Control of the width of active Western Antarctic Siple Coast ice streams by internal melting at their margins

Why do ice streams exist on the Western Antarctic Ice Sheet, like near the Siple Coast? And what sets their width? They are streams of typically 30-80 km horizontal width, flowing rapidly towards the Ross Sea at hundreds of m.yr. The streams are bordered by slow moving ice, frozen to the bed, but no structure in the bed has been cited as controlling the location of the stream margins. We examine, and find evidence supportive of, the hypothesis that the stream width is set by the development of significant internal melting, i.e., development of temperate ice conditions, within the sheet. We first show, from published ice sheet deformation data and from thermal modeling based on temperature-dependent flow and conduction properties, that most existing Siple Coast ice stream margins are indeed in a state of partial melt, with temperate ice being present over a substantial fraction of the sheet thickness. We then propose and quantify approximately, if incompletely, a possible related mechanism of margin formation, that is, of locking the sheet to the bed outboard of that temperate zone. Shear heating of the temperate ice continually generates melt which percolates toward the bed below. If that develops a channelized marginal drainage of Röthlisberger type, standard theory argues that the high, nearly lithostatic, pore pressure elsewhere (i.e., near the bed within the fast moving stream) is somewhat alleviated within the channel. That results in a higher Terzaghi effective normal stress, which we quantify approximately, acting along the bed just outboard of the channel, and hence creates high resistance against frictional shear and locks the ice outboard to the bed, naturally forming an ice stream margin. Our estimates are that the effective normal stress acting on the bed just outside such a channel, and hence the till shear strength, is of order 10 to 60 times larger than that inferred to be present at the bed within the fast-moving stream.