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VOLTAMMETRIC PUMP PROFILING OF O2, H2S AND OTHER
DISSOLVED REDUCED SULFUR SPECIES IN THE
OXIC/ANOXIC WATER COLUMN OF THE BLACK SEA. S.K.
Konovalov and A.S. Romanov, MHI, NAS,
Ukraine G.W. Luther III, CMS, UD, USA G.
Friederich, MBARI, USA J.W. Murray, School of
Oceanography, UW, USA
BACKGROUND
RESULTS
Oxic/anoxic conditions have existed in the Black
Sea for millennia. This makes the Black Sea an
extremely important area for the investigation of
the conditions, which are responsible for redox
processes in this and other marine
ecosystems. NSF supported the R/V KNORR cruise to
the Black Sea from May 23 to June 10 of 2001 to
investigate chemo-denitrification reactions in
suboxic environments (Fig.1). There were several
main objectives of the cruise but a major 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 between the oxic
surface water and the sulfide containing deep
water where O2 (lt 10 mM) and H2S (lt 3 mM) and
have negligible gradients. This zone was
discovered on the KNORR cruises to the Black Sea
in 1988 and raised a number of questions about
the interaction of oxygen with sulfide and the
overall redox budget. Recently, S. Konovalov
demonstrated that the lateral flux of O2
generated due to an influx of the Mediterranean
waters to the Black Sea through the Bosporus
Strait should be extremely important for the
budget of H2S. He suggested that H2S should be
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,
thiosulfate, etc. A highly sensitive method
voltammetric analysis with solid-state Au/Hg
microelectrodes, recently developed in the
laboratory of G. Luther provided the possibility
to simultaneously analyze sea water for the
presence of O2, H2S and other reduced species of
sulfur. We combined these voltammetric methods
with the pump profiling system, developed by G.
Friederich, to continuously analyze seawater in
an electrochemical flow cell to minimize the lag
time between sampling and analysis and to improve
the vertical resolution to 1.5 m.
Stations of the KNORR cruise (Fig.1) covered a
wide range of oceanographic conditions specific
to the Black Sea oxic/anoxic environment. There
were stations located in the anticyclonic and
cyclonic gyres, in the center of the Black Sea
and at the shelf break, near the Bosporus
Strait. Oxygen Voltammetric pump profiling
throughout the oxic layer demonstrates a
progressive decrease in the intensity of oxygen
(and peroxide) signals (Fig.2). Local maxima are
found in the vertical profiles of O2 (Fig.3) and
reveal the presence of a lateral flux of O2
generated by intrusions of the Bosporus Plume
waters 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,
allows detecting the narrow layers of the lateral
intrusions of O2 (Fig.4). We have been able to
demonstrate that the suboxic layer exists as
found in 1988 in the offshore areas of the Black
Sea, and these areas are unaffected by the
Bosporus lateral influx of O2 (Fig. 5 and
6). Sulfide and other reduced species of
sulfur Voltammetric profiles of the vertical
distribution of sulfide in the central part of
the sea collected with a time interval of 6 days
are very consistent and confirm that the
distribution of sulfide is linear versus depth
(Fig.7). There is no systematic difference
between the voltammetric and volumetric data
obtained below sigma-t 16.4 (Fig.8 and 9). BUT
the voltammetric data are systematically lower as
compared to volumetric data above sigma-t 16.4
(Fig.8). This suggests the presence of other
substances that reduce I2 (e.g. organic matter)
and increase the H2S results of the volumetric
analysis. Some 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 O2 into the layer of
the main pycnocline and the upper part of the
anoxic zone should intensify sulfide oxidation.
The vertical profiles of sulfide demonstrate that
the onset of H2S in the southern part of the sea
is located deeper, as compared to the central and
northern part (Fig.10). Thus, more sulfur species
with intermediate oxidation states are
expected. Actually, we have found data that
suggests elemental sulfur exists at the depth of
H2S onset. The S8 signal is broader and has a
slight shift in the potential relative to the H2S
signal due to the very high scan rate used.
Polysulfide was not detected in waters from the
northern and southern periphery of the deep part
of the sea (Fig.11 and 12). However, the presence
of polysulfide was detected in waters from the
central part of the sea (Fig.13, 14 and 15)
suggesting a gradient from H2S to Sx2- to S8 to
sulfate in the upward direction.
OBJECTIVES
? To trace the exact location of the onset of
sulfide and the vertical structure of the suboxic
zone versus sigma-t and depth throughout the area
of the 2001 KNORR expedition to the Black Sea
using sensitive voltammetric techniques. ? To get
high-resolution vertical profiles of sulfide in
the upper layer of the anoxic zone using the pump
profiler with voltammetric techniques in the flow
cell (without sample manipulation). ? To obtain
detailed information on sulfur speciation,
primarily, near the Bosporus Strait.
MATERIALS AND METHODS
We applied both traditional volumetric
(Winklers for O2 and iodometric back titration
for H2S) and recently developed voltammetric
methods. To minimize contamination in the
volumetric analysis of O2, narrow neck glass
flasks well-dried and flushed with Ar-gas were
used. Zero-sulfide samples were taken from the
suboxic zone. Thoroughly calibrated glassware and
Metrohm-765 titrator were used in volumetric
analyses. A DLK-60 Electrochemical Analyzer, from
Analytical Instrument Systems, Inc., and a
solid-state Au/Hg 0.1 mm diameter working
electrode, Ag/AgCl reference electrode and Pt
counter electrode were used for voltammetric
analysis. We usually scanned the potential range
from 0.1 to 1.8V using a linear sweep and/or
cyclic mode at 4V/s. We also applied
preconditioning at 0.9 V for 2 sec to clean the
surface of the Au/Hg electrode and a deposition
at 0.1V for 20s. These conditions provided the
low detection limit of 3 nM for sulfide and about
3 mM for oxygen.
ACKNOWLEDGMENTS
S. K. K. and A. S. R. were supported by a CRDF
grant UG2-2080 Voltammetric Determination of
Sulfide and Other Reduced Dissolved Species of
Sulfur in the Black Sea. NSF supported the
American participation.