Sr isotopic characteristics in two small watersheds draining silicate and carbonate rocks: implication for studies on seawater Sr isotopic evolution

• Main Authors: W. H. Wu, H. B. Zheng, J. H. Cao, J. D. Yang
• Format: Article
• Language: English
• Published: Copernicus Publications 2014-02-01
• Series: Hydrology and Earth System Sciences
• Online Access: http://www.hydrol-earth-syst-sci.net/18/559/2014/hess-18-559-2014.pdf

We systematically investigated the Sr isotopic characteristics of a small silicate watershed, the Xishui River a tributary of the Yangtze River, and a small carbonate watershed, the Guijiang River a tributary of the Pearl River. The results show that the two rivers have uncommon Sr isotopic characteristics compared with most small watersheds. Specifically, the silicate watershed (Xishui River) has relatively high Sr concentrations (0.468 to 1.70 μmol L⁻¹ in summer and 1.30 to 3.17 μmol L⁻¹ in winter, respectively) and low ⁸⁷Sr/⁸⁶Sr ratios (0.708686 to 0.709148 in summer and 0.708515 to 0.709305 in winter). The carbonate watershed (Guijiang River) has low Sr concentrations (0.124 to 1.098 μmol L⁻¹) and high ⁸⁷Sr/⁸⁶Sr ratios (0.710558 to 0.724605). <br><br> As the ⁸⁷Sr/⁸⁶Sr ratios in the Xishui River are lower than those in seawater, the ⁸⁷Sr/⁸⁶Sr ratio of seawater will decrease after the river water is transported to the oceans. Previous studies have also shown that some basaltic watersheds with extremely high chemical weathering rates reduced the seawater Sr isotope ratios. In other words, river catchments with high silicate weathering rates do not certainly transport highly radiogenic Sr into oceans. Therefore, the use of the variations in the seawater ⁸⁷Sr/⁸⁶Sr ratio to indicate the continental silicate weathering intensity may be questionable. <br><br> In the Guijiang River catchment, the ⁸⁷Sr/⁸⁶Sr ratios of carbonate rocks and other sources (rainwater, domestic and industrial waste water, and agricultural fertilizer) are lower than 0.71. In comparison, some non-carbonate components, such as sand rocks, mud rocks, and shales, have relatively high Sr isotopic compositions. Moreover, granites accounted for only 5% of the drainage area have extremely high ⁸⁷Sr/⁸⁶Sr ratios with an average of greater than 0.8. Therefore, a few silicate components in carbonate rocks obviously increase the Sr isotopic compositions of the river water.