碳酸盐地区湖泊CO2来源的C13、C14同位素示踪

通过对泸沽湖、阿哈湖和草海的C13、14C和化学分析的系统研究，建立湖泊－流域体系的碳循模型，阐明喀斯物高原湖泊的CO2源汇效应、CO2来源以及转移、释放的动力学过程；碳酸盐侵蚀作用的产物在湖泊内的地球化学行为和归宿；流域侵蚀与湖泊体系的耦合对区域CO2平衡的总体效应。项目实施为研究湖泊在全球环境变化中的作用奠定坚实的基础。

The source, fate and biogeochemical cycle of dissolved CO2 in lakes is not, so far, clearly understood. Therefore, lake is usually disputed as a source or sink of atmospheric CO2, which should be more complex in carbonate area due to the dissolution of carbonate absorbing CO2 in the watershed. Through lots of investigation in this project, it is concluded that dissolved CO2 in surface lake in Yunnan-Guizhou plateau is supersaturated to atmospheric CO2 with the average partial pressure of 639±122μatm, and the average flux of 10.8±2.7 mmol m-2.day-1. The partial pressure and flux are lower than the average values in lakes covering northern globe due, in part, to the background of carbonate. The partitioning of retained carbon between the sediments and atmosphere appears not to be relative to lake alkalinity, but to be a function of TP and TN. To combine with the analysis of inflow, outflow, underground water and spring around, the mass balances of DIC and POC in Lake Hongfeng, a stratified, mesotrophic-eutrophic lake, is modeled. Dissolved CO2 in lake originates from CO2 in the watershed, which is transferred as DIC after dissolving carbonate. About 9～10×108 mol CO2 is annually absorbed in the weathering of rocks in the watershed of Lake Hongfeng. It is revealed that 2.29×109 mol C of DIC is annually brought into Lake Hongfeng through the river inflow from the watershed, comparatively, 1.25×109 mol C of DIC is produced by the mineralization of organic carbon within the lake. The flux of CO2 is approximately 1.7×108 mol a-1, equivalent to 7 percent of the amount of DIC input via river inflow, and 20 percent of the amount of CO2 absorbed in the watershed. The carbon retained in the sediments is about 13 percent of the total DIC from the watershed.Although there is a heavy seasonal change of CO2 resulting from the balances and transformation of photosynthesis and respiration in lakes, the change of C isotope of DIC is seasonally not obvious. However, two types can be distinguished in the relationship ofδ13CDIC and ΣCO2. One is a negative correlation. It implies that photosynthesis and respiration dominatingly control the processes of carbon cycle within the lake. The other is the positive correlation between theδ13CDIC and ΣCO2, which can be explained by three processes: ① the precipitation of carbonate; ② the dissolution of carbonate due to the acid other than carbonic acid (CO2); ③ the upward diffusion and mix of HCO3- whose δ13CDIC is more positive due to the methanogenesis on the surface sediments.There is a large seasonal change about C isotope of POC(δ13CPOC) in lake. 13C is most depleted in fall, slightly increase in winter andδ13CPOC has a highest value in summer. Also, C isotope of POC (δ13CPOC) varies with its sources, on basis of which POC in Lake Hongfeng is though to be autochthonous as production of photosynthesis within the lake.δ13CPOC is positively correlated with δ13CDIC in summer when photosynthesis exceeds respiration, andδ13CPOC is negatively correlated with δ13CDIC in autumn and winter as respiration exceeds photosynthesis. Although POC in lake is produced from DIC via photosynthesis, the amplitude of seasonal change of δ13CPOC is much larger than those of DIC. Obviouslyδ13CPOC is decided by the isotopic fractionation of photosynthes seriously affected by some factors which vary greatly with the season. The partial pressure of CO2 (pCO2) and temperature are linearly correlative with εδ13CCO2-POC, which imply pCO2 and temperature can be reflected by the C isotope of organic matter in sediments.By comparison, dissolved CO2、CO、CH4、N2O、O2（DO）and pH、SO42-、NO3--、NO2- all exhibit very concordant fluctuations in the different depth of different sites in lakes. It shows the tight relations among the biogeochemical cycles of C,N S. It is particularly suggested from the peaks of trace gases at the thermocline that the transformation of redox-reduction due to the decomposition of sedimenting particulate trigger many processes of environmental changes including nitrification/