Abstract

Jakobshavn Isbrae and many other outlet glaciers of present and past ice sheets lie in deep troughs which often have several over deepened sections. The subglacial drainage system of such glaciers is heavily influenced by two effects caused by the pressure dependence of the melting point of water. The melting point decreases with increasing water pressure, this enhances wall-melt in downward sloping channels and diminishes wall-melt in upward sloping channels. Thus the first effect is the well known shutdown of channels on steep adverse bed slopes of overdeepenings and the associated high water pressure/low effective pressure. The second effect is a 2D effect and has not received much/any attention so far: the orientation of a channel will be deflected from the direction of the (negative) hydraulic potential gradient (which drives the water flow) towards the steepest slope of the bed. This leads to the enhanced formation of side channels dipping into the trough at about a 45° angle. This efficient connection between the margin and the trough equalizes the hydraulic potential and increases the water pressure in the trough even further.

I investigate these two effects with the 2D subglacial drainage system model GlaDS using Jakobshavn Isbrae as an example. I compare model runs with the pressure melt term disabled and enabled. With the term disabled the main channel situated in the trough is continuous and produces a large depression in the hydraulic potential and consequently high effective pressure in the trough (1-2MPa). Conversely, with the term enabled the main channel becomes discontinuous on steep adverse bed slopes and many side channels form on the margins of the trough. This leads to a hydraulic potential in the trough which is higher than in the surrounding area and consequently the effective pressure is low (0-1MPa).

Low effective pressure leads to reduced basal drag and thus to more basal sliding. This work suggests that the pressure dependence of the melting point of water may be one of the leading causes for the fast flow of Jakobshavn Isbrae and other glaciers in deep troughs.