| Summary: | Arctic glaciers are predicted to lose mass rapidly in the next century, leading to the release of organic matter to downstream ecosystems, crucially including nutrients and pollutants. The aim of this study was to collect a comprehensive record of organic carbon (OC) fluxes on a glacier of well-known glaciological characteristics, providing a benchmark example of the biogeochemical impact of glacial melt. Foxfonna, a small Svalbard glacier with net mass losses of -0.74 ± 0.10 m w.e. per annum in the period 1990–2012, represents a rapid melt scenario. The supraglacial OC fluxes associated with atmospheric deposition, glacier ice melt, runoff and biological activity, were quantified in a supraglacial catchment (1.3 km2) during two balance years (2011–2012) on Foxfonna. The three fluxes related to physical processes ranged from 0.4–0.7 Mg OC per annum and majorly exceeded the biological growth flux. Collectively, these fluxes contributed to a net annual OC storage change of +0.65 ± 0.12 Mg OC per annum (0.37 ± 0.11 Mg per annum particulate OC), implying retention. This flux would account for the entire 1.14 ± 0.23 Mg OC store found in supraglacial debris (cryoconite) in c. 3 years. On the glacier surface, hydrological processes were found to arrange the timing of the release of different organic matter types, liberating the dissolved OC (DOC) from snowpack during meltwater percolation, followed by cells at the time of snowpack saturation with meltwater. Both these organic matter types enriched the OC content of superimposed ice, delaying their ultimate release from the glacier. The particulate OC remaining on the glacier surface in the end of the ablation season can contribute to surface darkening and further melt. This work has shown that physical processes will govern the timing and intensity of the release of various OC types from future shrinking glaciers in multiple ways.
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