Changes in the margin of Ice Stream C, Antarctica

We present results from satellite imagery ice-motion surveys and ice penetr ating radar studies of part 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 w as not a single event. but a sequence involving stagnation of ice and migra tion of the ice-stream boundary Ground-based studies confirm the inference from imagery that a series of former shear zones exist, decreasing in age t owards the ice-stream center A region of ice-stream trunk, including a form er margin, lies sheared and folded between the (recent) inner and (older) o uter margins of the area. Ice-motion and topographic surveys give some cons traint on the time of shutdown of the outer margin. The results provide a f orum for discussing shutdown mechanisms. Possible causes tor the stepwise m igration 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 th e basal till, or a freezing of the till layers on the northern side.