Water Levels in Crevasses Reveal Subglacial Water Pressure Variations at Helheim Glacier, South East Greenland

Supraglacial lake drainage events are common on the Greenland Ice Sheet. Observations on the west coast of Greenland show that lakes at higher elevations typically drain later than those at lower elevations. We observe a system at Helheim Glacier where, for the majority of years between 2007 and 2014, the reverse is true. Here, a lake drainage event precedes the consecutive filling and draining of multiple areas of crevasses at lower elevations by between ten and thirty days. Furthermore, the presence of a proglacial plume typically coincides with increased water levels in the lower crevasses. Combining a linear elastic fracture mechanics model with existing models of surface melt production, we show that hydrofracture driven by surface melt occurs much earlier in the year than the observed peaks in water volume. From this we conclude that, following the initial hydrofracture, the crevasses remain hydraulically connected to the bed for a large portion of the summer. The observed water level fluctuations in the crevasses can therefore be explained by a transient high pressure event passing through the subglacial system following the lake drainage. These observations have wide ranging implications for our understanding of the dynamics of tidewater glaciers. For calving, full depth fractures may exist in the ice many kilometres from the terminus providing initiation points for subsequent calving events. This suggests a more nuanced role for hydrofracture in the calving process than previously theorised. For subglacial hydrology, these transient events can have major impacts on the subglacial system. Additionally, these water filled crevasses should provide a useful proxy for basal water pressures in areas which are notoriously difficult to access. We expect that these results are not specific to Helheim Glacier or the south east region of Greenland, and will have important implications for tidewater glaciers globally.