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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
Stations network of the KNORR cruise
(May 23, 2001 - June 10, 2001)
28°
OBJECTIVES
32°
36°
250
Leg#2, STN#2
Pump Prof iling
CV-4000-20s
1m, 12.444
53m, 14.354
59m, 14.814
64m, 15.066
69m, 15.239
88m, 15.785
40°
200
Odessa
6
7
8
4
3
45°
Constanta
Kerch
9 10
45°
Novorossiisk
Sevastopol
11
12
The Black Sea
Sochi
Varna
2
13
34 5 6
2
1
42°
1
7
8
Zenguldak
9
10
14
Istanbul
28°
150
12
13
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.
Fig.3
15
50
Oxygen
Voltammetry
Pump Prof iling
0
100
200
300
400
Concentration, mM
-0.4
(C) 1995. MHI, Department of Oceanology
-0.8
-1.2
-1.6
40°
Potential v/Ag\AgCl, V
15.6
The "classic" suboxic zone
at the stations which are out
of direct inf luence of
the Bosporus lateral f low
16
100
LEG#1 - STN#10
Voltammetric Data
Winkler CTD#47
Winkler CTD#55
13
Sigma-t
Leg#1-STN#6-CTD#17
Leg#1-STN#6-CTD#31
Leg#1-STN#6-CTD#34
Leg#2-STN#2-CTD#3
Leg#2-STN#2-CTD#12
Leg#2-STN#12-CTD#24
Leg#2-STN#11-CTD#25
Leg#2-STN#12-CTD#27
Leg#2-STN#13-CTD#30
Leg#2-STN#13-CTD#32
10 uM (suboxic level)
Sulfide
15.2
Sigma-t
12
Oxygen
Volumetric Titration
Leg#2-STN#2
Leg#2-STN#13
Fit 1: Linear
120
Depth, m
14.8
14
15
Fig.7
140
160
Sulf ide
Voltammetry
Pump Prof iling
180
Fig.4
16
16.4
0
40
80
120
200
Fig.5
Concentration, mM
0
100
200
300
0
400
10
20
30
40
50
Concentration, mM
Concentration, mM
15.8
Oxygen
Volumetric Titration
15.2
15.6
Stations which are directly
af f ected by the lateral
Bosporus f low
16
16
Sigma-t
Leg#1-STN#5-CTD#12
Leg#1-STN#7-CTD#38
Leg#1-STN#10-CTD#47
Leg#1-STN#10-CTD#55
Leg#2-STN#14-CTD#33
Leg#2-STN#14-CTD#35
Leg#2-STN#14-CTD#40
10 uM (suboxic level)
LEG#2 - STN#2
Voltammetric Data
Volumetric CTD#3
Volumetric CTD#12
16.2
Fig.8
40
80
1000
1500
2000
16.6
120
0
Concentration, mM
10
20
30
40
0
50
16
50
Current, nA
Sigma-t
60
Leg#2-STN#3
Leg#2-STN#12
Leg#2-STN#2
Leg#2-STN#13
Leg#1-STN#7
Leg#1-STN#10
Leg#2-STN#14
16.4
Leg#2, STN#12
CTD#27
102m, 15.49
123m, 15.82
133m, 15.98
138m, 16.06
144m, 16.11
149m, 16.15
155m, 16.21
180
Sulf ide
Voltammetry
Pump Prof iling
16.2
Fig.11
40
30
20
200
300
400
420
Leg#2, STN#14
Pump Prof iling
152m, 16.29
153m, 16.34
154m, 16.35
155m, 16.36
158m, 16.38
159m, 16.39
160m, 16.39
165m, 16.41
Fig.12
300
180
60
50
Current, nA
300
100
Concentration, mM
Concentration, mM
420
Fig.10
Voltammetric L1-S6-CTD#18
Voltammetric L1-S6-CTD#35
Volumetric L1-S6-CTD#18
Volumetric L1-S6-CTD#35
Volumetric L2-S13-CTD#31
16.4
16.4
0
Fig.9
500
Depth, m
14.8
Fig.6
 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.
14
Samsune
36°
RESULTS
Leg#2-STN#12
Leg#1-STN#6
Leg#2-STN#2
Leg#2-STN#3
Leg#2-STN#13
Leg#1-STN#7
Leg#1-STN#10
Leg#2-STN#14
16
42°
Fig. 1
Fig.2
100
Numbers in red for Leg#1
Numbers in blue for Leg#2
32°
(C) 1995. MHI, Data Base Laboratory
Current, nA
5
Sigma-t
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 chemodenitrification 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 (< 10 mM) and H2S (< 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 solidstate 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.
Sigma-t
BACKGROUND
40
30
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.
20
16.6
0
MATERIALS AND METHODS
10
20
30
40
10
50
Concentration, mM
10
-0.4
-0.8
-1.2
-1.6
-0.4
Potential v/Ag\AgCl, V
420
Fig.13
180
60
50
40
30
20
300
Leg#1, STN#6
Pump Prof iling
113m, 16.11
122m, 16.19
125m, 16.21
128m, 16.24
132m, 16.27
135m, 16.30
Fig.14
180
60
50
40
30
20
-1.6
Leg#1, STN#6,
Experiments
4V/s & 20s preconditioning at -0.1V
1V/s & 20s preconditioning at -0.1V
14
Current, nA
Leg#2, STN#2
Pump Prof iling
88m, 15.79
114m, 16.19
116m, 16.20
118m, 16.21
121m, 16.23
123m, 16.26
-1.2
Potential v/Ag\AgCl, V
420
Current, nA
300
Current, nA
We applied both “traditional” volumetric (Winkler’s 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.
-0.8
4
12
3
10
2
8
Fig.15
10
10
-0.4
-0.8
-1.2
-1.6
Potential v/Ag\AgCl, V
-0.4
-0.8
-1.2
-1.6
Potential v/Ag\AgCl, V
ACKNOWLEDGMENTS
6
-0.4
-0.8
-1.2
Potential v/Ag\AgCl, V
1
-1.6
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.