Abstract
Abstract. We document the density and hydrologic properties of bare, ablating ice in a mid-elevation (1215 m a.s.l.) supraglacial internally drained catchment in the Kangerlussuaq sector of the western Greenland ice sheet. We find low-density (0.43–0.91 g cm−3, μ = 0.69 g cm−3) ice to at least 1.1 m depth below the ice sheet surface. This near-surface, low-density ice consists of alternating layers of water-saturated, porous ice and clear solid ice lenses, overlain by a thin (< 0.5 m), even lower density (0.33–0.56 g cm−3, μ = 0.45 g cm−3) unsaturated weathering crust. Ice density data from 10 shallow (0.9–1.1 m) ice cores along an 800 m transect suggest an average 14–18 cm of specific meltwater storage within this low-density ice. Water saturation of this ice is confirmed through measurable water levels (1–29 cm above hole bottoms, μ = 10 cm) in 84 % of cryoconite holes and rapid refilling of 83 % of 1 m drilled holes sampled along the transect. These findings are consistent with descriptions of shallow, depth-limited aquifers on the weathered surface of glaciers worldwide and confirm the potential for substantial transient meltwater storage within porous low-density ice on the Greenland ice sheet ablation zone surface. A conservative estimate for the ∼ 63 km2 supraglacial catchment yields 0.009–0.012 km3 of liquid meltwater storage in near-surface, porous ice. Further work is required to determine if these findings are representative of broader areas of the Greenland ice sheet ablation zone, and to assess the implications for sub-seasonal mass balance processes, surface lowering observations from airborne and satellite altimetry, and supraglacial runoff processes.
Highlights
Each summer a vast hydrologic network of lakes and rivers forms on the surface of the western Greenland ice sheet ablation zone in response to surface melting (Chu, 2014; Smith et al, 2015)
This is much lower than typical glacier ice densities of 0.83– 0.90 g cm−3 (Cuffey and Paterson, 2010), but is consistent with previous findings of ice densities < 0.50 g cm−3 in the upper few tens of centimetres of weathering crust (Müller and Keeler, 1969; Schuster, 2001)
Free draining liquid water was not observed in the extracted ice samples and there was no subsurface water table observed within this upper weathering crust layer, for example when penetrated with the depth probe or when the material was removed with the shovel
Summary
Each summer a vast hydrologic network of lakes and rivers forms on the surface of the western Greenland ice sheet ablation zone in response to surface melting (Chu, 2014; Smith et al, 2015). The assumption of efficient meltwater delivery is reflected in regional climate and surface mass balance models for Greenland that instantaneously credit ablation zone surface runoff to the ocean with no physical representation of hydrologic processes taking place on the bare ice surface (Smith et al, 2017). Hydrologic processes in the weathering crust are similar to those of meltwater transport, storage, and refreezing in snow and firn (Cox et al, 2015; Forster et al, 2014; Harper et al, 2012; Machguth et al, 2016). The presence of weathering crust in Greenland, has gone largely undocumented, and little is known about its effect on hydrologic processes in the bare ice ablation zone, where > 85 % of ice sheet surface meltwater runoff is generated (Machguth et al, 2016)
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