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

Abstract

Robust estimations of methane (CH4) oxidation in marginal seas remain elusive, making CH4 budgets particularly uncertain. Here, we investigate the CH4 benthic source and bottom layer oxidation across the entire Baltic Sea using concentration and stable isotope (δ13C-CH4) profiles along oxygen and salinity gradients. CH4 concentrations were highest near the seafloor under low oxygen conditions. Comparison with previous local-scale studies implies increasing deep water CH4 concentrations in the last decade. High CH4 concentrations and δ13C-CH4 values (−54.4 ± 17.7‰) in bottom waters indicate benthic sources. Oxygen, salinity, and 224Ra (benthic tracer) explained the CH4 distribution. Methane oxidation estimated from δ13C-CH4 fractionation removed 49 ± 33% of benthic-produced CH4 before reaching the surface, leading to small water–air fluxes (10.0 ± 9.2 μmol m−2 d−1). Overall, bottom layer CH4 oxidation was highly effective attenuating CH4 emissions to the atmosphere.