Deformation properties of subfreezing glacier ice: Role of crystal size, chemical impurities, and rock particles inferred from in situ measurements

To improve understanding of the deformation properties of subfreezing polyc rystalline glacier ice and, in particular the role of crystal size, chemica l impurities, and rock particle impurities, we analyze in situ strain rates of the basal layers of Meserve Glacier, Antarctica. Strain rates were moni tored on the walls of a subglacial tunnel (where down flow shear stress is relatively uniform) and ice properties were measured (texture, fabric, and impurity content). We propose a simple empirical model describing strain ra te variations due to variations in crystal size and impurity content, and w e use all relevant Meserve data to constrain odel parameters. We conclude t hat there is a direct dependence of strain rate on crystal size, which refl ects an important role for a grain-size-sensitive deformation mechanism suc h as grain boundary sliding or diffusion. Chemical impurities are found to enhance the grain-size-sensitive deformation and are found to be an importa nt control on strain rate variations in the very impure ices of Meserve Gla cier. However, the per molar sensitivity of strain rate to chemical impurit y content is shown to be very low, such that in the ice age ices of the Gre enland ice sheet there is probably an immeasurable contribution of chemical impurities to strain rate enhancement, though we cannot exclude chemical e nhancements as high as 1.3 there. Our analyses detect no direct theologic e ffect of rock particles in the Meserve ices, which suggests that rock conte nt is not directly responsible for the low viscosity of dirty basal layers.