Combined diurnal variations of discharge and hydrochemistry of the Isunnguata Sermia outlet of the Greenland Ice Sheet give in sight on sub glacial conditions

Abstract

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.