Geochemical indicators of external additions to soils in Big Bend National Park, Texas

Soil genesis in semiarid climates must consider the influences of aeolian and other forms of sediment deposition, particularly the emplacement of dust and dissolved constituents in rain and sediment trapped by overland flow. The purpose of this study was to explore the possibility of using Ti-indexed elemental concentrations of the fine earth, whole clay fractions, and rock fragments extracted from the soil to propose that the soil's elemental composition is incompatible with its development exclusively from the underlying igneous parent material. The soil, a member of the Brewster series (Loamy-skeletal, mixed, superactive, thermic Lithic Haplustolls), is located in the Chisos Mountains of Big Bend National Park, Texas. The soil profile descriptions, chemical properties and clay mineralogy are consistent with a limited degree of soil weathering. However the total Ti-indexed concentrations of the macro-elements and the rare earth elements indicate a substantial elemental depletion, suggesting that weathering has been prolonged and intensive. Titanium concentrations in the clay fraction are substantially greater than those of the whole soil, suggesting that external clay additions inflate both the Ti content of the whole soil and the elemental loss estimates. The evolution of these soils is, in part, predicated on external clay additions, with sediment accumulation caused by the trapping of overland flow the most plausible scenario.