Geochemical Characteristics Of Major Ions And Carbon Isotopes And Its Sources Analysis In The Upper Reaches Of Xi River, China

River system is important to global biogeochemical cycles, and land erosion is the main input channel to the ocean. Recent studies suggest that vertical carbon ?C? fluxes ?1.8 Pg C yr-1? released into the atmosphere are twice the lateral fluxes ?0.9 Pg C yr-1? to the ocean, accounts for 43% of the C degassing flux from inland waters including rivers, lakes and wetlands, and acts as an important role in the atmospheric C budgut. At the local scale, riverine C is a significant source of energy to riverine ecosystems ?e.g., for aquatic biota? and closely influences the geochemical nature of the aquatic environment. The karst drainage basin in southwest China is characterized by outstanding geochemical sensitivity and ecological fragility, where natural weathering is the main force of the provenance of substance in the water, while, the human disturbance make the geochemistry and C sources in the river more complex. Relying solely on major ion composition is not enough for distinguishing different C sources in the river, while stable C isotopes can help to trace the source of the matters in the rivers. In this paper,?¹³C was used to compare and analyze the the characteristics of C sources, further explore the primary mechanisms and processes controlling these variations, and then clarify the role in C budget of the upper reaches throughout the whole Xi River and world's rivers.The results have been summarized as follows.1. Ions composition and ion analysis suggested that the ions were influenced mainly by the carbonate weathering. Sulfuric acid and agricultural fertilizer act as agents during the weathering, especially in the Beipan River. The Nanpan River was affected more by sewage and industrial effluents. In contrast, the discharge from coal industry was also obvious in the Beipan River.2.?¹³CPOC and C/N differed between the two rivers, suspended soil organic C was an important source of POC. Higher terrestrial plants supplied a portion of the POC in the Beipan River. In contrast, the POC in the Beipan River was affected more by phytoplankton. The leaching from catchment soil may be an important source of DOC in the Beipan River. DOC within the Nanpan River was partly derived from the degradation of soil C within the water column. In addition, organic C in tributaries to the Beipan River may also have been derived from intense upland soil erosion. a process that resulted in the lowest DOC/POC ratios. The interception of C by hydrological projects positioned along the Nanpan River led to higher DOC/POC ratios. The correlations between DOC and three Cl-?NO3- and SO42-pollution-indicating ions suggest that anthropogenic activities not only affect chemical composition, but also complicate the organic C sources within the water column.3. Carbonate dissolution and soil CO₂ were regarded as the primary sources of DIC on the basis of 813CDIC values which varied along the Nanpan and Beipan Rivers, from -13.9%o to -8.1‰. The logPCO₂ values in the waters were positively correlated with the DIC contents and negatively correlated to the saturation index of calcite ?Sic? and?I3CDIC values. Factors controlling pCO₂, which strongly affected carbonate dissolution, differed between the two rivers. pCO₂ and DIC concentrations in the highly urbanized upper reaches of the Nanpan River were typical higher than in other carbonate-dominated areas of the upper Xi River. In contrast, the enhancement of photosynthesis, the transport of soil CO₂, and organic C through hydrologic conduits influence the pCO₂ along the Nanpan River. Within the Beipan River, the oxidation of organic C is the primary process that maintains pCO₂ levels. The pCO₂ within the Beipan River was more affected by sulfuric acid, scenic spot, and groundwater. The contents and ?¹³C values in the Nanpan River were generally lower than those in the Beipan River, indicating that chemical and physical weathering contributes more marine carbonate detritus to the PIC along the Beipan River.4. At the whole Xi River scale, Nanpan River exhibited a higher evasion flux than the Beipan River. The upper reaches of the Xi River emitted larger fluxes than the lower reaches. At the world's rivers scale, the values of CO₂ evasion from the Nanpan and Beipan Rivers are lower than those measured for tropic rivers, but generally higher than for temperate rivers, and close to other subtropical rivers.