Abstract
Abstract. 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). 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. 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.
Highlights
IntroductionBy analyzing the Sr isotopic compositions of marine limestones and their shells, the evolution curve of the seawater 87Sr/86Sr ratio in the Phanerozoic was well established (Veizer and Compston, 1974; Brass, 1976; Burke et al, 1982; Elderfield and Gieskes, 1982; DePaolo and Ingram, 1985; Palmer and Elderfield, 1985; DePaolo, 1986; Hess et al, 1986; Richter and DePaolo, 1987, 1988; Raymo et al, 1988; Veizer, 1989; Capo and DePaolo, 1990; Hodell et al, 1991; Richter et al, 1992; Veizer et al, 1999; Korte et al, 2006; Melezhik et al, 2009)
As the silicate rocks in the southern Himalayas have both high 87Sr/86Sr ratios and high Sr concentrations, the rise of the Sr isotopic composition of seawater since the Cenozoic can be used as a proxy of intensified silicate weathering (Palmer and Elderfield, 1985; Raymo et al, 1988; Edmond, 1992; Krishnaswami et al, 1992; Richter et al, 1992; Harris, 1995; Blum, 1997; Galy et al, 1999; Chesley et al, 2000; English et al, 2000; Bickle et al, 2005)
If much of the radiogenic Sr in the rivers is derived from carbonate weathering, changes in the seawater Sr isotopic composition would not be a proxy of the continental silicate weathering intensity
Summary
By analyzing the Sr isotopic compositions of marine limestones and their shells, the evolution curve of the seawater 87Sr/86Sr ratio in the Phanerozoic was well established (Veizer and Compston, 1974; Brass, 1976; Burke et al, 1982; Elderfield and Gieskes, 1982; DePaolo and Ingram, 1985; Palmer and Elderfield, 1985; DePaolo, 1986; Hess et al, 1986; Richter and DePaolo, 1987, 1988; Raymo et al, 1988; Veizer, 1989; Capo and DePaolo, 1990; Hodell et al, 1991; Richter et al, 1992; Veizer et al, 1999; Korte et al, 2006; Melezhik et al, 2009). As the silicate rocks in the southern Himalayas have both high 87Sr/86Sr ratios and high Sr concentrations, the rise of the Sr isotopic composition of seawater since the Cenozoic can be used as a proxy of intensified silicate weathering (Palmer and Elderfield, 1985; Raymo et al, 1988; Edmond, 1992; Krishnaswami et al, 1992; Richter et al, 1992; Harris, 1995; Blum, 1997; Galy et al, 1999; Chesley et al, 2000; English et al, 2000; Bickle et al, 2005). If much of the radiogenic Sr in the rivers is derived from carbonate weathering, changes in the seawater Sr isotopic composition would not be a proxy of the continental silicate weathering intensity
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