Environmental Engineering Reference
In-Depth Information
any given year [10, 36]. An excellent example of the vertical profiles of O
2
and
H
2
S in the center of the western gyre is shown plotted versus depth and density
in Fig. 2.
Figure 2.
Oxygen and sulfide data from the center of the western gyre (Leg 7; Stn 12; Pump
Cast #10) during R/V Knorr 2003. The data are courtesy of G. Luther (U. Delaware) and S.
Konovalov (MHI, Sevastopol, Ukraine). The vertical scales are depth on the left and density on
the right. Depth is the traditional scale but density is frequently used in the Black Sea to facilitate
comparisons of different locations.
The suboxic zone in the Black Sea [10, 22, 36, 40] is an important bio-
geochemical transition zone between the oxic surface layer and sulfidic deep
waters. This layer, where O
2
and H
2
S do not overlap, was first observed during
the 1988 Knorr Black Sea Expedition [35, 38]. Its boundaries were chosen from
the vertical distribution of oxygen and sulfide observed in the central gyre. It
was defined as the region between the depth where oxygen decreases to near
zero (O
2
<
10µM) and the depth where sulfide first appears (H
2
S
>
0.3µM) [36,
40]. After its discovery, these distributions were confirmed by others [2, 57, 72]
and the processes controlling its origin and variability have been extensively
discussed.
When the suboxic zone was first observed, Murray et al, [36] suggested that
it might be a new feature resulting from reduced fresh water input from rivers
and a resulting change in the ventilation of the shallow Black Sea. Subsequent
research has demonstrated that the suboxic zone is most likely a permanent
feature of the Black Sea (at least since the early 1960s) [8, 40]. The balance
between oxygen injected due to ventilation of the thermocline with surface
water and oxygen consumed by oxidation of organic matter governs the depth
of the upper boundary of the suboxic zone [25]. The injection of oxygen into the
