Rapid clay transformations in Delaware salt marshes

Abstract

Salt marshes are the major areas for net sedimentation in many estuaries such as the Delaware Bay, and their diagenetic chemistry is harsh and extreme with large seasonal excursions in chlorinity (1–50 ppt), pH (4–6), and Eh (−240+120). Such diagenesis is driven by organic matter decomposition using redox cycles of S and Fe materials imported primarily as tidal sea water SO 4 and Fe silicates, respectively. Important and quantitative changes in clay mineralogy occur within a decade at the redox boundary in a high marsh sediment near Lewes, Delaware. The clay mineralogy consists initially of a micaceous illite and chlorite mixture accumulating at the salt marsh surface. It is comprised of relic glacial sediments deposited on the continental slope during their net tidal movement from the sea to land. Once buried, these detrital clays are transformed into a new assemblage containing an illite/smectitic mixed layer mineral of poor crystallinity. Using curve decomposition techniques on complex X-ray traces, it is estimated that this new phase constitutes 45–55% of the clay fraction. The redox boundary where the sharp transition occurs is only about 20 a old as determined by 210Pb and 137Cs geochronology, and, thus, the clay mineral transformation is rapid. The occurrence of the new, abundant clay mineral is very abrupt (less than 1 cm at 12 cm in depth) and, thus, may itself occur in as little as three years. Once formed, the new mixed layer phase remains stable during the subsequent 40 a of burial from the time of formation at the oxic/anoxic boundary. Slow transformations of unstable primary clay reactants such as illite and chlorite are a common process of soil formation. However such rapid clay reactions have rarely been documented in either subaerial or submerged soil settings. The formation of a smectite mineral product of high chemical reactivity for a significant portion of the clays in a soil is unusual. In fact, the abrupt change in clay mineralogy in the salt marsh occurs precisely at the sharp evolution in salt marsh geochemistry from oxidized to reducing conditions where there is extensive redox cycling of Fe and S phases. A large seasonal oscillation in interstitial pH and Eh probably contributes to the rapid clay transformation. Such clay transformations may have important implications for the retention of other trace elements entering the salt marsh by atmospheric fallout and tidal cycles, or the release of such metal inventories after burial.