| Summary: | International audience Climate change is rapidly altering northern watersheds by disrupting large amounts of water and materials stored in glaciers, snow cover and permafrost. Permafrost thaw enhances the connectivity between surface and deeper water pathways and increases the suprapermafrost groundwater (SPGW) contribution to Arctic river discharge. SPGW, which can originate from permafrost thaw or infiltrated rain waters, participates to the transfer of water and carbon in Arctic catchments. However, the coupling of the water and carbon cycles in SPGW are not quantified, while it can modulate the permafrost climate feedback. Here, we investigate the origin and water transit time of SPGW and quantify and characterize dissolved organic matter (DOM) in the Zackenberg valley (Northeastern Greenland). We aim to quantify SPGW fluxes contribution to river water and carbon exports. In August 2021, we collected water samples from the glacio-nival Zackenberg river, its tributaries and a small nival headwater stream (Graense). SPGW was sampled from wells along transects from soils to river channels. Water samples were analysed for water isotopes (2H, 18O), dissolved gases (222-Rn, SF6, CFCs), dissolved organic carbon (DOC) concentration, radiocarbon ages (Δ 14 C), and DOM optical properties. A significant contribution of young-SPGW to rivers is estimated from dissolved gases. Contrasted DOC concentrations, DOM properties and Δ 14 C are observed between SPGW, hyporheic and river waters, with consistently higher concentrations and older DOC in subsurface flows. DOM optical properties evolve significantly from soils to rivers and along stream channels. Hyporheic waters displayed an enrichment in proteinaceous organic compounds illustrating their role as hotspots for microbial activity where intense DOM processing occurs. This dataset provides unique insights into SPGW role in hydrological and biogeochemical cycles in Arctic permafrost ecosystems.
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