Controlling factors of riverine CO2 partial pressure and CO2 outgassing in a large karst river under base flow condition

• pH and HCO 3 − decreased while the pCO 2 increased along the flow direction. • Spatial distribution of pCO 2 were controlled by the weathering products mixing. • The CO 2 diffusion rate in Xijiang River varied from 260 to 6354μatm. • High pH derived from chemical weathering in karst terrains prevent CO 2 outgassing. Riverine CO 2 outgassing constitutes the important vertical conduit in global carbon cycle and affects climate change. In order to investigate the factors that control riverine pCO 2 and CO 2 diffusion rate (FCO 2 ), river water samples were collected in the mainstream and 42 tributaries of Xijiang River, a karst river during the dry season in 2015. The pCO 2 varied from 260 to 6354μatm with a mean value of 1765μatm, indicating most of the samples were oversaturated with respect to CO 2 compared to atmosphere. The pH, pCO 2 , and HCO 3 − exhibited clearly spatial distribution pattern that pH and HCO 3 − decreased along the flow direction while the pCO 2 increased along the flow direction. Theoretical calculation indicated that the water temperature fluctuation, bio-degradation/respiration, photosynthetic activities and anthropogenic acidic inputs were not the reasonable explanations. The lithologic differences essentially predominated the riverine pH, pCO 2 , and HCO 3 − , the upper reaches of Xijiang River flow through karst carbonate terrain and thus exhibited high pH, HCO 3 − concentrations meanwhile low pCO 2 , the spatial distribution of pH, pCO 2 , and HCO 3 − became visible as the continuous addition of silicate weathering products with low pH and dissolved inorganic carbon (DIC) concentrations. The mixing calculation indicated the DIC concentrations of silicate weathering should lower than 0.5 mmol/L. The calculated FCO 2 of Xijiang River ranged between −38.6 and 1579.8 mmol/m 2 /D, with a mean value of 574.8 mmol/m 2 /D and median value of 410.9 mmol/m 2 /D. FCO 2 exhibited obviously lower values in karst terrain compared that in silicate terrains, because carbonate weathering results in high pH of karst rivers, which caused the low relative proportion of pCO 2 in DIC. More estimations of CO 2 outgassing rate from the karst rivers are required due to the unique characteristics and geochemical process in karst landform.