| As the atmospheric CO2concentration is rising continuously, it has very important influence on global climate change, and carbon cycle issue has received more and more attention from the scientific community. The existing research results showed that the carbon source and carbon sequestration of earth carbon pool is not balanced, about20%of atmospheric CO2disappears without a trace, which is the so-called "missing carbon sink". So looking for this part of the "missing sink" has become a highly contested issue debated amongst the scientific community. Since the "missing sink" was discovered, scientists of the carbon cycle research have done a lot of work to give the reasonable explanation to it from all aspects. Although some people think that this unknown carbon sink may be from terrestrial vegetation or soil, but there is no direct scientific evidence. At the same time, domestic and foreign scholars begin to focus on carbonate rock weathering process and its role in the carbon cycle in academic circles. But the general view of academia is that just silicate chemical weathering carbon sink function controls the long time scale climate change, while the weathering of carbonate rocks has a source and sinks conversion processes, the carbon sink effect in the middle-long time scale is zero. The reason may be regard the geological processes (including karstification) as a slow and long time scale process.Therefore, the analysis of the CO2cycle mechanism are concentrated in the ocean, vegetation, atmosphere, hydrosphere, pedosphere. However, the geological process is supposed to be a long time scale cycle, without taking the carbon absorption and its dynamic change of the lithosphere into account.However, modern karstology researches results show that carbonate rock, not only actively participate in the global carbon cycle, but also is extremely rapid response to the external environment. And recent studies indicate that CO2in water is much more stable than the original understanding, which is not the previously thought of equilibrium constants6.35, but3.45. If the CO2stability enhance when water carbonate rock corrosion, it will have a great influence on the carbon sink in Karst area. But this study is limited to the laboratory, it lack of practical research on the stability of CO2and the change of karst dynamic system in the field. Recent research shows that, if starting from the angle of the Earth System Science, take the hydrosphere, lithosphere and biosphere into consideration together, dissolved inorganic carbon (DIC) derived from karst geological will be used by rivers or lakes’aquatic plant photosynthesis, and il will finally form the organic matter and buried in the seabed. Research shows that, the epikarst system actively participating in the global carbon cycle with the biological participation, its amount of absorbtion carbon accounted for20%of the "missing sink". Thus, the epikarst system absorbing and releasing carbon is an important part of the natural carbon cycle, which is a part of the "missing sink", it plays an important role, which is not to be ignored of the atmospheric CO2. It has an important significance on study the characteristics of carbon cycle, and to improve the global carbon cycle model and to solve the missing sink.Based on the present situation of domestic and foreign research and the needs of the project which is depended on, we selected Xue yu cave underground river basin, which is located in Fengdu,Chongqing as the research object.We use MS-5multi parameter water quality online recorder which produced by USA HACH company to monitor routine hydrochemistry characteristic index of groundwater and surface water for continuous monitoring for a period of one year (January to December of2013), and then comparing both. We Selecte representative months of four seasons in a year (January, April, July, November) to do four times diel monitoring for the length of36to72hours of field monitoring experiment to find out the difference between day and night, and the influence of aquatic plant photosynthesis on karst carbon cycle in different seasons.The contents and conclusions of this paper include the following two parts:First, starting from the point view of the Earth System Science and karst dynamic theory, karstification is the important part of the global carbon cycle, weathering of carbonate in karst zone has the absorption of atmospheric CO2effect.The epikarstification carbon cycle is mainly carbon sink. Therefore, a reasonable evaluation of karst sink effect on atmospheric CO2is very necessary and important. This paper study on annual variations of Xueyu cave Underground River can effectively recognize chemical weathering processes of the study area. And then based on this, we use the water chemical-runoff method to quantitatively estimate the weathering intensity of rocks. Through the analysis of pH, SC and Ca2+, Mg2+, HCO3-,K+, Na+. Cl-of Xue yu cave underground river in2013,we found that:Xue yu cave underground river pH value is between7.5-8.1, the average value is7.8, which is the neutral and alkaline water. The SC range from333μs/cm to450μs/cm, the average value is379μs/cm. The concentration of HCO3-range from152.5mg/Lto250.1mg/L, and Ca2+concentration is62mg/L-94mg/L. The HCO3-and Ca2+are the main ions of underground river, and the cations are mainly Ca2+and Mg2+, which accounted for93.9%of the total cations, followed by