Closing the Balances of Ice, Water and Sediment Fluxes Through the Terminus of Gepatschferner

Abstract

The terminus of Gepatschferner (46°52′ N, 10°46′ E) was subject to detailed glaciological investigations in the joint project PROSA. Both direct and geodetic methods were applied. Specifically, ice surface lowering measured at ablation stakes determined mass loss at the glacier surface, ice surface velocity was measured directly at the same stakes with differential GPS, and geodetic radar and vibroseismic soundings came into operation to investigate ice thickness and thickness of subglacial sediments. Multiple high-resolution airborne laser scanning (ALS) surveys document total volume changes. In contrast to a differentiated examination of the balances of ice water and sediments, the combination of these balances was an appropriate approach for investigating glacier mass exchanges and identifying dominant processes within the glacial system. The calculation or estimate of the fluxes of ice, water and sediments entering the narrow terminus at an elevation of 2875 m and leaving it at the glacier snout at 2200 m was based on glacier motion, surface and basal melt rates and on the lateral mass transport to the glacier from rock face and moraine bedrock erosion recorded from repeated terrestrial laser scans. In the course of the investigations on Gepatschferner, multiple rockfall events and the rapid evacuation of subglacial sediments were observed. The highest mass fluxes within the glacial system of Gepatschferner were associated with these extreme or episodic events, which exceeded the normal annual processes by multiple orders of magnitude. The relevant geophysical processes in this study period were thus not representative of long-term averages, if these ever existed. They did, however, display an interesting spectrum of naturally occurring situations. In that period, the mean velocity through the cross section at 2875 m was 22.5 m per year. Below that profile, the ice loss at the terminus corresponds to a mean surface lowering of 3.61 m per year between 2012 and 2015.