Soil genesis, optical dating, and geoarchaeological evaluation of two upland Alfisol pedons within the Tertiary Gulf Coastal Plain

Much of the Gulf Coastal Plain is mantled by sandy A and E soil horizons overlying finer-grained Bt horizons. Researchers have applied optical stimulated luminescence (OSL) dating to a variety of depositional settings, such as hillslopes and terraces, in an attempt to understand depositional versus pedogenic origins of the sandy horizons, and archaeological burial processes. These investigations have documented a range of integrity conditions from pristine to highly bioturbated. However, little work has been conducted in upland summits. Thus, we integrated field morphological descriptions, constituent mass balance reconstruction, and optical dating to reconstruct the timing and pedogenic pathways of two summit Alfisols on the Tertiary Gulf Coastal Plain in Texas. Investigations reveal that the sandy A and E horizons and the Bt horizons in each pedon are genetically related, with textural contrasts resulting from pedogenic processes. Optical dating revealed highly complex age structures strongly influenced by bioturbation, weathering, and the presence of coarse clay plus total silt particle-sizes. Pedogenic mixing is extensive in the A and E horizons, which effect apparent OSL ages <10,000 years due to admixture of recently and partially bleached grains to the matrix. Decreased mixing occurs within the argillic horizon of each pedon, and ages increase systematically with depth. In all horizons pedogenic weathering and mixing have altered the K-40, U and Th content and environmental dose rates, with concomitant changes in OSL ages. Apparent maximum age calculations provide insight into the most probable oldest equivalent dose group and suggest that the investigated soils began to weather no later than 90-112 ka, and 40-62 ka, respectively, which greatly precedes human entry of into North America. High overdispersion of equivalent dose values in the sandy horizons reflects on-going pedoturbation, which varies between pedons and may result in differential archaeological preservation potential. (C) 2012 Elsevier B.V. All rights reserved.