallizes, sediments become trapped in an icy matrix. Subsequently incorporated into the basal glacial ice, these sediments account for 15--60 % by volume of the accreted material (Lawson and others, J. Glac., 1998). Further investigations have shown that glaciohydraulic supercooling is more globally widespread. These occurences suggest that supercooling may have been an important process beneath the Laurentide Ice Sheet for mobilizing sediments.

With discharges in modern environments exceeding 0.1 m³ s^-1 m^-1, one would expect high bedload transport in the glaciohydraulic system. However, recent field studies have shown that bedload transport is supply-limited (Pearce and others, Geology, 2003). This limitation is either caused by a lack of clasts at the glacier's sole, by constricted hydraulic arteries that cannot pass larger clasts, or a combination of these processes. Because there are few larger clasts in the flow, few of the larger clast sizes are represented in the basal ice. We investigate ice condensation, sediment mobilization, and the corresponding flow constrictions that may limit the movement of basal sediment using a numerical model of transient water flow up an overdeepened subglacial water system. The model is based on the conservation laws of continuum mechanics with empirical condensation and sediment mobilization rules. Implications of the model are wide ranging for larger--scale geomorphology and glaciogenic sediment deposits.