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and of reduced sulfur compounds in surface sediments of the hydrogen
sulfide zone (δ
34
S = -31.8; Table 5), it is evident that the flux of dissolved
sulfide from bottom sediments into the water column does not influence
the balance of sulfur compounds in the Black Sea. The bulk of hydrogen
sulfide of the water column is produced during sulfate reduction in the
water column itself. Hence, the results of the isotopic/geochemical analy-
sis of the Black Sea sulfur compounds confirm previous sulfur balance
estimations [67, 68].
At the end of the section on the isotopic composition of reduced and oxidized
sulfur compounds let us consider other than sulfate reduction mechanisms of
sulfur isotopes fractionation.
Many investigators of the sulfur isotopic composition of pyrite in marine
sediments with oxic near-bottom waters suggested that in the production of
isotopically light pyrite (δ
34
S
pyrite
>
45‰), disproportionation of
elemental sulfur, thiosulfate, and sulfite (derived from the H
2
S oxidation with
bottom water oxygen) is of considerable importance [14]. In the Black Sea,
such conditions exist only in shelf sediments. Sediments at water depths below
180-200 m are permanently anoxic. We assume that the disproportionation
reactions in shelf sediments may occur, but their role in the formation of the
isotopic composition of sedimentary pyrite in the Black Sea anoxic zone are of
minor importance due to the absence of the H
2
S oxidation.
The arrival of terrigenous pyrite in littoral sediments is theoretically possible,
however, detailed microscopical observations have shown that the major portion
of sulfide minerals in sediments are represented by pyrite framboids as well as
pyrite precursors: hydrotroilite, mackinawite, and greigite [98]. Muramoto et
al. [78] performed a quantitative analysis of the flux of pyrite produced in the
upper part of the sulfide water column and in bottom sediments of the deep-sea
basin. The estimated flux (10 mg or 0.32 mmol/m
2
/year) comprised less than
2% of the annual bacterially produced hydrogen sulfide under m
2
34
SO
4
2
−
- δ
of bottom
sediments (16-18 g per year - Table 1).
4. THE ISOTOPIC COMPOSITION OF ORGANIC
CARBON IN THE BLACK SEA WATER COLUMN
The first data on the isotopic composition of carbon of Black Sea plankton
were published by Deuser [24]. He analyzed 10 samples of plankton from the
surface down to 100 m water depth at 7 stations taken during the cruise of R/V
“Atlantis II” in March-April 1969 (Fig. 3). Fry et. al. [36] published the results
on the isotopic composition of POC of 30 samples collected at station 2 during
the cruise of R/V “Knorr” in May 1988 (Fig. 3). Samples were collected from
the surface down to a depth of 240 m. Kodina et al. [58] published the data
