# Hydrogeochemistry of Fault-Related Hot Springs in the Qaidam Basin, China
## 4 Discussion
### Chemical Characteristics of the Hot Spring Gas
#### 4.2.3 Isotopic Characteristics of Stable Carbon  
The carbon isotope characteristics in hot springs are governed by various geological-tectonic environments and different hydrogeological conditions, mainly depending on the CO\({}_{2}\) source and the storage environment of groundwater. The carbon isotope composition of carbonaceous compounds in the hot spring gas contains rich and important information [81; 82]. The main types of CO\({}_{2}\) genesis in the hot springs are as follows: inorganic genesis and organic genesis. The CO\({}_{2}\) gases with different types of genesis  
Figure 7: Plot of \({}^{3}\)He/\({}^{4}\)He values vs. \({}^{4}\)He/\({}^{20}\)Ne ratios of the hot spring gases. present different carbon isotope characteristics. The \(\delta^{13}\)C\({}_{\rm CO2}\) values of the organic-genesis CO\({}_{2}\) are generally lower than \(-10\)%, mainly \(-30\)% to \(-10\)%, while the \(\delta^{13}\)C\({}_{\rm CO2}\) values of inorganic-genesis CO\({}_{2}\) generally range from \(-8\)% to \(3\)% [83]. Organic genesis is divided into two categories--biogenic and organic metamorphosis, which are usually transported from the outside. On the other hand, inorganic genesis occurs mainly through mantle magma and metamorphism. The \(\delta^{13}\)C\({}_{\rm CO2}\) values of metamorphic genesis from carbonate rocks are close to those of carbonate rocks (approximately 0% \(\pm\) 3%), while the \(\delta^{13}\)C\({}_{\rm CO2}\) values of volcanic-magmatic and mantle-derived genesis are in the range of \(-6\)%\(\omega\)\(\pm\) 2% [84; 85]. Previous studies of the carbon isotopic composition of hot spring gases on the Tibetan Plateau have revealed that regional differences in carbon isotope compositions can be identified. The carbon in the samples from Yangbajain is lighter than in the samples from the northern sites. Carbon dioxide 13C values are comparable to mantle values, which range from \(-3\) to \(-7\)% [86]. The \(\delta^{13}\)C\({}_{\rm CO2}\) values of the hot spring gases in the current study region ranged from \(-3.4\) to \(-6.4\)%\(\omega\), which is more consistent with previous study results. Therefore, it was tentatively concluded that the hot spring gases in the study region had volcanic-magmatic and mantle-derived genesis. However, the carbon isotopes of the CO\({}_{2}\) gases from various sources in the hot spring gases overlapped. Consequently, the main source of CO\({}_{2}\) could not be directly derived from the carbon isotopes. CO\({}_{2}\) in the hot spring gases has greater isotopic fractionation in physicochemical processes than helium [81]. The relationship between \(\delta^{13}\)C\({}_{\rm CO2}\) and the CO\({}_{2}\)/\({}^{3}\)He ratio is used frequently to determine the source of CO\({}_{2}\)[87]. The proportions of mantle CO\({}_{2}\) in the gas samples of the study region were 5.06 and 3.32, which indicated intrusion of the mantle source material. It was apparent that crustal marine limestone was the major contributor to the carbon inventory of the hot spring gas samples (>75%), followed by sedimentary organic and mantle sources (Figure 8, Table 4). The CO\({}_{2}\) released from the dissolution of the marl aquifer and the metamorphism and hydrothermal reactions of the basement lithologies were the main sources of carbon in the hot springs in the study region. This was further closely related to the geology of the region because the sedimentary basement of the study region mainly comprised marl. This suggested that crustal additions exerted a strong influence on the samples. Figure 8: A plot of CO\({}_{2}\)/\({}^{3}\)He values vs. \(\delta^{13}\)C\({}_{\rm CO2}\) values obtained for the hot spring gases.