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

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.(5) Comparison of the averageδ¹³C values of newly deposited carbonates from different caves with the degree of karst-desertification intensity above these caves indicates that theheavierδ¹³C correlates to more intensive karst-desertification. Therefore, speleothemδ¹³C value can reflect the degree of karst-desertification which basically reflects the vegetation coverage above the cave.(6) Speleothemδ¹³C in Zhijin cave turned heavier sharply around 1460AD. According to history records, army of Ming dynasty entered central-western Guizhou led to population increased greatly during that time. The forest was cut and the land was cultivated, which caused the first deforestation over the past 1000 years in this area. About 1900AD, speleothemδ¹³C values again turned heavier with a shift of 2.5‰, shown in both stalagmite and soda-straw records. Recorded by Zhijin County Annals, the population of Zhijin county increased rapidly from 1900～1910. Human activity has been increased continuously until the present. The above comparison shows that speleothemδ¹³C can reflect the vegetation decrease caused by the population increase, so that it can be used to reflect karst-desertification evolvement.(7) The generally positive correlation betweenδ¹⁸O andδ¹³C in studied stalagmites and soda-straws reflect the correspondence of vegetation to climatic conditions: when the summer monsoon was stronger, the climate was warmer and wetter (with lighterδ¹⁸O). Under such climate condition, the vegetation developed preferably; and vice versa. We must take climate condition into consideration even though human activity is main influencing factor to the karst-desertification evolvement. Through theδ¹⁸O andδ¹³C records of stalagmite and soda-straws in Zhijin cave, we can see that natural shifts could cause degeneration in vegetation but the shifts were not enough to cause karst-desertification during Holocene. Human activity is the main reason that caused the karst-desertification.(8) The study has discussed the climate and vegetation shifts in the past 800 years in central-western Guizhou according toδ¹⁸O andδ¹³C records of stalagmite ZJD-21 and soda-straw ZJD-9C sampled from Zhijin cave. Between 1200～1360AD and 1700～1860AD, the climate was wetter, and the vegetation was exuberance. Between 1360～1690AD and 1860～1960AD, the climate was drier, and the vegetation decreased. Theδ¹³C records of stalagmite and soda-straws in Zhijin cave reflect the large-scale immigrant in central-western Guizhou occurred in early Ming dynasty and after 1900. Human settlement and reproduction, and continuously increased population especially over the past 50 years led to karst-desertification.