K+, Na+; HCO3-accounted for80.3%of the total anions, and the correlation between HCO3-and Ca2+was significant (correlation coefficient is0.923), then deduce its underground water chemical type is HCOs-Ca. According to the analysis of the variation of underground water chemical characteristics in a hydrological year, carbonate rock weathering of the basin showed a seasonal change, due to the different temperature, rainfall in different seasons, which lead carbonate rock weathering intensity is different in different seasons,at lastlead to the differences of hydrogeochemical characteristics. From January to December, concentrations of Ca2+and HCO3-in the underground water variation trend is from low to high, then to low, which indicate that it increase in the rainy season, and decrease during the dry season. Ca2+、HCO3-in rainy season, reached to the highest concentration of94mg/L and250.1mg/L, While in the dry season reached the minimum value of62mg/L and152.5mg/L respectively. According to the continuous monitoring of Ca2+, HCO3-found that it showed positive relationship with water flow, which indicated that the precipitation and flow plays an important role in carbonate rock weathering, rainfall and flow increase weathering of carbonate rock, so as to enhance carbon sequestration effect. It tells us that carbonate rock weathering intensity is stronger in raining season than in dry season. Finally, using the water chemical-flow method, we calculate the basin carbonate weathering carbon sequestration is about5.19t/a, and the DIC derived from nitrate dissolved carbonate is about3.91t/a.Second, in the karst area, the karst groundwater tranform to surface water, a significant diurnal variations of Ca+and HCO3-concentration in the water were found, and is strongly related to illumination, and it shows a significant diurnal variation. Through day and night monitoring, we found, during monitoring period, no significant diurnal variation of groundwater water chemistry was discovered, which means that water chemical characteristics of groundwater is stable. While the surface water showed an obvious dynamic change. During the day, pCO2, Ca2+and HCO3-concentrations SC decrease, and at about16:00pm in the afternoon reached the lowest value. And the values of pH, DO were contrary to the former, the pH value, DO increase during the day, and reduce at night. On the other hand, in the evening, pCO2, Ca2+and HCO3-concentration、SC increase, and reached the maximum at around4:00am.The main reason of diel variation is because in the daytime, the pool aquatic plants photosynthesis absorbed CO2in water, and release O2, which causing the elevation of water DO, pH increased, SIc increased, pCO2decreased, concentrations of Ca2+、HCO3-and SC decreased.At night, the aquatic plant photosynthesis was replaced by the respiration, aquatic plants absorb O2, and release CO2, which lead to water DO decreased, pH values decreased, SIc decreased, pCO2increased, concentrations of Ca2+, HCO3-and SC increased. And through the correlation analysis, the pH and pCO2showed inverse correlation (correlation coefficient is0.914), the pH value of the water is greatly affected by the change of CO2concentration in water. The change of pH value will directly affect the karstification. At daytime, the pH value increased, the karstification is mainly deposited. At nighttime, pH value decreased, karstification is mainly controlled by dissolution,In addition, DO and pCO2in water show a significant inverse correlation (correlation coefficient is0.732),which indicate that aquatic plants photosynthesis and respiration has a very important influence on dissolved gas of pool water, thus affecting the carbon cycle in Karst water. At the same time, through the four times monitoring we found that:in the different seasons, changes amplitude of water chemistry during the day and night monitoring is not the same. In July, adequate light, high temperature, enhance the biological activity of aquatic plants, which is good for metabolism of aquatic plants. While in January, the solar radiation is weak, resulting in low temperature, which is not good for metabolism of aquatic plants, and cause biological activity weakened.Since different seasons have different temperature and different illumination, aquatic plant photosynthesis has different influence on the carbon cycle in karst water. Different seasons, aquatic plant efficiency of using DIC in water are different, according to calculation, in a hydrological year, photosynthesis of aquatic plants use water DIC produced organic carbon efficiency descending order is summer> spring>winter> autumn. Finally, after calculated, we found that:the summer use efficiency is the highest, reached to135.06kg/day, followed by spring, autumn and winter, they are10.03kg/day, kg/day and7.17kg/day8.75respectively. The loss of this part inorganic carbon will transform into organic carbon by the plant absorption, in the karst system it can form a long time scale nature carbon sequestration. Therefore, the dynamic biogeochemical behavior of aquatic plants has important effect on carbon cycle of the karst area. In addition, under the condition of rainfall, aquatic plant photosynthetic effect is reduced. In the rainfall condition, diurnal changes caused by aquatic plant photosynthesis and respiration is not obvious, rainfall makes this diurnal variation small. |
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