SNOWBALLS IN THE DEVIL’S ANUS AND OTHER ADVENTURES IN THE WORLD OF PEDOGENIC SULFATES

Gypsic soils occupy more than 2x10⁸ hectares worldwide. Pedogenic gypsum can form in any type of parent material, and is most common in arid and semi-arid climates. Accumulation of gypsum in soil can affect all physical and chemical characteristics and produce adverse effects on both agricultural and engineering uses. In this study, we examined pedogenic gypsum in Quaternary to modern soils from Nevada, California, New Mexico, northern Jordan; and in Eocene paleosols from northeastern Mexico. Methods used included field descriptions, petrographic microscope, SEM/EDS, XRD, and Electron Microprobe (EPMA/WDS) analyses. In non-gypsic parent materials, with increasing time (and sulfate input), pedogenic sulfate minerals form 0.5-3 mm spherical accumulations of crystals termed snowballs; stage II nodules; and massive, indurated horizons. The snowball morphology is a characteristic feature of sulfate minerals, and contrary to a previous study, soil microorganisms are not necessary for snowball formation. However, research in Jordan suggests that shallow accumulations of pedogenic gypsum may act as an oasis – wherein the hygroscopic nature of the gypsum attracts water vapor and creates micro-environments for organisms. This morphology is also found at a smaller scale: micro-snowballs (100-500 µm spherical accumulations of tabular crystals) form during intense evaporation and precipitation events. Soil morphology of pedogenic gypsum should not be the only criterion used to determine relative age of soil development because the morphology is not only time-dependant, but also controlled by the input of gypsum into the soil (determined by the mechanism of transport of gypsum to the site of pedogenesis and/or the nature of the parent material). Pedogenic sulfate minerals and other salts are not only important in modern and Quaternary soils, but can persist into the rock record and can be used for a multitude of paleoenvironmental, and tectonic applications. For example, Eocene paleosols in the Carroza Formation in Mexico (Devil’s Anus) contain pedogenic gypsum that accumulated during periods of uplift and exposure of the La Popa diapir. Snowballs, stage II nodules, and natric horizons are preserved and provide insight into the tectonic history of the diapir.