Optical televiewer borehole logging within a crevassed region of fast-moving Store Glacier, Greenland, revealed the presence of 35 high-angle planes that cut across the background primary stratification. These planes were composed of a bubble-free layer of refrozen ice, most of which hosted thin laminae of bubble-rich ‘last frozen’ ice, consistent with the planes being the traces of former open crevasses. Several such last-frozen laminae were observed in four traces, suggesting multiple episodes of crevasse reactivation. The frequency of crevasse traces generally decreased with depth, with the deepest detectable trace being 265 m below the surface. This is consistent with the extent of the warmer-than-modelled englacial ice layer in the area, which extends from the surface to a depth of ~400 m. Crevasse trace orientation was strongly clustered around a dip of 63° and a strike that was offset by 71° from orthogonal to the local direction of principal extending strain. The traces’ antecedent crevasses were therefore interpreted to have originated upglacier, probably ~8 km distant involving mixed-mode (I and III) formation. We conclude that deep crevassing is pervasive across Store Glacier, and therefore also at all dynamically similar outlet glaciers. Once healed, their traces represent planes of weakness subject to reactivation during their subsequent advection through the glacier. Given their depth, it is highly likely that such traces - particularly those formed downglacier - survive surface ablation to reach the glacier terminus, where they may represent foci for fracture and iceberg calving.

Surface crevasses on the Greenland Ice Sheet capture nearly half of the seasonal runoff, yet their role in transferring meltwater to the bed has received little attention compared to that of supraglacial lakes and moulins. Here, we present observations of crevasse ponding and investigate controls on their hydrological behaviour at a fast-moving, marine-terminating sector of the Greenland Ice Sheet. We map surface meltwater, crevasses, and surface-parallel stress across a ~2,700 km² region using satellite data and contemporaneous uncrewed aerial vehicle (UAV) surveys. From 2017-2019 an average of 26% of the crevassed area exhibited ponding at locations that remained persistent between years despite rapid advection rates. We find that the spatial distribution of ponded crevasses does not relate to previously proposed methods for predicting the distribution of supraglacial lakes (elevation and topography) or crevasses (von Mises stress thresholds), suggesting the operation of some other physical control(s). Ponded crevasse fields were preferentially located in regions of compressive surface-parallel mean stress, which we interpret to result from the hydraulic isolation of these systems, in contrast to unponded crevasse fields, which we suggest are able to drain into the wider supraglacial and englacial network. UAV observations show that ponded crevasses can drain episodically and rapidly, likely through hydrofracture. We therefore propose that the surface stress regime informs a spatially heterogeneous transfer of meltwater through crevasses to the bed of ice sheets, with potential consequences for processes such as subglacial drainage and the heating of ice via latent heat release by refreezing meltwater.

Calving and solid ice discharge into fjords account for approximately half of the annual net ice loss from the Greenland Ice Sheet, but these processes are rarely observed. To gain insights into the spatio-temporal nature of calving, we use a terrestrial radar interferometer to derive a three-week record of 8,026 calving events from Store Glacier, including the transition between a mélange-filled and ice-free fjord. We show that calving rates double across this transition and that the interferometer record is in good agreement with volumetric estimates of calving losses from contemporaneous UAV surveys. We report significant variations in calving activity over time, which obfuscate any simple power-law relationship. While there is a statistically significant relationship between surface melt and the number of calving events, no such relationship exists between surface melt and the volume of these events. Similarly, we find a 70% increase in the number of calving events in the presence of visible meltwater plumes, but only a 3% increase in calving volumes. While calving losses appear to have no clear single control, we find a bimodal distribution of iceberg sizes due to small sections of ice breaking off the subaerial part of the front and large capsizing icebergs forming by full-thickness failure. Whereas previous work has hypothesised that tidewater glaciers can be grouped according to whether they calve predominantly by the former or latter mechanism, our observations indicate that calving here inherently comprises both, and that the dominant process can change over relatively short periods.

Calving and solid ice discharge into fjords account for approximately half of the annual net ice loss from the Greenland Ice Sheet, but these processes are rarely observed. To gain insights into the spatio-temporal nature of calving, we use a terrestrial radar interferometer to derive a three-week record of 8,026 calving events from Store Glacier, including the transition between a mélange-filled and ice-free fjord. We show that calving rates double across this transition and that the interferometer record is in good agreement with volumetric estimates of calving losses from contemporaneous UAV surveys. We report significant variations in calving activity over time, which obfuscate any simple power-law relationship. While there is a statistically significant relationship between surface melt and the number of calving events, no such relationship exists between surface melt and the volume of these events. Similarly, we find a 70% increase in the number of calving events in the presence of visible meltwater plumes, but only a 3% increase in calving volumes. While calving losses appear to have no clear single control, we find a bimodal distribution of iceberg sizes due to small sections of ice breaking off the subaerial part of the front and large capsizing icebergs forming by full-thickness failure. Whereas previous work has hypothesised that tidewater glaciers can be grouped according to whether they calve predominantly by the former or latter mechanism, our observations indicate that calving here inherently comprises both, and that the dominant process can change over relatively short periods.

Distributed acoustic sensing (DAS) is a new technology in which seismic energy is recorded at high spatial and temporal resolution along a fibre-optic cable. We show analyses from the first glaciological borehole deployment of DAS to measure the englacial and subglacial seismic properties of Store Glacier, a fast-flowing outlet of the Greenland Ice Sheet. By characterizing compressional and shear wave propagation in 1030 m-deep vertical seismic profiles, sampled at 10 m vertical resolution, we detected a transition from isotropic to anisotropic ice consistent with a Holocene-Wisconsin transition at 83% of the ice thickness. We also infer temperate ice in the lowermost 100 m of the glacier, and identified subglacial reflections originating from the base of a 20 m-thick layer of consolidated sediment. Our findings highlight the transformative potential of DAS to inform the physical properties of glaciers and ice sheets.