INTRUSION OF ICE INTO POROUS-MEDIA BY REGELATION - A MECHANISM OF SEDIMENT ENTRAINMENT BY GLACIERS

Unlithified sediment at glacier beds should be entrained in ice as a r esult of melting above individual grains and refreezing below grains ( regelation). To test this hypothesis, a device that simulates subglaci al conditions was used to push ice downward through idealized and natu ral porous media in contact with a flat bed. Ice regelated toward the bed at speeds proportional to the gradient in ice pressure across the thickness of particles. Ice temperatures responded predictably to chan ges in driving stress and demonstrated that the ice-particle mixtures were at or very near the pressure-melting temperature. A theory of por e ice motion by regelation predicted the measured speed usually to wit hin a factor of two, although at low driving stresses the speed was le ss than the predicted value by as much as a factor of five. The low sp eeds were probably caused by imperfect temperature control that result ed either in heat loss from the ice-particle mixtures or in slightly s ubfreezing pore ice temperatures that were not detected by thermistors . Ice should intrude the pores of underlying sediment to a steady dept h at which the downward regelation speed equals the rate of basal melt ing. Departures from the steady state should cause release of debris f rom ice or further entrainment. An approximate calculation of the stea dy intrusion depth yields values comparable to typical thicknesses of debris in basal ice, a few centimeters to more than a meter. Intrusion of sediment by regelation is consistent with the isotopic composition of debris-bearing ice, if melting exceeds refreezing.