Stable carbon and hydrogen isotope measurements on Black Sea water-column methane

We report measurements of δ13C-CH4 and δ2H-CH4 at a central station in the Black Sea. We considered the Black Sea as a “biogeochemical bucket” and the single station as a basin-wide integrator of processes affecting methane. Considering the rapid (3.6–18 yr) turnover of methane and the similarity of these stable isotope distributions to the methane concentration and oxidation rate profiles [Reeburgh, Ward, Whalen, Sandbeck, Kilpatrick, Kerkhof, 1991. Black Sea methane geochemistry. Deep-Sea Research 38, S1189–S1210], it appears that methane is being added approximately as fast as it is being oxidized. Methane can be thought of as “running in place” in the Black Sea water column.Recent reports of extensive vents on the northern side of the Black Sea suggest that they might be a methane source capable of effectively balancing the Black Sea methane budget. Unfortunately, we have limited information on basin-wide seep fluxes and cannot identify them with stable isotope measurements. Methane oxidation (and accompanying isotope fractionation) is so extensive that the water-column stable isotope measurements provide little information on methane sources. Future measurements of 14C-CH4 should permit partitioning Black Sea methane sources into fossil and recent components.