Fracture Propagation to the Base of the Greenland Ice Sheet During Supraglacial Lake Drainage

Abstract

Surface meltwater that reaches the base of an ice sheet creates a mechanism for the rapid response of ice flow to climate change. The process whereby such a pathway is created through thick, cold ice has not, however, been previously observed. We describe the rapid (<2 hours) drainage of a large supraglacial lake down 980 meters through to the bed of the Greenland Ice Sheet initiated by water-driven fracture propagation evolving into moulin flow. Drainage coincided with increased seismicity, transient acceleration, ice-sheet uplift, and horizontal displacement. Subsidence and deceleration occurred over the subsequent 24 hours. The short-lived dynamic response suggests that an efficient drainage system dispersed the meltwater subglacially. The integrated effect of multiple lake drainages could explain the observed net regional summer ice speedup.

Recent Advances in Our Understanding of the Role of Meltwater in the Greenland Ice Sheet System

“…Furthermore, it seems likely that moulins will be hydraulically efficient once formed, with turbulent and high velocity flow, in order to generate the high frictional melt rates required to prevent the closure of the englacial channels (or 'pipes') at depth where ice is often > 500 m thick. This inference is supported by the fact that once open, moulins continue to efficiently drain large supraglacial streams with discharges commonly upwards of 5 m 3 s −1 [9,49]; repeated surface ponding would be more commonplace if moulins did not maintain efficient drainage following their initial opening each melt season.…”

“…A tracer test from a mouliñ 57 km inland, where ice is~1200 m thick, indicated subglacial routing through less efficient drainage [62]. Other observations however suggest that efficient drainage can reach well in to the interior of the ice sheet to distances extending to 80 km; observations during rapid supraglacial lake drainage events reveal dramatic horizontal and vertical acceleration of the ice as vast volumes of water drain to the bed [9,41,55]. The subsequent rapid (~24 h) drop in vertical ice bed separation suggests that the water has been evacuated down-glacier quickly and efficiently.…”

“…It seems likely however that these features are largely vertical through inference from a number of lines of evidence including (i) field-based ice-penetrating radar studies [50], (ii) surface uplift in close proximity to the lake that occurs immediately following rapid lake drainage events [9,41,55] and (iii) the expectation that the physics of hydrofracture will most efficiently drive crevasses vertically from the ice surface to the bed [48]. Furthermore, it seems likely that moulins will be hydraulically efficient once formed, with turbulent and high velocity flow, in order to generate the high frictional melt rates required to prevent the closure of the englacial channels (or 'pipes') at depth where ice is often > 500 m thick.…”

“…The lakes can either drain rapidly by hydrofracture [9,41,42] or more slowly by overtopping their topographic lip and draining supraglacially downstream [43][44][45]; a 5-year study of 2000 Greenland-wide supraglacial lakes estimated that 13% of the lakes were 'fast-draining' (< 2 days) [35], while a 10-year catchment-based study of~200 lakes estimated that 28% of the lakes drained 'rapidly' (< 4 days) [38]. Rapid lake drainage results in large volumes of water entering the subglacial drainage system in a few hours with rates of 8700 and 3300 m 3 s −1 recorded [9,41]. In order for surface lakes to drain, an appropriate stress regime in the ice is required with existing crevasses, which can then be exploited and expanded by ponded surface waters draining englacially via hydrofracture [46][47][48].…”

“…This was followed by observations of a close correspondence between surface melt rates and ice acceleration at a site where the ice was over 1 km thick [8]; the inference again was that surface meltwaters were accessing the ice bed interface, thereby perturbing the subglacial water pressure and enhancing ice motion through basal sliding. Confirmation of the unequivocal link between surface meltwater drainage and ice dynamic response came from observations of ice acceleration and surface uplift following the rapid drainage of two large supraglacial lakes in west Greenland through ice 980 m thick [9]. The realisation of this direct hydro-dynamic ice sheet coupling, and the potential implications for a mechanism by which the ice sheet might respond rapidly to climate warming, has seen a proliferation of research into the hydrology of the GrIS.…”