喀斯特洞穴系统中稳定同位素地球化学特征及其环境意义

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	喀斯特洞穴系统中稳定同位素地球化学特征及其环境意义
其他题名	Geochemistry Characteristic of Stable Isotope in Karst Cave System and Its Environmental Implications
	罗维均
	2007-07-30
学位授予单位	中国科学院地球化学研究所
学位授予地点	地球化学研究所
学位名称	博士
关键词	稳定同位素地球化学特征贵州省洞穴滴水环境意义
摘要	洞穴次生化学沉积物作为探索区域性、全球性气候变化规律和环境变迁史最重要的“信息库”之一，取得了不少有价值的研究成果。但同时，随着高分辨、短时间尺度气候-环境变化记录研究的开展与数据的进一步积累，也发现此类研究目前存在着一些问题与局限性。事实上，洞穴次生化学沉积物环境替代指标的形成经历了一系列复杂的过程，包含的影响因数相对较多。总的来说，其形成过程从大气降雨开始，穿越土壤层、洞穴盖板层到洞穴内滴水中CO2逸出、过饱和的CaCO3结晶和水中携带的不溶物沉淀。其中在不同的气候与环境条件下水与各层（如土壤层、表层喀斯特和潜水带等）中媒介之间发生的物理化学过程决定了环境替代指标的真实涵义。但目前有关这些替代指标的形成过程及其影响因子、适用性等方面的研究工作相当薄弱，尽管包括本文作者所在课题组在内的一些学者已认识到这一工作的重要性，零星做了一点工作，但还无法形成系统的认识。在我国西南部，缺少类似我国北方的巨厚黄土堆积，但岩溶发育，且连片分布，其面积居世界首位（东亚喀斯特片区的中心）。因此，在该地区气候环境变迁史研究中，环境记录“档案”——洞穴次生化学沉积物在该方面所起的作用具有与北方黄土一样的重要意义。该研究为利用洞穴次生化学沉积物的地球化学特征恢复古生态环境意义的研究奠定坚实的理论基础。基于此，我们选择我国西南喀斯特发育最为典型的贵州作为研究区域，通过对分布于境内不同地方100多个洞穴进行实地考察和综合评判之后，筛选出一个系列的4个喀斯特洞穴作为研究对象，分别选取一定数量采样点进行采样。在前人研究的基础上，更加深入系统地对该4个洞穴系统中的稳定同位素（碳和氧同位素）进行研究。分别系统地采集了4个洞穴系统中的大气、植物、土壤、土壤空气、土壤水、基岩、泉水、洞穴滴水、洞穴空气、塘水、潭水和滴水对应的洞穴次生化学沉积物等，现场测试pH值、电导率（Cond.）和气温（T）等，室内分别测试了稳定同位素和其他相关指标（如各种离子浓度等），以期开展不同生态环境条件下正在形成的洞穴次生化学沉积物及其对应的洞穴滴水的地球化学特征的比较研究，揭示它们对气候环境变化的响应机制，即稳定同位素等洞穴次生化学沉积物环境替代指标的现代形成过程以及各种影响因子。本研究主要取得以下几点认识： 1. 虽然缺乏同气候、等海拔和生物量在10~90 t/hm2之间的植被序列洞穴的对比，以及对土壤有机质和土壤微生物活性等的相关试验研究，但根据分析仍可获知，洞穴化学次生沉积物的碳同位素的生物量效应是由植物碳同位素及土壤CO2微生物分馏作用机制共同制约。也就是说，洞穴沉积物碳同位素信号所反映的信息包括植被碳同位素组成及其生物量所控制的土壤性质（如微生物等），并不一定指示地表植被的C3/C4比例变化，甚至也不一定指示乔、灌、草的比例，当然也不仅仅是植被覆盖变化的简单体现。总之，生物量效应在横向上是存在的，它至少部分控制着洞穴化学次生沉积物碳同位素的变化，在对石笋等碳同位素的高分辨率解译过程中是不容忽视的。其机理可能是，生物量的大小控制土壤性质（营养和含水量等），进而激发或抑制土壤微生物群落的生长和发育，从而影响土壤有机质的分解过程最终导致次生沉积物δ13C值的变化。 2. 贵州喀斯特地区δ13CSOC与源植物的δ13C差值最大达8‰以上，其主要受土壤水热条件、质地和pH值控制。由此可见，δ13CSOC动态总体上反映了植被破坏历史。不过由于未进行具体年限测定，植被受破坏的具体时间未知。因此，今后除了需对SOC含量等问题进行系统的研究外，14C定年以及喀斯特地区更多土壤剖面等的工作也有待进一步开展。 3. 洞穴水在下渗过程中普遍发生了混合作用，致使从上（地表）到下（洞穴）δ13CDIC值变化幅度变小。除了QXD和JJD中δ13CDIC值偏重的滴水外，基岩对滴水碳同位素组成的影响都相对较小，而各洞穴中滴水（沉积物）等碳同位素组成与洞穴上覆植被等有关，是生态环境的真实记录。LFD中各种洞穴水δ13CDIC值之间基本上都达到或接近同位素平衡，且与洞穴系统的生态环境有很好的相关关系（非常显著的时间变化）；QXD和JJD之间的洞穴水（除土壤水外）δ13CDIC动态极为相似，即同一洞穴同期不同滴水点滴水δ13CDIC存在很大差异，可能与上覆土壤受到严重破坏导致基岩贡献率相对较高有关，而其中的几个滴水与相应沉积物的同位素不平衡可能是沉积速率太慢或沉积间断所致；XND尽管土壤水与JJD的有相似的特征，但其洞穴滴水平均δ13CDIC之间基本一致且与相应沉积物接近同位素平衡。因此，4个洞穴中，LFD可能是最为适合进行高分辨率和短时间尺度（如季节甚至月份）研究的洞穴。另外，对于文中出现的滴水（上）δ13CDIC偏重于滴水（下）的现象以及土壤中水气平衡的影响因素等还有待今后进一步研究。 4. 研究的4个洞穴系统大气降雨尽管主要受东亚季风控制，但来源不完全一致，还不同程度地受西南夏季风等影响。LFD和QXD受东亚季风影响程度相对高于XND和JJD，西南季风的影响程度恰好相反。 5. 尽管地表蒸发作用对表层（10cm以上）土壤水δ18O有一定的控制作用，但是较深（如10cm以下）土壤水基本不受影响；同位素示踪结果显示，土壤水和洞穴滴水对大气降水均有响应，但响应时间不一，可能与上覆物质（植被、土壤和基岩等）结构构造等控制的水文学特征和有效大气降水量等有关；喀斯特土壤的高度异质性导致土壤水运移机制的不同，从而影响混合作用的过程，最终导致洞穴系统同期土壤水δ18O存在一定差异；雨水在下渗过程中的不同来源水混合作用，是不同类型水δ18O时间上变化幅度不同的主要原因；同一洞穴不同滴水δ18O之间的差异可能与水文学特征不同有关；水化学性质（如SIC）控制沉积物的沉积过程从而影响同位素平衡。由此可见，利用研究的4个洞穴系统中洞穴次生化学沉积物中氧同位素恢复和重建古降水量的高分辨率短时间变化是可行的。但我们也应注意到，在利用洞穴次生化学沉积物恢复和重建古降水量之前有必要寻找合适的方法（如利用Hendy法则等）判断是否达到同位素平衡。不是所有点沉积物都真实地记录着滴水（大气降水）同位素信号的，尤其在QXD和JJD中，一半左右的滴水点沉积物没有达到同位素平衡。这些均显示出在高分辨率短时间尺度古气候研究中，了解沉积物的形成过程对于准确解译洞穴过去环境变化具有重要意义。
其他摘要	Speleothem is one of the most important "information bases" in exploring regional and global climate and environmental changes in the past, and lots of valuable research results were made in this field. Meanwhile, with development of high resolution, short time scale climate change researches and data accumulation, we also found that there were still some problems and limitations. In fact, the forming processes of speleothem proxy index are very complex and effected by many factors. Generally speaking, the forming processes include the following steps: precipitation, coming through the soil layer and flat floor, going into the cave, leaking out of CO2 from drip water, depositing of saturated calcite and insoluble substances within the cave. The physical and chemical processes within the soil and bedrock (such as soil layer, epikarst and phreatic zone, etc.) controll the true meaning of proxy index in different climatic and environmental conditions. However, the forming processes, influencing factors and applicability of these indexes were quite unknown by far. Although a number of scholars, including us, have recognized the