| Since the Industrial Revolution, human activities with a large number of fossil (coal, oil) in the form of fixed carbon to re-release into the atmosphere, which has been lead to a continuing upward trend in atmospheric CO2concentration around the world. The resulting global warming has increasingly become a hot topic of public discussion and the environmental impact of this phenomenon on mankind's survival becomes more direct. Since the discovery of the existence of "missing carbon sink" scientists in the field of carbon cycle have done a lot of work concerning all aspects to reasonably explain and speculate it.. At the same time, many scholars at home and abroad, pay close attention to the process of rock weathering, especially the role of carbonate rock karst in the global carbon cycle. Carbonate is the world's largest carbon pool compared with other carbon forms. However, due to their slower rate of participation in the carbon cycle, and the research on carbonates in the global carbon cycle was less or even ignored. However, the modern karst study results indicate that the carbonate is actively involved in the global carbon cycle, and the response is extremely rapid. Due to the special structure of the aquifer, karst water is the world's largest groundwater body, and supplies about20%-25%of the world's population as a source of drinking water, and occupies an important position in the global water cycle. The distribution of carbonate rocks is wide in China, covering3,440,000km2, taking1/3of China's total land area, and1/7of world's carbonate rock outcrop area of22,000,000km2. However, for a long time, the global change is mainly attributable to human who does not attach great importance to the study of natural adjustment. People began suspecting the IPCC Report after he "Climate Gate" in the Copenhagen UN climate conference, and thinking calmly of the role of natural factors in global change. In view of this, in March2009. China Geology Survey Bureau started a "Geological Response Study on Global Change" project. In March2010, China Geology Survey Bureau also started "Research on China's Geological Carbon Sequestration Potential". These two research projects involve with the karstification carbon cycle research. Description of the geological carbon sequestration in China has risen to the national strategic level, in which karst carbon sinks are an extremely important part.The Ministry of Land and Resources started "Research on China's Geological Carbon Sequestration Potential" project in2010to reassess the role and strength of the geological carbon sinks, providing strong data for China's driving position in the international science and technology competition and climate change negotiation to respond to global climate change, fulfill its international commitments as a responsible stakeholde, having great practical significance. In the other hand, how can we use the karst carbon exchange mechanism of carbon sequestration strength of China's land area of karst, a more reasonable description of the mechanism of the karst process development, an accurate calculation of the amount of carbon sinks caused by the karst water circulation for the global carbon cycle, as well as the calibration of the model provide important parameters having important theoretical significance.This study is basic research of southwest China karst mountainous karst water cycle and its carbon sequestration mechanism. In order to clarify the karst water carbon sink strength of the role of the karst basin under different environmental conditions and its control factors, this study chose the Xiangxi River karst Watershed in Three Gorges reservoir area of middle reaches of the Yangtze River to study, based on the detailed investigation on the hydro geological conditions of the Xiangxi River Basin. The Xiangxi River Basin is divided into three sub-basins for which a comparative study has been made for the main stream region (main stream located in the backwater area of the Three Gorges reservoir area), and two typical underground river system were also selected for a groundwater study. From September2010to December2011, quarter by quarter, field water chemistry sampling and analysis were performed regularly for the three sub-basins, including the main stream of surface water and underground river. The strontium isotope (87Sr/S6Sr), dissolved inorganic carbon isotopic composition (δ13CDIC), deuterium and oxygen isotopic composition (δD of and δ18O) of the Xiangxi River Basin were sampled, tested and analyzed, and the Xiangxi River watershed sub-basins and underground river system and space on the karst carbon sequestration were then analyzed in a timely manner. Carbon sequestration effect of differences and the impact factors were analyzed to investigate the following key knowledge and conclusions:1. Hydro-geochemical characteristics are not only controlled by the hydro-geological conditions, but also influenced by weather, vegetation, and the soil.1) The regional characteristics of the type of water chemistry in Xiangxi River Basin:Water chemistry type of the Nanyang River and the Gufu River basin is HCO3-Ca.Mg type; the Gaolan River Basin is mainly HCO3-Ca.Mg type with HCO3-SO4-Ca-Mg type