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Southwestern University (2008)

Applications of Geochemical Proxies in Speleothem to the Study on Evolution and Impact Factor of Karst-desertification in Central Western Guizhou during Morden and Contemporary Period

Liu Zi Zuo

Titre : Applications of Geochemical Proxies in Speleothem to the Study on Evolution and Impact Factor of Karst-desertification in Central Western Guizhou during Morden and Contemporary Period

Auteur : Liu Zi Zuo

Grade : Doctoral Dissertation 2008

Université : Southwestern University

Résumé partiel
Karst environment is regarded as fragile as the surrounding environment of deserts. The treatment of karst-desertification and endangered karst environment is one of the targets of globally sustainable development. Located in the centre of the eastern Asia, China southwest karst region is one of the three continuous karst regions and is also a typical monsoon region. Regardless the plenty water and heat as well as the good seasonality in this area, karst-desertification is now very serious. Barren land, short in surface water storage and fragile ecosystem called not only local government but also the central government’s attention. In order to provide scientific foundation and basic data for recovering and repairing the environment in karst-desertification region, it is urgent to understand the formation and evolution of karst-desertification, as well as the impacts of both natural phenomena and human activity on karst-desertification.Guizhou province is in the core of southwest karst region, whose barren carbonate rock area reaches 130,000 km2, up to 73.8% of the total land of the province. Amongst the eight provinces of southwest China, Guizhou has the most serious situation in karst-desertification. Moreover, central-western Guizhou is the most serious karst-desertification region in Guizhou. Therefore, this area which is the studying area of my dissertation deserves the most need for investigation.Stalagmite has become one of the best geological archives for high resolution paleoclimate studies. With abundant cave speleothems widely distributed in central-western Guizhou, one can use speleothems for reconstructing the paleoclimate and studying the evolution of karst-desertification. In this dissertation, guided by Scientific Theory of the Earth System, the author aims to understand material exchange and energy transition among lithosphere, hydrosphere, atmosphere and biosphere in karst environment, to demonstrate the meaning of geochemistry indexes in speleothems, and to find out impact of human activity and climatic shift on karst-desertification by using geochemical proxies in speleothems.Between 2005 and 2007, the author took samples and monitored some dripping water spots in Jiangjun Cave in Qingzhen City, Zhengjia, Shijiangjun and Qiyun Caves in Anshun City, Zhijin Cave in Zhijin County, and Houzi Cave in Shuicheng County as well as their the surrounding areas. Part of the speleothem samples have been dated by 210Pb and 230Th/U methods. I have analyzed water samples forδD andδ18O, soda-straw and stalagmite samples forδ18O andδ13C, and soil and plant samples for organicδ13C. Comparing these results with instrumental records and historic records, as well as published stalagmite records, this thesis conducts the following conclusions :(1) Variation ofδ18O in dripping water at some cave sites reflects seasonal change of surface waterδ18O which is dominated by the seasonal variation in rainδ18O due to thin vadose zone and sufficiently mixture of rainfall and surface water. The low capacity of vadose zone at these sites allows rainwater quickly flowing into the cave and becoming dripping water. Thus, speleothem samples from these cave sites may record the seasonal change of precipitationδ18O.(2) The abundant measurements ofδ18O,δ13C and 210Pb of soda-straws from different caves show that only a few soda-straws whose growth conditions meet certain requests can be used to reconstruct paleoclimate. In a word, there should be no carbonate precipitation inside the straw before the water reaches the tip. One may find such a straw by observe the following features judged in the field : (a) the straw must be white and pure carbonate ; (b) it must be thin and transparent ; (c) no matter how long the soda-straw is, it must look like a straw-hollow, so that the diameter of the straw should be relatively constant throughout ; and (d) it muse be growing at the sampling time with full of water interiorly. If a soda-straw meets the above requests, it is of great significance to record climate and environment during modern and contemporary periods that is the advantage in terms of resolution comparing to stalagmites. Otherwise, we must be cautious to use it.(3) By comparingδ18O of stalagmite and soda-straw with the instrumental records of air temperature and rainfall, there is no obvious correlation between theδ18O and air temperature. However, theδ18O have similar trend with instrumental rainfall and historic moisture records. Relatively heavyδ18O corresponds to less rainfall or drier condition ; and vice versa. Hence, we can infer that rainfall is the dominant factor to influence speleothemδ18O in the studying area.(4) Shijiangjun Cave was developed in the strata of inter-bedded middle-upper Triassic limestone and dolomite bed rocks (δ13C=1.21‰). Houzi Cave was developed in the strata of pure middle Carboniferous and Permian limestone bed rock (δ13C=3.8‰). Karst-desertification is very serious at both sites. Rainfall is similar in both sites : the former is 1375mm/yr and the latter is 1190mm/yr. But, Houzi Cave has higher elevation (~2100m) and lower temperature than Shijiangjun Cave (~1200m). Therefore, C3/C4 plant ratio above Houzi Cave is higher than that above Shijiangjun Cave. Theδ13C of both modern and old carbonate deposits in Houzi Cave is much lighter than that in Shijiangjun Cave. It is obvious thatδ13C of speleothem are more influenced by surface vegetation than by bed rock

Mots clés : speleothem oxygen and carbon isotope karst-desertification paleoclimate central-western Guizhou province

Présentation (Dissertationtopic)

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