Environmental Engineering Reference
In-Depth Information
rise along the southwestern Black Sea shelf does not support the catastrophic
refilling of the Black Sea [2]. Studies by Gorur et al. [26] around the mouth
of the Sakarya River demonstrated that there was a gradual rise in the level
of the Black Sea from some time before 8,0 ka BP until it attained a surface
level of 18 m below present sea-level around 7,2 ka BP, when the most recent
Mediterranean influx occured. Recent sedimentological and palaeontological
studies in the Bosporus Strait suggest that the sill depth was probably not
the critical factor in the exchange between the Black Sea and Mediterranean
Sea at the Holocene/Pleistocene transition. Other alternative routes such as
the Gulf of Izmit-Sapanca and Lake-Sakarya Valley may have existed at that
time [40].
The evolution of the Black Sea anoxic zone is closely connected with the evo-
lution of the Black Sea stratification pattern, that is presently characterized by
the existence of a strong pycnocline separating the upper freshwater influenced
surface layer with a salinity of 17.5-18.5‰ and the deep water mass below ca.
150-200 m with a salinity of 22.3‰ at the bottom. Models for evolving Black
Sea salinity after the opening of Bosporus show that salinity in bottom waters
reached 90% of present-day values about 3,000 yrs or less after the Bosporus
opening [5, 10, 36, 54] or indicate that the freshwater content of the Black
Sea became depleted over a period of about 3,700 yrs after the opening [46].
Due to the stable stratification, anoxia developed below the pycnocline, which
corresponded to the deposition of an organic-rich sapropel after 7,800 yrs [94]
or 7,540 yrs B.P. [34] through the entire Black Sea area. Since that time bottom
waters in the Black Sea have remained anoxic. Development of anoxic condi-
tions over time after the Bosporus inflow was modelled by Dueser [18], who
suggested that present anoxic conditions were achieved within 2.0-4.0 ka after
Mediterranean waters reached the Black Sea. These data are in correspondence
with the evolution of the T-S structure of the Black Sea waters discussed earlier
and residence time of main seawater anions such as chloride, bromide, and
sulphate [82].
Our knowledge about the anoxic zone is of great importance for understand-
ing the functioning of the Black Sea ecosystem. The goal of this review is to
present an overview of the processes occurring in the anoxic zone of the Black
Sea with an emphasis on the sulphur cycle using data obtained during the last
ten years. Hydrogen sulphide,
H 2 S 1 , is the key chemical compound, which
defines the direction and origin of many biogeochemical cycles in the anoxic
zone. The chapter starts with describing the inventory of hydrogen sulphide,
its vertical and spatial dynamics, and the distribution of sulphur intermediates.
It is followed by the discussion of the Black Sea alkalinity, which is a cu-
Σ
1
Σ H 2 S=[H 2 S]+[HS ]+[S 2 ], where [HS ] represents ca. 80% under pH=7.5-7.65 in the Black Sea anoxic
interior
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