Abstract
AbstractSubglacial hydrology modulates basal motion but remains poorly constrained, particularly for soft-bedded Greenlandic outlet glaciers. Here, we report detailed measurements of the response of subglacial water pressure to the connection and drainage of adjacent water-filled boreholes drilled through kilometre-thick ice on Sermeq Kujalleq (Store Glacier). These measurements provide evidence for gap opening at the ice-sediment interface, Darcian flow through the sediment layer, and the forcing of water pressure in hydraulically-isolated cavities by stress transfer. We observed a small pressure drop followed by a large pressure rise in response to the connection of an adjacent borehole, consistent with the propagation of a flexural wave within the ice and underlying deformable sediment. We interpret the delayed pressure rise as evidence of no pre-existing conduit and the progressive decrease in hydraulic transmissivity as the closure of a narrow (< 1.5 mm) gap opened at the ice-sediment interface, and a reversion to Darcian flow through the sediment layer with a hydraulic conductivity of ≤ 10−6m s−1. We suggest that gap opening at the ice-sediment interface deserves further attention as it will occur naturally in response to the rapid pressurisation of water at the bed.
Highlights
A list of symbols is presented in Appendix A
The results show that hydraulic transmissivity was an order of magnitude lower during the early recovery phase than during the pumping test, with hydraulic transmissivity spanning the range (1.8 − 3.5) × 10−6 m s−1 equivalent to gap widths of 0.16 − 0.20 mm for gaps covering the whole of the glacier bed (φ = 1; Table 3)
We suggest that the theory of gap opening at the ice-sediment interface (Engelhardt and Kamb, 1997) may involve the same physical process as ice-sediment decoupling envisaged by Iverson and others (1995)
Summary
A list of symbols is presented in Appendix A. The nature of subglacial hydrology and basal motion on ice masses underlain by soft sediments are central questions in ice dynamics (e.g., Clarke, 1987; Murray, 1997; Tulaczyk and others, 2000). Despite abundant evidence for subglacial sediments beneath fast-moving outlet glaciers and ice streams draining the Greenland and Antarctic ice sheets (e.g., Alley and others, 1986; Blankenship and others, 1986; Christianson and others, 2014) and mountain glaciers (e.g., Humphrey and others, 1993; Iverson and others, 1995), softbedded processes remain poorly constrained (Walter and others, 2014; Alley and others, 2019). Direct evidence for gap-opening at the ice-sediment interface is limited to three observational studies (Engelhardt and Kamb, 1997; Lüthi, 1999; Iverson and others, 2007). Despite support from detailed analytical modelling (Schoof and others, 2012; Rada and Schoof, 2018) dynamic gap opening has yet to be fully developed for larger-scale numerical models of subglacial hydrology
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