Enhancement variations (Budd & Jacka, 1989; Paterson, 1991; Cuffey et al., 2000a) arise from grain-scale processes associated with two physical properties of polycrystalline ice: c-axis fabric strength and grain size. As noted previously, ice crystals deform easily by glide on basal planes, the planes normal to the c-axis. Preferred orientation of c-axes (also referred to as 'strong fabric') makes ice soft for shear resolved on these planes. The preferred orientation itself develops as a function of strain as crystals rotate when deforming to minimize interaction with neighbours. Most of the rapidly shearing deep ice in the ice sheets has been strained to very large values and consequently has strong vertically aligned fabrics, with partial broadening by recrystallization. In a simple shear flow, the capacity for enhancement variation between isotropic ice and ice with a favourably oriented perfect fabric is approximately a factor of ten (Azuma & Goto-Azuma, 1996). Typical actual documented enhancement contrasts that are relevant to large-scale, ice-sheet shear flow are approximately a factor of three (Dahl-Jensen & Gundestrup, 1987). This is much smaller than the potential factor of ten because ice deforming in shear in the high-stress deep layers of the ice sheets is never isotropic.
Both small grain sizes and high soluble impurity contents contribute to much larger enhancements of dirty basal layers of glaciers (Cuffey et al., 2000a). There is no convincing evidence that soluble impurities affect enhancement at the concentrations found in the main body of the ice sheets, but grains are probably fine enough in some cases to induce enhancement variations (Cuffey et al., 2000c). This needs more investigation. Viscosity variations due to grain size could result from grain size dependence of dislocation recovery processes (Montagnat & Duval, 2000), or possibly from deformation by grain-boundary sliding (Goldsby & Kohlstedt, 2001). Laboratory experiments (Jones & Glen, 1969) do show that some acids (such as HF and HCl) have the capacity to soften ice even at low concentrations. The soft ice-age ices of the Northern Hemisphere are alkaline, however, due to the abundance of rock microparticles.
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