[A-HW23] Isotope Hydrology 2021

[AHW23-02] Seasonal and spatial variations of chemical weathering in the Mekong Basin: From the headwaters to the lower reaches

★Invited Papers

*Hiroto Kajita¹, Yuki Ota², Toshihiro Yoshimura³, Daisuke Araoka², Takuya Manaka⁴, Ziyu Ouyang¹, Shinya Iwasaki³, Takuya Yanase¹, Akihiko Inamura², Etsuo Uchida⁵, Hongbo Zheng⁶, Qing Yang⁶, Ke Wang¹, Atsushi Suzuki², Hodaka Kawahata¹ (1.The University of Tokyo, 2.National Institute of Advanced Industrial Science and Technology, 3.Japan Agency for Marine-Earth Science and Technology, 4.Forestry and Forest Products Research Institute, 5.Waseda University, 6.Yunnan University)

Keywords:Mekong River, Chemical weathering, Carbon cycle, Seasonal variation, Spatial variation

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 River, 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, which 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.