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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 highresolution 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 C1 to C2-C3 and the presence of C5. [ABSTRACT FROM AUTHOR]

A steady state box model was developed to estimate the methane input into the Black Sea water column at various water depths. Our model results reveal a total input of methane of 4.7 Tg yr-1. The model predicts that the input of methane is largest at water depths between 600 and 700 m (7% of the total input), suggesting that the dissociation of methane gas hydrates at water depths equivalent to their upper stability limit may represent an important source of methane into the water column. In addition we discuss the effects of massive short-term methane inputs (e.g. through eruptions of deep-water mud volcanoes or submarine landslides at intermediate water depths) on the water column methane distribution and the resulting methane emission to the atmosphere. Our non-steady state simulations predict that these inputs will be effectively buffered by intense microbial methane consumption and that the upward flux of methane is strongly hampered by the pronounced density stratification of the Black Sea water column. For instance, an assumed input of methane of 179 Tg CH4 d-1 (equivalent to the amount of methane released by 1000 mud volcano eruptions) at a water depth of 700 m will only marginally influence the sea/air methane flux increasing it by only 3%. [ABSTRACT FROM AUTHOR]

A steady state box model was developed to estimate the methane input into the Black Sea water column at various water depths. Our model results reveal a total input of methane of 4.7 Tg yr-1. The model predicts that the input of methane is largest at water depths between 600 and 700m (7% of the total input), suggesting that the dissociation of methane gas hydrates at water depths equivalent to their upper stability limit may represent an important source of methane into the water column. In addition we discuss the effects of massive short-term methane inputs (e.g. through eruptions of deep-water mud volcanoes or submarine landslides at intermediate water depths) on the water column methane distribution and the resulting methane emission to the atmosphere. Our non-steady state simulations predict that these inputs will be effectively buffered by intense microbial methane consumption and that the upward flux of methane is strongly hampered by the pronounced density stratification of the Black Sea water column. For instance, an assumed input of methane of 179 TgCH4 d-1 (equivalent to the amount of methane released by 1000 mud volcano eruptions) at a water depth of 700m will only marginally influence the sea/air methane flux increasing it by only 3%. [ABSTRACT FROM AUTHOR]

Abstract: Four seep sites located within an ∼20km2 area offshore Georgia (Batumi seep area, Pechori Mound, Iberia Mound, and Colkheti Seep) show characteristic differences with respect to element concentrations, and oxygen, hydrogen, strontium, and chlorine isotope signatures in pore waters, as well as impregnation of sediments with petroleum and hydrocarbon potential. All seep sites have active gas seepage, near surface authigenic carbonates and gas hydrates. Cokheti Seep, Iberia Mound, and Pechori Mound are characterized by oil-stained sediments and gas seepage decoupled from deep fluid advection and bottom water intrusion induced by gas bubble release. Pechori Mound is further characterized by deep fluid advection of lower salinity pore fluids. The Pechori Mound pore fluids are altered by mineral/water reactions at elevated temperatures (between 60 and 110°C) indicated by heavier oxygen and lighter chlorine isotope values, distinct Li and B enrichment, and K depletion. Strontium isotope ratios indicate that fluids originate from late Oligocene strata. This finding is supported by the occurrence of hydrocarbon impregnations within the sediments. Furthermore, light hydrocarbons and high molecular weight impregnates indicate a predominant thermogenic origin for the gas and oil at Pechori Mound, Iberia Mound, and Colkheti Seep. C15+ hydrocarbons at the oil seeps are allochtonous, whereas those at the Batumi seep area are autochthonous. The presence of oleanane, an angiosperm biomarker, suggests that the hydrocarbon source rocks belong to the Maikopian Formation. In summary, all investigated seep sites show a high hydrocarbon potential and hydrocarbons of Iberia Mound, Colkheti Seep, and Pechori Mound are predominantly of thermogenic origin. However, only at the latter seep site advection of deep pore fluids is indicated. [Copyright &y& Elsevier]