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By comparison the suboxic layer of the central part of the sea suggests an
example of a stagnant biogeochemical structure. The oxygen concentration
remains at the level of 5 µM throughout the upper and middle part of the
suboxic zone and decreases to the analytical detection limit below σ θ = 15.9.
The maximum of particulate manganese is smaller than observed in the NW
and SW parts of the sea, and seems to be disconnected from the onset of
sulfide. The onset of dissolved manganese starts far below the upper boundary
of the suboxic zone suggesting that redox transformations of manganese cannot
generate a significant flux of electron-acceptors through the suboxic zone.
As a result of the Bosporus Plume there are significant lateral gradients along
density surfaces in the SW region. An example for sulfide is shown in Fig. 13
where we compare sulfide versus density for the western central gyre and the
SW region. In the SW region sulfide is consumed by direct reaction with the
injected O 2 or indirectly by reacting with Mn (III, IV) formed from Mn (II) by
the O 2 injection. As a result there are significant horizontal gradients for sulfide
on densities from about 16.2 to 16.5. Thus, there must also be significant lateral
fluxes of sulfide to the SW region driven by mixing along isopycnal surfaces.
This has yet to be modeled in detail but Konovalov and Murray [25] and Neretin
et al. [41] calculated that as much as 50% of the sulfide production in the
Black Sea appears to be oxidized by oxygen injected laterally by the Bosporus
Plume.
6. TEMPORAL VARIABILITY
The thickness of the suboxic zone varies temporally (Fig. 14) mostly on time
scales of 5 to 10 years [25]. The vertical distribution of O 2 is determined by a
balance between oxygen consumption due to respiration of sinking particulate
organic matter (enhanced by increasing eutrophication during the 1970s and
1980s) and the input of O 2 by ventilation of the CIL and layers below [25].
Ventilation of the CIL sets the upper oxygen concentration and fundamentally
determines the steepness of the vertical gradient and thus the downward flux
of oxygen. Thus, the upper boundary of the suboxic zone varies with time. The
first appearance of sulfide has been much less variable. It may be that sulfate
reduction is less susceptible to changes in POC flux, which may not be as large
at this depth. In addition, the first appearance of sulfide may not be as dependent
on changes in ventilation because it is deeper. During the 1980s the suboxic
zone thickened and then the oxycline moved deeper in the late 1980's and early
1990's (Fig. 14). These changes corresponded to a series of warmer winters
when there was probably less ventilation of the CIL followed by a series of
severe winters that produced favorable colder climate conditions for ventilating
the CIL. The thickness of the suboxic zone decreased as the input of oxygen
increased. During this period the temperature minimum (T min ) of the CIL was
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