Isotopic Evidence for Clay Mineral Weathering and Authigenesis in Cryoboralfs

Abstract

Clay mineral distribution and weathering were studied in two Cryoboralf pedons developed on texturally different tills in northeastern Alberta, Canada, to elucidate authigenic mineral formation in these soils. The crystalline clay minerals were identified by x-ray diffraction analyses, and quantified by analyses for cation exchange, surface area and elemental content. The total non-crystalline clay component was determined using selective dissolution treatments. The crystalline clay suite of the parent materials consisted of an admixture of smectites, kaolinite, dioctahedral mica, and chlorite. Beidellite and montmorillonite were both identified as components of the smectite group. Kaolinite was negatively enriched in the E horizons in all clay size separates, whereas mica and smectites were enriched in the Bt horizons as a result of lessivage. Routine chemical and mineralogical characterization suggested apparent neoformation of beidellite in the E horizons of these pedons. As the detrital crystalline clays in these soils are isotopically distinct from any neoformed clay species, the 18O content of both crystalline and poorlycrystalline clay species was used to explain the observed clay mineral variations within the sola. The 18O content of the crystalline clay separates indicated the observed inter-horizon variability in content of the individual phyllosilicate mineral species was a result of physical translocation rather than authigenesis. The interpretation of neoformation of beidellite in E horizons was not supported by the 18O data. Changes in measured 18O content of clay separates following selective dissolution provided evidence, however, of authigenic poorly crystalline components. Intra-profile variation in the 18O signature of the authigenic clays is explained in terms of evaporative effects on 18O enrichment of the soil water.