water in its southeast sub-basins; the Xiangshui Cave is HCO3-Ca Mg type; and the Xianglong Cave is HCO3-Ca type.2) Xiangxi River Basin's carbonate dissolution-precipitation balance is the dominant factor in controlling the stability of the water chemistry. Most parts of the basin are saturated and over-saturated with calcite and dolomite, and the content of dissolved pCO2in middle and upper reaches of the Xiangxi river domain is higher than that of atmosphere; and in the main stream near the Yangtze River, the content of pCO2in the water is close to or even lower than that of atmosphere,. The water promotes CaCO3nucleation and solid phase precipitation through CO2degassing and exchanges with atmosphere for CO2by means of continuous degassing process. This is relevant to the over long time of exchange with atmosphere due to slow rate of river flow affected by backwater of the Three Gorges Reservoir.3) New understanding on the water cycle way within the Basin getting from analysis of the environmental isotope results:upstream water of Nanyang river basin is from different sources, subject to the special terrain and climate and other factors, like low temperature, in (Shennongjia Forest) mountains, and the middle and lower reaches of the river flow increased gradually, groundwater flowing into the river basin and mixing with the surface water with a certain amount of evaporation; Upstream tributaries of the Gao Lan River watershed are from different sources, and the middle and lower reaches of river basin is mainly affected by the altitude effect and evaporation; the underground runoff pathways and groundwater residence time of the Xiangshui Cave (underground river outfalls) are relatively long in a closed geological environment, having a strong water-rock reaction; the underground runoff pathways of the Xianglong Cave (underground river outfalls) are relatively short with fast rate of flow, having a weak water-rock interaction.4) From the analysis of the strontium isotope composition, we know that the87Sr/86Sr in Xiangxi River Basin comes mainly from the weathering or dissolution of carbonate rocks. The weathering of carbonate rocks plays a major role on the basin water chemistry composition which is also affected by the dissolution of silicate minerals. The strontium isotope composition in part of the Gaolan River Basin is higher, reflecting the significant impact of clastic weathering on water chemistry of the basin. Two underground rivers are significantly controlled by the different end members. The Xianglong Cave is near the limestone end member, while the Xiangshui Cave is affected by two end members, namely limestone and dolomite..2. The main control factor of karst carbon sink in the area is water cycle condition; but the impacts of the soil can never be ignored.1) The order of dissolved inorganic carbon (DIC) concentration in different sub basins and underground stream in Xiangxi River Basin from high to low is underground water outfall (Xiangshuidong and Xianglongdong)>Gaolan River Basin (Kongzi River Basin)> Gufu River Basin> Nanyang River Basin> Gaolan River Basin (Xiayang River Basin).2) The extent of different basins controlled by water cycle is various:①The DIC concentration of samples from Nanyang River Basin (except Honghua village) in wet season is less than it in dry season. It is controlled by rainfall in rainy season.②The DIC concentration of samples from Kongzi River Basin in Gaolan River Basin is not influenced by flows. Instead, the concentration in dry season is less than it in wet season. The DIC concentration of samples from Xiayang River Basin in wet season is higher than it in dry season; it is much affected by flows and rainfall capacity.③The concentration of DIC in Xiangshuidong and Xianglongdong, both underground stream, shows different characteristics in dry season and wet season. The DIC concentration of Xiangshuidong in dry season is obviously higher than it in wet season, reflecting that it is affected by flow. There is dilution effect distinctly. Meanwhile, there are fewer differences between the concentration of Xianglongdong in dry season and wet season; it is not affected by flow.3) The distribution of DIC concentration of surface water and groundwater in Xiangxi River Basin has obvious territorial difference due to the impact of basin lithology and hydrogeologic condition.①The DIC concentration of Kongzi River Basin in Gaolan River Basin is the most due to the high carbonatite proportion of area in Kongzi River Basin. The carbonatite proportion of area in Nanyang River Basin reachs up to79.82%, though the DIC concentration there is less. The runoff modulus of this kind of strata karst water is the highest, and the karst reaction is very intense. The DIC concentration improves because of the water-rock interaction over long term.②The carbonatite proportion of area is high in Nanyang River Basin, with Pts in the lead. Its proportion in carbonatite is43.73%. The proportions of∈1,∈2O1and Z3d are18.58%,13.15%and9.55%. The runoff modulus of strata karst water is relatively low, leading to less DIC concentration in this river basin. It reflects that high DIC concentration is related to not only the emersion area proportion of carbonatite in river basin but also the intense and time of carbonatite karstification in strata of different times, or hydrogeologic condition.