Abstract
Marine‐terminating outlet glaciers of the Greenland Ice Sheet make significant contributions to global sea level rise, yet the conditions that facilitate their fast flow remain poorly constrained owing to a paucity of data. We drilled and instrumented seven boreholes on Store Glacier, Greenland, to monitor subglacial water pressure, temperature, electrical conductivity, and turbidity along with englacial ice temperature and deformation. These observations were supplemented by surface velocity and meteorological measurements to gain insight into the conditions and mechanisms of fast glacier flow. Located 30 km from the calving front, each borehole drained rapidly on attaining ∼600 m depth indicating a direct connection with an active subglacial hydrological system. Persistently high subglacial water pressures indicate low effective pressure (180–280 kPa), with small‐amplitude variations correlated with notable peaks in surface velocity driven by the diurnal melt cycle and longer periods of melt and rainfall. The englacial deformation profile determined from borehole tilt measurements indicates that 63–71% of total ice motion occurred at the bed, with the remaining 29–37% predominantly attributed to enhanced deformation in the lowermost 50–100 m of the ice column. We interpret this lowermost 100 m to be formed of warmer, pre‐Holocene ice overlying a thin (0–8 m) layer of temperate basal ice. Our observations are consistent with a spatially extensive and persistently inefficient subglacial drainage system that we hypothesize comprises drainage both at the ice‐sediment interface and through subglacial sediments. This configuration has similarities to that interpreted beneath dynamically analogous Antarctic ice streams, Alaskan tidewater glaciers, and glaciers in surge.
Highlights
Over the last two decades the Greenland Ice Sheet (GrIS) has been the focus of considerable scientific attention due to its recent mass loss and the uncertainty regarding its future response to atmospheric and oceanic forcing
Here we present findings from a suite of boreholes drilled to the bed of Store Glacier, a fast-flowing tidewater outlet glacier that drains the western sector of the GrIS
Borehole-based measurements of (i) englacial temperature and tilt; and (ii) subglacial water pressure, electrical conductivity (EC), and turbidity were obtained during the summers of 2014 and 2016 from a site located 30 km from the terminus of Store Glacier
Summary
Over the last two decades the Greenland Ice Sheet (GrIS) has been the focus of considerable scientific attention due to its recent mass loss and the uncertainty regarding its future response to atmospheric and oceanic forcing. The fast flow of marine-terminating outlet glaciers is generally attributed to rapid basal motion, which relies upon a subglacial hydrological system sustained at high pressure over a large area of the bed to reduce friction and, where present, enhance the deformation of subglacial sediments (e.g., Kamb et al, 1994) These conditions are similar to those observed beneath ice streams and glaciers in surge (e.g., Engelhardt et al, 1990; Kamb et al, 1985), but direct evidence for subglacial material properties and conditions beneath fastflowing marine-terminating glaciers remains limited (Humphrey et al, 1993; Walter et al, 2014). The issue of whether these studies’ findings are representative of conditions beneath outlet glaciers flowing several time faster remains to be answered
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