Abstract: Background, aim, and scope Chemical weathering of continental silicate rocks and carbonate rocks is closely related to global climate change. Study of weathering in watersheds is an important way to understand how continental weathering responds to global climate change. Previous researches focused on large river basins, and few of them worked on small river basins. In particular, the influencing factors of chemical weathering in river basins in high and cold regions are still uncertain. The Nyang River Basin on the Tibetan Plateau with relatively simple lithology and less human disturbance was selected as the research object. A hydrological year sampling analysis was performed to explain the seasonal variation characteristics of rock chemical weathering rate and its influencing factors in the alpine region. Materials and methods In the present study, a hydrological year (2017—2018) sampling analysis was conducted at the lowest stream of Nyang River Basin. Annual variation of chemical compositions of surface river water were analyzed, including the major cations (Na+, K+, Mg2+, Ca2+), soluble silicon and major anions (F-, Cl-, SO42- , NO3- and HCO3- ). Based on chemical composition analyses, contributions of four end-members (silicate, carbonate, hotspring and atmosphere) to riverine cations were estimated. Coupled with discharge data calculated by a hydrological model and water temperature measured in the field, weathering rates of silicate and carbonate as well as their responses to discharge and temperature were also investigated. Results Results show that the water ions of the Nyang River are mainly derived from the weathering of carbonate rocks and the weathering of silicate rocks, contributing 60% and 29% of cations to the river water, respectively, and the weathering rates are 0.20—19.00 t·km-2·month-1 and 0.09—0.80 t·km-2·month-1, with annual averages of 11.90 t·km-2·a-1 and 4.38 t·km-2·a-1, respectively. In a hydrological year, the weathering of carbonate rocks is obviously affected by seasonal changes, while the response of silicate rock weathering to seasonal changes is insignificant. In general, the weathering rate increases in the rainy season and decreases in the dry season. Discussion Discharge is an important factor controlling chemical weathering in the basin. The increase of discharge promotes the weathering of carbonate and silicate rocks, but the dissolution dynamics of rocks will limit the effect of discharge on the weathering rate. The increase of discharge can continue to effectively promote the weathering of carbonate rocks because of the large dissolution rate, while for the silicate rocks with small dissolution rate, the effect of discharge on weathering is weakened when discharge continues to increase. Different dissolution dynamic characteristics of rocks are important reasons for the different responses of carbonate rock weathering and silicate rock weathering to seasonal changes. Increased temperature can effectively promote rock weathering in the Nyang River Basin by increasing the dissolution rate of rock minerals. Temperature can also import an effect on weathering of the watershed by influencing discharge. Conclusions From above, it can be concluded that, in Nyang River basin, discharge can be a primary control on chemical weathering, and temperature is relatively minor control, but temperature can also affect glacier activities as well as discharge in this alpine area. The different dissolution kinetics of carbonate and silicate account for the discrepant weathering behaviors of the two kinds of rocks during a hydrological year. Recommendations and perspectives The climatic factors in the alpine area like Nyang River Basin interact with each other and also affect physical and chemical weathering. For this reason, the longer time monitoring and higher sampling density in alpine watersheds will help to better understand the response of weathering to climate change.
Keywords: Nyang River, chemical weathering, seasonal variations, climate factor