Physical Conditions of Fast Glacier Flow: 1. Measurements From Boreholes Drilled to the Bed of Store Glacier, West Greenland

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.

Rapidly changing subglacial hydrological pathways at a tidewater glacier revealed through simultaneous observations of water pressure, supraglacial lakes, meltwater plumes and surface velocities

“…The borehole record from Kronebreen supports this idea and further suggests that consistently high basal water pressure may be exclusively associated with lake-terminating glaciers, tidewater glaciers and glaciers undergoing surging. Similar observations were concluded by Doyle et al (2017), with boreholes drilled to the bed at Store Glacier, Greenland showing consistently high water levels. Rapid drainage events have also been observed at other marine-terminating glaciers (e.g.…”

“…However, first-hand investigations of the role of subglacial hydrology at the terminus region of tidewater glaciers remain virtually absent (e.g. Kamb et al, 1994;Doyle et al, 2017).…”

Physical Conditions of Fast Glacier Flow: 2. Variable Extent of Anisotropic Ice and Soft Basal Sediment From Seismic Reflection Data Acquired on Store Glacier, West Greenland

“…In July 2014 a thermistor string was installed at the onset of the seismic survey (i.e., CMP 870 of the along‐profile) using hot‐water drilling (Figure a). In all seven boreholes drilled in 2014 and 2016, connection to the subglacial hydrological system was confirmed by the rapid drainage of borehole water when the drill stem reached a depth of 605–611 m below the surface Doyle et al (). R327 at 611 ± 5 m depth can thus accurately be interpreted to represent the ice‐bed interface.…”

“…Considering the interval velocity of 1,839 ± 1,620 m/s and the weakness of R327 that occasionally disappears in the central part of the along‐profile (22 m offset, 1 ∘ < θ < 16 ∘ ), we interpret the subglacial material as unconsolidated sediments. This interpretation was supported in 2016, when the hot‐water drilling team was able to penetrate, haltingly and over some hours, some tens of meters into this sequence (Doyle et al, ). The reflection coefficient R ( θ ) (Figure b) can be tracked from an angle of 4 to 16 ∘ where it has a positive but weak polarity.…”

Author response to reviewer comments on manuscript tc-2018-56

“…This presents an issue for identifying rapid lake drainage with confidence since hydrofracture usually occurs in hours Doyle et al, 2013;Tedesco et al, 2013). An alternative approach involves tracking lakes at high temporal (sub-daily) resolution but at lower spatial resolution (∼ 250-500 m) using MODIS imagery (Box and Ski, 2007;Sundal et al, 2009;Selmes et al, 2011Selmes et al, , 2013Liang et al, 2012;Morriss et al, 2013;Fitzpatrick et al, 2014;Everett et al, 2016;Williamson et al, 2017Williamson et al, , 2018. However, this lower spatial resolution means that lakes < 0.125 km 2 cannot be confidently resolved Williamson et al, 2017) and even lakes that exceed this size are often omitted from the satellite record (Leeson et al, 2013;.…”