Rheology of ice at the bed of Engabreen, Norway

A three-dimensional finite-element model is used to analyze field data collected as dirty basal ice flowed past an instrumented obstacle at the bed of Engabreen, a temperate glacier in northern Norway. The ice is modeled as an incompressible power-law fluid, with viscosity eta = (B/2) IID(1-n)/2n, where IID is the second invariant of the stretching tensor, and B and n are two parameters. Using measurements obtained in 1996 and 1997: two values of B are obtained, one using the measured normal stress difference across the obstacle, and the other using the measured bed-parallel force over the instrument. These two values are not equal, probably owing to small frictional forces at the bed unaccounted for in the numerical model. Hence, B ranges between 1.9 x 10(7) and 3.2 x 10(7) Pa s(1/3) in 1996, and between 2.2 x 10(7) and 4.1 x 10(7) Pa s(1/3) in 1997. These values are smaller than measured elsewhere for clean glacier or laboratory ice. Field measurements of water content, fabric and texture of the basal ice suggest that unbound water between thin sediment layers and lamellae of clean ice may act as a lubricant and significantly weaken the ice. Near-isotropic fabrics indicate that preferred fabric orientation does not enhance the deformation.