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

Abstract

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.