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Pore water evidence for a dynamic sedimentary iron cycle in salt marshes

Pore water evidence for a dynamic sedimentary iron cycle in salt marshes

Limnology & Oceanography 29(1): 47-63

Dynamic transformations of Fe occur seasonally at Great Sippewissett Marsh, Massachusetts [USA]. Small changes in the dissolved Fe concentration in porewater represent only a small fraction of the Fe involved in transformation reactions during the year. During the growing season, salt marsh grasses oxidize the sediment, and a large percentage of sedimentary FeS2 is converted to an oxidized Fe mineral. Over the fall and winter there is a net increase in FES2 as the grass is anaerobically decomposed. When oxidation rates in summer are high enough to neutralize the alkalinity produced by SO42- reduction and substantially lower the pH, oxidized Fe minerals become increasingly soluble and Fe levels in the porewater increase. If large amounts of soluble Fe are lost by tidal flushing, Fe availability may limit FeS2 formation in later years. S2- concentrations in the porewater would then increase, leading to depressed growth of Spartina alterniflora. For most of the year the porewaters of Great Sippewissett were undersaturated with respect to all FeS minerals and supersaturated with respect to FeS2. Thus, FeS2 formation at Great Sippewissett probably occurs directly by reaction of polysulfides with Fe and not by reactions of FeS with elemental S. Porewaters were always undersaturated with respect to Mn minerals. Porewaters taken from marshes at Sapelo Island, Georgia [USA] in fall were supersaturated with respect to FeS2 at all depths and appear to be saturated for Fe monosulfides below 12 cm at all sites.

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