Miocene paleoenvironments and paleosol pH proxies.
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Lukens, William Ellis, 1987-
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Paleoenvironmental analysis is an interdisciplinary endeavor that draws on sedimentology, stratigraphy, geochemistry, and geostatistics. This dissertation includes two paleoenvironmental case studies and the development of a new suite of proxies for soil pH. The first case study was performed at Coffee Ranch, the site of a late Miocene (6.6 Ma) bonebed in the Texas Panhandle. Using sedimentology, stratigraphy, and paleopedology, the depositional environments at Coffee Ranch were reconstructed as a fluvial system with seasonally-variable discharge, alluvial paleosols with seasonal water deficit, and intermittent eolian deposits. Climate state was constrained using a combination of paleosol morphological properties, micromorphological associations, pedotransfer functions for climate variables, and elemental mass-balance trends, all of which were consistent with a mean annual precipitation (MAP) nearly double modern values and a much warmer and less seasonally variable mean annual temperature (MAT) than modern conditions. An early Miocene (ca. 17.5 Ma) fossil site located at Ngira, western Kenya, was also analyzed using fluvial geomorphologic and paleopedologic techniques. The Ngira fossil locality preserves a large number of vertebrate fossils, but has a conspicuous lack of primate remains, unlike nearby contemporaneous fossil sites that contain some of the world’s best preserved early Miocene fossil apes. Using pedotransfer functions for climate variables, as well as elemental mass-balance and paleosol morphological indices, the site was reconstructed as having a strongly seasonal, dry subhumid climate. Stable carbon isotopes from pedogenic carbonates are consistent with a surprisingly large amount of C4 biomass, which was used in conjunction with the presence of microcharcoal grains and rodent community paleoecology to interpret an open-canopy habitat for the site. Finally, a soil database was analyzed, with the goal of producing predictive models for soil pH using elemental oxides as input variables. It was found that large-scale vegetation and climate patterns correlate with soil pH; however, elemental ratios traditionally used to predict MAP and MAT were shown to more closely predict pH than climate. Three new pH pedotransfer functions were applied on succession of middle and late Triassic paleosols from western Pangea, and the results indicate that a paleosols became progressively more alkaline as the megamonsoon climate system gradually diminished.