③The DIC concentration of Xiayang River Basin in Gaolan River Basin is low. The main lithology is gneiss, marble and so on. It is quite different from the lithology of upper middle in Gaolan River Basin (Kongzi River Basin). The DIC is rooted in various rock chemical weathering type.4) The DIC concentration of3sub-basins in Xiangxi River Basin and2underground streams is range greatly in a water year, especially between a wet season and a dry season. It is related to the different effect of CO2in soil on the DIC concentration in different hydrological season.①The inorganic carbon concentration in wet season is lower than it in dry season. It is rooted in karstification, and is less affected by CO2in soil.②The inorganic carbon concentration in wet season does not decrease with the raising flow. It shows that DIC in water comes from not only karstification but also maybe the dissolution of CO2in air and soil.3. Using the stable isotope δ13CDIC as the indexes, this paper analyzed the source of the inorganic carbon sink and the biogeochemistry behavior characteristics of the carbon in the river system, judged the contribution of the atmosphere(soil) CO2and carbonate rocks in inorganic carbon sink during the water cycle;1)The value of δ13CDIC in the karst water is evidently influenced by season. In high water season, the value was more positive and in low water season, the value was more negative, which indicated the important affection of decomposition of organic matter and biological effect. In different seasons, there was a negative linear correlation between the DIC concentration and δ13C in the Xiangxi river basin, which was related to the photosynthesis of the phytoplankton:①The effect of photosynthesis surpass decomposition of organic matter and aquatic respiration, moreover, due to high temperature, the dissolving capacity of CO2was reduced, then the CO2was escaped from the river to the atmosphere, so that the DIC in river was decreased;②In the winter,δ13CDIC showed the negative correlation, accompanied by elevated DIC concentrations, lower water temperature, weakening of the photosynthesis, declining in production capacity of water, a lot of organic matter began to decompose, which lead to the negative changes of δ13CDIC while elevated DIC concentrations, due to lower water temperature, the dissolution capability of the water to atmospheric CO2was enhanced, which lead to an increase of DIC concentrations, coupled with poor rains, conspiring to make the DIC concentrations peaked.2) The temporal and spatial distribution of the δ13CDIC in Xiangxi River Basin reflects the basin river system dissolved inorganic carbon to form the nature of its source and sink characteristics related to the watershed characteristics and river hydrodynamic conditions. The DIC concentration and δ13C show a negative correlation in Nanyang River Basin water in summer and winter, which shows that the DIC of biogenesis has a principal effect, the DIC was low but the δ13C was high in Nanyang River Basin, which shows the geological characteristics of the regional carbonate significantly;②The DIC concentration was high but the δ13C was low slightly in Gufu River Basin and Gaolan River Basin, this shows that the soil CO2had a big contribution to DIC.3) There was a high DIC concentration in the two subterranean rivers (Xiangshui Cave and Xianglong Cave), which show the Karst geological features evidently of the two caves; but there was a large different in δ13CDIC, the value of δ13CDIC had a small negative correlation in winter and summer in Xianglong Cave, but in Xiangshui Cave, the value of δ13CDIC only lower in summer, while higher in winter, which was nearly the same with the value in surface water of Xiangxi River Basin. In summer, inorganic carbon contribution by plant root respiration and soil organic matter oxidation in the two underground river recharge area is higher; In winter, only the recharge area of Xianglong Cave is still driven by CO2gas dissolved in the soil carbonate system and continues to react and evolution, with a strong microbial activity.4) The value of δ13CDIC in Nanyang River Basin had a positive correlation with its distance from the mouth of the river generally, but in Gaolan River Basin, the situation was opposite. This showed that photosynthesis may be the important factor in control of the δ13CDIC value in Nanyang River Basin, due to the prosperous mining activities in Gaolan River Basin upstream, fast-flowing, short residence time, a large number of land-based sources of organic carbon, which imported in the river, decomposes slowly and quickly imported into the downstream segment. Only in the estuary location, a large number of land-based sources of organic matter was able to strongly decompose due to slowed water flow and increased amount of water, releasing a flood of light carbon, showing a trend that the δ13CDIC became smaller and lightweight carbon ratio became bigger along the road to the estuary. 