Seasonal Evolution of the Subglacial Hydrologic System Modified by Supraglacial Lake Drainage in Western Greenland

Abstract

The impact of summer surface melt on Greenland Ice Sheet dynamics is modulated by the state of the subglacial hydrologic system. Studies of ice motion indicate that efficiency of the subglacial system increases over the melt season, decreasing the sensitivity of ice motion to surface melt inputs. However, the behavior of the subglacial hydrologic system is complex and some characteristics are still poorly constrained. Here we investigate the coevolution of subglacial hydrology and ice motion in the Pâkitsoq region of western Greenland during the 2011 melt season. We analyze measurements from 11 Global Positioning System stations, from which we derive ice velocity, longitudinal strain rates, and basal uplift, alongside observations of surface ablation and supraglacial lake drainages. We observe ice acceleration after the onset of local surface melting, followed by gradual ice deceleration, consistent with increasing subglacial efficiency. In the study area, supraglacial lake drainages cooccur with a change in regional strain rate patterns and ice deceleration, suggesting that lake drainages contribute to rapid subglacial reorganization. At lower ice surface elevations (below ~900 m above sea level), ice motion is correlated with both total basal uplift and its rate of change, while at higher elevations (~900–1,100 m above sea level), ice motion correlated only with the basal uplift rate. This pattern suggests that continued cavity growth or subglacial sediment dynamics may be important in the apparent increase in subglacial drainage efficiency at higher elevations in the ablation zone. Our results further suggest that transient subglacial behavior is important in the seasonal evolution of ice motion.

The Influence of Hydrology on the Dynamics of Land-Terminating Sectors of the Greenland Ice Sheet

“…However, observations of the subglacial environment and constraints on crucial model parameters are sparse, leading to uncertainty in inferences from models: modelled R-channels can form under conditions representative of the upper ablation area if a large (∼1 m 2 ) initial conduit size is used (e.g., Hewitt, 2011;Gulley et al, 2012). Such conditions may occur due to uplift during rapid lake drainage (Das et al, 2008;Pimentel and Flowers, 2010;Andrews et al, 2018) or may be facilitated if persistent surface-to-bed hydrological connections (Catania and Neumann, 2010) enable erosion of preferential flow pathways into the substrate (Gulley et al, 2012;Beaud et al, 2018). In addition, the spatial pattern of surfaceto-bed connections (moulins) strongly influences the spatial organisation of modelled subglacial channels, with a high density of moulins producing widespread and rapid channelisation beneath ice up to 815 m thick (Banwell et al, 2016).…”

“…Large inputs of meltwater to the bed during rapid lake drainage events also appear to influence the subglacial drainage system. Observations indicate ice sheet uplift during and immediately following rapid lake drainage (e.g., Das et al, 2008;Doyle et al, 2013;Andrews et al, 2018), followed by lowering within a few hours (e.g., Das et al, 2008;Bartholomew et al, 2012) to days (Doyle et al, 2013;Andrews et al, 2018), and sustained flow deceleration (Andrews et al, 2018). The uplift of the ice sheet-surface implies that the initial influx of runoff exceeds the capacity of the local subglacial drainage system (e.g., Das et al, 2008;Stevens et al, 2015) and likely spreads out radially from the point of input as a "blister" (Tsai and Rice, 2010;Dow et al, 2015).…”

“…The uplift of the ice sheet-surface implies that the initial influx of runoff exceeds the capacity of the local subglacial drainage system (e.g., Das et al, 2008;Stevens et al, 2015) and likely spreads out radially from the point of input as a "blister" (Tsai and Rice, 2010;Dow et al, 2015). The subsequent subsidence suggests that this runoff can be evacuated relatively rapidly, possibly in a turbulent sheet (Adhikari and Tsai, 2015;Dow et al, 2015), whilst the subsequent slow-down implies persistent, efficient drainage formation at low elevations (below ∼900 m) or changing pressures within inefficient parts of the drainage system at high elevations (see section Modulation of Ice Flow by the Weakly-Connected Subglacial Drainage System; Andrews et al, 2018). Whether efficient channels develop during rapid lake drainage is uncertain and will depend on local ice thickness and hydropotential gradient.…”

“…Whether efficient channels develop during rapid lake drainage is uncertain and will depend on local ice thickness and hydropotential gradient. Large and turbulent fluxes of subglacial runoff would theoretically cause rapid channel opening (Röthlisberger, 1972;Schoof, 2010), which would be followed by sustained flow deceleration (Andrews et al, 2018); however, the short duration of drainage events may limit channel growth (e.g., Dow et al, 2015). A modelled lake drainage event through 1,200 m thick ice (Dow et al, 2015) did not generate substantial channel growth; however, the sensitivity of this finding to key properties (such as ice thickness and pre-drainage basal hydrology) remains unclear.…”

Challenges in predicting Greenland supraglacial lake drainages at the regional scale

“…We next calculate principal strain rates from the 2011-2012 GPS network data from Andrews et al (2014Andrews et al ( , 2018. These data were recorded at 15-second intervals at eleven GPS stations separated by 2 to 20 km within a local network.…”