The dynamics of ice stream margins: englacial melting, water weakening, and drainage

The margins of fast moving ice streams are characterised by steep velocity gradients, which cannot be explained by a temperature-dependent viscosity alone. In ice stream margins, ice is expected to reach its melting point. Laboratory data suggest that water in the ice-grain matrix decreases its viscosity but the magnitude of weakening and its consequences for ice stream dynamics are poorly understood. Here we investigate how the coupling between temperate ice properties, ice mechanics, and drainage of melt water from the ice stream margin alters the dynamics of ice streams. We consider the steady-state ice flow, temperature, water content, and subglacial water drainage in an ice stream cross-section. Temperate ice dynamics are modelled as a two-phase flow, with gravity-driven water transport in the pores of a viscously compacting and deforming ice matrix. We find that the dependence of ice viscosity on melt-water content focuses the temperate ice region and steepens the velocity gradients in the ice stream margin, consistent with observations. This localizes heat dissipation there which in turn increases the amount of melt water delivered to the ice stream bed. This process is controlled by the permeability of the temperate ice and the sensitivity of ice viscosity to melt-water content, both of which are poorly constrained properties. We discuss the implications of this process for the temporal evolution of ice streams.