Dissolved Fe2+ and ΣH2S behavior in sediments seasonally overlain by hypoxic-to-anoxic waters as determined by CSV microelectrodes

Variability of dissolved Fe2+ and Sigma H2S concentrations in porewaters were studied, using cathodic stripping voltammetry (CSV) microelectrodes, in sediments overlain by hypoxic waters in the summer from the southeastern region of Corpus Christi Bay, Texas (CCB) and the Mississippi River Bight (MRB), Louisiana. These measurements were complimented by sediment microcosm studies where oxygen concentrations in the overlying water were manipulated. Sulfate reduction rates, benthic oxygen demand, total reduced sulfide, porewater sulfate, and total organic carbon were also determined. Fe2+ and Sigma H2S were the major dissolved redox-reactive dissolved species in these sediments. During hypoxic conditions, an upward migration of porewater Fe2+ and Sigma H2S occurred, with Fe2+ reaching much higher maximum concentrations than Sigma H2S. Statistically significant (p < 0.05) differences in both Fe2+ and Sigma H2S occurred between sediments at the CCB and MRB study sites. Although both sites were Fe-dominated, reactive and dissolved iron were higher while Sigma H2S was lower at the MRB site. However, there were no statistically significant (p > 0.05) difference in regard to Sigma H2S between microcosm and field monitoring studies. Results indicated that, for Fe2+ and Sigma H2S, relatively large and rapid changes occurred in both the concentrations and distributions of these important porewater constituents in response to relatively short-term changes in overlying water oxygen content. Model calculations indicated that conditions in the sediments can be responsible for the induction of hypoxic conditions in the formation of hypoxic overlying waters in < 6 days at CCB and similar to 20 days at MRB.