Environmental Engineering Reference
In-Depth Information
upper part of the sulfide zone by water of Bosporus origin (the Bosporus Plume)
is also an important control for the depth of the first appearance of sulfide [25].
Redox processes involving nitrogen-manganese-iron-sulfur are important for
cycling of those elements in the lower part of the suboxic zone and they also
play a role in determining the depth of the lower boundary [49].
There are several reasons why the Black Sea is an important location for
geochemists to study.
1. It is a classic anoxic ocean basin and is considered a possible analog for the
earth's ancient ocean. The ocean was initially totally anoxic. As atmospheric
oxygen increased during the Proterozoic, the ocean contained an oxic surface
layer and anoxic deep water from about 2.5 billion yrs. to 0.6 billion yrs. [3,
19]. After 0.6 b.y. the ocean was mostly oxic as it is today.
2. It has a well developed suboxic zone at the interface between the oxic and
sulfidic layers where many important redox reactions involving Fe, Mn, N and
other intermediate redox trace elements (e.g. Co, As, Sb, I, Ce) occur. Similar
redox reactions take place in organic rich sediments throughout the world's
oceans but they are easier to study in the Black Sea because they are spread
out over a depth scale of 10s of meters (rather than cm or mm scales as in
sediments).
3. The chemical distributions have been shown to occur on similar density
(or depth) horizons from year to year, making the chemical reactions and the
sequence of microbes that mediate them easy to study in a predictable way on
repeated cruises.
4. The Black Sea is an ideal site to study how variability in climate affects
air-sea physical forcing and resulting changes in chemical fluxes and resulting
biological distributions.
2. HOW DOES THE BLACK SEA WORK?
In order to understand the suboxic zone it is necessary to describe the phys-
ical framework where it exists. Like the open oceans the Black Sea has wind
driven circulation with gyres, eddies, deep water thermohaline circulation and
shallower ventilation into the thermocline. Neuman [42] described the surface
circulation of the Black Sea as consisting of two large cyclonic (counterclock-
wise) central gyres that define the eastern and western basins (Fig. 3). These
gyres are bounded by the wind-driven Rim Current [47] which flows along
the abruptly varying continental slope all the way around the basin. The Rim
Current exhibits large meanders and filaments that protrude into the regions of
the central gyres. The geostrophically calculated currents along the axis of the
Rim Current typically have speeds of 25 cm s 1 . Inshore or coastal of the Rim
Current there are several anticyclonic (clockwise currents) eddies [43]. Some of
these eddies are permanently controlled by topography (e.g. the Sakarya Eddy
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