Englacial and basal temperature data for the Greenland ice sheet (GrIS) are sparse and mostly limited to deep interior sites and ice streams, providing an incomplete representation of the thermal state of ice within the ablation zone. Here we present 11 temperature profiles at five sites along a 34 km east-west transect of West Greenland. These profiles depict ice temperatures along a flowline and local temperature variations between closely spaced boreholes. A temperate basal layer is present in all profiles, increasing in thickness in the flow direction, where it expands from �3% of ice height furthest inland to 100% at the margin. Temperate thickness growth is inconsistent with modeled heat contributions from strain heating, heat conduction, and vertical extension of the temperate layer. We suggest that basal crevassing, facilitated by water pressures at or near ice overburden pressure, is responsible for the large temperate ice thicknesses observed. High-temperature kinks at 51-85 m depth are likely remnants from the thermal influence of partially water-filled crevasses up ice sheet. Steep horizontal temperature gradients between closely grouped boreholes suggest the recent thermal influence of a moulin. These profiles demonstrate the ability of meltwater to rapidly alter ice temperatures at all depths within the ablation zone.

Abstract. In order to examine daily cycles in meltwater routing and storage in the Isunnguata Sermia outlet of the Greenland Ice Sheet, variations in outlet stream discharge and in major element hydrochemistry were assessed over a 6-day period in July 2013. Over 4 days, discharge was assessed from hourly photography of the outlet from multiple vantages, including where midstream naled ice provided a natural gauge. pH, electrical conductivity, suspended sediment, and major element and anion chemistry were measured in samples of stream water collected every 3 h.Photography and stream observations reveal that although river width and stage have only slight diurnal variation, there are large diurnal changes in discharge shown by the doubling in width of what we term the "active channel", which is characterized by large standing waves and fast flow. The concentration of dissolved solutes follows a sinusoidal diurnal cycle, except for large and variable increases in dissolved solutes during the stream's waning flow. Solute concentrations vary by ∼ 30 % between diurnal minima and maxima. Discharge maxima and minima lag temperature and surface melt by 3-7 h; diurnal solute concentration minima and maxima lag discharge by 3-6 h.This phase shift between discharge and solute concentration suggests that during high flow, water is either encountering more rock material or is stored in longer contact with rock material. We suggest that expansion of a distributed subglacial hydrologic network into seldom accessed regions during high flow could account for these phenomena, and for a spike of partial silicate reaction products during waning flow, which itself suggests a pressure threshold-triggered release of stored water.

Abstract. In order to examine daily cycles in meltwater routing and storage in the Isunnuguata Sermia outlet of the Greenland Ice Sheet, variation in outlet stream discharge and in major element hydrochemistry were assessed over a six day period in July, 2013. Discharge was assessed from hourly photography of the outlet from multiple vantages, including where mid-stream naled ice provided a natural gauge. pH, electrical conductivity, suspended sediment, and alkalinity were measured in samples of stream water collected every three hours. Element and ion concentrations were subsequently measured in a laboratory setting. Photography and stream observations reveal that although river width and stage have only slight diurnal variation, there are large changes in discharge shown in the portion of the width characterized by standing waves and fast flow. Width of this active channel approximately doubles over a diurnal cycle. Together with changes in flow over the naled, these features allow an observationally based relative record of stream discharge in this unconstrained alluvial setting. Peaks in discharge were offset by 3–7 hours from peak melt of the interior ice surface. Concentration of dissolved solutes follows a sinusoidal diurnal cycle, except for large and variable increases in dissolved solutes during the stream’s waning flow. Diurnal changes in solute concentration average 31 % of the base value. Diurnal solute concentration minima and maxima lag peak and minimum stream discharge by 3–6 hours. This phase shift between discharge and solute concentration suggests that during high flow, water is either encountering more rock material or is stored in longer contact with rock material. We suggest that expansion of a distributed subglacial hydrologic network into seldom accessed regions during high flow could account for these phenomena, and for a spike of partial silicate reaction products during waning flow, which itself suggests a pressure threshold-triggered release of stored water.