Dynamics of the Law Dome Ice Cap, Antarctica, as Found From Bore-Hole Measurements

Abstract

The internal dynamics of the Law Dome ice cap have been investigated by measuring the deformation of three bore holes located on an approximate flow line. Bore holes BHC1 (300 m deep) and BHC2 (344 m) were drilled in the coastal area to within several metres of bedrock and BHQ (418 m) was drilled about half-way towards the dome centre to about 50% of the ice thickness. Detailed measurements of orientation (inclination and azimuth), diameter, and temperature were taken through each bore hole over a 1 year span for BHC1 and BHC2 and a 10 year span for BHQ. The orientation data were reduced to obtain ∂u/∂z, a measure of the shear strain-rate. Changes in the depth of features located by bore-hole diameter measurements were used to obtain vertical velocity profiles. Other measurements discussed are temperatures, oxygen isotopes, crystal structure, surface velocities, and surface and bedrock topography.At the coastal sites, the ∂u/∂z profiles show two maxima in the lower third of the ice sheet. Flow due to the measured deformation accounts for about 55% of the surface velocity, the remainder being due to deformation and sliding in the basal zone. The vertical velocity profiles show mostly firn compression. The deeper ∂u/∂z maximum occurs in ice from the Wisconsin period which appears to deform more rapidly than the Holocene ice immediately above. The upper ∂u/∂z maximum may be related to the stress history of the ice, which can also explain the presence of significant shear strain and crystal-fabric development at only half the ice thickness at the BHQ site.