BACKGROUND

1. 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 (< 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 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 H2StoSx2-toS8 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 (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. 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.