Assessing Soil Genesis by Rare‐Earth Elemental Analysis

The soils of the St. Francois Mountains in Missouri are developed in loess and rhyolite residuum. Loess contributions may be expected to modify the texture, clay mineralogy, and nutrient availabilities, awarding characteristics significantly different from soils developed entirely in rhyolite residuum. The objective of this investigation is to assess the validity of using the rare‐earth elements (REEs) as a geochemical indicator to discriminate between two contrasting parent materials. Elemental analysis of the whole soil, the rhyolite residuum, and a representative loess deposit were analyzed by instrumental neutron activation analysis (INAA) and X‐ray fluorescence (XRF). Whole soil Ti contents are consistent with the local loess and incompatible with rhyolite residuum as the only parent material. The REE signatures of the whole soil are consistent with a mixture of local loess and rhyolite residuum. The clay fractions show a preferential REE accumulation, suggesting the REE may co‐illuviate with clay across soil profile boundaries. Neodymium, and to a smaller extent Eu, show dramatic associations with illuviated clay, altering the REE signatures and their interpretation. However, interhorizon transfers of the REE because of lessivage are not sufficient to render the use of the REE signatures ineffective for recognizing loess as a parent material. The migration potential of the REE must be further investigated before the method may be accepted as a diagnostic tool for recognizing lithologic discontinuities, especially for extremely weathered soils.