Response of subglacial sediments to basal freeze‐on 2. Application in numerical modeling of the recent stoppage of Ice Stream C, West Antarctica

Abstract

[1] Ross ice streams supply over 90% of the ice volume flowing out of the Ross sector of the West Antarctic ice sheet (WAIS). Stoppage of Ice Stream C (ISC) ca. 150 years ago appears to have pushed this sector of WAIS from negative into positive mass balance [Joughin and Tulaczyk, 2002]. We propose an explanation for the unsteady behavior of ISC using a new numerical ice-stream model, which includes an explicit treatment of a subglacial till layer. When constrained by initial conditions emulating prestoppage geometry, dynamics, and mass balance of ISC, the model yields a rapid ($100 years) stoppage of the main ice-stream trunk. The stoppage is triggered by basal freeze-on, which consolidates and strengthens the subglacial till. Our numerical simulations produce results consistent with a number of existing observations, for example, continuing activity of the two tributaries of ISC. The model always yields rapid stoppage unless we specify icestream width that is smaller than its prestoppage values (maximum of $80 km). We conjecture that if ISC was active for at least a few thousand years before slowdown, its width was significantly smaller than today to sustain the long active phase. Ice-stream width is a key control that helps determine whether ice-stream flow is sustainable over a long term. Our work indicates that the recent stoppage of Ice Stream C could have been part of inherent ice-stream cyclicity, and it leaves open the possibility that other active ice streams may evolve in the future toward rapid shutdowns.

Sensitive response of the Greenland Ice Sheet to surface melt drainage over a soft bed

“…Lateral water fluxes in the sediment layer are ignored 61 . We further note that without an explicit representation of the channelized subglacial drainage system, we cannot account for variation in effective pressure related to such a system.…”

“…When a large volume of water is transiently delivered to the bed, excess pore-water pressure and vertical hydraulic gradients drive water flow downwards and into the sediment (equation (3)). The hydraulic gradient at the ice-sediment interface is calculated from equation (2) 61 , with the rate of water entering the sediment layer (v top w , m s À 1 ) estimated as follow:…”

The physics of premelted ice and its geophysical consequences

“…The rapid motion is accomplished by a combination of sliding and sediment deformation-plastic flow of ice itself is of secondary importance. The activity of ice streams is known to be transient on time scales ranging from days ͑Bindscha-dler, Bindschadler, Vornberger, Joughin, et al, 2004͒ to centuries ͑Conway et al, 2002Bougamont et al, 2003;Ng and Conway, 2004͒. Models of ice stream flow highlight the role of pore fluid pressure in controlling the effective stress that resists deformation of the glacial till ͑Kamb, 1991; Tulaczyk et al, 2000͒.…”

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).…”