Chemical weathering under mid- to cool temperate and monsoon-controlled climate: A study on water geochemistry of the Songhuajiang River system, northeast China

摘要

For the first time, the river water geochemistry of the Songhuajiang Basin, the third largest river system in China, has been studied in order to understand chemical weathering and associated CO_2 consumption rate in a mid- to cool temperate and monsoon-controlled climatic zone. The major ion compositions of the river waters are characterized by the dominance of Ca~(2+) and HCO3-, accounting for 46% and 74% of major cations and anions, respectively. The average total dissolved solids (TDS, 116mgL~(-1)) and total cation concentration (TZ~+, 1388μEqL~(-1)) of the river waters are similar to those of global major rivers. The chemical weathering rates of carbonate, silicate and evaporites in the whole Songhuajiang Basin are estimated to be approximately 5.15, 2.23 and 0.40tkm~(-2)a~(-1), respectively. The total rock weathering rate for the whole Songhuajiang Basin is approximately 7.78tkm~(-2)a-1, which is at the lower end of the spectrum for global major rivers, and is comparable with that of the Amur and the Congo-Zaire River. The estimated CO2 consumption rates for the whole Songhuajiang Basin are 53.4×10~3molkm~(-2)a~(-1) and 66.6×103molkm~(-2)a~(-1) by carbonate and silicate weathering, respectively.As a sub-basin, the 2nd Songhuajiang has the highest (18.9tkm~(-2)a~(-1)) while the Nenjiang River Basin has the lowest total rock weathering rate (5.03tkm~(-2)a~(-1)), which indicates important controls of regional climate and lithology. Compared to the large rivers of China, total rock weathering rates increase from north to south, supporting the idea that the climate acts as major control on global chemical weathering. The CO_2 consumption rate by silicate weathering within the whole Songhuajiang River Basin, though under mid- to cool temperate climate, is in the same order of magnitude as that of Huanghe (82.4×10~3molkm~(-2)a~(-1)) and is not much lower than that of Changjiang (112×10~3molkm~(-2)a~(-1)), which suggests that the role of Songhuajiang River weathering in long-term climate change cannot be neglected compared to those of the large rivers of China.