Heterogenous Thermochemical Stability of Sedimentary Organic Carbon in High‐Arctic Svalbard Fjords

Abstract The modern carbon cycle in Arctic fjords is being disrupted by Arctic warming as permafrost thaw and glacier retreat intensify. Consequently, the erosion and export of terrestrial organic carbon (OC) to the ocean have accelerated. However, it is still largely unknown how this carbon cycle perturbation impacts the fate of mobilized OC in sediments. Here, we applied the ramped‐temperature pyrolysis oxidation (RPO) technique to investigate the thermochemical decomposition potential of sedimentary OC in Svalbard fjords. The thermochemically labile, moderate, and refractory OCs are classified by specific energy thresholds. Our results show that sedimentary OC in Svalbard fjords exhibits overall high thermochemical heterogeneity. Fractions of refractory OC, as defined thermochemically, increase on a per‐fjord basis from northern to southern fjords and are generally higher near the head of each fjord. The spatial pattern of refractory OC is most likely attributed to the delivery and sorting of petrogenic OC derived from bedrock erosion. By compiling published data sets with our results, we estimate an OC burial rate of 7.3 ± 6.2 × 10 11 gC yr −1 in Svalbard fjords, with 1.2 ± 1.1 × 10 11 gC yr −1 and 2.7 ± 2.4 × 10 11 gC yr −1 attributed to labile and refractory components, respectively. Although a thermochemical moderate fraction still dominates the sediment OC pool, this work highlights that Svalbard fjords also serve as significant reservoirs of both labile and refractory sedimentary OC.