Environmental Engineering Reference
In-Depth Information
3. HISTORY OF STUDY OF THE SUBOXIC LAYER
(SOL)
The presence of hydrogen sulfide in the deep waters of the Black Sea was
first described by Andrusov [1] in his report of the scientific expedition to the
Black Sea on the Russian gunboat “Chernomorets” in 1890. He proposed that
the reason for this occurrence is that the Bosporus restricts the exchange of deep
waters between the Black and Mediterranean Seas. He also proposed that the
hydrogen sulfide originated due to reaction of non-mineralized organic matter
with sulfurous salts of the sea water.
A layer of co-existence of oxygen and hydrogen sulfide (C-layer or S-layer)
was observed in some of the earliest hydrochemical studies in the Black Sea.
The origin of this term is that C is the Cyrillic letter for S and is the first letter
of the Russian word for overlap. It was assumed that oxidation of hydrogen
sulfide took place mainly by direct reaction with oxygen within this layer
[4, 60, 61]. Sorokin [61] devised a special analytical technique to determine
oxygen in the presence of sulfide. During the 1988 RV “Knorr” Expedition it
was found that when special care was taken to avoid contamination the oxygen
concentrations measured with the standard and micro Winkler techniques were
significantly lower than found earlier [10, 36] . Bezborodov and Eremeev [4] and
Broenkow and Cline [6] showed that the error connected with contamination of
reagents with oxygen and consumption of oxygen during the standard sampling
procedure can reach 0.15 ml
−
1
(6.6 µM), and after this correction the C-layer
practically disappears. This absence of oxygen at the hydrogen sulfide boundary
was subsequently confirmed by [2, 32, 72].
The disappearance of oxygen above the hydrogen sulfide zone was also
confirmed by Stunzhas [68] who used a specially designed membrane-less
oxygen sensor for studies of the Black Sea suboxic layer.
4. DISTRIBUTIONS
Here we describe the distributions of the key biogeochemical species in the
region of the suboxic zone. The density values that were characteristic of many
water column features during the 1988 Knorr Expedition are shown in Table
2. These values have served as a benchmark for subsequent cruises to evaluate
the stability of the characteristic features. Since 1988 there has been some
spatial and temporal variability but the general picture of the distributions has
remained the same.
4.1 OXYGEN - SULFIDE
An example of the oxygen and sulfide distributions versus depth (left) and
density (right) in the center of the western gyre is shown in Fig. 2 (R/V Knorr
2003 Leg 7, Stn 12). Oxygen is close to atmospheric saturation (
∼
330 µM) in
