Determining the strontium isotopic composition of rivers draining the Himalaya and Tibet has been subject of sustained attention because of the control these rivers exert on seawater (Palmer and Edmond, 1989). This study is focussed on the major chinese rivers because they are characterized by intense weathering and erosion, compared with other large rivers of the world (Zhang et al., 1990). The results obtained are placed in a global context in order to increase our knowledge of chemical weathering, erosional processes and the chemical composition of the continental crust.
Here, we report preliminary results for Sr isotopes, and major and trace element concentrations in both the dissolved and particulate phases transported by the Huanghe river, the Changjiang river and the Pearl river, sampled during November 1993 and August 1994. During the dry season, the 87Sr/86Sr ratios of the soluble phase range between 0.7097 and 0.7113, compared with 0.7106 and 0.7111 during wet season. For both seasons, the lowest isotopic ratios found are for the Pearl river.
The range of Sr isotopic ratios for the insoluble phase are between 0.7147 (Huanghe river) and 0.7219 (Changjiang river) with small differences dependent on the season. The 87Sr/86Sr ratios of soluble versus insoluble phase displays a clear negative correlation because of the low isotopic variation of the Huanghe river between both phases.
87Sr/86Sr ratios and chemical element ratios (e.g. Ca/Na, Ca/Sr, Cl/Na) of the soluble phase display good correlations which are most simply interpreted as mixing curves. A mixing model between four end-members (rainwater, water draining silicates, limestones and evaporites) similar to that of Négrel et al. (1993) in the Congo basin, resolved by an inversion scheme, allows us to estimate the proportion of each element originating from each reservoir. The main result is, in the large rivers of China, the Sr isotopic composition is dominated by carbonate dissolution, especially for the Pearl river where the Sr derived from the silicate rocks represents only 11±3% of the total Sr.
The weathering rate for each lithology can be calculated, and in particular the net consumption of CO2 by the silicate weathering. For the Pearl river, we obtain a value of about 0.7.103 mol/km2/y, which is much lower than estimates of Congo and Amazon (Gaillardet et al., 1995).
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