Temporary and net sinks of atmospheric CO₂ due to chemical weathering in subtropical catchment with mixing carbonate and silicate lithology

Abstract

The study provided the major ion chemistry, chemical weathering rates and temporary and net CO2 sinks in the Bei Jiang, which was characterized as a hyperactive region with high chemical weathering rates, carbonate and silicate mixing lithology, and abundant sulfuric acid chemical weathering agent of acid deposition and acid mining drainage (AMD) origins. The total chemical weathering rate of 85.46 t km−2 a−1 was comparable to that of other rivers in the hyperactive zones between the latitudes 0 and 30∘. A carbonate weathering rate of 61.15 t km−2 a−1 contributed to about 70 % of the total. The lithology, runoff, and geomorphology had a significant influence on the chemical weathering rate. The proportion of carbonate outcrops had a significant positive correlation with the chemical weathering rate. Due to the interaction between dilution and compensation effect, a significant positive linear relationship was detected between runoff and total carbonate and silicate weathering rates. The geomorphology factors such as catchment area, average slope, and hypsometric integral value (HI) had nonlinear correlation with chemical weathering rate and showed significant scale effect, which revealed the complexity in chemical weathering processes. Dissolved inorganic carbon (DIC) apportionment showed that CCW (carbonate weathering by CO2) was the dominant origin of DIC (35 %–87 %). SCW (carbonate weathering by H2SO4) (3 %–15 %) and CSW (silicate weathering by CO2) (7 %–59 %) were non-negligible processes. The temporary CO2 sink was 823.41×103 mol km−2 a−1. Compared with the temporary sink, the net sink of CO2 for the Bei Jiang was approximately 23.18×103 mol km−2 a−1 of CO2 and was about 2.82 % of the “temporary” CO2 sink. Human activities (sulfur acid deposition and AMD) dramatically decreased the CO2 net sink, even making chemical weathering a CO2 source to the atmosphere.