Chemical weathering in the upper reaches of Xijiang River draining the Yunnan–Guizhou Plateau, Southwest China

Abstract

Chemical and strontium isotopic compositions of Nanpanjiang River and Beipanjiang River in the upper reaches of Xijiang River draining the Yunnan–Guizhou Plateau, Southwest China, were measured with a main purpose to understand the rock weathering and associated CO 2 consumption rates in the karstic landform-dominated area. The chemical ion composition of the river water is characterized by dominance of Ca 2+, Mg 2+ and HCO 3 −, and SO 4 2−, which account for more than 90% of total ion concentration. 87Sr/ 86Sr ratios of Nanpanjiang River water range between 0.7080 and 0.7140, while those of most Beipanjiang River water vary within a limited range from 0.7075 to 0.7085. Weathering of carbonate rocks dominate chemistry of major ions especially in the water of Beipanjiang River, whereas weathering of silicate minerals in the upper reaches of Nanpanjiang River basin is obvious in addition to carbonate weathering, mainly according to the Sr isotopic composition of the river water. Analyses of the stoichiometry of the river water show that the water chemistry is controlled by carbonate dissolution not only by carbonic but also by sulfuric acid, and indicate that sulfuric acid plays an important role in carbonate weathering. The sulfate in river water is mainly from atmospheric input by coal-combustion industries and from oxidation of sulfide minerals during weathering of coal-containing strata and coal mining in the catchment. The chemical weathering rates of silicate and carbonate and associated CO 2 consumption rates by both carbonic and sulfuric acid and by only carbonic acid are respectively estimated. The results show that chemical weathering rates of carbonate and silicate weathering in Beipanjiang River basin are higher than those of Nanpanjiang River basin, showing linkages of geology and geography in the catchment to crustal weathering. The involvement of sulfuric acid in carbonate weathering greatly enhances the carbonate weathering but lowers the CO 2 consumption rates, indicating that sulfide acid is an important agent to rock weathering and the role of sulfide acid should further be clarified in view of the regional and global budget of CO 2.