magnitude of this work and have done some sporadic work, establishing of systemic knowledge in this field have been unaccomplished. In Southwestern China, there is a vast distribution of karst area, and its area ranks first in the world (as the center of the East Asia karst area). Therefore, when studying the regional climate change history, environmental record "archives" -- speleothem plays an important role as loess in the North of China, which would lay a theory basis for further reconstruction of the ancient ecological environment by using geochemical characteristics of speleothem. Hence, we chose Guizhou province as a research region, where there is a most typical karst landscape in the Southwest of China. We chose four karst-cave systems for studying, after exploring and evaluating all factors of more than 100 caves within this region. On the basis of previous researches, we studied stable isotopes (carbon and oxygen isotope) in these cave systems systematically. All kinds of samples, such as atmosphere, vegetation, soil, soil air, soil water, bedrock, spring water, drip water, cave air, pool water, deep pool water and speleothem corresponding drip water etc. were systematically collected in four cave systems; and conductivity, pH value and temperature etc. were measured in site. Stable isotope and other related indexes (such as ion concentration, etc.) were measured in laboratory respectively. All these work was done for comparative studying the geochemistry characteristic of modern speleothem and drip water under the different ecological and environmental conditions, aiming to reveal their response mechanism to climate and environment change, in other words, to reveal what the form processes of speleothem proxy indexes are and how various controlling factors of modern environment affects speleothem proxy indexes. The following conclusions were made in this study: 1. Despite the lack of comparing data about the same climate condition, equal elevation and/or biomass between 10 and 90t/hm2, and the lack of related testing of soil organic matter and soil microbial activity in the studied region, it could be concluded that biomass effect of modern speleothem carbon isotope were controlled by vegetation carbon isotope and soil microbial fractionation to carbon isotope together according to our analysis. In other words, carbon isotope signals of speleothem reflected at least following informations, including carbon isotopic composition of vegetation and the soil nature (e.g. microbes) controlled by biomass, is not necessarily an indication of surface vegetation C3/C4 ratio changes, and even not necessarily an indication of proportion of arbor, shrubs, grass etc. and of course not only simple a reflection of changes in the vegetation cover. In short, the biomass effect of carbon isotope is prevalent in space, it at least partially controls speleothem δ13C changes, and should not be overlooked in the process of interpreting the speleothem high-resolution carbon isotope. The mechanism maybe is, the biomass would control soil properties (such as nutrition and water content etc.), and further stimulate or dampen the growth and development of soil microbial community, then affect the decomposition process of soil organic matter, finally lead to the changes of speleothem δ13C. 2. Difference between δ13CSOC and that of corresponding plants is above 8‰ respectively in the above four karst cave systems. It maybe be caused by their soil humid-thermal conditions, texture and pH value. It can be seen that δ13C dynamic reflect on the whole history of vegetation damage. However, because of the absence of dating, specific time of vegetation damage still was unknown. Therefore, more work should be carried out in the future, such as SOC content, 14C dating, other soil profile, and so on. 3. The mixing of cave waters is prevalent in the process of infiltration, which leades that the fluctuation of δ13C is minor from up (surface) to down (the caves). Contribution rate of bedrock to drip water DIC is relatively small except for drip