USING COMPARATIVE MICROMORPHOLOGY AND GEOCHEMISTRY TO DISTINGUISH PALEOCATENA RELATIONSHIPS IN BARITE-BEARING PALEOSOLS

A modern barite-bearing soil toposequence in College Station, TX is examined in order to interpret lateral variability in barite-bearing Morrison Fm. (Jurassic) paleosols associated with lacustrine deposits in north-central Wyoming, for which subtle topographic and hydrological differences are inferred to have existed across the paleolandscape. Barite is generally considered an uncommon mineral in soils and paleosols, but in recent years has become increasingly recognized in aquic soil orders that develop on stable, low-gradient landscapes. In spite of increasing reports, the paleoclimate and paleohydrologic significance of barite is still not well understood. Primary objectives of this study are to document the micromorphology and elemental trends of the barite-bearing soil toposequence, and to compare these data with Morrison barite-bearing paleosols to evaluate processes controlled by hydrology and topography. Geochemical mass-balance for the toposequence indicates that under seasonally reducing conditions Ba and S are mobilized and concentrated in clay-rich horizons. Epiaquatic conditions above the barite-bearing Bt horizon in both paleosols and soils result in the leaching of cations, specifically Ca²⁺, with transfer of the Ca²⁺ to hydric soils lower on the topography, where higher amounts of humic acids prevent barite precipitation and drive the precipitation of gypsum instead. Soils lower on the topography and closer to water bodies contain reducible, diffuse FeMn concentrations throughout the profile, with little or no clay translocation. Organic matter is completely oxidized in soils that develop higher on the topography, allowing the oxidation of HS^- to sulfate and subsequent barite precipitation. Clusters of typic calcite nodules precipitate above the barite-bearing Bt horizon in both soils and paleosols. The upper profiles of these soils contain typic Fe₂O₃ nodules, clay skins, and weak sepic-plasmic fabric. This study demonstrates the utility of integrated modern-ancient analog studies for differentiating subtle differences in topography and hydrological conditions with barite-bearing paleosols.