Environmental Engineering Reference
In-Depth Information
Figure 1.
Stations sampled for sulfur isotopic composition of hydrogen sulfide and sulfate.
1-2 [36]; 3 - [94]; 4 - [93]; 5-6 [79]; 7 - our data.
plemented with substrates utilized by sulfate reducers (these substrates would
increase the rates of sulfate reduction), our experiments were carried out without
the addition of any organic substrate under conditions close to in situ conditions.
To determine rates, a method with the use of Na
35
SO
4
of high sensitivity was
developed. It was first proposed in the 1950s [43] and later specifically modified
to study marine sediments recovered during Pacific expedition in 1973 [44].
The lowest SRR in sediments on the continental slope off California and
in the South China Sea measured with this method, varied between 7 and 125
nM/day per 1 kg. These sediments contained isotopically light pyrite (δ
34
S=
-44.5
-45.8‰). In the sediments sampled at smaller depths on the upper part
of the slope, SRR increased to 1.16 - 1.74 µM/day, and δ
÷
34
S of pyrite varied
from -24.2 to -26.9‰ [64]. The analysis of these data suggests that at lower
SRRs, which are characteristic for oceanic deep-sea sediments, higher isotope
fractionation between sulfate and sulfide is observed [45, 64]. Similar, in the
deep-sea water column of the Black Sea very low SRRs (Table 3) play a crucial
role producing isotopically light hydrogen sulfide (Tables 2 and 3).
An alternative way of explaining the extremely light isotopic composition
of sulfides is the effect of isotopic fractionation during microbial dispropor-
tionation of S
0
,S
2
O
3
2
−
and SO
3
2
−
[14, 16, 37, 57, 82]. The fact that the
disproportionation is performed by many microorganisms in nutrient-rich media
