Oxygen

1. 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 MHI LOGO UW 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 (Winkler’s 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”.