Age, genesis, and archaeological geology of the sandy mantle on the Gulf Coastal Plain of Texas.
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Ahr, Steven Wayne, 1970-
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Alfisols exhibiting strong textural contrasts between sandy A-E horizons (sandy mantle) and clayey Bt horizons on the Gulf Coastal Plain have invoked debate among pedologists, geologists, and archaeologists about parent material uniformity, the age and origin of the sandy mantle, and the scientific value of buried archaeological sites. This dissertation addresses these issues, as well as climate-related variability in the mass-balance of soil constituents, across a 700-mm mean annual precipitation (MAP) gradient. Five Alfisol pedons located on upland summit settings of the Gulf Coastal Plain of Texas were sampled, described, and analyzed. Depth trends in clay-free particle-size classes and Ti and Zr failed to reveal lithologic discontinuities in each pedon, suggesting that the A-E and Bt horizons are genetically related and the textural contrasts resulted from pedogenic, rather than depositional, processes. Thus, any buried archaeological materials would have moved down the profile under the influence of bioturbation and gravity. Optical dating of the soil profiles illustrates complex age structures. Mixing in the A-E horizons effects apparent OSL ages <10,000 years due to admixture of recently and partially bleached quartz grains. Decreased mixing occurs within Bt horizons, and ages increase systematically with depth. Pedogenic weathering alters 40K, U and Th content and environmental dose rates, with concomitant changes in OSL ages. Apparent maximum age calculations from the probable oldest equivalent dose group in soil parent materials suggest the soils began weathering at least by 90-112 ka, and 40-62 ka, which greatly precedes human entry into North America. Mass-balance trends revealed nearly 100% weathering losses of Ca, Mg, Na, Al, and Fe from the A-E horizons, with redistributions and variable gains in Bt horizons. Elemental flux is positively correlated with net mass flux of fine clay and MAP, likely due to increased weathering-related concentrations of negatively-charged clay particles in higher-MAP soils, which facilitate cation adorption and retention. This has implications for predicting pedogenic response to global climate change and for estimating important soil properties in Alfisol-like paleosols in the geologic record.