Borehole-based insights into the nature and timing of hydrological-regulation at a fast-moving Greenlandic outlet glacier

Enhanced ice velocity around the margins of the Greenland Ice Sheet is facilitated by the presence and pressure of subglacial meltwater. However, annual and longer-term velocity may be moderated by hydrological regulation, which reduces late melt-season and winter ice velocities following summers with relatively high surface melting and associated meltwater flux to the bed. While detail is lacking, hydrological regulation is likely driven by variations in the efficiency of subglacial drainage pathways and by associated variations in post-melt-season water retention at the glacier bed. Spatial variations in these processes may be viewed in terms of domains of differing degrees of hydrological connection to large subglacial meltwater channels. To date, a ‘well connected’ domain and an ‘isolated’ domain have been characterized, and an intermediate ‘weakly connected’ domain proposed. Generally, changes in the extent and/or pressurisation of the isolated domain are proposed as the driver of hydrological regulation. Yet, identifying the precise nature and timing of hydrological regulation, as well as the role of the weakly-connected domain, remain elusive.

             Here, we investigate the nature of the onset of hydrological regulation through a field experiment ~30 km from the terminus of Sermeq Kujalleq/Store Glacier, a fast-moving Greenlandic outlet glacier. We recorded simultaneous high-resolution time series of surface meltwater discharge, surface ice velocity and subglacial water pressure in two boreholes drilled at different distances from a substantial moulin. Analysis of the magnitude and timing of diurnal cycles and longer-term trends in all four records reveals that initially, in July, one borehole intersected the isolated subglacial domain and the other a ‘weakly-connected’ domain, with the latter showing a gradual decline in water pressure through the summer melt season. Transition to a winter state over the period ~10^th – 20^th August was marked by (i) a decrease in surface melting; (ii) a decrease in the amplitude of diurnal water pressure cycles in both boreholes, and (iii) a decrease in surface velocity. This transition was accompanied by an almost instantaneous (<1 d) switch in the borehole hitherto intersecting the weakly-connected domain to the isolated domain, evidenced by a 180° phase shift in the timing of its diurnal water pressure cycle. After this transition, diurnal cycles in all records diminish and both subglacial water pressure and ice surface velocity increase gradually through the winter.

                We conclude that ice velocity at our study location is at least partly governed by water pressure within a weakly-connected subglacial drainage domain. Water pressure here declines gradually through the melt season but increases after transition to hydraulic isolation, a transition that occurs over a period of only some days in the autumn. At the scale represented by records from individual boreholes the transition can occur over just some hours.