Development of the Crystal Structure within the Law Dome Ice Cap, Antarctica (Abstract)

Abstract

Fourteen shallow and medium-depth cores have been drilled from the Law Dome ice cap, between the summit and the coast near Casey Station. Measurements of their crystal and other physical properties are reviewed briefly. The variations along the cores in crystal size, orientation, fabric type and strength, and bubble dimensions, are used to define the internal structure of the ice cap locally at the bore-hole sites. Surveys of bore-hole deformation and the shape and movement of the ice cap are used to define relations between the structure and the variables: stress, temperature, strain-rate and accumulated strain. The relations and the survey data are incorporated in a numerical model in order to deduce the internal structure of the ice cap along a flow line linking the bore-hole sites. The results of the model in turn depend on the crystal anisotropy of the calculated structure.

The main results are provided by the medium-depth bore holes located at the summit, near the margin, and about half-way along the flow line. The major features of the internal structure are determined by the predominant shear deformation in the ice cap. There is horizontal continuity in the properties and structure within the group of bore holes near the margin of the ice cap. There are distinct differences, between the coastal and the inland ice cores, in the changes in properties with depth. Near the margin a strong single-maximum fabric develops within the upper 60% of the ice thickness; crystal size initially increases with depth, then shows a marked decrease at about 50% thickness. For the inland cores, a strong single-maximum fabric also develops, but at a greater total depth and a much shallower fraction of the thickness. A similar decrease in crystal size was not observed.

The broad-scale trends of the properties are reproduced by the model. The finer-scale deviations in the properties can be explained by the effects of longitudinal strain and of past changes in surface conditions, such as the effect of surface melting. A complex stress distribution, related to flow over rough bedrock, needs to be invoked to explain the pronounced multi-layer structure in the lower part of the ice cores from near the margin. A series of time lines is modelled, following the flow along the ice-particle trajectories, to produce the stress, temperature and deformation histories of the ice in the cores. These provide the basic data for a reconstruction of past changes in the ice cap.