Radiocarbon and Stable Carbon Isotopes Constrain Rapid Mixing and Deep Meltwater Influence on Dissolved Inorganic Carbon in an Arctic Fjord

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Increased freshwater discharge from Arctic glaciers modifies the carbonate chemistry of coastal fjords. We investigated the carbon isotopic composition (Δ 14 C and δ 13 C) of dissolved inorganic carbon (DIC) in Kongsfjorden, Svalbard, to evaluate how freshwater inputs and water‐mass exchange shape fjord carbon dynamics. DIC concentrations were positively correlated with salinity and Δ 14 C values, consistent with dilution by freshwater inputs associated with aged carbon sources. No significant difference was observed between average Δ 14 C‐DIC values in surface (+14 ± 3‰) and deep waters (+17 ± 2‰) across all stations, suggesting rapid vertical mixing. Although primary production and organic matter remineralization can modify DIC concentration and its isotopic composition, their influence during the sampling period in June accounted for less than 3% of the total DIC pool. The small vertical gradients in carbon isotopes are also consistent with deep‐water mixing, potentially associated with subglacial discharge–driven circulation, which partly offsets surface dilution of DIC. These observations highlight the dominance of rapid mixing in regulating DIC dynamics and isotopic distribution in this Arctic fjord.