Water geochemistry of the Xijiang basin rivers, South China: Chemical weathering and CO2 consumption

The Xijiang River, the mainstream of the Zhujiang (Pearl) River, which is the second largest river in China in terms of discharge, flows through a large carbonate rock region in South China. The chemical and Sr isotopic compositions of the Xijiang waters were determined during the high-flow season in order to understand the chemical weathering processes, associated CO2 consumption and anthropogenic influences within the carbonate-dominated basin. The major ion compositions of the river waters are characterized by the dominance of Ca2+, Mg2+, HCO3-; and are significantly rich in SO42-. The SO42- is mainly derived from the oxidation of sulfide minerals and acid precipitation caused by coal combustion. Chemical and Sr isotopic compositions of the river waters indicate that four reservoirs (carbonates, silicates. evaporites and anthropogenic inputs) contribute to the total dissolved loads. The chemical weathering rates of carbonates and silicates for the Xijiang basin are estimated to be approximately 78.5 and 7.45 ton km(-2) a(-1), respectively. The total chemical weathering rate of rocks for the Xijiang basin is approximately 86.1 ton km(-2) a(-1) or 42 mm ka(-1), which is much higher than global mean values. The budgets of CO2 consumption by carbonate and silicate weathering are estimated to be 284 x 10(9) and 54.3 x 10(9) mol a(-1). respectively. It would appear that H2SO4 is involved as a proton donor in weathering reactions in the Xijiang basin; calculated results show that the contribution of cations from rock weathering induced by H2SO4 accounts for approximately 11.2%. Results from this study show that the flux of CO2 released into the atmosphere is approximately 0.1 x 10(6) mol C km(-2) a(-1) or 0.41 x 10(12) g C a(-1) produced by H2SO4-induced carbonate weathering in the basin. When extrapolated to the entire surface area of carbonate in SW China, the flux of CO2 released to the atmosphere by H2SO4-induced carbonate weathering is about 1.41 x 10(12) g Ca-1. (C) 2010 Elsevier Ltd. All rights reserved.