Methane-consuming microbes inhabiting marine methane seeps have recently been found to have the capacity to assimilate inorganic nitrogen, suggesting a previously unaccounted role in the global nitrogen cycle. Despite ex-situ experimental observations, definitive evidence of this process under in-situ conditions remains elusive, hindering the complete understanding of the controlling factors and magnitude of this process. We present the isotopic variations of organic carbon δ13Corg and total nitrogen δ15N values in two sediment cores collected from the gas hydrate-bearing Håkon Mosby Mud Volcano, SW Barents Sea (72°N, ∼1260 m water depth). We identified a stratigraphic interval containing methane-derived carbonates directly overlying a gas hydrate layer at 67 cm and typified by δ13Corg and δ15N as low as −42.0‰ and 1.2‰, respectively. Stable isotope mixing models confirm in-situ nitrogen uptake by methanotrophic consortia, contributing to up to 49.1 wt% of the local bulk sedimentary organic matter – a finding calling for reevaluation of the role of methane seeps in the oceanic nitrogen cycle.
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