Response of subglacial sediments to basal freeze‐on 1. Theory and comparison to observations from beneath the West Antarctic Ice Sheet

Abstract

[1] We have constructed a high-resolution numerical model of heat, water, and solute flows in sub-ice stream till subjected to basal freeze-on. The model builds on quantitative treatments of frost heave in permafrost soils. The full version of the Clapeyron equation is used. Hence, ice-water phase transition depends on water pressure, osmotic pressure, and surface tension. The two latter effects can lead to supercooling of the ice base. This supercooling, in turn, induces hydraulic gradients that drive upward flow of pore water, which feeds the growth of segregation ice onto the freezing interface. This interface may progress into the till and form ice lenses if supercooling is sufficiently strong. Hence, the ice segregation process develops a stratified basal ice layer. In our model, a high basal temperature gradient ($0.054°C m À1 ) triggers ice stream stoppage, and the loss of basal shear heating leads to relatively high basal freeze-on rates ($3-5 mm a À1 ). In response, the subglacial till experiences comparatively rapid consolidation. Till porosity can decrease from 40% to <25%, and till strength can increase from $3 kPa to >120 kPa, approximately within one century. Basal supercooling arising from redistribution of solutes and ice-water interfacial effects amounts to ca. À0.35°C below the pressuremelting point. Fine-grained till is in our model associated with widely spaced, thick ice lenses. Coarse-grained till yields thinner ice lenses that are more closely spaced. Our model results compare favorably, although not in all details, with available observational evidence from borehole studies of West Antarctic ice streams.

Evidence of rapid subglacial water piracy under Whillans Ice Stream, West Antarctica

“…Any sediment present at SLW likely records some of that variability. Christoffersen and Tulaczyk (2003) indicated that the presently occurring freezing at the base in lower WIS tends to induce erosion of the subglacial sediments, evidenced by packages of accreted sediment in boreholes drilled into KIS (Engelhardt, 2004). The only areas not directly exposed to sediment erosion in this environment are the beds of subglacial lakes.…”

A robust and energy-conserving model of freezing variably-saturated soil

“…Newman and Wilson (1997) proposed a theoretical formulation for unsaturated soils using soil-freezing and soil-water characteristic curve data to combine the heat and mass transfer relationships into a single equation for freezing or frozen regions of the soil. Christoffersen and Tulaczyk (2003) constructed a high-resolution numerical model of heat, water, and solute flows in sub-ice stream till subjected to basal freeze-on. They proposed a formulation of the equilibrium relation without assuming zero ice pressure through the full version of the generalized Clapeyron equation (Mikkola and Hartikainen, 2001), which enables them to model segregation ice onto the freezing interface and so develop stratified basal ice layers.…”

Ice stream or not? Radio-echo sounding of Carlson Inlet, West Antarctica

“…Kamb Ice Stream (formerly known as Ice Stream C) ceased fast flow about 150 yr ago (Retzlaff and Bentley, 1993) and the main trunk is now stagnant. This ice stream is thought to have thinned to the point where basal melting changed to basal freezing which removed water from the basal till causing stiffening of the substrate, a decrease in lubrication and eventual stoppage (Tulaczyk et al, 2000a, b;Bougamont et al, 2003;Christoffersen and Tulaczyk, 2003;Hulbe and Fahnestock, 2004;Catania et al, 2006).…”