High-flux Gas Seepage fuels Shallow Gas Hydrate Deposits in the Eastern Black Sea

In order to quantify gas hydrates and to elucidate their dynamics, we investigated a high-flux seepage site in the anoxic Eastern Black Sea. Pressure and non-pressure near-surface sediment cores, CH4-derived carbonates, ROV-based seafloor images, and gas venting from the seafloor were collected at the Batumi seep area (BSA) in about 845 mbsl. Late glacial to Holocene sediments were recovered with the Dynamic Autoclave Piston Corer (DAPC) and with gravity corers. In gravity cores, hydrates were absent in the uppermost Black Sea Unit 1, but occurred as layers of massive aggregates in deeper sections of Unit 2. In Unit 3, disseminated gas hydrates occurred throughout the entire section recovered. Gas from degassing DAPC cores and from dissociated hydrates as well as vent gas collected with our Gas Bubble Sampler were strongly dominated by CH4 (> 99.9 mol-% of light hydrocarbons, LHC). LHC ratios (C1/[C2 + C3] >1000) and stable isotopic compositions of CH4 (δ13C = -53.5‰; D/H around -175‰) indicated a predominant microbial LHC origin. CH4 in vent gas was virtually devoid of 14C, suggesting that the contribution of CH4 from degradation of fresh organic matter is minimal. Of all gas types collected, vent gas seemed to be least affected by molecular fractionation during sediment migration and hydrate precipitation. Thus, its properties might resemble that of gas in deep reservoirs. LHCs in DAPC cores restricted to top sediments (Units 1 and 2) were characterized by relative CH4 depletion most probably due to the anaerobic oxidation of methane. Gas in DAPC cores additionally comprising Unit 3 material and from dissociated hydrates contained highest CH4 portions due to preferential incorporation in hydrates. X-ray diffraction showed structure I hydrates to prevail at the BSA. Similar crystal sizes of shallow hydrates both at BSA (mean 405 µm) and Hydrate Ridge (412 µm) in contrast to larger grain sizes of deeply buried hydrates at Hydrate Ridge (510 µm) suggest that those from BSA are relatively young in age. Quantitative degassing of DAPC cores yielded volumetric gas/bulk sediment ratios of up to 20.3 indicating substantial hydrate presence. Nominal CH4 concentrations surpassed equilibrium concentrations in Units 2 and 3, resulting in constant hydrate fractions of 6% of pore volume in Unit 2 and mean values of 27% pore volume in Unit 3. About 14.7 kt of hydrate-bound CH4 are estimated to accumulate in sediments down to 265 cm bsf over the entire BSA covering about 0.5 km2. Stable O isotope data and petrology of carbonates suggest that hydrate dissociation accompanied by high gas/fluid discharge in overlying sediments occurs episodically separated by phases of hydrate build-up and reduced fluid flux. From the rough seafloor topography, hydrate crystal grain sizes and carbonate data we conclude that vigorous dissociations and/or upfloating of shallow-buried hydrates are a typical feature of the BSA. This process might constitute an important factor for altering the carbon budget of the Black Sea.