Seasonal drainage-system evolution beneath the Greenland Ice Sheet inferred from transient speed-up events

Abstract

The transport of meltwater from the surface to the bed of the Greenland Ice Sheet is well understood to result in elevated surface velocities, although this relationship remains poorly resolved on a seasonal scale. Transient speedups associated with supraglacial lake drainages, which generally occur in the early-to-mid summer melt season, have been studied in detail. However, the connection between basal hydrology and ice dynamics is less well understood in the late melt season, after most lakes have ceased draining and meltwater input to the bed is through widely distributed moulins. Here, we use a Global Positioning System (GPS) array to investigate transient speed-up events in response to runoff across the 2011 and 2012 melt seasons and use these data to infer the evolution of subglacial conditions beneath the ice sheet in western Greenland. We find no relationship between the magnitude of runoff and the amplitude of speed-up events; we do observe a general trend of increasing velocity responses and decreasing variability in the velocity response across the GPS array as the melt season progresses. Early-season transient speed-ups (frequently associated with lake drainages) produce highly variable speed-up and pronounced uplift across the array. The variability across the array during a lake drainage corresponds with the bedrock topography and persists on annual timescales. By contrast, late-season melt events produce longer, higher amplitude, and more uniform velocity responses, but do not produce large or coherent uplift patterns. We interpret our results to imply that by the late melt season, most subglacial channels and/or connective flow pathways between cavities are closing or have closed, sharply lowering basal transmissivity. At the same time, moulins formed throughout the melt season remain open, producing pervasive and widely distributed surface-to-bed pathways. The result is that small magnitude, late-season melt events can rapidly supply meltwater to the bed and overwhelm the subglacial system, decreasing frictional coupling. This late-season response contrasts with early-season lake drainage events when surface-to-bed pathways are not yet open, and therefore, similarly small magnitude melt events do not have the same impact. Finally, we show that due to their extended duration and amplitude, late-season melt events accommodate a larger fraction of the annual ice motion than early-season lake drainages but their net influence on ice-sheet motion remains small (2 %–3 % of annual displacement).