| 其他摘要 | Carbon cycling in carbonate basins directly links carbon reservoir of carbonate to the carbon reservoirs of terrestrial ecosystem, marine and atmospheric CO2, therefore is of significance to understand the local, regional and global carbon cycles. Chemical weathering is one of important interactions between carbonate and terrestrial ecosystem and atmospheric CO2, then influence the export of carbon from land to marine, and has potential impacts on marine carbon cycle. Carbonate is dissolved by CO2 and other acids in chemical weathering and release dissolved inorganic carbon (DIC）(mainly HCO3-, CO2, and CO32-) that is conveyed into seas through rivers. To recognize the impact of carbonate weathering on the global carbon cycle, it is the primary issue to distinguish between sources and fluxes of DIC, and the influence of climatic, biological and anthropic factors on the geochemical processes. If only understanding clearly them, it is possible to assess accurately the impact of carbonate weathering on the global carbon cycle. In generally, the carbon of various sources could have the differences of carbon isotope composition (δ13C), and that provides a dependable approach of geochemistry for identifying the various sources of DIC and analyzing the influencing factors. In this study, water samples were collected monthly from 54 sampling sites for groundwater, 12 sites for river waters, 7 sites for surface water of upstream reservoirs and 10 sites of stratified water of lakes (5 sampling sites for Hongfeng and Baihua, respectively) in the karst mountainous Honghong-Baihua watershed in southweastern China in whole year of 2013. The concentration and carbon isotope of DIC were determined for the samples, and spatial and temporal variations of DIC in the groundwater, the river water, and the lake water were discussed based on the obtained data. The following conclusions were drawn: 1. In the groundwater in the basin, mean concentrations of DIC (4.06mmol/L ±) and (3.50mmol/L±) were high, and δ13CDIC values were low (-9.86‰ and -9.7‰, respectively) in residential areas and cultivated land. The mean concentration DIC (2.59±0.73 mmol/L) and mean value δ13CDIC (-9.7±1.7‰) of woodland groundwater were low. The result showed that: the impacts of agricultural activities and sewage on DIC content and δ13CDIC varaitions in groundwater were significant. In comparison, the effect of human activities on the woodland groundwater DIC was light. However, for August’s samples, information of human activities in DIC concentration and isotope of groundwater in each land use types were screened due to the abundant rainfall and high temperatures.. 2. The DIC concentrations in the river water varied in a relatively small ranges with an average of 2.28±0.74mmol/L. However, the δ13CDIC was highly sensitive to the seasonal changes in rainfall and temperature. 3. The DIC concentrations of river water flowing into the lake were higher than that of the surface water and bottom water in Hongfeng and Baihua Lake and the water flowing out of the lakes. The DIC concentration in the lakes had small changes with increasing depth in spring and winter, but there were significant changes in summer and autumn. The DIC content increased with increasing water depth, and there were much difference in surface and bottom water. The δ13CDIC values of the upper lake water were slightly positive in summer and negative in winter. In the vertical profile, δ13CDIC values gradually get negative with water depth. The degree (amplitude) of δ13CDIC change in different depths were different, there were typically small ranges in the surface (0 ~5m) and intense changes in 5 ~10m, and keep relatively stable after 10m, 10m is inflection point that δ13CDIC values shift from drastic changes to stable. These evidences show that temperature could influence the DIC in the lakes through regulating the thermal stratification and biologic and microbio activities. |
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