Abstract. We present detailed records of lake discharge, ice motion and passive seismicity capturing the behaviour and processes preceding, during and following the rapid drainage of a ∼ 4 km 2 supraglacial lake through 1.1-km-thick ice on the western margin of the Greenland Ice Sheet. Peak discharge of 3300 m 3 s −1 coincident with maximal rates of vertical uplift indicates that surface water accessed the ice-bed interface causing widespread hydraulic separation and enhanced basal motion. The differential motion of four global positioning system (GPS) receivers located around the lake record the opening and closure of the fractures through which the lake drained. We hypothesise that the majority of discharge occurred through a ∼ 3-km-long fracture with a peak width averaged across its wetted length of ∼ 0.4 m. We argue that the fracture's kilometre-scale length allowed rapid discharge to be achieved by combining reasonable water velocities with sub-metre fracture widths. These observations add to the currently limited knowledge of in situ supraglacial lake drainage events, which rapidly deliver large volumes of water to the ice-bed interface.

Changing climate in northern regions is causing permafrost to thaw with major implications for the global mercury (Hg) cycle. We estimated Hg in permafrost regions based on in situ measurements of sediment total mercury (STHg), soil organic carbon (SOC), and the Hg to carbon ratio (RHgC) combined with maps of soil carbon. We measured a median STHg of 43 ± 30 ng Hg g soil−1 and a median RHgC of 1.6 ± 0.9 μg Hg g C−1, consistent with published results of STHg for tundra soils and 11,000 measurements from 4,926 temperate, nonpermafrost sites in North America and Eurasia. We estimate that the Northern Hemisphere permafrost regions contain 1,656 ± 962 Gg Hg, of which 793 ± 461 Gg Hg is frozen in permafrost. Permafrost soils store nearly twice as much Hg as all other soils, the ocean, and the atmosphere combined, and this Hg is vulnerable to release as permafrost thaws over the next century. Existing estimates greatly underestimate Hg in permafrost soils, indicating a need to reevaluate the role of the Arctic regions in the global Hg cycle.

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Amplitude-versus-angle (AVA) analysis of a seismic reflection line, imaged 13km from Russell Glacier terminus, near the western margin of the Greenland ice sheet (GrIS), suggests the presence of sediment at the bed. The analysis was complicated by the lack of identifiable multiples in the data due to a highly irregular and crevassed ice surface, rendering deeper seismic returns noisy. A modified technique for AVA processing of glacial seismic data using forward modelling with primary reflection amplitudes and simulated multiple amplitudes is presented here. Our analysis demonstrates that AVA analysis can be applied to areas with noisy seismic returns and indicates that sediment underlies the seismic study site. Our data are inconsistent with the common assumption that the GrIS is underlain only by hard bedrock, but consistent with the presence of subglacial sediment with porosity between 30% and 40%. As analysis and modelling of ice-sheet dynamics requires a sound knowledge of the underlying basal materials, subglacial sediment should be taken into account when considering ice dynamics in this region of the GrIS.Peer reviewe