Volume 30 Issue 9 - September 2, 2016 PDF
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Evidence for stable Sr isotope fractionation by silicate weathering in a small sedimentary watershed in southwestern Taiwan
Hung-Chun Chao1, Chen-Feng You2,3,*, Hou-Chun Liu2,3, Chuan-Hsiung Chung2,3
1 Department of Earth and Environmental Sciences, National Chung Cheng University,
Chiayi, Taiwan.
2 Earth Dynamic System Research Center, National Cheng Kung University, Tainan,
Taiwan.
3 Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan.
 
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Radiogenic Sr isotopes (87Sr/86Sr) are robust for provenance identification in hydrology, affected mainly by the age of background lithologies and the degree of chemical weathering. However, there is limited knowledge concerning the fractionation mechanism of stable Sr isotopes (88Sr/86Sr) in rivers. In this study, river water was collected on a weekly to monthly basis throughout dry and wet seasons. Furthermore, to study the variations of radiogenic and stable Sr isotopes during intense weathering, a major flooding event (2000 mm precipitation in three days, Typhoon Morakot), water was captured within a small drainage catchment system (161 km2) along the Hou-ku River in southwestern Taiwan. For a better constraint on the end member compositions, bedload sediments, suspended particles, and several host rocks were sampled for a systematic investigation. The carbonate and silicate phases of these solids were chemically separated. Dissolved major elements indicate that the watersheds were predominated by silicate weathering. Stable Sr isotopes show no significant variation (δ88Sr = 0.24 – 0.31 ‰) temporally and spatially with an average of 0.28 ‰. Additionally, all solids showed lower δ88Sr values than the river water while the host rocks had higher δ88Sr values (δ88Sr = 0.20 – 0.26 ‰) than the residual weathering products (δ88Sr = 0.08 – 0.22 ‰), indicating preferential leaching of heavy Sr into the hydrosphere and leaving light Sr in the residual solids. Results of laboratory acid leaching experiments reveal that dissolution of high δ88Sr value minerals occurred at an early stage of weathering. The variation of weathering intensity does not alter stable Sr isotopes in silicate weathering dominated river water, which contains higher stable Sr isotopes than the associated sediments. The silicatic sedimentary rocks preferentially released higher stable Sr isotopes into the hydrosphere during chemical weathering, thus leaving lower stable Sr isotopes in the residual solids.
Figure 1, Plot of Mg/Na versus Ca/Na in logarithmic scale for all water samples in this study. All samples fall on or close to the end member of silicate weathering.
Figure 2, Plot of (a) 87Sr/86Sr; (b) d88Sr versus 1/Sr for river water samples in this study. The composition of mud volcano fluid is shown for reference. The bars denote the range of the reproducibility (2r). For panel (a), the error bar is smaller than the symbol. The d88Sr values show no variation with Sr concentration.
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