Environmental Engineering Reference
In-Depth Information
of about 5.2 Tg yr
−
1
. Lein and Ivanov [47] have estimated the total sulphide
burial in the Black Sea of 2.4 Tg yr
−
1
including about 1 Tg yr
−
1
that is buried
in the anoxic zone. Using these data and integrated over the upper 20 cm of
sediments sulfate reduction rates, Neretin et al. [63] concluded that the annual
sulphide flux into the water column from sediments of the anoxic zone is in
the range between 3 and 5 Tg per year. The value is likely to be overestimated
due to spatial differences in pyrite burial rates and possible sulphide diffusion
downward into the deeper sediment layers [33]. The limnic Late Pleistocene
clays with high reactive iron and low dissolved sulphide concentrations are
often exposed to the surface on continental slopes of the Black Sea bordering
the Caucasus and Anatolia [104]. These sediments may serve as a sink for
hydrogen sulphide formed in the deep sea.
9.2 Hydrogen Sulphide Production in the Black Sea
Water Column
Measurements of SRRs in Black Sea water column are summarized in Table
5 [64]. Generally, a maximum in sulfate reduction rates is observed in the
upper (200-300 m down to 600-700 m) part of anoxic column and in the layers
adjacent to the bottom. The highest rate measured for the upper anoxic zone
so far was 1569 nmol l
−
1
day
−
1
[29]. The lowest SRRs in the water column
are reported by Albert et al. [3] and do not exceed 3.5 nmol l
−
1
day
−
1
. With a
sensitivity of the radiotracer method of about 0.2-0.6 nmol l
−
1
day
−
1
[3, 49],
reduction of sulfate in the intermediate zone (600(700)-2000 m) comprising
the main part of the Black Sea hydrogen sulphide pool has not been detected
[49, 83]. SRRs in these layers are 1-2 orders of magnitude lower than in the
proximity to the upper anoxic boundary [3, 28].
Seasonal variability of sulfate reduction in the water column is still unre-
solved. The only data acquired in wintertime show measurable SRR in the upper
200-300 m only at three of nine sampling stations [86]. The authors attributed
the absence of SR at the other stations during this season to the disappearance
of anaerobic bacterial community in the central parts of the basin due to intense
mixing at the chemocline during winter convection. Careful examination of the
published data and methods suggests that the variability in the SRR is not an
analytical artifact. Existing data sets are characterized by significant seasonal
variations and yield the average sulphide production in the water column of
41
31 (95%CI) Tg yr
−
1
.
Sulfate reduction in the deeper parts of the anoxic zone is probably particle-
associated. Vertical distribution of this process is related to sinking rate and size
of the particles, and can have a seasonal pattern and spatial patchiness. Con-
centrations and turnover times of potentially important substrates for sulfate-
reducers (acetate, lactate, formate) in the anaerobic layers are well above the
±
