Measuring methane from the seafloor to the atmosphere: an integrated experiment in the Black Sea

Methane is an important greenhouse gas and an energy resource. Methane in sea water can originate from microbially-mediated organic matter (OM) degradation processes at shallow depth  within the sediments, or from thermal cracking of refractory OM at deeper depth. On continental margins, this methane is stored in specific sedimentological bodies or as gas hydrates, or is released at the seafloor as submarine geological seeps followed by its oxidation in the water mass. However, methane released at the seafloor may not entirely be oxidized in the water column and a fraction of it may ultimately reach the atmosphere. The factors that govern the magnitude of methane transfer through the water column to the atmosphere remain poorly known. It has been identified that the amount of methane transferred to the atmosphere is strongly dependent on sites, and the thickness of the water column plays a critical role.

The Black Sea shelf and margin are known to host a large number of strong methane seepages. It has therefore been identified as a perfect candidate to investigate the fate of methane released from the seafloor to the atmosphere. This area can also act as a proxy for investigating the fate of methane in potential scenarios of hydrate destabilization in a changing climate, which can become a societal problem in the future. In the frame of ENVRIplus H2020 project (www.envriplus.eu) we developed a joint pilot experiment to measure methane transfer from the seafloor to the atmosphere, in a pilot study involving European research infrastructures ICOS, Eurofleets, EMSO and ACTRIS. We investigated the influence of depth by mapping CH4 concentration and bubble distribution at two different sites, at 60m and 100m water depth, respectively. The pilot experiment developed joint monitoring strategy for methane detection at various levels starting from the seafloor and moving across the water column, the water/air interface and the atmosphere. An EK80 echosounder was used to identify emission areas through massive bubble plumes. The methodology applied integrates (1) sampling from the geosphere, hydrosphere and atmosphere for laboratory measurements of methane concentration by well-proven standard methods together with δ13CH4 analysis, (2) in situ measurements of methane concentration into the water column and the atmosphere, and (3) the deployment of a seafloor observatory for a short monitoring period (4-5 days) to evaluate the temporal variability of gas fluxes.

During the cruise we found several occurrences of bubble plumes extending near the surface. Our measurements indicate that dissolved methane concentration drastically decreases from the seafloor to the water surface, highlighting its degradation and dispersion along the pathway to the atmosphere. The atmospheric data suggests a consistent input of marine methane to the atmosphere at the shallower site,. Our study highlights the observational challenges both for the measurement of methane from in situ and laboratory methods, and for the estimation of sea surface fluxes.