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Carbon sequestration resulting from carbonate rock weathering is closely linked to the global carbon cycle and has turned out to be important in the adjustment of atmospheric CO 2 levels. Traditional karst dynamic models based on water-rock-gas interactions underestimate carbon sink fluxes related to carbonate rock weathering because they ignore the utilization of dissolved inorganic carbon (DIC) by aquatic organisms. In this study, a new model based on water-rock-gas-organism interactions was applied in the Pearl River basin, China, to recalculate atmospheric CO 2 consumption and to develop an accurate estimation model for carbon sink fluxes at catchment scale. Stable carbon isotope (d 13 C) and C/N ratios were used in the counting processes. Data were collected from published literature as well as through field investigation and laboratory analysis. Results show that the Pearl River carbon sink in the Pearl River is 4.31 9 10 9 kg/a, i.e., 15.8 9 10 9 kg of atmospheric CO 2 per year. Of this, the carbon sink resulting from carbonate rock weathering amounts to 2.14 9 10 9 kg/a, i.e., 49.7 % of the total. The three largest tributaries of Pearl River, Dongjiang, Beijiang, and Xijiang, respectively absorb 0.5 9 10 9 , 1.19 9 10 9 , and 2.62 9 10 9 kg of carbon from the atmosphere annually, accounting for 12, 28 and 60 % of the river's total carbon sink. When compared with the results of previous researches that disregarded the role of aquatic organisms, the new calculation method provides a carbon sink flux value that is 1.2-3.3 times higher, and 1.6 times higher on an average. To improve the calculation accuracy of atmospheric CO 2 consumption in global karstic rivers, further research is needed regarding carbon sequestration mechanisms that involve aquatic organisms.

The hydrogeochemical characteristics of groundwater are an important element in the study of the spatial and temporal variation of groundwater resources, which is crucial to water resources utilization, ecological environmental protection, and human development. Water samples were collected at eight observation of Nandong Karst Water System (NKWS) sites in each month of 2019, and the main ions and isotopes of the water samples were examined. The hydrogeochemistry characteristics of groundwater and its differences with surface water were explored by using the methods of multivariate statistics, Gibbs model. Results showed that the water chemistry types of groundwater were mainly HCO3 – Ca and HCO3 –Ca· Mg. The analysis of hydrogen and oxygen isotope showed that the initial recharge source of surface water and groundwater were atmospheric precipitation, and the measured hydrogen and oxygen isotopes of surface water were heavier due to the strong evaporation effect. The natural and anthropogenic processes contributed to the chemical composition of surface water and groundwater in the study area. However, the main factor affecting the quality of surface water and groundwater was the input of anthropogenic contaminants. In terms of natural factors, the main chemical ions of surface water and groundwater were mainly controlled by water–rock action originating from weathering and hydrolysis of rocks and soils. Ca2+, Mg2+, and HCO− 3 mainly originated from natural dissolution of carbonate rocks. K+, Na+, SO2− 4, and Cl− were partly from atmospheric precipitation. For human activities, Na+ and Cl− were partly from domestic water for local residents. SO2− 4 in surface water mainly came from mining. NO− 3 in groundwater mainly came from chemical fertilizers, and NO− 3 in surface water were mainly from human waste and domestic sewage.

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