The age of microbial carbonate structures grown at methane seeps in the Black Sea with an implication of dating of the seeping methane

The 14C-dating of microbial carbonate structures growing at methane seeps in anoxic waters of the Black Sea has shown a gradual increase with depth of the age of carbonates of these buildups. Comparing the radiocarbon age of the base and middle parts of the microbial structures gives an approximate time of origin of the deepest and shallowest microbial buildups as about 5300 and 2900 years before present, respectively. These dates correspond to the first appearance of hydrogen sulfide in the deepest Black Sea waters and to the stabilization of the upper boundary of the anoxic zone around the present-day level. Stable carbon isotope analyses were used to evaluate contribution of carbon from methane and from the seawater bicarbonates to carbonate material of the microbial structures. The results have shown that 48–60% of carbon in the carbonates is derived from the seeping methane. The given percentage allows calculation of the age of methane, which is assessed to be in the range of about 8500–10,600 years before present for methane seeping from the shallowest microbial structures. These time limits correspond to the last glacial period, when the Black Sea was a freshwater lake. Accordingly, the age of methane emanating from the deepest microbial structures may be assessed as 29,200–36,500 years, suggesting an older sedimentary source of methane seeping from the abyssal Black Sea sediments.