This study describes the chemical composition and fluxes of two ~2000 m3/s glacial floods which emerged from the IcelandicMýrdalsjökull and Vatnajökull glaciers into the Múlakvísl and Kaldakvísl rivers in July 2011.Water samples collected during both floods had neutral to alkaline pH and conductivity from 100 to 900 μS/cm. The total dissolved inorganic carbon (DIC), present mostly as HCO3 −, was ~9 mmol/kg during the flood peak in the Múlakvísl but stabilized at around 1 mmol/kg; a similar

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... iour was observed in the Kaldakvísl. Up to 1.5 μmol/kg of H2S was detected. Concentrations of most of the dissolved constituents in the flood waters were comparable to those commonly observed in these rivers. In contrast, the particulate suspended material concentration increased dramatically during the floods and dominated chemical transport during these events. Waters were supersaturated with respect to a number of clays, zeolites, carbonates, and Fe hydroxides. The most soluble elements were Na, Ca, K, Sr, Mn, and Mg, whereas the least soluble were Ti, Al, and REE. This is consistent with the compositions of typical surface waters in basaltic terrains and the compositions of global rivers in general. The toxic metal concentrations were below drinking water limits, suggesting that there was no detrimental effect of flood waters chemistry on the environment. Increased concentration of DOC, formate, and acetate in the flood waters suggests substantial subglacialmicrobiological activity in the melt water prior to the floods. Reaction path modelling of the flood water chemical evolution suggests that it experienced subglacial water–rock interaction for at least a year in the presence of limited amounts of acid gases (e.g. SO2, HCl and HF). This suggests that the heat source for glaciermeltingwas geothermal rather than volcanic.