Preliminary results of marine methane flux measurement to the atmosphere from the Western Black Sea

The global ocean is a net source of CH₄ to the atmosphere. Among the natural processes, marine emissions are significant contributors with large uncertainties that deserves effort to improve current estimates, and eventually predict their trajectories in a changing climate. Oceanic CH₄ emissions to the atmosphere can either be transported from seafloor or in situ produced in surface waters. Seafloor emissions include both CH₄ emanating from CH₄ hydrate degradation and from free gas in the sediment. Ultimately, CH₄ enters the atmosphere across the sea-air interface either from bubbles rising from the seafloor or by diffusion of dissolved gas. Estimates of global marine emissions diverge widely due to very large uncertainties linked to limited data coverage, seasonal and methodological differences and the difficulty to capture the environmental factors that lead to high variability of the emissions.

As the world’s largest natural anoxic waterbody, the semi-enclosed Black Sea (BS) is very sensitive to human and climate perturbations. It is characterized by widespread seafloor CH₄ emissions from the shallow coast to the deep basin. One of the major issues that arises on the BS methane dynamics is the determine to what extent and in which quantity part of the urge amount of dissolved methane stored in the anoxic bottom water layer is transferred to the atmosphere.

During the GHASS2 (Gas Hydrates, fluid Activities and Sediment deformations in the black Sea) cruise in September 2021, CH₄transfer to the atmosphere has been investigated in the Western sector of the BS at sites with water depth ranging from 60 m to 1200m. CH₄ partial pressures were measured in the surface water and in the atmosphere using optical spectrometers, respectively the SubOcean membrane inlet laser spectrometer (Grilli et al., 2021, https://doi.org/10.3389/feart.2021.626372) and an ICOS-calibrated commercial analyzer (Picarro G2401). We have also developed an open-path setup dedicated to shipborne measurement composed by an open-path CH₄ analyzer Li-7700, a H₂O-CO₂ analyzer 7200RS from LiCor, a Gill 3D sonic anemometer, and an inertial navigation sensor (Lord).  An inox structure was specifically designed to protrude by 1m the front mast of the R/V Pourquoi Pas? to install the open-path sensor.

We present preliminary flux estimates comparison obtained from partial pressure gradient by the diffusive method with the experimental eddy covariance set-up. We also discuss our preliminary results in comparison with previous reports for the area and conclude on the respective challenges and relative basin-scale representativity of the various measurement techniques.