Strong feedbacks between hydrology and sliding of a small alpine glacier

Abstract

[1] We report the spatial and temporal pattern of sliding on the 7-km-long Bench Glacier, Alaska. Using five continuously recording GPS antennas following motion of the surface ice, distributed at 1 km spacing along the glacier center line, we documented surface ice motion over 50 days during summer 2002. Surface speeds in two previous winters constrain the motion component associated with ice deformation, allowing isolation of the sliding speed history. We observed two speedup events bracketing 2 weeks of steady slow sliding. The first event was not associated with a meteorological trigger, was more subtle than the second, and propagated up-glacier at a rate of several hundred meters per day. The second event coincided with a warm up-valley wind, which triggered considerable melt of the glacier surface. Sliding speeds in this event reached 0.3 m d À1 and began almost simultaneously at all sites in the ablation area. Both the horizontal and vertical displacement time series can be explained by growth and collapse of cavities in the lee of bumps in the bedrock bed. Cavities grow during rapid sliding and decay by viscous creep. We posit that effective pressure, averaged over some large area of the bed, is inversely proportional to the sliding speed. This effective pressure then controls the collapse rate of cavities, whose dimensions are estimated from a plausible, stepped-bed geometry. This model explains well the horizontal and vertical surface displacement history through the first event and beginning of the second event. The vertical record demands a substantial and abrupt drop in water pressure that departs from the posited sliding-effective pressure relationship. We argue that this pressure drop reflects establishment of efficient subglacial drainage, also manifested in a nearly simultaneous step increase in water discharge in the exit stream. The establishment of an efficient conduit system terminates sliding; its maintenance inhibits further sliding over the remainder of the summer.

Quantitative estimates of velocity sensitivity to surface melt variations at a large Greenland outlet glacier

“…Observations of surface displacement in combination with other hydrological and meteorological measurements used as a proxy for basal water pressure suggest a similar correlation (e.g. Björnsson, 1998;Anderson and others, 2004;Bartholomaus and others, 2008). A common interpretation of these studies is that an increase in basal water pressure causes or sustains cavitation, which leads to increased bed separation and consequently increased sliding speed.…”

“…Similarly large effects have been observed on alpine glaciers (e.g. Iken and Bindschadler, 1986;Anderson and others, 2004) and tidewater glaciers outside Greenland (e.g. Kamb and others, 1994;Meier and others, 1994).…”

The annual glaciohydrology cycle in the ablation zone of the Greenland ice sheet: Part 1. Hydrology model

“…dS/dt) in alpine glaciers (Kamb and others, 1994;Fountain and Walder, 1998;Anderson and others, 2004;Bartholomaus and others, 2008). This conceptual model of basal sliding has recently been extended to the Greenland ice sheet (Bartholomew and others, 2010).…”

“…This increases pressure in subglacial cavities and conduits, enhancing basal sliding (e.g. Fountain and Walder, 1998;Anderson and others, 2004;Bartholomaus and others, 2008). Following the onset of melt, both meltwater input and subglacial transmissivity generally increase with time, the former a direct response to meteorological forcing and the latter a response to the widening of the ice-walled conduits due to the dissipation of energy created by viscous friction (Rö thlisberger, 1972;Nye, 1976).…”

“…total glacier water input minus output; Fountain and Walder, 1998;Anderson and others, 2004;Bartholomaus and others, 2008) and changes in flotation fraction (the ratio of subglacial water pressure to basal ice pressure; Iken and others, 1983;Kamb and others, 1994). Thus, both the total amount of water storage, which is related to englacial water-table elevation, and its rate of change influence basal sliding velocity.…”