Micromorphology of Argillic Horizons in an Upland/Tidal Marsh Catena

Apparent sea level in Chesapeake Bay has been rising since the close of the Pleistocene, resulting in tidal marshes overlying former upland soils. Soils were examined across one such marsh onto the adjacent upland to evaluate the soils for evidence of clay illuviation and effects due to submergence. Thin sections of Bt and Btg horizons of soils throughout the transect showed illuvial features confirming the presence of an argillic horizon beneath the marsh. Soil morphology and particle‐size distributions indicated that argillic horizons of marsh and boundary soils were relict features. Kaolinite, vermiculite, mica, quartz, and smectite were the dominant clay minerals of the argillic horizons. Minimal change in mineralogy was observed across the catena. Bright, first‐order interference colors were observed in many of the illuvial and argillaceous features of marsh and boundary argillic horizons. Using x‐ray diffraction, differential scanning calorimetry, and microprobe and statistical analysis, no clay mineralogical or elemental differences could be ascertained between the features with bright interference colors and those of a more typical color. Areas of clay, lacking orientation and lying adjacent to voids, indicated that dispersion may have disrupted some illuvial features. Cycles of oxidation and reduction in localized zones of marsh and boundary argillic horizons has caused Fe 3+ to concentrate as Fe nodules, ferrans, and neoferrans. Pyrite framboids were observed in the marsh argillic horizon. As a result of inundation, soils that were formerly Ultisols are now Alfisols and Histosols.