Strain-rate enhancement at Dye 3, Greenland

Ice at depth in ice sheets can be softer in bed-parallel shear than Glen's flow law predicts. For example, at Dye 3, Greenland, enhancement factors of 3-4 are needed in order to explain the rate of borehole tilting. Previous authors have identified crystal fabric as the dominant contributor, but the role of impurities and crystal size is still incompletely resolved. Here w e use two formulations of anisotropic flow laws for ice (Azuma's and Sachs' models) to account for the effects of anisotropy, and show that: the measu red anisotropy of the ice at Dye 3 cannot explain all the detailed variatio ns in the measured strain rates. The jump in enhancement across the Holocen e-Wisconsin boundary is larger than expected from the measured fabrics alon e. Dust and soluble-ion concentration divided by crystal size correlates we ll with the residual enhancement, indicating that most of the "excess defor mation" may be due to impurities or crystal size. While the major features of the deformation at Dye 3 are explained by anisotropy and temperature, re sults also suggest that further research into the role of impurities and cr ystal size is warranted.