Seasonal variation of chemical weathering and its controlling factors in two alpine catchments, Nam Co basin, central Tibetan Plateau

Abstract

To investigate the seasonal variation of chemical weathering and its controlling factors in the central Tibetan Plateau (TP), hydrochemistry of stream water in two alpine catchments, the Niyaqu and Qugaqie, was studied at an unprecedented temporal resolution from July 2016 to September 2017. Chemical compositions of river water in the two catchments were dominated by Ca2+ and HCO3−. SO42− in the stream water mainly derived from sulfide oxidation but sulfuric acid played a minor role in rock weathering compared with carbonic acid. The contributions of atmospheric input, silicate weathering, carbonate weathering and sulfide oxidation to riverine solutes of the Niyaqu, which overlies tuff, sandstone, shale, and carbonate and is mainly recharged by groundwater, were about 5%, 12%, 65%, and 18%, respectively; these contributions to dissolved loads of the Qugaqie, where lithology consists of granite and sandstone and nearly half the stream water originates from glaciers meltwater, were 10%, 35%, 34% and, 20%, respectively. Contribution of different sources to riverine solutes in the Niyaqu were almost invariant between different seasons, while the percentage of these sources in the stream water of the Qugaqie showed significant seasonal variations, especially for the weathering of silicate and carbonate: average value of the former increased from 29% in the monsoon season to 45% during the nonmonsoon period, on the contrary, average value of the latter decreased from 37% to 29% in the corresponding periods. Carbonate weathering rates (CWR) and silicate weathering rates (SWR) in the study areas exhibited distinct seasonal variability. During the monsoon season, the average CWR and SWR values in the Niyaqu were 34.3 t/km2/y and 6.8 t/km2/y, respectively, and their corresponding values in the Qugaqie were 16.7 t/km2/y and 11.0 t/km2/y, respectively, which means that the average value of CWR in the Niyaqu during the monsoon season is comparable with the CWR (average for one year) in catchments located in the eastern TP, and the SWR in the Qugaqie during the monsoon season is close to the SWR (average for one year) of some basins at the south slope of the Himalaya. Chemical denudation rates decreased sharply during the nonmonsoon period, indicating little chemical exhumation within that time due to cold and arid weather. Runoff was the most important factor that controls chemical weathering in both the Niyaqu and Qugaqie, followed by physical erosion rates, and the influence of temperature seems to be insignificant in the Niyaqu while at the same time crucial in the Qugaqie.