FRACTIONATION OF STABLE ISOTOPES OF CARBON AND SULFUR DURING BIOLOGICAL PROCESSES IN THE BLACK SEA - Past and Present Water Column Anoxia - page 363

of the Black Sea during project EROS 21 [63]. Four large rivers (Danube,
Dniper, Dniestr, and Bug) affected the region of our investigations. Therefore
samples were collected during flood period in spring (April - May) and in mid-
summer (July - August). We analyzed samples having the isotopic composition
of particulate organic carbon (POC) of the terrigenous and planktonic origin
[50, 70]Furthermore, we studied the influence of autotrophic microorganisms
located at the upper sulfide boundary on the carbon isotopic composition of
POC in the central Black Sea [50].
The third task of the review is to discuss the effects of the stable carbon
isotope fractionation during microbial processes of organic matter consump-
tion. During the “Atlantis II” cruise (March-April, 1969), depletion in 13 Cof
dissolved inorganic carbon (DIC) with depth in the water column of the Black
Sea was discovered [25].
During last 25 years three important discoveries were made which stimulated
interest in the origin of methane and its cycle in the Black Sea. In 1988, a
group of Ukrainian scientists discovered numerous methane jets from bottom
sediments in the western Black Sea - cold methane seeps [83]. Two years later it
was proved that the intense microbial oxidation of methane with the formation
of large (up to 3-4 m high) carbonate constructions occurs at the outlets of
methane seeps under strictly anaerobic conditions [48]. Around this time the
process of anaerobic oxidation of methane was discovered in the water column
of the Western gyre in the Black Sea [84]. Finally, methane gas hydrates were
found in deep-sea sediments of the Black Sea [33, 60, 61].
The isotopic composition of methane carbon and hydrogen is one of the
characteristics of the low-temperature (microbial) vs. high-temperature genesis
of methane [99]. One of the tasks of the review is to summarize the data on the
isotopic composition of methane carbon in the Black Sea.
2. FRACTIONATION OF STABLE SULFUR
ISOTOPES DURING SULFATE REDUCTION IN
THE BLACK SEA WATER COLUMN
Prior to large-scale investigations of the isotopic composition of Black Sea
sulfur compounds [36, 75, 93, 94, 98] and our investigations of sulfate reduction
rates using 35 S [46, 65-68], most researchers believed that most part of dissolved
sulfide is not produced in the water column, but diffuses from bottom sediments
[62, 90].
Sorokin, who was the first to use Na 35 SO 4 to determine sulfate reduction rate
(SRR) in the water column and bottom sediments of the Black Sea expressed
the same point of view [91]. Sorokin's erroneous assumption resulted from
the use of Na 35 SO 4 with insufficiently high specific activity. He was able to
measure SRR only in the samples with active sulfate reduction: in the surface
sediments, in the upper layer of the H 2 S zone, and in the near-bottom water.