Chemical weathering rates and controlling mechanisms of glacial catchments within different climate regimes in the Tibetan Plateau

Abstract

Background: Continental weathering plays an important role in regulating atmospheric CO₂ levels. Chemical weathering in glacial areas has become an intensely focused topic in the background of global change compared with other terrestrial weathering systems. However, research on the weathering of the glacial areas in the Yarlung Tsangpo River Basin (YTRB) is still limited.

Methods: In this article, the major ions of the Chaiqu and Niangqu catchments in the YTRB have been investigated to illustrate the chemical weathering rates and mechanisms of the glacier areas in the YTRB.

Results: Ca²⁺ and HCO${}_3^{-}$ dominate the major ions of the Chaiqu and Niangqu rivers, accounting for about 71.3% and 69.2% of the TZ⁺ of the Chaiqu (the total cations, TZ⁺ = Na⁺ + K⁺ + Ca^{2 +} + Mg²⁺, in µeq/L), and about 64.2% and 62.6% of the TZ⁺ of the Niangqu. A Monte Carlo model with six end-members is applied to quantitatively partition the dissolved load sources of the catchments. The results show that the dissolved loads of the Chaiqu and Niangqu rivers are mainly derived from carbonate weathering (accounting for about 62.9% and 79.7% of the TZ⁺, respectively), followed by silicate weathering (about 25.8% and 7.9% of the TZ⁺, respectively). The contributions of precipitation and evaporite to the Chaiqu rivers are about 5.0% and 6.2%, and those to the Niangqu rivers are about 6.3% and 6.2%. The model also calculated the proportion of sulfuric acid weathering in the Chaiqu and Niangqu catchments, which account for about 21.1% and 32.3% of the TZ⁺, respectively. Based on the results calculated by the model, the carbonate and silicate weathering rates in the Chaiqu catchment are about 7.9 and 1.8 ton km^-2 a^-1, and in the Niangqu catchment, the rates are about 13.7 and 1.5 ton km^-2 a^-1. The associated CO₂ consumption in the Chaiqu catchment is about 4.3 and 4.4 × 10⁴ mol km^-2 a^-1, and about 4.3 and 1.3 × 10⁴ mol km^-2 a^-1 in the Niangqu catchment. The chemical weathering rates of the glacier areas in the YTRB show an increasing trend from upstream to downstream. Studying the weathering rates of glacier catchments in the Tibetan Plateau (TP) reveals that the chemical weathering rates of the temperate glacier catchments are higher than those of the cold glacier catchments and that lithology and runoff are important factors in controlling the chemical weathering of glacier catchments in the TP. The chemical weathering mechanisms of glacier areas in the YTRB were explored through statistical methods, and we found that elevation-dependent climate is the primary control. Lithology and glacial landforms rank second and third, respectively. Our results suggest that, above a certain altitude, climate change caused by tectonic uplift may inhibit chemical weathering. There is a more complex interaction between tectonic uplift, climate, and chemical weathering.

Keywords: Chemical weathering; Glacial catchment; Monte Carlo inversion model; River chemistry; Yarlung Tsangpo river basin.