Processes and Rates of Pedogenesis in Some Maryland Tidal Marsh Soils
Griffin, Ted | Rabenhorst, M. C.
Tidal marsh profiles from various locations around the Chesapeake Bay were dated using ²¹⁰Pb geochronology. These soils, which were classified as Typic and Terric Sulfihemists, Typic and Histic Sulfaquents, and Histic Hydraquents, were characterized with respect to bulk density, organic matter content, S and Fe speciation, and pyrite content. These characteristics were interpreted in the context of marsh accretionary activity and soil forming processes. Rates of vertical accretion ranged between 0.35 and 0.75 cm yr⁻¹, indicating that the marshes examined are keeping pace with published rates of sea level rise. Generally, those marshes accreting most rapidly contained a higher mineral content and a higher reactive Fe (FeS²-Fe plus “free” Fe) content. Pyrite S and organic S were the dominant S species forming from the microbial reduction of SO²⁻₄. Redox conditions, Fe reactivity, organic matter content, and SO²⁻₄ concentrations were the major factors controlling S and Fe transformations. Models developed to define different marsh environments with respect to pyrite formation and the degree of pyritization illustrate that in those marshes receiving low rates of mineral sediments (i.e., slowly accreting marshes), the amount of reactive Fe is limiting FeS₂ formation, the FeS₂ content is low, degree of pyritization (Py) is high, and the dominant S species is organic S. Where the reactive Fe content is high (i.e., rapidly accreting marshes) and SO²⁻₄ reduction is not retarded, the FeS₂ content is high and Py is moderate to high. If reactive Fe is high and SO²⁻₄ reduction is impeded for some reason, then FeS₂ formation becomes limited by the SO²⁻₄ reduction rate, and the FeS₂ content and Py will be variable.
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