Environmental Engineering Reference
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the predominant alkalinity forms and serve as a buffer system determining pH.
Other alkalinities such as ammonium, silicate, and phosphate alkalinities are
formed in response to pH conditions. Depending on the total H 2 S concentra-
tion, the role of sulfide alkalinity in the total alkalinity increases from 2% in the
Cariaco Trench and 7% in the Black Sea to 24% in the Framvaren Fjord. The
total alkalinity increase
A T with depth in the Black Sea is supported by 27%
increase in the sulfide component (
A S ). Overall the total alkalinity increase
in the anoxic zone of the Black Sea (
A T ) is explained by 99.6% increase in
the carbonate (
A C ) and sulfide (
A S ) alkalinities.
C T /S T ratio of 2.09-2.17 and C T = 3240 + 2.13S T calculated using
new alkalinity and sulphide data show that bacterial sulfate reduction is the
only significant process of the anaerobic mineralization of organic matter in
the anoxic zone of the Black Sea. Insignificant deviations of 10-15% from the
“theoretical” C/S ratio of 2.0 can be explained by methodological problems with
alkalinity and sulphide measurements or by fermentation reactions resulting
in CO 2 generation in addition to sulfate reduction. The conclusion that sulfate
reduction is the dominant process of organic matter mineralization in the anoxic
water column is valid not only for the Black Sea, but also for Cariaco Trench
and Framvaren Fjord, where C/S ratios are in the range of 2.00-2.19 [103].
The
9. SULPHIDE BUDGET IN THE BLACK SEA
The sulphur budget for the Black Sea has been considered in several papers
[3, 8, 48, 49, 64, 82]. The main budget components to be considered are: sul-
phide production in sediments and sulphide flux at the sediment/water interface,
sulphide production in the water column, sulphide oxidation at the oxic/anoxic
interface and in the basin interior by dissolved oxygen of the modified Mediter-
ranean water, and iron sulphide formation in the water column.
9.1 Sulphide Production in Deep-Sea Sediments
Based on measurements by Sorokin [83], Deuser [17] calculated an average
annual sulphide production in Black Sea sediments of 3.6 Tg. No sulfate re-
duction was measured below the uppermost 5 cm of sediment [83]. Sulphate
reduction presence throughout the whole Holocene and upper Pleistocene sed-
imentary sequences was challenged by Vainshtein and co-authors [91]. Recent
data on the presence of the anaerobic methane oxidation have shown that
sulphate-reducing bacteria can be active in deep sediments of the Black Sea
[33]. Lein et al. [49] calculated the average hydrogen sulphide production in the
anoxic sediments of the Black Sea of about 560 mmol m 2 yr 1 , or 5.9 Tg yr 1 .
This estimate is higher than Deuser's because the whole Holocene sequence
was considered. Recent measurements by Albert and co-authors [3] gave an
average sulphide production in the upper 20 cm (including 2 cm fluffy layer)
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