Regolith production rates calculated with Uranium-series isotopes at Shale Hills Critical Zone Observatory: implications on chemical weathering and landscape evolution
Abstract
It is essential to understand the rates and mechanisms of regolith formation as it contributes to important Critical Zone processes such as nutrient cycling, carbon sequestration, erosion, and acid rain mitigation. Over the long term, the rates of weathering and erosion also combine to control the evolution of landscapes. Uranium-series isotopes offer a powerful tool to investigate regolith production rates and residence times within a weathering system because of their well-documented fractionation behavior during chemical weathering and transport by water. Here, we present a study of U-series isotopes (238U, 234U and 230Th) in soils developed on shale bedrock at the Shale Hills catchment in central Pennsylvania. (234U/238U) and (230Th/238U) activity ratios in samples collected from soil profiles along a planar hillslope in the catchment range from 0.934 to 1.072 and from 0.920 to 1.096, respectively. These values show significant U-series disequilibrium and are explained by a dual process of U-series isotopes during weathering: a loss of 234U, 238U, and 230Th during water-rock interactions and a gain of 234U and 238U as U precipitated from circulating soil water. Regolith production rates calculated with U-series isotopes for these soil profiles range from ~15 to 45 m/Myr and decrease systematically with increasing distance from the ridge top to the middle slope and to the valley floor. Soil residence times within these profiles increase from the ridge to the valley floor. The regolith production rates at Shale Hills vary as an exponential function of soil thickness, similar to the soil production functions derived from many other studies. With the local soil production function at Shale Hills, a hillslope soil transport model is used to predict the landscape evolution and change of soil thickness along the planar transect. The simulation results suggest that both the landscape and soil thickness along the planar hillscope at Shale Hills are currently at a transient state. U-series disequilibrium isotopes thus provide an independent means to determinate the time scales of shale weathering and regolith formation for this watershed. The regolith production rates provide an important reference value along both the lithology and climate gradients in the network of Critical Zone Observatories, for the case of shale weathering under a temperate climate. Furthermore, this study also enhances our understanding of the behavior of U-series isotopes during shale weathering and element transport, the response of soil production to anthropogenic and climate perturbations, as well as the evolution of soil thickness and landscape for this first-order catchment.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMEP51E..08M
- Keywords:
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- 1039 GEOCHEMISTRY / Alteration and weathering processes;
- 1120 GEOCHRONOLOGY / Isotopic disequilibrium dating;
- 1625 GLOBAL CHANGE / Geomorphology and weathering