Small-catchment perspective on chemical weathering and its controlling factors in the Nam Co basin, central Tibetan Plateau

Abstract

Very little data is available on chemical weathering and its controlling factors in small catchments of the central Tibetan Plateau (TP) despite being the main part of the plateau. Hydrochemistry of river water in 24 small catchments in the Nam Co basin draining different strata and recharge forms was investigated monthly during the 2018 monsoon season. The results show that carbonate dissolution is the most important geochemical process controlling the evolution of hydrochemistry of most river water in the Nam Co basin (the average contribution to riverine solutes reaches 46%) even though its contribution shows significant spatial distribution: low values occur in some granite regions (e.g. 27% in H10) but is high in catchments where carbonate rocks are prevalent (e.g. 68% in H23). The contribution of silicate weathering is low, with an average of 11% in catchments located on the east and south banks draining sandstone/mudstone, while this value increases to 32% in catchments on the south bank which pass through granite, and on the west bank. The contribution of sulfide oxidation in most rivers on the south bank is unexpectedly high (40%) but is much lower in other rivers (13%). This spatial difference is attributed to the influence of topography, particularly slope. The carbonate weathering rate (CWR) in small catchments of the Nam Co basin shows spatial heterogeneity and is significantly and positively correlated with the lithologic coefficient which proposed to indicate the degree of rock weatherability and negatively correlated with mean slope and mean altitude. The spatial distribution of the silicate weathering rate (SWR) is not as significant as the CWR and is mainly governed by runoff depth. Chemical weathering rates in the glacierised catchments are 1.3 times higher than those in the non-glacierised regions which share similar lithology. Thus, glaciation enhances chemical weathering in the study area. The area-weighted average value of CWR and SWR in the study area is 23.2 t/km2/y and 5.2 t/km2/y, respectively. Chemical weathering rates in the Nam Co basin, central TP are relatively low when compared with those in catchments located on the edge of the TP, indicating the carbon sequestration effect caused by this supergene geological process in this area may be insignificant.