Ice tectonic deformation during the rapid in situ drainage of a supraglacial lake on the Greenland Ice Sheet

Abstract

Abstract. We present detailed records of lake discharge, ice motion and passive seismicity capturing the behaviour and processes preceding, during and following the rapid drainage of a ∼ 4 km 2 supraglacial lake through 1.1-km-thick ice on the western margin of the Greenland Ice Sheet. Peak discharge of 3300 m 3 s −1 coincident with maximal rates of vertical uplift indicates that surface water accessed the ice-bed interface causing widespread hydraulic separation and enhanced basal motion. The differential motion of four global positioning system (GPS) receivers located around the lake record the opening and closure of the fractures through which the lake drained. We hypothesise that the majority of discharge occurred through a ∼ 3-km-long fracture with a peak width averaged across its wetted length of ∼ 0.4 m. We argue that the fracture's kilometre-scale length allowed rapid discharge to be achieved by combining reasonable water velocities with sub-metre fracture widths. These observations add to the currently limited knowledge of in situ supraglacial lake drainage events, which rapidly deliver large volumes of water to the ice-bed interface.

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

“…Channel formation could however be preconditioned by basal topography, the presence or otherwise of debris-laden basal ice [78] and permeable remnants of previous drainage systems [70, 83•]. In addition, following both initial lake and moulin drainage to the bed, there is often a substantial uplift of~0.1-1 m driven by ice bed separation [9,41,55] suggesting that it may be more realistic for models, driven by surface inputs, to initiate channels with considerably larger dimensions.…”

“…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].…”

“…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.…”

“…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.…”

Force Balance along Isunnguata Sermia, West Greenland

“…These landterminating outlet glaciers have been a focus of field-based research investigating subglacial hydrology (e.g., Chandler et al, 2013;Meierbachtol et al, 2013), lake drainage dynamics (Doyle et al, 2013;Dow et al, 2015), and meltwater influences on ice motion (e.g., Palmer et al, 2011;Bartholomew et al, 2012). The inland extent of the study domain reaches elevations of ∼2300 m, and is selected based on the fidelity of InSAR velocity measurements, which degrade toward the ice sheet interior where control points are either based on balance velocity or are 100 s of km away from bedrock reference points (Joughin et al, 2010).…”