Abstract
Abstract. A leading hypothesis for the mechanism of fast supraglacial lake drainages is that transient extensional stresses briefly allow crevassing in otherwise compressional ice flow regimes. Lake water can then hydrofracture a crevasse to the base of the ice sheet, and river inputs can maintain this connection as a moulin. If future ice sheet models are to accurately represent moulins, we must understand their formation processes, timescales, and locations. Here, we use remote-sensing velocity products to constrain the relationship between strain rates and lake drainages across ∼ 1600 km2 in Pâkitsoq, western Greenland, between 2002–2019. We find significantly more extensional background strain rates at moulins associated with fast-draining lakes than at slow-draining or non-draining lake moulins. We test whether moulins in more extensional background settings drain their lakes earlier, but we find insignificant correlation. To investigate the frequency at which strain-rate transients are associated with fast lake drainage, we examined Landsat-derived strain rates over 16 and 32 d periods at moulins associated with 240 fast-lake-drainage events over 18 years. A low signal-to-noise ratio, the presence of water, and the multi-week repeat cycle obscured any resolution of the hypothesized transient strain rates. Our results support the hypothesis that transient strain rates drive fast lake drainages. However, the current generation of ice sheet velocity products, even when stacked across hundreds of fast lake drainages, cannot resolve these transients. Thus, observational progress in understanding lake drainage initiation will rely on field-based tools such as GPS networks and photogrammetry.
Highlights
Most surface meltwater produced in the ablation zone of the western Greenland Ice Sheet descends to the ice sheet bed through moulins, vertical englacial conduits that deliver surface melt to the subglacial environment (Smith et al, 2015)
We examine time-varying principal strain rates at moulin sites on the ice sheet surface in an effort to quantify their influence on the date that a moulin begins delivering significant meltwater fluxes to the bed
We subdivided the moulins into two distinct populations: moulins that drain supraglacial lakes (Morriss et al, 2013) and moulins that are fed by supraglacial rivers, independent of lakes (Andrews, 2015)
Summary
Most surface meltwater produced in the ablation zone of the western Greenland Ice Sheet descends to the ice sheet bed through moulins, vertical englacial conduits that deliver surface melt to the subglacial environment (Smith et al, 2015). The water flux through moulins drives the evolution of the subglacial hydrologic system on diurnal and seasonal timescales (Schoof, 2010; Hoffman et al, 2011; Hewitt, 2013; Werder et al, 2013; Andrews et al, 2014). The subglacial hydrological system evolves in response to two primary aspects of surface meltwater input to the ice sheet bed: (1) the locations where moulins deliver surface water to the bed and (2) the temporal evolution, including diurnal to seasonal variability, of that water flux to the bed (Schoof, 2010; Banwell et al, 2016). Meltwater is collected in these lakes through the melt season and may abruptly drain to the bed, presum-
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