INFLUENCE OF SUBGLACIAL GEOLOGY ON THE POSITION OF A WEST ANTARCTIC ICE STREAM FROM SEISMIC OBSERVATIONS

Ice streams drain much of the interior West Antarctic Ice Sheet and bu ffer the main ice reservoir from oceanic influences(1,2). The slow-flo wing interior feeds the floating Ross Ice Shelf with ice via fast-flow ing ice streams' that are believed to modulate sea-level change throug h their control of inland ice storage. Understanding ice-stream behavi our, and predicting the response to climate change(4), requires a bett er knowledge of the subglacial geology(5,6). It is known that a thawed ice-bed and high-pressure basal water are necessary, but not sufficie nt, conditions to cause ice streaming(7,8). Moreover, it has been hypo thesized that a soft sedimentary bed is also required, because of its intrinsic low frictional resistance to flow(9), and owing to its high erodibility so as to generate till that can deform and lubricate ice m otion(10,11), or to bury rough features and smooth the bed for sliding . Here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill. The ice stream is within the b asin and the ice outside the basin is slow-flowing. The basin fill pre sents an order-of-magnitude lower frictional resistance to ice flow th an the subglacial material outside the basin. We conclude that the ice stream position is dependent on subglacial geology.