Towards a chemical weathering rate chronometer: A U-series investigation of catena development in Kruger Park, South Africa

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Scientific Merit: We propose a pilot study that will make a significant contribution towards the development of a new chemical weathering rate chronometer and address fundamental issues involving chemical weathering and soil development on hillslopes. We will measure U-series isotopes on carefully selected, well-characterized set of soil profiles sampled from the South African savanna. The soils comprise an exceptional example of a topographic catena sequence, where soils have differentiated through long-term downslope redistribution of solutes and weathering products. These sequences are only now being quantified by modern geochemical and geomorphic approaches. This represents a major step forward for geochemical studies of weathering profiles, which have traditionally focused on ridgetops or geomorphically stable surfaces, which represent a small fraction of Earth?s surface. Uranium-decay-series isotopes have emerged in the last decade as a powerful tracer of Earth processes on timescales shorter than one million years. The tool has been underexploited for studying surface processes, but recent studies have shown great promise. Most of these studies have approached weathering from the riverine perspective, seeking to quantify landscape-integrated weathering and erosion rates from U-series disequilibria in suspended sediments and river water. These studies inherently depend on an understanding of the processes that produce disequilibria by fractionating U-series elements (U, Th, and Ra) in soils, but this understanding is lacking. The few studies that have been done on U-series isotopes in soils often lack the geomorphic, pedogenic, and geochemical characterization required to develop robust interpretations of U-series data. They will measure trace element (including U, Th, and Nb) concentrations on 43 samples from 9 soil pits comprising a 1km transect of a well-characterized catena sequence. These data will be used to select 20 samples for full U-series disequilibria (234U/238U, 230Th/234U, and 226Ra/230Th) analyses. These isotope ratios are sensitive to trace element redistribution on timescales of roughly 1 ma, 500 ka, and 10 ka, respectively. These time-sensitive data, plus additional soil physical (textural, hydrologic) and chemical (mass balance) data will be used to develop and test box models of catena differentiation. Theit approach is in contrast to prior applications of U-series data to weathering profiles that have sought to determine soil ages, an approach that is overly sensitive to model assumptions. Instead, soil age here is constrained by existing cosmogenic 10Be data. The 10Be-derived ~220ka soil age in this environment is well-suited to the applicable time scale of U-series data. Broader Impact: This project will support lab costs for work initiated during the PI?s sabbatical at the Woods Hole Oceanographic Institution, where he will begin a new collaboration with WHOI scientists Bernhard Peucker-Ehrenbrink and Ken Sims. Importantly, the data collected and ideas developed here are intended to be applicable beyond a single set of soils in South Africa, and should improve our understanding of hillslope-scale weathering processes in general, and provide much-needed constraints on weathering fractionations for future studies of U-series isotopes in river studies. Based on data collected in this pilot proposal, the PI intends to submit a larger proposal that will support a future PhD student at BU to further develop U-series applications in the weathering environment.