Riverine water chemistry and rock weathering processes of Qingyi River basin, a subtropical basin in east China

To investigate the rock weathering processes in silicate-dominated subtropical basin in east China, we analyzed major ion compositions of rivers and precipitation samples in the Qingyi River basin in the lower reaches of the Yangtze River. In this study, the characteristics of weathering processes in the Qingyi River basin were identified, and the rock weathering rates and consumption rates of atmospheric CO₂ were estimated based on water chemistry and the forward model. The results showed that the anthropogenic influences on rock weathering was not significant, which means the rock weathering in the study area was mainly induced by carbonic acid while the influence of sulfuric acid and nitric acid could be neglected. The cations of rivers were mainly contributed by weathering of carbonates (59.2%), followed by weathering of silicates (17.9%). Atmospheric precipitation and evaporites contributed 9.6% and 5.6%, respectively. Spatially, the carbonate weathering rates and silicate weathering rates decreased as order of tributary Huishui river in the upstream mountainous areas (32.04 t·km^-2·a^-1 and 20.97 t·km^-2·a^-1) > main stream of Qingyi river (24.12 t·km^-2·a^-1 and 8.91 t·km^-2·a^-1) > tributary Zhanghe river in the downstream areas (13.68 t·km^-2·a^-1 and 2.85 t·km^-2·a^-1). Similarly, the CO₂ consumption rates from carbonates weathering and silicate weathering followed the order of tributary Huishui river (5.86×10⁵·mol·km^-2 a^-1 and 3.29×10⁵·mol·km^-2 a^-1) > main stream of Qingyi river (2.45×10⁵·mol·km^-2 a^-1 and 2.43×10⁵·mol·km^-2 a^-1) > tributary Zhanghe river (0.77×10⁵·mol·km^-2 a^-1 and1.39×10⁵·mol·km^-2 a^-1). In conclusion, carbonate weathering induced by carbonic acid was dominant in the Qingyi River basin, with chemical weathering rates slightly lower than similar silicate-dominated subtropical basins in east China. The rock weathering rates in the study area differed spatially. In particular, silicate weathering in upstream mountainous areas accounted for more carbon sink of the whole Qingyi River basin, which is of great importance for the regional carbon cycle.

Key words: subtropical; Qingyi River basin; rock weathering; atmospheric CO₂ consumption; carbon sink