Self-regulation of fast-moving glaciers in Greenland: from borehole observations to spaceborne measurements

Subglacial drainage systems exert control on glacier motion; however, the nature and evolution of these drainage systems are not well established. Here, we report the co-evolving state of friction, water pressure and water flows at the base of Sermeq Kujalleq (Store Glacier), a fast-moving glacier in west Greenland. Seismic records from a centreline location on a major subglacial drainage axis show stick-slip impulsive events (icequakes) to be far more frequent in winter than in summer. In contrast, the amplitude of low-frequency tremor from subglacial water flows are low in winter but high in summer. Additional insight into this basal environment is gained through boreholes, which show a strong anti-phase relationship between water pressure recorded in water-filled cavities that are either connected with or isolated from surface melt inputs.

Collectively, the observations show a winter-system of largely unconnected cavities switching rapidly to a system of linked or partially linked cavities as soon as meltwater reaches the bed. The formation of a channel occurs later in the summer season and is seen in our data as a distinct slow-down in glacier speed. The return to the winter system of mostly unconnected cavities is seen from a switch to in-phase water pressure in borehole records. Reduced seismic tremor at this point in time is consistent with linked cavities becoming isolated, while more frequent stick-slip events suggest the glacier bed is stronger after the melt season has ended. We hypothesise glacier motion is governed by the extent to which cavities are either isolated (strong bed) or linked (weak bed), and that channelisation strengthens the bed by capturing water from the latter.

To upscale our findings we use spaceborne measurements of glacier velocities to look for evidence of channelisation in the basal drainage system more widely. Out of 54 glaciers examined in west Greenland, we report 45 glaciers with strong self-regulation and a hydro-dynamic behaviour similar to Sermeq Kujalleq (Store Glacier). We found a statistically robust correlation between latitude and the elevation to which channelised systems could be traced on tidewater glaciers, with channels extending to 1,500 m or higher beneath tidewater glaciers in the southwest. For land-terminating glaciers in the same sector we found no evidence of channelisation above 1,000 m elevation and there was no statistical correlation with latitude. Contrary to the current consensus: that the additional runoff generated in warmer and longer summers is routed away with little or no impact on the ice sheet, our study shows this self-regulation is only strong for marine-terminating glaciers. High melt combined with poor drainage in the land-terminating setting make the southwest sector of the Greenland more vulnerable to climate change than previous work and the latest IPCC report has suggested.