Observation of microbial carbonate build-ups growing at methane seeps near the upper boundary of the gas-hydrate stability zone in the Black Sea

Extensive dredge sampling carried out in May-June 2004 in the deeper part of the Dnepr paleo-delta area (NW Black Sea) yielded for the first time chimney-shaped carbonate microbial build-ups, which occur at methane seeps close to upper boundary of the gas-hydrate stability zone (~ 700 m). Carbonate samples taken with a benthic trawl represent fragments of the uppermost, middle and lowest parts of the build-up; they are similar morphologically to those found previously at the shallower and deeper methane seeps in the Black Sea. At the same time, the perforated, plate-like carbonates in the lowest parts of the build-up provide first indications that gas channels are formed during the earliest growth phase of these microbial structures. Stable carbon isotope analyses of the carbonates from the uppermost fragments gave the 5I3C values ranging from -33.7 to -36.6 %o, while the 813C values of the lowest fragments are significantly lighter, varying between -42.0 and -44.6 %o. Oxygen isotopic values also show differences between the samples from the uppermost part of the build-ups, which are composed of a mixture of aragonite and Mg-calcite (5180 = 0.7 to 0.94 %o), and the only Mg-calcite cemented thin slabs of lowest carbonates (5180 = 1.35 to 1.57 96o). The isotope data for carbon and oxygen suggests that carbonates formed as a result of anaerobic microbiological oxi¬dation of methane supplied as a shallower-sourced fluid component from below. The difference in 513C and 5I80 values found in the upper and lowest parts of the build-ups may indicate that more carbon derived from seawater and less hydrate water are involved to the chimney formation during its growth, but this may be also a record of the long-term changes in the near-bottom environments related to evolution of salinity, temperature and anoxic conditions in the Black Sea