Geochemical evidence for nitrogen fixation by methane-oxidizing consortia in gas hydrate-bearing sediments at Håkon Mosby Mud Volcano, SW Barents Sea

Anaerobic oxidation of methane (AOM) occurs in the sediments of all continental margins and acts as an effective biofilter consuming ~ 80 % of upward migrating methane before entering the ocean. It has been previously shown that microbial communities performing AOM are able to fix and share nitrogen, but definitive geochemical evidence for nitrogen fixation in seep-impacted sediments are still missing, limiting our understanding of nitrogen cycling in these peculiar environments and of their contribution to global N2 fixation. Here we report the pore water (sulfate and dissolved inorganic carbon) and sediment geochemistry (XRF core logs, organic carbon and total nitrogen systematics) of two gravity cores collected during CAGE20-3 expedition at Håkon Mosby Mud Volcano, SW Barents Sea. Gravity core 367-GC intercepted a shallow gas hydrate layer 67 cm bsf, whereas core 369-GC was collected in a background area 16 km away from the outer volcanic rim. XRF data of core 367-GC indicated the presence of a carbonate-rich interval above the gas hydrate layer. The isotopic composition (δ13C, δ18O) of two micro-concretions isolated from that interval yielded δ13C of –31.8 ‰ and –31.6 ‰, and δ18O of 6.1 ‰ and 5.8 ‰, indicating the incorporation of methane-derived carbon and heavy oxygen released by the underlying gas hydrates. Foraminiferal tests display δ13C values as low as –18.0 ‰ consistent with secondary overgrowth influenced by AOM. The modern sulfate-methane transition is located ~30 cm above this AOM-impacted interval, suggesting a recent increase in methane flux from the underlying gas hydrate layer. The carbonate-rich interval strikingly matches with depleted δ13C values in organic matter (δ13Corg