Water flow through sediments and at the ice-sediment interface beneath Sermeq Kujalleq (Store Glacier), Greenland

Abstract

Subglacial 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 ≤ 10−6 m 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.

Reorganisation of subglacial drainage processes during rapid melting of the Fennoscandian Ice Sheet

“…Future work will seek to address some of these limitations by including, for example, coupled ice dynamics which are allowed to vary in response to effective pressure (e.g., as in Ehrenfeucht et al, 2023). Initial sensitivity testing of velocity forced to change seasonally does indicate that changes in velocity throughout the year is important for repressurising the system each winter to more closely match borehole records (e.g., Doyle et al, 2018Doyle et al, , 2022. Considering each of these limitations our results therefore describe the seasonal evolution of channelised and distributed drainage system beneath a necessarily idealised representation of our study site in the FLDIL.…”

“…Yet our knowledge of basal topography is limited to spatial resolutions in the order of 10 2 m (e.g., Morlighem et al, 2017Morlighem et al, , 2020 and direct observations of hydraulic connectivity are sparse, especially at the ice sheet scale (Greenwood et al, 2016). Boreholes and instrumentation placed within and beneath ice are one means of direct observation of hydraulic properties (e.g., Hubbard et al, 1995;Mair et al, 2003;Meierbachtol et al, 2013;Doyle et al, 2018Doyle et al, , 2022, however, these are difficult to implement and impractical to use beyond a handful of sites per field campaign. Instead, much of our insight into subglacial hydrology has come from indirect proxies for drainage efficiency including proglacial discharge (e.g., Willis et al, 1996;Cowton et al, 2013), geophysical investigation (e.g., Chu et al, 2016;Killingbeck et al, 2020), and ice surface velocity time-series analysis (e.g., Mair et al, 2002;Tuckett et al, 2019;Wallis et al, 2023) from which the influence of channelised drainage has been inferred extending up to 50 km inland of the Greenland Ice Sheet (GrIS) margin (Sole et al, 2013;Chandler et al, 2013Chandler et al, , 2021.…”

“…Instead, much of our insight into subglacial hydrology has come from indirect proxies for drainage efficiency including proglacial discharge (e.g., Willis et al, 1996;Cowton et al, 2013), geophysical investigation (e.g., Chu et al, 2016;Killingbeck et al, 2020), and ice surface velocity time-series analysis (e.g., Mair et al, 2002;Tuckett et al, 2019;Wallis et al, 2023) from which the influence of channelised drainage has been inferred extending up to 50 km inland of the Greenland Ice Sheet (GrIS) margin (Sole et al, 2013;Chandler et al, 2013Chandler et al, , 2021. Without a strong control on the bed characteristics beneath contemporary ice sheets, estimating and defining an appropriate set of basal parameters remains a fundamental challenge to the accurate representation of basal hydrology in ice sheet models (Dow et al, 2020;Doyle et al, 2022;Hager et al, 2022;McArthur et al, 2023).…”

Shear dilation of subglacial till results in time-dependent sliding laws

“…However, water pressure in the subglacial environment is not constant over time; it fluctuates on timescales from hourly (tidal forcing, daily surface melt) to yearly (summer melt season) to multi-decadal, accompanied by a response in the glacier surface speed [8][9][10][11]. Nor are these pressure changes necessarily spatially uniform [12,13]. Increasingly, observations [9], simulations [14,15] and experiments [16] have shown the potential for transient adjustment of basal dynamics to introduce hysteresis and lag between changes in effective pressure and changes in sliding speed, in settings dominated by either hard-bedded or soft-bedded sliding.…”