Abstract
Chemical weathering in the Himalayan river basins is among the highest in the world and has received vast research attention related to past climate change. Many early estimates of chemical weathering are based on a small number of water property data that ignore those spatial and seasonal variations. Therefore, this study analyzed spatial and seasonal variations in chemical weathering in the Mekong Basin, where the geology, climate, and hydrologic cycle of the basin vary significantly from the lower to upper reaches and from dry to rainy seasons. We separately estimated the origins of dissolved elements and potential CO2 consumption rates using the numerous chemical compositions of river water throughout the entire basin and in both seasons. The CO2 consumption rate in the rainy season is three to five times that in the dry season that may be due to the high temperature and precipitation. Despite the low temperatures and dryness of the upper and middle basins, the CO2 consumption rate is approximately twice that in the lower reaches; this can be attributed to active physical denudation in steep mountainous areas which increases the surface area for water–rock interactions. The total CO2 consumption obtained by combining each season and basin was 48‒70 × 109 mol/a and 148‒159 × 109 mol/a for silicate and carbonate weathering, respectively, which are almost half the values of previous estimates. Our results suggest that seasonally and spatially separated evaluations are important for generating estimates of chemical weathering in large Himalayan rivers.
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Acknowledgements
We would like to thank students and researchers at Yunnan University for their great help in the fieldwork. We also thank for valuable suggestions that helped to improve this manuscript by Editor Dr. Marc Benedetti and anonymous reviewers. This research was partly supported by the Graduate Research Abroad in Science Program (GRASP) of the University of Tokyo for H. Kajita. It was also jointly supported by the Second Tibetan Plateau Scientific Expedition and Research (STEP) (2019QZKK0704), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB26020301), and National Natural Science Foundation of China (NSFC) (U1902208, 41991323) to H. Zheng.
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HK and HK contributed to the study conception and design. Sample collection in the upper reaches was performed by HK and ZO. Sample collection in the middle and lower reaches was performed by HK, HK, YO, DA, SI, TY, and AS. On-site management during fieldwork was performed by KW, QY, HZ, and EU. Laboratory analyses were conducted by HK, YO, TY, and AI. The first draft of the manuscript was written by HK. All authors read and approved the final manuscript.
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Kajita, H., Ota, Y., Yoshimura, T. et al. Seasonal and Spatial Variations of Chemical Weathering in the Mekong Basin: From the Headwaters to the Lower Reaches. Aquat Geochem 26, 137–159 (2020). https://doi.org/10.1007/s10498-020-09374-y
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DOI: https://doi.org/10.1007/s10498-020-09374-y