GETTING STUCK: SEDIMENT ENTRAINMENT AND GLACIER SLIDING

Glacier sliding takes place when liquid water lubricates the slip surface. It is commonly assumed that slip only occurs along portions of the glacier base that are at the pressure melting temperature. In fact, the equilibrium temperature is slightly lower along an ice-liquid interface that curves down into pore openings and is disjoined from mineral surfaces by thin premelted films. The offset to the equilibrium temperature, or undercooling, produced by these surface energy and wetting interactions plays a central role in governing the degree to which glacier ice can infiltrate the underlying sediments. No such infiltration can take place as long as the undercooling is below a threshold that is inversely proportional to the size of pore openings. For example, with a characteristic pore opening of 4 microns diameter the threshold undercooling is 0.03 ^oC, and it is achieved when the effective stress is 30 kPa. When the undercooling at the ice-sediment boundary exceeds this level, a layer of sediment-rich ice is entrained with a steady "freeze-on thickness" that can be predicted for any particular rate of basal melting and at modest rates of freezing that are below a threshold controlled by mineral surface properties and the pore-size distribution through their influence on changes in ice saturation and permeability with undercooling. This threshold freezing rate decreases with effective stress. For typical sediment constitutive behavior and thermal conditions, the threshold freezing rate is approximately 5 mm/a when the effective stress is 60 kPa and the corresponding maximum steady freeze-on thickness is 1 m; the threshold is 1 mm/a at 100 kPa and 2 m thickness. If the threshold freezing rate is exceeded, the freeze-on thickness subsequently undergoes a monotonic increase even if the freezing rate drops later on. This freeze-on behavior implies that spatial variations in effective stress produce gradients in the freeze-on thickness that translate into enhanced roughness along the underlying sliding surface. Enhanced roughness increases the total basal resistance until it matches the driving stress and slip arrests. Renewed sliding is expected only after sufficient basal melting allows the entrained thickness to be mobilized once more.