Monitoring of Shallow-Water Methane Seeps at Cape Fiolent (Black Sea).

From 2019 to 2021, integrated studies of new shallow-water methane bubble gas seeps were carried out in the coastal zone near Cape Fiolent (Southwest Coast of Crimea). The studies included determination of the hydrocarbon and isotope composition of bubble gas, measurement of the methane and nutrient concentration in the water in the areas of gas seeps, estimation of the value of bubble flows, and measurement of the hydrophysical parameters over the seep sites compared to background areas. A seasonal type of Cape Fiolent methane seeps was noted, and the durations of its active gas emission phases differed in different years. The increased pore water silica concentration at the seep sites and their localization in the vicinity of freshwater slope springs may indicate its association with submarine freshwater discharge in the area. However, no significant desalination of either pore water or the bottom water layer above the seeps was recorded. Dissolved methane concentrations in pore water at seep sites were two orders of magnitude higher compared to the background areas, reaching 448 μmol/L. High values were also obtained for surface water directly above the bubble gas emission points (maximum 353 nmol/L). Multihour monitoring of the hydrophysical parameters above the active seeps showed a decrease in dissolved oxygen compared to the background sites. The maximum difference in O2 concentrations was 3 mg/L. The carbon isotope composition of bubble gas methane δ13C-CH4 (–62.84 to –38.27‰) and carbon dioxide δ13C-CO2 (–16.83 to –10.17‰) corresponded to a mixture of isotopically heavy gas and near-surface isotopically light gas of microbial origin. The question remains open: what are the reasons for the change in the summer active and cold season passive gas emission phases? [ABSTRACT FROM AUTHOR]