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Strontium isotope compositions of river waters as records of lithology-dependent mass transfers : the Garonne river and its tributaries (SW France)

Semhi , Khadija and Clauer, Norbert and Probst, Jean-Luc Strontium isotope compositions of river waters as records of lithology-dependent mass transfers : the Garonne river and its tributaries (SW France). (2000) Chemical Geology, vol. 168 (n° 3-4). pp. 173-193. ISSN 0009-2541

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Official URL: http://dx.doi.org/10.1016/S0009-2541(00)00226-6

Abstract

The relation of lithology in the drainage basin to the dissolved load of the Garonne river and its main tributaries, in southwestern France, was evaluated by determining 87Sr/86Sr ratios, and concentrations of major and trace elements during a 2-year-long survey. In the upper drainage basin, the Garonne river waters were isotopically varied at two observation points: 0.71131+/−0.00030 (2σ) for 84+/−18 ppb (2σ) and 0.71272+/−0.00044 for 86+/−10 ppb, respectively. In the lower drainage basin, the Garonne river waters were isotopically identical at three observation points at 0.71020+/−0.00024 for 125+/−22 ppb. By contrast, the tributaries (Lot, Truyère, Aveyron, Arriège, Gers and Salat) are widely varied in their 87Sr/86Sr ratios and Sr concentrations from 0.70836+/−0.00049 to 0.71058+/−0.00057, and from 18+/−8 to 280+/−116 ppb. The Sr isotope ratios and concentrations suggest a dominant supply of two reservoirs of Sr, one of which is with low 87Sr/86Sr ratios and high Sr contents that is typically characteristic of carbonate rocks, and the other with high 87Sr/86Sr ratios and low Sr concentrations that is characteristic of felsic rocks. Locally as in the Lot waters draining the Massif Central and within the Pyrénées mountains, a third source of Sr from mafic rocks may be involved. Mass-balance calculations based on the mean 87Sr/86Sr ratios and contents of the dissolved Sr, and on the mean discharges integrating the 2-year survey, suggest that contribution of the silicate reservoir amounts 3–8% of the total dissolved Sr flux. Mass-balance calculations also suggest that variation in the supply of Sr from either of the two major reservoirs does not exceed the analytical uncertainty at about +/−5%. The 87Sr/86Sr ratios of HCl and NH4Cl leachates of suspended loads of the Garonne river are different from that of the associated dissolved Sr. This leaching-related supplementary Sr represents less than 10% of the total amount of Sr transported by the Garonne waters. The Sr isotope characteristics of the leachates are probably records of an intermediate pedogenic episode in the weathering-erosion process occurring in the Garonne drainage basin.

Item Type:Article
Additional Information:Thanks to Elsevier editor. The definitive version is available at http://www.sciencedirect.com The original PDF of the article can be found at Chemical Geology website : http://www.sciencedirect.com/science/journal/00092541
Audience (journal):International peer-reviewed journal
Uncontrolled Keywords:
Institution:French research institutions > Centre National de la Recherche Scientifique - CNRS
French research institutions > Institut de Recherche pour le Développement - IRD
Other partners > Université Louis Pasteur-Strasbourg I - ULP (FRANCE)
Université de Toulouse > Université Paul Sabatier-Toulouse III - UPS
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Deposited By:Florence Amor

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