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

Abstract. We examine, and find evidence to support, the hypothesis that the width of Siple Coast ice streams in West Antarctica is set by the development of significant internal melting (i.e., development of temperate ice conditions) within the ice sheet at the margins. We first illustrate, from published ice sheet deformation data and from sim-ple 1-D thermal modeling based on temperature-dependent flow and conduction prop-erties, that most existing Siple Coast ice stream margins are in a state of partial melt, with temperate ice being present over a substantial fraction of the sheet thickness. We show that, although the margins sustain high lateral strain rates, they can support (and hence transmit to the cold ridges) notably less lateral shear stress than would the some-what less rapidly deforming ice located inboard, away from the ridge. We then propose, and quantify approximately, a possible related mechanism of margin formation, that is, of locking the sheet to the bed at the margin. Shear heating of the temperate ice con-tinually generates melt, which percolates toward the bed below. If this develops a chan-nelized marginal drainage of Röthlisberger type, the standard theory argues that the high, nearly lithostatic, basal pore pressure elsewhere (i.e., near the bed within the fast mov-ing stream) is somewhat alleviated within the channel. This results in a higher Terza-ghi effective normal stress, which we quantify approximately, acting along the bed near the channel, thereby creating high resistance against frictional shear and possibly lock-ing the ice 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, can be twenty to sixty times larger than that inferred to be present at the bed within the fast-moving stream. Reduced basal fluid pressure, and hence en-hanced strength, over a broad region near the margin is shown sufficient to remove the strong stress concentration at the slipping to locked bed transition, and provides a sta-bilizing mechanism for the margin. 1