Chemical weathering inferred from riverine water chemistry in the lower Xijiang basin, South China Huiguo Sun a,b, ⁎, Jingtai Han a , Dong Li a , Shurong Zhang b , Xixi Lu b a Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR China b Department of Geography, National University of Singapore, Singapore abstract article info Article history: Received 9 December 2009 Received in revised form 20 May 2010 Accepted 7 June 2010 Available online 10 July 2010 Keywords: Water geochemistry Carbonate weathering Silicate weathering CO 2 consumption Lower Xijiang basin Seasonal sampling was conducted on 13 sites involving the lower stem of the Xijiang river and its three tributaries to determine the spatial patterns of the riverine water chemistry and to quantify the chemical weathering rates of carbonate and silicate of the bedrock. Results indicate that the major ions in the Xijiang river system are dominated by Ca 2+ and HCO 3 - with a higher concentration of total dissolved solids, characteristic of the drainages developed on typical carbonate regions. Obvious spatial variations of major ion concentrations are found at various spatial scales, which are dominantly controlled by the lithology particularly carbonate distribution in the region. The four selected rivers show similar seasonal variations in major ions, with lower concentrations during the rainy season. Runoff is the first important factor for controlling the weathering rate in the basin, although increasing temperature and duration of water–rock interaction could make positive contributions to the enhancement of chemical weathering. The chemical weathering rates range from 52.6 to 73.7 t/km 2 /yr within the lower Xijiang basin and carbonate weathering is over one order of magnitude higher than that of silicates. CO 2 consumption rate by rock weathering is 2.0 × 10 11 mol/yr, of which more than 60% is contributed by carbonate weathering. The flux of CO 2 released to the atmosphere–ocean system by sulfuric acid-induced carbonate weathering is 1.1 × 10 5 mol/km 2 /yr, comparable with the CO 2 flux consumed by silicate weathering. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Chemical weathering is a key part of earth surface processes that links geologic cycling of solid earth to the atmosphere and the ocean. Rock weathering consumes CO 2 , mainly from atmospheric/soil origin, and produces aqueous HCO 3 - and CO 3 2- , then transport into the sea by rivers. On a geological time scale, the flux of CO 2 consumed by carbonate dissolution is balanced by the CO 2 flux released to the atmosphere by carbonate precipitation in the oceans, while chemical weathering of silicate rocks acts as a net sink for atmospheric CO 2 . Consumption of atmospheric CO 2 plays an important role in effect on the long-term global air temperature (Caldeira, 1995; Gaillardet et al., 1999; Amiotte- Suchet et al., 2003). Inversely, the atmospheric temperature determines the chemical weathering rate and the associated consumption of the atmospheric CO 2 . The temperature-dependence of weathering rate constitutes a negative feedback on atmospheric CO 2 (walker et al., 1981; Berner et al., 1983). The tendency and processes of global climate change are basically determined by the competing status of these factors. Hence, quantifying chemical weathering rates and clarifying the controlling factors are essential for understanding of Earth's climatic evolution. As weathering products of terrestrial rocks are transported mainly by rivers, riverine water chemistry is often used for evaluation of chemical weathering rates in a catchment (Meybeck, 1987; Gaillardet et al., 1999; Mortatti and Probst, 2003). Rivers integrate various processes taking place in a basin, both natural and anthropogenic. A large number of studies have demonstrated that natural factors (lithology, temperature, runoff, relief and vegetation), particularly lithologic variations are dominant controls of water chemistry for both carbonate and silicate rivers (Gaillardet et al., 1999; Horowitz et al., 1999; Grasby and Hutcheon, 2000; Millot et al., 2002). However, the relative importance of these factors remains debatable. Some studies suggest that runoff controls the chemical weathering (Millot et al., 2002; Tipper et al., 2006), while others consider that temperature rather than runoff is more important (White and Blum, 1995; Dessert et al., 2001; Dalai et al., 2002). There are also studies emphasizing the factors such as physical erosion and water/rock contact time (Krishnaswami et al, 1999; Oliva et al., 2003; Hagedorn and Cartwright, 2009). With increasing influence of human activity, anthropogenic disturbance has been detected from water geochemistry in many rivers (Meybeck, 1998, 2003; Roy et al., 1999; Xu, 2004). As a subtropical–tropical river featured with high temperature, plentiful rainfall, intense continental erosion and high population Science of the Total Environment 408 (2010) 4749–4760 ⁎ Corresponding author. Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, PR China. Tel.: + 86 10 82998385; fax: + 86 10 82998122. E-mail address: shg@mail.iggcas.ac.cn (H. Sun). 0048-9697/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2010.06.007 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv