Examining the utility of barium isotopes as a tracer of large-scale seafloor methane venting

Periods of increased sedimentary barium (Ba) burial rates have been interpreted as evidence for climate-driven release of methane (CH₄) from sediments in the geological past, yet such interpretations remain ambiguous. Here, we present the first Ba isotope measurements from an active methane seep to directly evaluate whether Ba isotopes in seawater or authigenic carbonates can record Ba release associated with seafloor methane venting. Dissolved Ba concentration ([Ba]) and isotope composition (δ^138/134Ba) were measured in seawater profiles (1 m to 60 m above the seafloor) and in authigenic carbonates collected across the Regab pockmark on the Congo Margin. Seawater over the active methane venting site shows an increase of around 8% in dissolved [Ba] compared to ambient seawater at the Reference Site, indicating detectable benthic Ba release during methane venting. However, the δ^138/134Ba values of these samples (+0.23‰ to +0.42‰) are indistinguishable from ambient seawater, demonstrating that Ba released during methane venting does not impart a resolvable isotopic shift to the water column. An additional isotopically light Ba input may arise from the dissolution of Fesingle bondMn oxyhydroxide phases at the sediment water interface, but this interpretation is based on a single data point and requires further investigation. Authigenic carbonates at sites of high methane flux exhibit markedly lower δ^138/134Ba values (−0.30‰ to +0.20‰) that are markedly lower than seawater, coral carbonates, and published porewater values, indicating a potential Ba isotope fractionation linked to cold-seep carbonate formation. Overall, these findings suggest that Ba isotopes are unlikely to be a reliable tracer of Ba mobilisation associated with methane venting and challenges the interpretation of sedimentary Ba isotope records as indicators of past large-scale methane release.