Sediment-water CH4 fluxes across Baltic Sea bottom water oxygen gradients

Methane (CH₄) is produced mostly in anoxic sediments through anaerobic degradation of organic matter. Here, we used both ex-situ sediment core and in-situ chamber lander incubations to quantify sediment-water CH₄ fluxes under anoxic to oxic bottom water conditions in the Baltic Sea. Sediments acted as a source of CH₄ into the water column with fluxes up to 13 mmol m^-2 d^-1. Strong spatial variability in sediment-water CH₄ fluxes was observed with highest fluxes in the anoxic Western Gotland basin, followed by the Gulf of Finland and Gulf of Riga, and near-zero fluxes in the oxic Bothnian Bay. Sediment-water CH₄ fluxes were negatively correlated with bottom water oxygen concentration, and positively correlated with sediment organic carbon content.

We incorporated observational data into a physical-biogeochemical model (BALTSEM-CH₄ v1.0) to perform extrapolations. Sediments release 5 - 60 Gg CH₄ yr^-1 to the water column of the Baltic Sea. These large benthic CH₄fluxes are largely counteracted by efficient CH₄ oxidation in the water column (3 - 50 Gg CH₄ yr^-1). Both observations and model results indicate that water column oxidation prevents the high sediment-water CH₄ fluxes from reaching the atmosphere.