Marine methane fluxes to the atmosphere in the Western Black Sea: comparing eddy covariance and diffusive fluxes

The global ocean is a net source of CH₄ to the atmosphere. Large uncertainties remain on marine emissions that deserves effort to improve current estimates, and eventually predict their trajectories in a changing climate. Ocean CH₄ emissions can either be CH₄ emanating from seafloor sediments or in situ production in surface water linked to primary productivity. Sediment input into the water column can be either CH₄ emanating from hydrate dissociation or free gas rising through the sediment. Ultimately, CH₄ enters the atmosphere across the sea-air interface either from bubbles rising from the seafloor or by diffusion from dissolved gas. Estimates of global marine emissions diverge widely due to very large uncertainties linked to limited data coverage, 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 characterized by widespread seafloor CH₄ emissions from the shallow coast to the deep basin. The evolution of the anoxic properties of the BS is strongly linked to the amount of CH₄ discharged and the supply of organic matter from the connected large rivers. Therefore, it is crucial to estimate the BS CH₄ budget and understand the transfer mechanism to the atmosphere to better understand the impact of climate change. 
During the GHASS2 (Gas Hydrates, fluid Activities and Sediment deformations in the black Sea) cruise in September 2021, CH₄ transfer to the atmosphere has been investigated at water depths ranging from 60 m to 1200m in the Western sector of the BS. 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 model G2401). We report eddy covariance measurements using  an open-path CH₄ analyzer Li-7700 and a H2O-CO2 analyzer 7200RS from LiCor, a Gill 3D sonic anemometer, and an inertial navigation sensor (Lord). 
We compare flux estimates obtained from partial pressure gradient by the diffusive method under various schemes with the experimental eddy covariance set-up, applying available corrections for ship movement and interference with airflow. We also compare our results with previous reports for the area and conclude on the respective challenges and relative basin-scale representativity of the various measurement techniques.