Environmental Engineering Reference
In-Depth Information
mulative result of the ongoing organic mineralization processes in the anoxic
zone dominated by sulphate reduction. We finally present the sulphur budget of
the Black Sea. Physiochemical dynamics of the oxic/anoxic interface and the
sulphur isotopic composition of hydrogen sulphide are discussed elsewhere in
this volume (Ivanov and Lein; Murray and Yakushev).
2. H
2
S INVENTORY
The total sulphide inventory of the contemporary Black Sea is about 4.6
x10
3
Tg, the main part residing between 500 and 2000 m [61]. The average
dissolved sulphide concentrations at different depths are given in Table 1. The
H
2
S concentrations below 30 µM for the period after 1989 given in Table
1 were obtained using methylene blue photometric method [13]. Iodometric
titrations were used for higher concentrations. The averaged data for the period
before 1996 represent basin-wide averages, whereas more recent data were
obtained in the north-eastern part of the sea only (Fig. 2). The lower sulphide
concentrations given by Skopintsev [82] are explained by the underestimation
of sulphide concentrations determined from water samples collected with metal
bottles routinely used before 1980s [66].
The H
2
S vertical distribution is quasi-linear above 500-600 m. Dissolved
sulphide concentration increases gradually with depth for every station in the
Black Sea (Fig. 3). The vertical gradient above 500 m is about 0.5 mmol m
−
4
and
decreases with depth (Table 1). The vertical sulphide gradient at the boundary
between the entire anoxic water mass and the bottom convective layer (ca.
1700-1750 m) increases sharply and is only two times less the vertical gradient
in the upper 500 m. The H
2
S distribution in this zone is controlled by the density
gradient and can be twice as large as the average gradient of 0.24 mmol m
−
4
given in Table 1. H
2
S concentrations do not change with depth in the bottom
convective layer.
The average H
2
S concentration in the Black Sea water column is about 270
µM. The H
2
S concentration in bottom waters changes between 367 and 400
µM, the average is 376 µM.
3. H
2
S VERTICAL DISTRIBUTION AND MIXING
PROCESSES IN THE ANOXIC ZONE
Vertical distributions of temperature, salinity, and density in the Black Sea
correlate with the sulphide vertical distribution. As a consequence, the H
2
S
vertical distribution versus salinity (Fig. 4a) and temperature (Fig. 4b) is con-
sistent with the θ-S curve (Fig. 4a) for deep waters. The evidence suggests that
thermohaline structure of the water column controls the vertical distribution of
hydrogen sulphide in the sea [62]. Identifiable on the both, θ-H
2
S and S-H
2
S
diagrams, the boundaries of three water masses in the anoxic water column
