Hydrogen sulfide in oxic seawater can be found as uncomplexed dissolved species ("free sulfide. H2S+HS+S2), as dissolved metal sulfide complexes ("complexed sulfide"), and as particulate metal sulfides. The biogeochemical cycle of sulfide species in the surface ocean includes sea-air exchange, reactions with trace metals, production via carbonyl sulfide (OCS) hydrolysis and emissions from phytoplankton, and removal via oxidation and particle sinking. In this fashion, an understanding of the cycling of OCS and hydrogen sulfide species is relevant to the global sulfur cycle and to the cycling of trace metals. Analytical methods have been developed to determine OCS and hydrogen sulfide speciation (uncomplexed/complexed; particulate/dissolved) at picomolar concentrations, the rate of OCS hydrolysis in seawater has been measured, and phytoplankton has been identified as important sources of dissolved and particulate sulfide in the surface ocean. This research will quantify the remaining portions of the hydrogen sulfide cycle, with the specific objectives being (1) to determine the formation constants of sulfide complexes with Cd, Ni, and Zn, and use these in thermodynamic complexation models to help elucidate the chemical speciation of dissolved sulfide; (2) to determine the rate of oxidation of uncomplexed sulfide by iodate; (3) to measure the flux of particulate sulfide production by phytoplankton; (4) to quantify the rates of total dissolved and particulate sulfide production by phytoplankton; (5) to evaluate whether OCS, a precursor of uncomplexed sulfide and a radiatively important gas, can be produced during the regeneration of particulate organic matter.
