Submarine groundwater discharge and gas hydrate dissociation fuel organic matter formation in Arctic fjord sediments

Arctic fjord sediments store significant amounts of organic matter (OM), contributing to the global carbon cycle, but are increasingly influenced by climate change. This study investigates OM distribution and sources in sediments from 24 stations across three West Spitsbergen fjords (Hornsund, Isfjord, and Kongsfjord-Krossfjord), focusing on relationships with porewater chloride concentrations and stable water isotopes. Freshening of porewater indicated by low chlorinity was detected in Isfjord and Hornsund. In Hornsund and Tempelfjord (Isfjord), water isotope signatures point to a meteoric origin, likely via submarine groundwater discharge (SGD). In contrast, Ekmanfjord (Isfjord) shows isotopic evidence of gas hydrate dissociation as the freshening source. Both Isfjord sites contain OM with low δ¹³C_org < –26.8 ‰) and δ¹⁵N_tot (< 3.1 ‰) values, which are inconsistent with known OM sources in Svalbard fjords. We propose that carbon released from SGD and gas hydrate dissociation, along with methane oxidation and nitrogen assimilation by methanotrophic microbes, contribute to OM formation. These findings suggest that Arctic fjord sediments may help mitigate climate change by supporting microbial processes that consume methane, a potent greenhouse gas.