waters which δ13CDIC are relative positive in Qixing cave and Jiangjun cave(hereinafter QXD and JJD respectively). And drip water (speleothem) δ13CDIC is correlative with surface vegetation δ13C in all cave systems. It records the true ecological environment information. In Liangfeng cave (hereinafter LFD) system, all kinds of cave waters almost reach or nearly approach the carbon isotope equilibrium, and there is a good correlation between their δ13CDIC and that of the ecological environment (which changes significantly with time). Except for soil water, δ13CDIC of all cave waters are very similar between QXD and JJD, where there is a great difference among different drip waters in same cave and same period. This may result from the fact that the contribution rate of DIC by bedrock is relatively higher, due to overlaying soil damaged badly. Moreover, the carbon isotope equilibrium was not reached between drip water DIC and its corresponding speleothem, which maybe can owe to slower deposition rate in some drip site. In Xiniu cave (hereinafter XND), the carbon isotope of soil water DIC is similar to that of JJD, but other waters, average drip water δ13CDIC are almost equal among different drip sites, and they all close to isotopic equilibrium between drip water and corresponding speleothem respectively. Therefore, in the four caves, LFD may be most suitable for the carbon isotope research at high-resolution and a short time scale (seasonal or monthly). In addition, the reason why some δ13CDIC of drip waters (up) is more positive than drip waters (down), and what factors control isotope equilibrium between soil water and soil CO2(g), still need be further researched. 4. Precipitation is mainly controlled by the East Asian summer monsoon in the four cave systems, but their sources are not exactly the same. Southwestern summer monsoon (India summer monsoon) may also be one of factors to some extent. Moreover, the East Asian summer monsoon affects LFD and QXD more than that of XND and JJD, but Southwestern summer monsoon acts as a reverse way. 5. Though δ18O of surface soil water (above 10cm) is controlled by the surface evaporation to some extent, the deeper (under 10cm) is nearly not affected by it. Results of isotopic trace showed that responses of soil water and drip water to precipitation were very sensitive, but their response time was different. This maybe results from their different characteristics of hydrology and effective rainfall which are controlled by physical property of overlying material (vegetation, soil and bedrock). The soil high heterogeneity would lead to the differences of soil water movement mechanisms in karst areas, and then affect the process of mixing, hence lead to a certain difference of δ18O for soil waters in some cave systems in same period. The mixing of different source waters in the process of rainwater infiltration is the reason for different amplitude of δ18O among different types cave waters. The differences among different drip waters in same cave maybe owe to their hydrological characteristics. Moreover, the processes of isotopic equilibrium would also be affected by chemical properties (such as SIC) of drip water. Therefore, it would be feasible to rehabilitate and reconstruct paleo-precipitation at a high-resolution and a short time scale by analyzing those speleothem δ18O in above four caves. But it would be noted that it would be necessary to find some suitable ways to judge the isotopic equilibrium before a speculating by using speleothems to resume and reconstruct peoleo-precipitation (such as Hendy test). In above cave systems, especially in QXD and JJD, drip water of many (nearly a half of them) drip sites did not reach oxygen isotopic equilibrium, apparently, isotope signal of drip water (precipitation) was not recorded completely.
页数	122
语种	中文
文献类型	学位论文
条目标识符	http://ir.gyig.ac.cn/handle/352002/3282
专题	研究生_研究生_学位论文环境地球化学国家重点实验室
推荐引用方式 GB/T 7714	罗维均. 喀斯特洞穴系统中稳定同位素地球化学特征及其环境意义[D]. 地球化学研究所. 中国科学院地球化学研究所,2007.

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