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VOLTAMMETRIC PUMP PROFILING OF OXYGEN, SULFIDE
AND OTHER REDUCED DISSOLVED SPECIES OF SULFUR IN
THE OXIC/ANOXIC WATER COLUMN OF THE BLACK
SEA S.K. Konovalov, MHI, NAS, Ukraine L.W.
Luther III, CMS, UD, USA G. Friederich, MBARI,
USA J.W. Murray, School of Oceanography, UW, USA
LOGO UW
LOGO MHI
LOGO MBARI
LOGO CMS, UD
BACKGROUND
RESULTS
Oxic/anoxic conditions exist in the Black Sea on
the time scale of milleniums. This makes the
Black Sea an extremely important for
investigation of the conditions and processes,
which are responsible for oxic/anoxic balance or
dis-balance in this and other marine
ecosystems. NSF supported the KNORR cruise to the
Black Sea from May 23 to June 10 of this year to
investigate chemo-denitrification reactions in
suboxic environments (Fig.1). There were several
main objectives of the cruise but the first one
was to study the biogeochemical cycling of
nitrogen, manganese, iron and sulfur species in
the suboxic zone of the water column. The
suboxic zone is the part of the water column at
the contact between the oxic surface water and
the sulfide containing deep water. This zone was
discovered by J. Murray during the cruise of
KNORR to the Black Sea in 1988 and raised a
number of questions on interaction of oxygen and
sulfide and the overall redox budget. Recently,
S. Konovalov demonstrated that the lateral flux
of oxygen generated due to influx of the
Mediterranean waters to the Black Sea through the
Bosporus Strait should be extremely important for
oxidation of sulfide. This suggested that sulfide
is intensively oxidized in the vicinity of the
Bosporus and might result in elevated
concentrations of intermediate reduced species of
sulfur, such as elemental sulfur, poly-sulfide,
sulfate, etc. Highly sensitive methods of
voltammetric analysis at solid-state Au/Hg
microelectrodes, recently developed in the
laboratory of G. Luther, provided a possibility
to simultaneously analyze sea water for the
presence of oxygen, sulfide and other reduced
species of sulfur. We combined these voltammetric
methods with the pump profiling system, developed
by G. Friederich, to continuously analyze sea
water in the flow cell minimizing the lag time
between sampling and analysis and improving
vertical resolution to 1.5 m.
Oxygen Stations of the KNORR cruise (Fig.1)
covered a wide range of oceanographic conditions
specific for the Black Sea oxic/anoxic
environment from the shelf to the deep part.
There were stations located in the regions
anticyclonic and cyclonic gyres, in the center of
the central part and at the shelf break, near the
Bosporus Strait and far from it. Voltammetric
pump profiling throughout the oxic layer
demonstrates progressive decrease in the
intensity of both oxygen and peroxide signals
(Fig.2). Local maximums on the vertical profiles
of oxygen (Fig.3) reveals the presence of the
lateral flux of oxygen generated by intrusions of
the Bosporus Plum into the layer of the main
pycnocline. Results of voltammetric and
volumetric analysis appear to be very similar,
but voltammetric pump profiling, due to a higher
vertical resolution, allow to detect the layers
of lateral intrusions of oxygen (Fig.4). We have
been able to demonstrate that the suboxic layer
exists in its initially defined form in the area
of the Black Sea that is not affected by the
Bosporus related lateral flux of oxygen (Fig. 5
and 6). Sulfide and other reduced species of
sulfur Voltammetric data on the vertical
distribution of sulfide in the central part of
the sea collected with the time interval of 6
days are very consistent and confirm that the
profile of sulfide is linear versus depth scale
(Fig.7). There are no systematic difference
between voltammetric and volumetric data obtained
below sigma-t 16.4 (Fig.8 and 9), BUT
voltammetric data systematically lower as
compared to volumetric data above sigma-t 16.4
(Fig.8). This suggests the presence of other
substances that reduce iodine and increase the
results of volumetric analysis. Intermediate
products of sulfide oxidation were expected to
exist in a higher concentration in the southern
part of the sea, where the lateral flux of oxygen
into the layer of the main pycnocline and upper
part of the anoxic zone intensifies the rate of
oxidation of sulfide. The vertical profiles of
the distribution of sulfide clearly demonstrate
that the onset of sulfide in the southern part of
the sea is located clearly deeper, as compared to
the central and northern part (Fig.10),
suggesting a higher concentration of intermediate
products of oxidation of sulfide. Elemental
sulfur probably exists at the depth of sulfide
onset following a broader signal and a slight
shift in the potential of this signal due to the
very high rate of scanning. But nothing like
polysulfide has been detected both at the
northern and southern periphery of the deep part
of the sea (Fig.11 and 12). On the other hand,
the presence of polysulfide is highly possible
for the central part of the sea (Fig.13, 14 and
15) suggesting an evolution from sulfide through
polysulfide and elemental sulfur to sulfate in
the upward direction.
OBJECTIVES
? To trace the exact location of the onset of
sulfide and vertical structure of the suboxic
zone versus sigma-t throughout the area of the
2001 KNORR expedition to the Black Sea using the
low level voltammetric technique. ? To get
high-resolution vertical profile of sulfide in
the upper ??0.5 0.7 layer of the anoxic zone
using the pump profiling voltammetric technique
in the flow cell. ? To get/extend information on
the sulfur speciation, primarily, in the vicinity
of the Bosporus Strait.
MATERIALS AND METHODS
We applied both traditional volumetric
(Winklers for oxygen and iodometric back
titration for sulfide) and recently developed
voltammetric methods for analysis of sea water.
Well-dried and flushed with Ar-gas narrow neck
glass flasks were used in the volumetric analysis
of oxygen to minimize contamination. The
reference zero-sulfide samples were taken from
the suboxic zone. Thoroughly calibrated glassware
and Metrohm-765 was used in volumetric
analyses. DLK-60 Electrochemical Analyzer,
Analytical Instrument System, Inc., and a
solid-state Au/Hg 0.1 mm diameter working,
Ag/AgCl reference and Pt counting electrode were
used for voltammetric analysis. We usually
scanned the potential range from 0.1 to 1.8V
using linear sweep and/or cyclic voltammetry at
4V/s. We also applied preconditioning at 0.1V
for 20s. Altogether, these conditions provided
the low detection limit of 3 nM of sulfide and
about 3 uM of oxygen.
ACKNOWLEDGMENTS
The CRDF grant supporting this study is CRDF
UG2-2080 Voltammetric Determination of Sulfide
and Other Reduced Dissolved Species of Sulfur in
the Black Sea.