Concentration and form of dissolved sulfide in the oxic water column of the ocean

Abstract

During May through July 1987, we analyzed for hydrogen sulfide in the oxic water column of the eastern Mediterranean Sea (May–June) and the northwest Atlantic Ocean (July). Cathodic stripping square wave voltammetry was used to detect and quantify sulfide species. This is a direct method requiring no sample preparation. Our measurements show a consistent profile of 2 nM in the Mediterranean Sea, except for increases that occur at the oxygen maximum in the photic zone and above the hypersaline anoxic brine of the Bannock Basin (2.9 mM sulfide maximum). The measurements in the Atlantic Ocean are lower and more variable as a result of different source waters. Sulfide increases are noted near the surface and in cold pool waters along the East Coast of the United States. Experimental manipulation of seawater samples suggests that the sulfide is bound covalently to a metal. The metal (s) is likely to have an electron configuration consistent with inert complexes, which do not dissociate readily to form free sulfide. Free sulfide reacts with iodate and oxygen in seawater. Sulfide in an inert metal complex should not dissociate and react with iodate and oxygen readily. We propose that the kinetic stability of sulfide-metal complexes is the primary control on the distribution and reactivity of sulfide in the oxic water column of the ocean, rather than thermodynamic considerations alone. This kinetic effect results from high ligand field stabilization energies associated with the electron configuration of the metal (e.g. d 3 and d 6 low spin octahedral metal complexes).