GLACIAL ABRASION CONTROLLED BY SLIDING VELOCITY AND HYDRAULIC CONDITIONS: NEW OBSERVATIONS

It is thought that glacial erosion is proportional to ice sliding velocity raised to some power. Although there are physical foundations behind such laws, there are currently very few observations confirming their theoretical basis. Here we quantify spatial and temporal variations in ice sliding velocity and erosion using a multi-disciplinary approach. We focus on the Franz Josef glacier, New Zealand. This fast-flowing glacier directly slides on its underlying bedrock, with observed surface velocities up to about 4 m/day. We show through a time series analysis of suspended sediment during the austral summer that the glacier remains supply-limited, i.e. erosion rates remain lower than the transport capacity, and convert the sediment discharge measurements into erosion rates. Furthermore, we invert the spatial distribution of erosion from the provenance of suspended sediments using a new approach based on Raman Spectroscopy of Carbonaceous Material. By comparing the pattern of erosion rates to sliding velocities, quantified using optical satellite imagery and an ice flow model, we find that erosion rates are proportional to sliding velocity raised to a power of two. Interestingly, this matches theoretical predictions for abrasion proposed by Hallet (1979). More importantly, our new observations reveal that the magnitude of erosion is highly sensitive to rainfall events. During these events, water can easily reach the ice-bedrock interface, increase sliding and, in turn, erosion rates. These results demonstrate that glacial abrasion strongly depends on the hydraulic conditions at the ice-bedrock interface.