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We systematically investigated the Sr isotopic characteristics of a smallsilicate watershed, the Xishui River a tributary of the Yangtze River, and asmall carbonate watershed, the Guijiang River a tributary of the Pearl River.The results show that the two rivers have uncommon Sr isotopiccharacteristics compared with most small watersheds. Specifically, thesilicate 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 insummer 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) andhigh 87Sr/86Sr ratios (0.710558 to 0.724605). As the 87Sr/86Sr ratios in the Xishui River are lower than thosein seawater, the 87Sr/86Sr ratio of seawater will decrease afterthe river water is transported to the oceans. Previous studies have alsoshown that some basaltic watersheds with extremely high chemical weatheringrates reduced the seawater Sr isotope ratios. In other words, rivercatchments with high silicate weathering rates do not certainly transporthighly radiogenic Sr into oceans. Therefore, the use of the variations inthe seawater 87Sr/86Sr ratio to indicate the continentalsilicate weathering intensity may be questionable. In the Guijiang River catchment, the 87Sr/86Sr ratios of carbonaterocks and other sources (rainwater, domestic and industrial waste water, andagricultural fertilizer) are lower than 0.71. In comparison, somenon-carbonate components, such as sand rocks, mud rocks, and shales, haverelatively high Sr isotopic compositions. Moreover, granites accounted foronly 5% of the drainage area have extremely high 87Sr/86Srratios with an average of greater than 0.8. Therefore, a few silicatecomponents in carbonate rocks obviously increase the Sr isotopiccompositions of the river water

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