The ice thickness distribution of a debris-covered glacier: Tasman Glacier, New Zealand

The ice thickness distribution of mountain valley glaciers is an important physical constraint for modelling their flow. Ice thickness measurements are used to calculate the geometry and ultimately the driving stress of a glacier. This information is all required if realistic models are to forecast the response of glaciers to climate forcings. For New Zealand's Tasman Glacier, two factors complicate its response to climate: 1) A layer of insulative rocky debris covers the lower half of the glacier, retarding surface melt, and 2) the glacier has recently entered a period of iceberg calving into a proglacial lake, introducing complex mechanical processes. These complications, along with the uncertainty of the current bed topography of the Tasman Glacier, make future predictions of its retreat behaviour difficult. The bed of the Tasman Glacier has not been fully imaged but ice thickness measurements obtained through seismic and gravity surveys have provided constraints for parts of the glacier. This study applies a range of geophysical methods (gravity and refraction seismics) to measure and model the ice thickness distribution of the lower Tasman Glacier. We surveyed orthogonal to glacier flow to obtain 12 transects within the lower 5 km of the glacier. Two-dimensional and three-dimensional gravity models generally indicate a U-shaped valley with ice thicknesses of 92-722 m from the present day terminus to the most upstream transect respectively. These results were used as input data to a simple mass flux model to assess its performance in estimating ice thickness and volume for the Tasman Glacier. The mass-flux model estimated a volume of 14.96 km3 for the Tasman Glacier, but generally underestimated ice thickness with an RMSE of 148 m between the modelled and the gravity-derived ice thickness. This discrepancy could be reduced by constraining ice thickness for a larger area of the glacier and providing a more recent DEM to the mass flux model. Studies such as this highlight the importance of constraining ice thickness in order to improve glacio-dynamic models and global volume estimates.