Large methane oxidation across an oxygen gradient in Baltic Sea deep waters

Abstract Robust estimations of methane (CH 4 ) oxidation in marginal seas remain elusive, making CH 4 budgets particularly uncertain. Here, we investigate the CH 4 benthic source and bottom layer oxidation across the entire Baltic Sea using concentration and stable isotope (δ 13 C‐CH 4 ) profiles along oxygen and salinity gradients. CH 4 concentrations were highest near the seafloor under low oxygen conditions. Comparison with previous local‐scale studies implies increasing deep water CH 4 concentrations in the last decade. High CH 4 concentrations and δ 13 C‐CH 4 values (−54.4 ± 17.7‰) in bottom waters indicate benthic sources. Oxygen, salinity, and 224 Ra (benthic tracer) explained the CH 4 distribution. Methane oxidation estimated from δ 13 C‐CH 4 fractionation removed 49 ± 33% of benthic‐produced CH 4 before reaching the surface, leading to small water–air fluxes (10.0 ± 9.2 μ mol m −2 d −1 ). Overall, bottom layer CH 4 oxidation was highly effective attenuating CH 4 emissions to the atmosphere.