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.
Abstract. Supraglacial lakes represent an ephemeral storage buffer for meltwater runoff and lead to significant, yet shortlived, episodes of ice-flow acceleration by decanting large meltwater and energy fluxes into the ice sheet's hydrological system. Here, a methodology for calculating lake volume is used to quantify storage and drainage across Russell Glacier, West Greenland, between 2002 and 2012. Using 502 MODIS scenes, water volume at ∼ 200 seasonally occurring lakes was derived using a depth-reflectance relationship, which was independently calibrated and field validated against lake bathymetry. The inland expansion of lakes is strongly correlated with air temperature: during the record melt years of 2010 and 2012, lakes formed and drained earlier, attaining their maximum volume 38 and 20 days earlier than the 11 yr mean, as well as occupying a greater area and forming at higher elevations (> 1800 m) than previously. Despite occupying under 2 % of the study area, lakes delay the transmission of up to 7-13 % of the bulk meltwater discharged. Although the results are subject to an observational bias caused by periods of cloud cover, we estimate that across Russell Glacier, 28 % of supraglacial lakes drain rapidly (< 4 days). Clustering of such events in space and time suggests a synoptic trigger mechanism. Further, we find no evidence to support a unifying critical size or depth-dependent drainage threshold.
The dynamic response of the Greenland Ice Sheet (GrIS) depends on feedbacks between surface meltwater delivery to the subglacial environment and ice flow. Recent work has highlighted an important role of hydrological processes in regulating the ice flow, but models have so far overlooked the mechanical effect of soft basal sediment. Here we use a threedimensional model to investigate hydrological controls on a GrIS soft-bedded region. Our results demonstrate that weakening and strengthening of subglacial sediment, associated with the seasonal delivery of surface meltwater to the bed, modulates ice flow consistent with observations. We propose that sedimentary control on ice flow is a viable alternative to existing models of evolving hydrological systems, and find a strong link between the annual flow stability, and the frequency of high meltwater discharge events. Consequently, the observed GrIS resilience to enhanced melt could be compromised if runoff variability increases further with future climate warming.
Abstract. The concept of a positive feedback between ice flow and enhanced melt rates in a warmer climate fuelled the debate regarding the temporal and spatial controls on seasonal ice acceleration. Here we combine melt, basal water pressure and ice velocity data. Using 20 years of data covering the whole ablation area, we show that there is not a strong positive correlation between annual ice velocities and melt rates. Annual velocities even slightly decreased with increasing melt. Results also indicate that melt variations are most important for velocity variations in the upper ablation zone up to the equilibrium line altitude. During the extreme melt in 2012, a large velocity response near the equilibrium line was observed, highlighting the possibility of meltwater to have an impact even high on the ice sheet. This may lead to an increase of the annual ice velocity in the region above S9 and requires further monitoring.
We present surface velocity measurements from a high-elevation site located 140 km from the western margin of the Greenland ice sheet, and~50 km into its accumulation area. Annual velocity increased each year from 51.78 ± 0.01 m yr À1 in 2009 to 52.92 ± 0.01 m yr À1 in 2012-a net increase of 2.2%. These data also reveal a strong seasonal velocity cycle of up to 8.1% above the winter mean, driven by seasonal melt and supraglacial lake drainage. recently argued that ice motion in the ablation area is mediated by reduced winter flow following the development of efficient subglacial drainage during warmer, faster, summers. Our data extend this analysis and reveal a year-on-year increase in annual velocity above the equilibrium line altitude, where despite surface melt increasing, it is still sufficiently low to hinder the development of efficient drainage under thick ice.
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