Fractionation of stable isotopes of carbon and sulfur during biological processes in the Black Sea

The paper presents literature and authors' own data on the isotopic composition of sulfur and carbon compounds in the water column and bottom sediments of the Black Sea. The fractionation factors of stable isotopes have been compared with the rates of sulfate reduction, photo- and chemosynthesis, methanogenesis, and anaerobic oxidation of methane, In the water column and bottom sediments, the inverse relationship between S-32 and S-34 fractionation and sulfate reduction rate (measured in situ with the use of (NaSO4)-S-35) Was observed. The isotopic composition of hydrogen sulfide in the water column (delta S-34 = -40.0 parts per thousand) differs greatly from delta S-34 of the reduced sulfur compounds of bottom sediments; this confirms the hypothesis that H,S forms in the water column itself. Seasonal dynamics of delta C-13 of phytoplankton-produced organic carbon was revealed; it was demonstrated that some changes in the isotopic composition of POC occur in the chemocline as a result of the photo- and chemosynthetic activity of microorganisms. The data on the isotopic composition of the three main sources of the Black Sea methane are presented. delta C-13 of the biogenic methane produced in bottom sediments reaches -67.6 parts per thousand; delta C-13 of methane from cold methane seeps reaches -65.8 parts per thousand; delta C-13 of methane from mud volcanoes ranges from -30.0 parts per thousand to -75.0 parts per thousand. The large-scale process of microbial oxidation of methane results in the production of methane-derived carbonates (delta C-13 values range from -27.2 parts per thousand to -45.6 parts per thousand). Using the data on the rates of methanogenesis and anaerobic oxidation of methane as well as the data on the isotopic composition of methane, the balance between the methane flux into the water column and its oxidation has been calculated. It was found that the annual methane production and oxidation in the anoxic zone of the Black Sea are 62.9 and 77.7 center dot 10(10) mol m(-2), respectively. About 80% of methane production is concentrated in the water column and 20% of methane is produced in mud volcanoes and cold seep areas (10% each).