Incorporation of rheological properties into ice sheet flow models

In most current ice sheet models the rheological properties of ice are usually assumed to be either isotropic or some effects of anisotropy are taken into account by the use of a simple constant enhancement factor. Studies of ice cores have shown that in natural ice sheets approximately isotropic ice only exists at shallow depths typically less than several hundred metres. The large amounts of deeper ice have developed strong anisotropic crystal orientation fabrics. The results from both field and laboratory studies have indicated that the anisotropy due to flowinduced crystal orientations has a considerable effect on ice flow rate, therefore, the assumption of isotropic ice is no longer appropriate when anisotropy of the ice crystal fabric develops and we need to take account of the interaction between crystal orientation and rates of deformation. This project aims to incorporate into an ice sheet rnpdel the effect of ice fabric anisotropy on the flow of ice. A model for anisotropic ice flow in a polar ice sheet is developed. It is based on laboratory measurements of ice rheology including ice cores and combined with borehole measurements in the ice sheet. In the model the shear flow of ice is enhanced as the ice passes through a range of stress regimes from predominantly vertical compression to predominantly vertical shear stress, as it flows down through the ice sheet. The enhanced flow, characterised by an increase in strainrates compared to the secondary creep rate for isotropic ice, is primarily based on laboratory measurements of tertiary creep under combined compression and shear stresses. To test this model two different flow lines in the Antarctic ice sheet have been studied. One flow line is from Law Dome which is an isolated ice cap with shallower and warmer ice. For comparison, the second, longer flow line, the I.A.G.P traverse line inland of Casey in the interior of East Antarctica, was studied to explore different conditions for deeper, colder ice and larger scale bedrock features. A simple application of the model to a number of boreholes with ice cores in Law Dome has been made, including a new borehole drilled in Law Dome Summit South where shear strain rates were measured as well as ice core fabrics through the 1200 m depth to the bed. In the study areas a large amount of glaciological work involving field surveys and ice core drilling provides sufficient data for model inputs and for model verification. The results from the model were compared with observations. The implications of this work for other regions, including the whole Antarctic ice sheet, were also investigated. It is concluded that in regions of high bedrock roughness the occurrence of a high shear layer of strongly anisotropic ice above the predominant bedrock perturbations needs to be taken into account.