Soil formation and erosion in central Texas: insights from relict soils and cave deposits

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Cooke, Mary Jennifer

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Cave deposits containing sediments of eroded soils and relict soils preserved on modern uplands provide some of the few insights into understanding the Quaternary evolution of central Texas landscapes and environments. This study investigates an ancient soil-erosion event on the Edwards Plateau of central Texas by using a new approach that relies on the strontium isotope composition of the ancient soil as an indicator of ancient soil thickness. I found the strontium isotope composition of soil on the Edwards Plateau today varies with soil thickness. I also found that variations in strontium isotope compositions of fossil plants and animals provide a record of temporal changes in soil thickness. Furthermore, I present stable carbon, oxygen, and nitrogen isotope data measured on sediments and fossils from a central Texas cave-fill deposit that support the hypothesis that increases in aridity and increases in precipitation intensity may have facilitated massive, late Quaternary soil erosion on the Edwards Plateau. The origin of the former thick soil cover that was removed by the erosion is poorly understood. This study uses the geographic distribution and the textural and mineralogical properties of relict thick soils along with their neodymium isotope compositions, and rare earth element concentrations to determine the silicate source of the former thick soils. I propose that the Del Rio Clay, a locally-eroded, stratigraphically-higher, clay-rich strata provided the silicates to form thick soils overlying the silicate-pure Edwards Limestone. Four important conclusions of this study are that: 1) in areas where the soil and bedrock are distinct in their strontium isotope composition, the strontium isotope composition of the soil varies with soil thickness, 2) the strontium isotope composition of terrestrial fossils can be a useful tool to reconstruct temporal changes in soil thickness, 3) climate variability can facilitate massive soil erosion with regional environmental consequences, and 4) soil formation from now-eroded bedrock of a higher stratigraphic unit may be more common than previously recognized. Thus, in light of future changes in climate and land use, soil conservation strategies should be considered in regions where soils rest on relatively pure limestones, because they are likely a “non-renewable” resource.