Environmental Engineering Reference
In-Depth Information
show the presence of oxygen intrusions at depths 158 and 190 m, well below
the sulphide onset. Hydrogen sulphide disappearance at these depths is ac-
companied by decreased ammonium and Mn(II) concentrations and increased
turbidity values [7]. The authors attribute the presence of oxygen intrusions
below the upper H
2
S boundary to the existence of strong vertical and horizon-
tal water mixing caused by the meandering of the MRC along the steep and
complicated topography of the Sakarya Canyon.
The presence of modified saline Mediterranean waters below to 500-600
m (from T/S analysis) was first reported in 1960s [9], although no evidence
of oxygen intrusions was discussed. Detailed studies in the western Black Sea
have shown that a dense Mediterranean water after passage over a Bosporus sill
flows over a wide continental shelf and entrains ambient Black Sea water. The
Mediterranean effluent follows along the canyon and the shelf and produces
a delta-like structure on the shelf region. The vertical spread in the interface
region between shelf and continental slope is the result of mixing of Bosporus
waters with CIL [15, 68]. Upon passage over the shelf, Mediterranean Water
(MW) is altered to “shelf-modified MW” (SMMW) (terminology suggested
by Ozsoy et al. [68]) resulting in a T-S signature distinctly different from the
initially warm, saline characteristics of MW. Cascades of dense SMMW sink
down to 500-600 m on the continental slope where they are distinguishable
from the ambient waters as a cold anomaly [68].
The role of oxygen intrusions below the upper sulphide boundary on depth
profiles of chemical parameters in the western Black Sea is discussed in details
by Murray and Yakushev (this volume). Recently Schippers et al. [80] explained
the observed Mn(II) oxidation rates in the western Black Sea as a result of lateral
oxygen intrusions with the modified Bosporus waters. The authors reported that
intrusions were observed far below the oxic/anoxic interface up to the density
surface of 18.4 kg m
−
3
. In Section 9.3 we discuss quantitatively the possible
maximum impact of oxygen carried by Bosporus intrusions on the sulphide
inventory in the Black Sea.
7. INTERMEDIATE SULPHUR SPECIES IN THE
BLACK SEA WATER COLUMN
Inorganic sulphur species are important intermediates in the sulphur cycle.
In euxinic environments, the most commonly detected S species are elemental
sulphur, S
0
, and thiosulphate, S
2
O
3
2
−
. Sulphite, SO
3
2
−
, is usually of minor
importance. Polythionates, S
n
O
6
2
−
(n=2-5), have not been reliably determined
in the anoxic water due to analytical difficulties. Furthermore, polythionates
are not stable in the presence of H
2
S yielding S
2
O
3
2
−
and polysulphides [111].
In the presence of hydrogen sulphide, elemental sulphur usually reacts with
HS
−
and S
2
−
to form polysulphides, S
n
2
−
(n=2-6). In the presence of iron and
