Dissolved Fe2+ and ∑H2S Behavior in Sediments Seasonally Overlain by Hypoxic-to-anoxic Waters as Determined by CSV Microelectrodes

Abstract

Variability of dissolved Fe 2+ and P H 2 S 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. Fe 2+ and P H 2 S were the major dissolved redox-reactive dissolved species in these sediments. During hypoxic conditions, an upward migration of porewater Fe 2+ and P H 2 S occurred, with Fe 2+ reaching much higher maximum concentrations than P H 2 S. Statistically significant (p<0:05) differences in both Fe 2+ and P H 2 S occurred between sediments at the CCB and MRB study sites. Although both sites were Fe-dominated, reactive and dissolved iron were higher while P H 2 S was lower at the MRB site. However, there were no statistically significant (p > 0:05) difference in regard to P H 2 S between microcosm and field monitoring studies. Results indicated that, for Fe 2+ and P H 2 S, 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 $20 days at MRB.

Artificially induced migration of redox layers in a coastal sediment from the Northern Adriatic

“…Experimental studies have shown that cores incubated under hypoxic conditions successively released dissolved manganese (Mn(II)), iron (Fe(II)) and sulfide S(-II)) to the overlying water column. This demonstrates that bottom-water oxygenation impacts the vertical extent of redox zones (Kristiansen et al, 2002;Sell and Morse, 2006).…”

“…After closure, oxygen was quickly depleted and sulfide began to accumulate (see above). Also, laboratory experiments using cores sampled in other environments showed a clear relationship between oxygen concentration in the bottom waters and sediment-to-water-column fluxes of Mn(II), Fe(II) and S(-II) (Kristiansen et al, 2002;Sell and Morse, 2006). The kinetics experiment performed by Kristiansen and co-workers clearly showed that reduced compounds accumulated more in the overlying water during more severe hypoxia.…”

Effects of progressive oxygen depletion on sediment diagenesis and fluxes: A model for the lower St. Lawrence River Estuary

“…However, if the decreasing oxygen trend continues, at some point the sediment will be affected (Kristiansen et al 2002;Sell and Morse 2006).…”

Role of microbial populations in the release of reduced iron to the water column from marine aggregates

“…However, under low oxygen conditions, Shewanella oneidensis was shown to simultaneously reduce Fe(III), NO 3 -and NO 2 -(DiChristina 1992). Fe(II) has also been found under suboxic conditions in sediment porewaters (Sell & Morse 2006) as well as in the oxygen minimum zone in the water column where NO 2 -was also found (Moffett et al 2007). The simultaneous respiration of NO 3 -and O 2 has also been shown to occur within both phytodetritus (Wolgast et al 1998) and coastal sediment (Brandes & Devol 1995).…”