Mantle origin of methane in the Black Sea

Abstract

A new distribution map of gas seeps and mud volcanoes has been compiled of the Black Sea. It has been derived from the published coordinates for ca. 5000 gas seeps and 80 mud volcanoes. An analysis of the stable isotopic composition of methane has been performed for authigenic carbonates and sediments. The δ13C values of carbonates and sediments form 2 distinct tight groups depending of the geological environments. The diagrams of values of δ13C vs. δD and δ13C vs. C1/(C2 + C3) have been applied to assess earlier classifications of methane. The origin of methane from most of samples has turned out to be uncertainly determined. Based on seismic reflection data, the feeder channels of gas releases and mud volcanoes penetrate to the Pre-Mesozoic basement beneath the Polshkov High, Andrusov and Tetyaev Ridges and Sorokin Trough at a depth of up to 12 km. In the Central Black Sea the feeder channel of mud volcano reaches the mantle surface where a depth is 19 km. The highest concentration of gas seeps distribution is observed at the triple junction of the mantle faults in the NW Black Sea. Assessments of different mechanisms have been made for their ability to produce an unprecedented methane concentration in the Black Sea water column. There occurs thermogenic methane in the sediments from gas seepage and mud volcanoes areas. The thermogenic methane results from post genetic alternation of biogenic methane of the Paleogene—Neogene sediments. Biological methane produced from organic matter plays negligibly small role in accumulating the world’s largest quantities of anaerobic methane in the Black Sea. Nonorganic methane is most likely to form the tremendous reservoir of dissolved gas in the Black Sea water column below 150—200 m. The seeming scarcity of abi-otic methane is accounted for its recycling by microbial activity, misclassificating origin and producing by chemical syntheses, with its biological methane δ13C values.