Colloquium: Mark D. Behn

Mark D. Behn, Associate Scientist, Department of Geology and Geophysics, Woods Hole Oceanographic Institution

Abstract - During the summer months, supra-glacial melt water lakes form across the ablation zone of the Greenland Ice Sheet.  Many of these lakes grow in size up to 2–5 km in diameter before draining rapidly (in minutes to hours) via hydro-fracture.  These drainage events deliver large volumes of water to the bed and create conduits (e.g., moulins) that provide surface-to-bed drainage channels that remain open for the remainder of the summer melt season.  This rapid influx melt water in turn influences ice sheet dynamics by modulating basal water pressure and frictional sliding at the ice-bed interface.  In my talk, I will present GPS and seismic data from a network of stations operational from 2011–2014 surrounding a supra-glacial lake system in the Jakobshavn-Isbrae region of the Greenland Ice Sheet.  These data are used to (1) invert for the opening history of the hydro-fracture beneath the lake in space and time, and (2) evaluate the acceleration of the ice in response to the rapid influx of surface meltwater to the bed.  Our data show that ice sheet response is strongly modulated by the seasonal evolution of the subglacial hydrologic system.  Lake drainages that occur early in the summer melt season have a larger and more prolonged effect on ice dynamics compared to those occurring later in the melt season.  We speculate that this reflects a less efficient early season hydrologic system, which cannot rapidly evacuate the influx of melt water associated with lake drainage, allowing basal water pressure to remain high and promoting sliding.  Finally, I will discuss how these individual drainage events scale up to influence regional dynamics along the western margin of the Greenland Ice Sheet.