Ice-bed interface conditions in the accumulation zone of the Grenzgletscher

Ice streams are river-like features of ice sheets that move much faster than the surrounding, ice. This contrast in velocity (100s m/yr vs 10s m/yr) results from ice flow being dominated by basal sliding with a fully temperate bed within ice streams, whereas ice is sliding little or not at all outside, where the bed is below the melting point. Here, we present initial results from an ERC-funded project, PHAST, which seeks to unravel the physics driving ice stream formation and dynamics. As part of this project, we seek to characterize observationally the onset of basal sliding at cold/temperate basal transition at an easily accessible alpine glacier (Grenzgletscher). Previous research has found a cold bed in the accumulation zone and a temperate bed in the ablation zone. However, the location of the cold/temperate basal transition is not known. Using a micro vibrator Elvis 7 (p-wave generator) we collected two active seismic profiles at a 3720 m high plateau on the Grenzgletscher; one parallel (250 m) and one (325 m) perpendicular to ice flow. The parallel profile shows a surprising lack of structure below the 328m deep ice-bed contact, suggesting it is likely to be bedrock. However, at the downstream end of the profile there is some stratification, which could be eroded sediments. As there is no polarity reversal at the ice-bed contact we find no indication of water at the bed. These initial results suggest that the cold/temperate basal transition is located further downstream. However, further analysis of this data, alongside passive seismics and ground-penetrating radar data, will help us identify the transition with more confidence – assisting a drilling campaign to be undertaken in 2026.