FRACTIONATION OF STABLE ISOTOPES OF CARBON AND SULFUR DURING BIOLOGICAL PROCESSES IN THE BLACK SEA - Past and Present Water Column Anoxia - page 361

FRACTIONATION OF STABLE ISOTOPES
OF CARBON AND SULFUR DURING
BIOLOGICAL PROCESSES IN THE BLACK SEA

Mikhail V. Ivanov 1 and Alla Yu. Lein 2
1 Winogradsky Institute of Microbiology RAS, Pr. 60-letija Oktaybrya 7/2, 117312 Moscow,
Russia
2 Shirshov Institute of Oceanology RAS, 37 Nakhimovsky prosp., 117997 Moscow, Russia

Abstract 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 32 S and 34 S fractionation and sulfate reduction
rate (measured in situ with the use of Na 35 SO 4 ) was observed. The isotopic
composition of hydrogen sulfide in the water column (δ

34 S = -40.0‰) differs

greatly from δ

34 S of the reduced sulfur compounds of bottom sediments; this
confirms the hypothesis that H 2 S forms in the water column itself. Seasonal
dynamics of δ

13 C 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 mi-
croorganisms. The data on the isotopic composition of the three main sources of
the Black Sea methane are presented. δ

13 C of the biogenic methane produced in

bottom sediments reaches -67.6‰; δ

13 C of methane from cold methane seeps

13 C of methane from mud volcanoes ranges from -30.0‰ to
-75.0‰. The large-scale process of microbial oxidation of methane results in
the production of methane-derived carbonates (δ

reaches -65.8‰; δ

13 C values range from -27.2‰
to -45.6‰). Using the data on the rates of methanogenesis and anaerobic oxida-
tion 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 77.7·10 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).

34 S and

Keywords: Black Sea, water column, bottom sediments, isotopic composition (δ

13 C), hydrogen sulfide, sedimentary sulfides, particulate organic carbon (POC),

δ

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