Quantifying in-situ gas hydrates at active seep sites in the eastern Black Sea using pressure coring technique

In the eastern Black Sea, we determined methane (CH4) concentrations, gas hydrate volumes, and their vertical distribution from combined gas and chloride (Cl-) measurements within pressurized sediment cores. The total gas volume collected from the cores corresponded to concentrations of 1.2-1.4 mol CH4 kg(-1) porewater at in-situ pressure, which is equivalent to a gas hydrate saturation of 15-18% of pore volume and amongst the highest values detected in shallow seep sediments. At the central seep site, a high-resolution Cl- profile resolved the upper boundary of gas hydrate occurrence and a continuous layer of hydrates in a sediment column of 120 cm thickness. Including this information, a more precise gas hydrate saturation of 22-24% pore volume could be calculated. This volume was higher in comparison to a saturation calculated from the Cl- profile alone, resulting in only 14.4 %. The likely explanation is an active gas hydrate formation from CH4 gas ebullition. The hydrocarbons at Batumi Seep are of shallow biogenic origin (CH4 > 99.6 %), at Pechori Mound they originate from deeper thermocatalytic processes as indicated by the lower ratios of C-1 to C-2-C-3 and the presence of C-5.