5) Calculating the contribution rate of DIC sources by using13C mass balance and thermodynamic equilibrium fractionation, although the results were different, the overall trend was the same:the average ratio of three sub-basins derived from soil CO2were Nanyang River, Gufu River and Gaolan River in dry season, which reflected that the DIC of Nanyang River Basin and the Gufu River Basin water were more from the soil CO2in the dry season; the average ratio of the source of soil CO2in wet season were the Gufu River, Gaolan River and Nanyang River, the DIC in Nanyang River Basin in the wet season was more from karstification, also, the inorganic carbon in Gaolan river Basin in the wet season was from karstification, but the more came from the atmosphere and the dissolution of CO2in the soil.4. Combining the characteristics of the Xiangxi River Basin, petrochemistry weathering carbon sequestration estimation model (GEM-CO2) and water chemical equilibrium model method, after analysis and comparison, were applied to karst carbon sequestration estimates in the Xiangxi River Basin. Where, according to the calculation of the weathering carbon sequestration estimation Model (GEM-CO2), the carbon sequestration of Xiangxi River Basin under the process of rock chemical weathering is23665.7t/a, the three major sub-basins'weathering carbon sequestration accounted for22.217%,38.064%and34.085%. that of the Xiangxi River Basin respectively. The carbon sequestration of carbonate in Xiangxi River Basin contributes to99.03%of its rock weathering carbon sequestration, in which the karst carbon sequestration makes the greatest contribution, playing a dominant role. The total outcrop area of the carbonate rocks in the study area is2135.11km2. Estimating by using the water chemical equilibrium model, the Xiangxi River Basin in2011absorbs6.59×107kg CO2. The difference of karst water CO2content in various times of formation of the Xiangxi River Basin is not significant, but the CO2absorption modulus of karst water among all layers differs more significantly, reflecting that the underground runoff conditions is a key factor, and indicating that the carbon absorption capacity of the karst system not only affected by the water chemistry conditions, but also strictly controlled by the groundwater dynamic conditions.5. Quantitative correlation analysis on DIC and meteorological factors (temperature and rainfall), DIC and six categories of factors (vegetation cover, land use, slope, soil thickness, soil type and parent rock lithology) shows that DIC, in addition to meteorological factors, is also affected by the six categories of factors. It is a result of multiple factors.Through comparative analysis, we found that the thickness of soil is not subject to seasonal influence, and differences in the contribution rate can't be determined. Regions with high vegetation cover and the high and steep slope are mainly affected by the type of land use,, respectively under the impact of rainfall and temperature.A small number of factors, being affected by the complex land use type, show no impact of rainfall and temperature changes,, the higher of the vegetation cover, the higher of their contribution rate. Soil type is mainly affected by soil characteristics itself, mainly divided into two types that are subject and not subject to seasonal impact., The contribution of brown calcareous soil is higher than yellowish brown soil. And the contribution rate of other soil types can't be determined. With regard to the host rock lithology, metamorphic rocks are affected by temperature changes, and the remaining types are affected by rainfall. The carbonate rocks were subdivided into three types, mainly included①∈2O1, O, Z3d and Pts carbonate,②supplemented with P, and T, and③two carbonate subcategories, containing a small amount of Z2∈1and∈1. The overall performance of the main carbonate mainly affected by the seasonal variations of rainfall impact, but its sub-carbonate contribution rate can't be judged.The main innovations:1. Combine the characteristics of karst watershed analysis results, the strength and mechanism of carbon sequestration of the karst water in the water cycle process of karst surface water and underground river system under different environmental conditions are analyzed and explained. And the role of vegetation, soil and other factors in the karst water cycle and the carbon sequestration process is probed.2. By using a number of hydrochemistry indicators and characteristics of isotopic composition, carbon sequestration strength of both surface water and groundwater and sources of inorganic carbon are analyzed. And using the carbon stable isotope, the carbon sources of carbon sequestration are traced, screening the respective contribution and change of atmospheric CO2and carbonate rocks to the carbon sequestration in the water cycle.3. Rock weathering carbon sequestration model and hydrochemistry balance method are used to estimate the carbon sequestration of rock weathering in the Xiangxi River Basin, and evaluate the the contribution of karst carbon sequestration.4. Quantitative analysis of correlation between multiple possible influencing factors and karst carbon sequestration (DIC) is performed to determine the influencing factors of karst carbon sequestration.
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