Gas seepage in the Dnepr paleo-delta area (NW-Black Sea) and its regional impact on the water column methane cycle

Methane concentrations and carbon stable isotopic ratios (δ 13C CH4), oxygen concentrations and hydrographic parameters (CTD) were investigated in the water column of the north-western Black Sea during the summers of 2003 and 2004. Water samples were collected along a transect which crosses three methane seep areas in 90, 220, and 600 m water depth. These active seeps strongly influence methane distribution in the overlying anoxic and oxic water column. Methane concentration and stable isotope (δ 13C CH4) patterns indicate that water column stratification and microbial methane oxidation efficiently hamper the transfer of methane to the sea surface. Only the shelf seep site in 90 m water depth acts as a direct source of atmospheric methane. Microbial methane oxidation and/or gas stripping seem to cause oxygen depletion above the two shallower seep areas. The methane flux from the 90 m site into the water column is estimated to have been 0.599 × 10 6 mol yr − 1 (9.6 t yr − 1 ) in 2003 and 0.347 × 10 6 mol yr − 1 (5.6 t yr − 1 ) in 2004. Comparison of results from the two years shows different water column methane inventories in the deep part of the transect, implying a variable methane source strength at the 600 m deep site. The flux from this area is estimated to have been 11.35 × 10 6 mol yr − 1 in 2003.