Diffusive, Linearly-Viscous Deformation of Temperate, Watery Ice at Stresses of 50-180 kPa

Where ice deforms rapidly in ice streams and outlet glacierseither near their beds or at depth in their lateral marginsit is commonly at its melting temperature and thus coexists with significant liquid water at crystal boundaries. Hence, the need to model accurately the deformation of this watery ice is acute. We present results from ice-deformation experiments conducted to tertiary creep on the watery, temperate ice relevant to fast glacier flow. Rings of lab-made polycrystalline ice are sheared in confined compression at either controlled stresses or strain rates, with liquid water content measured calorimetrically. The size of the apparatus and the durations of experiments allow stresses, strain rates, water contents, and ice grain sizes that fall within or span ranges expected in temperate glacier ice, eliminating the common need to extrapolate experimental parameter values to glacially relevant ranges. At stresses of 180-240 kPa, we find that the stress exponent in Glens law is n=3.1. However, at the smaller stresses that include most ice at glacier beds and most ice-stream shear margins, 50-180 kPa, we find that sufficiently watery temperate ice (0.71.9% liquid water) is linearly viscous (n=1.0). C-axis orientations of ice grains after shearing cluster sub-normal to the plane of shear, and a strong shape-preferred orientation of grains develops, with grain long axes rotated antithetic to shear an average of 15 from the shear plane. To explain these observations, we propose a diffusive, micro-deformation mechanism that will be especially efficient in warm ice with sufficient liquid water: pressure-melting and refreezing at grain boundaries.