Environmental Engineering Reference
In-Depth Information
the average isotopic composition of organic carbon in deep-sea sediments
(-25.3‰ and -24.4 - -25.7‰, respectively) (Table 12, No. 4-6).
Microbiological, hydrochemical and isotopic data (Table 13) point to the in-
tense organic matter mineralization in shallow sediments of the Black Sea. The
fact that the total alkalinity increases with depth is a direct evidence of ongoing
mineralization processes. The alkalinity increase was maximal in shallow-water
sediments at stations 11 and 16 where the rate sulfate reduction was high and
minimal in deep-sea sediments at station 28 (129 m water depth) (Table 13).
Mineralization of organic matter by sulfate-reducing bacteria in shelf sediments
occurs under anaerobic conditions. It is well known that an excess of calcium
ions as well as alkalinization are caused by the uptake of SO
4
2
−
ion during
sulfate reduction. Both processes shift the carbonate equilibrium toward the
precipitation of the early diagenetic calcium carbonate. A decrease in calcium
concentration along the depth profile in littoral sediments is illustrated by the
example of station 16 (Table 13). Precipitation of early diagenetic carbonates
can be proved by considerable lightening of the isotopic composition of total
carbonates. Precipitation was evident in sediments with high rates of sulfate
reduction (stations 11 and 16). Less isotopically light carbonate minerals were
discovered at the open shelf stations (station 1, Table 13) and in sediments from
the shelf edge (station 28, Table 13).
Active anaerobic decomposition of organic matter accompanied by the chan-
ge in the isotopic composition of DIC was discovered in the water column as
well. According to [25], the isotopic composition of DIC is lighter with depth
in the sulfide zone: -4.0‰ at 500 m, - 5.5‰ at 1000 m, - 6.5‰ at 1500 m,
and -6.9‰ in the near-bottom water at 2200 m. Since δ
13
C values change
simultaneously with an increase in the H
2
S content, it is obvious that both
processes result from activity of sulfate reducing bacteria.
While microorganisms play the main role in the distribution and redistrib-
ution of stable sulfur isotopes in the Black Sea, the isotopic composition of
organic carbon depends on the balance of allochthonous organic matter dis-
charged by rivers and autochthonous organic matter synthesized by plankton.
In spring, an increase in the inflow of terrigenic material leads to lighter car-
bon isotopic composition of POC in the water column and in upper layers of
shallow sediments. It becomes heavier during the periods of intensive blooms
of phytoplankton in early spring and summertime.
The average δ
13
C of organic carbon in the central part of the Black Sea is
-25.0‰ (Table 12, No. 5 and 6). This value is about the same as the isotopic
composition of POC of the deep-water zone (-25.3‰) (Fig. 5). The relative
content of hydrocarbons of different origin determine the carbon isotope com-
position of total POC [96]. It has been found that organic matter in upper
deep-sea sediments consists of 56% of hydrocarbons of the terrigenous origin
