INTERPRETATION OF DYNAMIC LATE QUATERNARY CLIMATE AND LANDSCAPE VARIABILITY BASED UPON PALEOSOL MACRO- AND MICROMORPHOLOGY AND STABLE ISOTOPES OF SOIL ORGANIC MATTER, OWL CREEK, CENTRAL TEXAS, USA

The late-Quaternary portion of the central Texas Owl Creek strata are characterized by fluvial and colluvial deposition punctuated by landscape stability and soil formation. Analysis of paleopedological features in buried soils, or paleosols, provides valuable insight into past soil-forming processes and potentially climate. Integration of macro-scale (field) and micromorphological-scale (thin-section) observations with geochemical data was the methodological approach taken in this study. Data analysis included optically stimulated luminescence (OSL) dating, field descriptions, micromorphology, particle size, stable carbon isotopes of soil organic matter (SOM) and pedogenic carbonate, bulk geochemistry (XRF), and x-ray diffraction (XRD). In a key stratigraphic exposure of Owl Creek, OSL dating constrains deposition to between 44,010±3485 and 8570±710 years, corresponding to marine oxygen isotope stages 3 and 2. The oldest paleosol (Ab3) is characterized by a thick layer of carbonate rich-silty yellowish brown clay loams resting on cross-bedded Pleistocene gravels.The overlying paleosol (Ab2) is a thin (60 cm thick), brown-colored silty clay characterized by prominent calcium carbonate rhizoliths. The uppermost buried soil (Ab1) is a well-developed carbonate-rich silty clay with strongly developed prismatic to wedge structure and clay films. The Ab1 soil well-developed paleosol may correspond to the widespread Royalty paleosol mapped in central Texas. Micromorphological observations suggest that the youngest paleosol is most strongly developed and indicates a period of prolonged landscape stability. Each paleosol has highest total organic C within the associated A horizons. Stable C isotopes of soil organic matter indicate mixed C3 and C4 plant contributions to the soil carbon pools, suggesting moisture and temperature conditions becoming more similar to that of the modern ecosystem. The upper paleosol is overlain by an alluvial unit encompassing the Pleistocene/ Holocene boundary and has gravel stringers within mixed detrital carbonate-silicate sand. This study suggests that absolute dating of fluvial channel and floodplain sediments, combined with stratigraphic, isotopic, and paleopedological methods, can be a useful archive of past late Pleistocene climates.