THE EFFECT OF BASAL ICE DEBRIS LOAD ON SLIDING REGIME AND THUS EROSION RATES

We report on a pattern of repeating earthquakes associated with the flow of David Glacier through the Transantarctic Mountains at it’s base. The seismic events (Mw=1.8) recurred regularly (approximately 20 minutes) for a 275-day span in 2001 and 2002. Before and after this 275-day period, the recurrence was orders of magnitude more infrequent and irregular. The events are likely caused by an asperity beneath David Glacier that regularly releases stress accumulated by the flow of the glacier. We suggest that the change in seismic behavior is due to changes in debris concentration of the basal ice over time. This results in the asperity experiencing changing rheological values over time, resulting in the observed behavior.

In order to study how changes in entrained debris affects deformation of ice we froze a number of ice samples with varying percentages of debris (1-60% by weight). The debris was collected by melting subglacial ice from Svartisen Subglacial Laboratory beneath Engabreen Glacier in Norway. We show than an increase in basal debris concentration can alter the velocity-strengthening/weakening boundary thus having large impacts on the ability of the system to stick slip or slide stably. These experiments (though over a much smaller range of debris concentrations) were also replicated with a sample of basal ice that was taken from the base of Engabreen glacier to observe the effects that fabrics contained within the ice have on the deformation. The stiffness of the shearing apparatus was reduced in order to allow stick-slip behavior to occur, which was successfully produced in the lab. Similarities between laboratory results and field observations are beginning to emerge which will help to scale from the one to the other.

The ability of the system to transition between stick and slip regimes results in periods where a significant part of the stored energy is released quickly, providing a mechanism to fracture the rock at the asperity. Fracturing of the asperity would result in brief periods of increased rates of erosion at the base of glacier. The seismic data from David Glacier show that this increased rate of seismicity only lasts for finite intervals, suggesting that this mechanism is strongly heterogeneous in space and time, but could encompass a significant percentage of the total subglacial erosion.