Changes in the Margin of Ice Stream C, Antarctica

We present results from satellite imagery, ice motion surveys, and ice-penetrating radar studies of a portion of the north margin of Ice Stream C, one of the ice streams draining the West Antarctic Ice Sheet to the Ross Embayment. Our studies suggest that the shutdown of Ice Stream C about 150 years ago was not a single event but a sequence involving stagnation of ice and migration of the ice stream boundary. Ground-based studies confirm the inference from imagery that a series of former shear zones exist decreasing in age towards the ice stream center. A region of ice stream trunk, including a former margin, lies sheared and folded between the (recent) inner and (older) outer margins of the area. Ice motion and topographic surveys give some constraint on the time of shutdown of the outer margin. The results provide a forum for discussing shutdown mechanisms. Possible causes for the stepwise migration of the north margin of Ice Stream C include a gradual decrease in ice flux, a reduction in the available water or hydrostatic pressure in the basal till, or a freezing of the till layers on the northern side. Introduction Drainage of the West Antarctic Ice Sheet (WAIS) is controlled by ice streams which deliver the majority of the mass flux to the bounding ice shelves. In the Ross Embayment, Ice Streams A through E drain the interior in a dynamic system that has changed in both flux and configuration over a range of temporal and spatial scales. Four ice streams are currently active, while Ice Stream C stagnated approximately 150 years ago (Retzlaf and Bentley, 1993; Shabtaie and Bentley, 1987), leading to questions about the conditions necessary to sustain rapid ice motion and the relationship of this event to other changes in the drainage configuration from the ice sheet. At the millennial scale, the system has lost approximately two-thirds of its mass since the end of the last glacial epoch (Bindschadler, 1998). In the nearer term, both ice stream velocities and ice stream widths are observed to fluctuate on a scale of years to decades at rates of 1-2 % per year for ice speed, and 0.1% per year for width (e. This suggests that ice streams may be constantly adjusting to local changes in basal lubrication and evolving at their margins. Additional evidence for change at an intermediate timescale has come from the morphology preserved in …