Effects of chemical weathering and CO2 outgassing on δ13CDIC signals in a karst watershed

As the important part of carbon cycle research, riverine dissolved inorganic carbon (DIC) has attracted continuous attentions for its close relationship with global climate change. In order to investigate the dominant factors of dissolved inorganic carbon species and its stable carbon isotopic signals in large watershed under base flow condition, 81 river water samples were collected in the Xijiang River during the dry season in 2015. The elemental ratios in river water revealed the hydro-chemistry in Xijiang River were mainly controlled by the carbonate weathering. The characteristics of the most samples were the high DIC concentrations and narrow range of δ¹³CDIC, which can be interpreted as the result of chemical weathering under open system based on the simulated calculation of soil respiration and mineral dissolution. The relatively lower DIC concentrations and δ¹³CDIC values have been observed in tributaries draining silicate terrains, this result may be controlled both by the mineral dissolution and CO₂ outgassing. Most samples had the over-saturated pCO₂ levels compared to the atmosphere, the respiratory quotient (RQ) demonstrated that high pCO₂ values were not from the biologic respiration, instead, the groundwater influx with the high soil pCO₂ increased pCO₂ level in river waters. The CO₂ concentration gradient between river water and atmosphere caused a violent CO₂ outgassing accompanied isotope fractionation. The outgassing significantly dropped the pCO₂ in river water and caused an enrichment of C¹³ in DIC. Based on the theoretical calculation and previous observation, the negative correlation between the δ¹³CDIC and pCO₂ in river water was caused by the CO₂ outgassing. Additionally, the isotope exchange between the DIC and atmosphere CO₂ can also partially increase the riverine CO₂. This study examined the impacts of soil respiration, minerals dissolution and CO₂ outgassing on DIC and δ¹³CDIC, and found that the riverine δ¹³CDIC is probably not reflect the mixing of source signals such as soil CO₂ and carbonate, instead, it is mainly controlled by the fractionation in minerals dissolution and CO₂ outgassing.