Measured basal water pressure variability of the western Greenland Ice Sheet: Implications for hydraulic potential

Abstract

The gradient of the hydraulic potential field at the ice‐bedrock interface beneath the Greenland Ice Sheet (GrIS) dictates the routing and energetics of subglacial water, thereby influencing drainage system characteristics and sliding dynamics. In the ablation zone of the GrIS, variable water pressure due to an active subglacial drainage system and basal topography with high relief potentially interact to drive unknown spatial patterns and temporal changes in the hydraulic potential field. Here we present a suite of water pressure measurements collected in 13 boreholes along a 46 km transect on the western GrIS to investigate the role of spatial and temporal basal water pressure adjustments in hydraulic potential gradient dynamics. All borehole sites show pressures with similar seasonality, having relatively steady and high values during winter, variable and irregular behavior during spring and fall, and diurnal cycles that can persist for multiple weeks during the peak melt season. Despite much higher variability during the melt season, the median pressure of the summer period is nearly the same as the median pressure of the winter period. However, time variability of water pressure due to basal drainage processes can force changes in the magnitude and orientation of the hydraulic potential field over diurnal periods. We find that the basal water pressure across the transect generally mimics the ice thickness field but with superimposed large pressure gradients that develop at shorter scales within the basal drainage system. This leads to a complex hydraulic potential field across regions of similar ice thickness.

Borehole measurements indicate hard bed conditions, Kangerlussuaq sector, western Greenland Ice Sheet

“…Time series of basal water pressure reveal substantial water pressure gradients between adjacent boreholes spaced tens to hundreds of meters apart. While a constant pressure gradient of ~20 kPa m −1 between holes just 20 m apart was observed for more than a month at site 46 km‐11 [ Wright et al ., ], more typically, the magnitude of gradients tended to be transient over periods of many weeks (Figure ).…”

“…Water pressure was measured in boreholes using downhole pressure transducers and was recorded for periods of up to several years [ Wright et al ., ]. Time series of pressure variations and gradients between boreholes yield information about the nature of basal water flow and therefore the nature of the substrate hosting the subglacial drainage system.…”

“…The region also contains a bifurcating trough of exceptional depth, which we have not fully penetrated with a drill hole. Measurements of ice temperature [ Harrington et al ., ], as well as borehole observations of hydrological conditions [ Wright et al ., ], reveal that all locations have basal ice at the pressure melting point and have an active water system at the bed.…”

Greenland subglacial drainage evolution regulated by weakly connected regions of the bed

“…Depressed ice speeds persist through fall and much of winter161415, which is inconsistent with a timescale of hours to days for channel collapse under the thick GrIS ice3616. Additionally, modelling has indicated that gentle surface slopes on the ice sheet should suppress channel formation1617, and low water pressures18 and depressed summer ice speeds19 characteristic of highly efficient channels are only observed near the ice sheet margin. Therefore, in interior regions there may be unrecognized drainage capacity elsewhere in the system.…”

Channelized, distributed, and disconnected: subglacial drainage under a valley glacier in the Yukon

“…Werder et al, 2013), still fail to reproduce direct borehole observations of subglacial conditions . These include the existence of disconnected areas that show no signs of flow-related changes in water pressure (Hodge, 1979;Engelhardt et al, 1978;Murray and Clarke, 1995;Hoffman et al, 2016), the development of widespread areas of high water pressure during winter Harper et al, 2005;Ryser et al, 2014a;Wright et al, 2016), large pressure gradients over short distances (Murray and Clarke, 1995;Iken and Truffer, 1997;Fudge et al, 2008;Andrews et al, 2014), sudden reorganizations of the drainage system (Gordon et al, 1998;Kavanaugh and Clarke, 2000), high spatial heterogeneity, boreholes exhibiting anti-correlated temporal pressure variations (Murray and Clarke, 1995;Gordon et al, 1998;Andrews et al, 2014;Lefeuvre et al, 2015;Ryser et al, 2014a), and englacial conduits (Fountain and Walder, 1998;Nienow et al, 1998b;Gordon et al, 1998;Fountain et al, 2005;Harper et al, 2010).…”