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

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