Weathering rates from top to bottom in a carbonate environment |
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| Authors: | Bernard Bourdon Sarah Bureau Morten B Andersen Eric Pili Amélie Hubert |
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| Institution: | 1. Laboratoire d''Hydrologie et de Géochimie de Strasbourg, EOST, Université de Strasbourg and CNRS, 1 rue Blessig, 67084 Strasbourg Cedex, France;2. Laboratoire de Biogéochimie des Ecosystèmes Forestiers, INRA, Champenoux, France;1. State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China;2. Department of Geology, University of Kansas, Lawrence, USA;3. Crawford Hydrology Laboratory, Western Kentucky University, Bowling Green, USA;4. Department of Geological Sciences, University of Florida, Gainesville, USA;5. Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin, China;6. School of Geographical Sciences, Southwest University, Chongqing, China;7. Institute of Geological Sciences, Freie Universität Berlin, Berlin, Germany;1. U.S. Geological Survey, Geology and Environmental Change Science Center, Box 25046, MS963, Denver Federal Center, Denver CO 80225-0046, United States;2. Department of Civil, Environmental and Architectural Engineering, University of Colorado, ECOT 441, Boulder, CO 80309-0428, United States;3. U.S. Geological Survey, Mineral and Environmental Resources Science Center, Box 25046, MS963, Denver Federal Center, Denver CO 80225-0046, United States;1. Université Paris-Sud, Laboratoire GEOPS, UMR 8148 – CNRS, F-91405 Orsay, France;2. Equipe de Géochimie des Isotopes Stables, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ Paris Diderot, UMR 7154 CNRS, F-75005 Paris, France;3. Equipe de Géochimie et Cosmochimie, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ Paris Diderot, UMR 7154 CNRS, F-75005 Paris, France;4. Institut Universitaire de France, France;5. Unité de Modélisation du Climat et des Cycles Biogéochimiques, Université de Liège, Bât B5c, 17 allée du Six Août, 4000 Liège, Belgium |
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| Abstract: | This study investigates U-series, Sr isotopes, major and trace elements in a chalk aquifer system located in Eastern France. Soil and rock samples were collected along depth profiles down to 45 m in four localities as an attempt to investigate the weathering processes in the soil, the unsaturated zone and the saturated zone of the aquifer. Interstitial water was extracted from soils and rocks by a centrifugation technique. U-series offer a powerful tool to calculate weathering rates because the relative mobility of the U- and Th-isotopes can be precisely measured and it does not require the determination of a reference state as in other approaches. As expected, the data show very large mobile element depletion in the soil with large 230Th excess relative to 238U, while the rocks show more limited but not insignificant mobile element depletion. The U-series data have been used to constrain weathering rates based on a 1-D reactive transport model. Weathering rates in the near surface are about 10–100 times faster than at depth. However, when integrated over the depth of the cores, including the unsaturated and the saturated zones, this underground weathering represents more than 30% of the total weathering flux, assuming congruent dissolution of carbonates. The (234U/238U) ratios in interstitial water are consistent with solid samples showing 234U depletion near the surface and an excess 234U at depth. A leaching experiment performed on chalk shows that the excess 234U in natural waters percolating through carbonate rocks results both from preferential 234U leaching and direct recoil in the interstitial water. A new approach was used to derive the recoil ejection factor based on BET measurements and the fractal dimension of chalk surface. Consideration of preferential leaching and recoil allows a more accurate modeling of weathering rates. |
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