Porewater evidence for a dynamic sedimentary iron cycle in salt marshes’

Dynamic transformations of iron occur seasonally at Great Sippewissett Marsh, Massachusetts. Small changes in the dissolved iron concentration in porewater represent only a small fraction of the iron involved in transformation reactions during the year. During the growing season, salt marsh grasses oxidize the sediment, and a large percentage of sedimentary pyrite is converted to an oxidized iron mineral. .Over the fall and winter there is a net increase in pyrite as the grass is anaerobically decomposed. When oxidation rates in summer are high enough to neutralize the alkalinity produced by sulfate reduction and substantially lower the pH, oxidized iron minerals become increasingly soluble and iron levels in the porewater increase. If large amounts of soluble iron are lost by tidal flushing, iron availability may limit pyrite formation in later years. Sulfide concentrations in the porewater would then increase, leading to depressed growth of Spartina alternijlora. For most of the year the porewaters of Great Sippewissett were undersaturated with respect to all iron monosulfide minerals and supersaturated with respect to pyrite (Fe!!&). Thus pyrite formation at Great Sippewissett probably occurs directly by reaction of polysulfides with iron and not by reactions of FeS with elemental sulfur. Porewaters were always undersaturated with respect to manganese minerals. Porewaters taken from marshes at Sapelo Island, Georgia, in fall were supersaturated with respect to pyrite at all depths and appear to be saturated for iron monosulfides below 12 cm at all sites. In salt marshes of the eastern United States, 50-90% of the production of the dominant grass, Spartina alterniflora, occurs belowground as roots and rhizomes (Valiela et al. 1976; Smith et al. 1979; Pomeroy and Wiegert 198 1). Most of this does not accumulate, but is decomposed anaerobically by sulfate reduction and associated fermentation reactions. The rate of sulfate reduction varies seasonally and can be extremely high (Howarth and Teal 1979; Howarth and Giblin 1983; Howarth and Hobbie 1982). Sulfide produced by sulfate reduction can affect marsh productivity (Morris 1980; Mendelssohn 198 1; King et al. 1982), energy flow (Howarth and Teal 1980), and trace metal transport (Boulegue l Financial support was provided by NSF grants DEB 78-03557 and DEB 81-04701, NOAA office of Sea Grant 04-8-MOI149, and by the University of Gcorgia Marine Institute. Part of a thesis submitted by A. Giblin for the Ph.D. degree at Boston University. 2 Present address: The Ecosystems Center, MBL, Woods Hole, Massachusetts 02543. et al. 1982). The reaction of sulfide with iron to form iron monosulfides or pyrite can modify the effect that sulfide has on marsh processes. One way to examine these interactions is by changes in porewater chemistry, which are sensitive indicators of diagenetic processes within sediments. When the quantitative relationship between several porewater constituents is known, a mass balance approach can be used to elucidate the reactions involved (Sholkovitz 1973). To examine the dynamics of the iron cycle in marshes, we measured porewater concentrations of dissolved iron throughout the year in stands of short Spartina altern$ora in Great Sippewissett Marsh, Massachusetts. We also measured other constituents which were produced by sulfate reduction or which could control or compete with iron precipitation. These components included sulfides, thiosulfate, polythionates, alkalinity, manganese, carbonate, phosphate, ammonium, pH, chlorinity, sulfate, and sili-