【作者】 刘敬华；

【导师】 陈发虎； 张平中；

【作者基本信息】 兰州大学 ， 自然地理学， 2008， 博士

【摘要】 重建高分辨率的历史时期气候序列是全球变化研究领域的热点,对短时间尺度气候变化形成原因及其影响进行探讨更成为地球科学研究中最为活跃的领域。洞穴化学沉积物的沉积特征及其所蕴含丰富的同位素、微量元素组成和年龄信息,是陆地环境中极为重要的古气候信息库,由于对气候变化的高度敏感性和适于精确定年等优势,洞穴石笋现已成为研究古气候记录与古生态环境变化极佳的环境信息载体,目前被广泛应用于季风气候的研究并取得了重要成果。万象洞位于青藏高原东缘与黄土高原西南的过渡带,靠近现代夏季风降水区的边缘,是典型季风系统交互作用的边缘地带,对亚洲季风系统的进退消长十分敏感,是研究石笋沉积记录与季风变化关系和古气候重建的理想区域;万象洞所在地——武都地区古代文明源远流长,各种史书典籍丰富,便于收集整理各类历史文献资料。本文基于万象洞石笋WX42-B高精度²³⁰Th测年结果和石笋方解石δ¹⁸O测试数据,定量重建了武都地区过去500多年来高分辨率的季风降水变化历史,同时在评价史料可靠性的基础上,建立了研究区内近500多年的旱涝变化序列,在两条序列相互校验和综合分析的基础上,获得如下认识:1.万象洞现代洞穴碳酸盐—水同位素体系研究表明,洞穴滴水主要来自于当地的大气降水,滴水的氧同位素组成特征反映了当地大气降水的同位素组成变化,且洞内次生碳酸盐沉积时与滴水达到了同位素平衡;根据Hendy检验,万象洞石笋氧同位素过去沉积记录也表明石笋在沉积过程中并没有明显的动力分馏效应。现代沉积过程分析和过去平衡检验结果,可知万象洞石笋氧同位素是在平衡分馏条件下沉积的,可以反映过去气候环境信息,指示了当时降水的同位素组成和洞穴温度的变化,适于古气候重建。2.通过与武都地区现代器测数据、亚洲季风指数的对比分析以及季节洞穴滴水氧同位素的研究表明,在短时间尺度上本研究区内季风强弱带来的降水量多寡是影响洞穴石笋δ¹⁸O值的最主要因素,万象洞石笋δ¹⁸O灵敏地记录了亚洲季风强弱变化以及季风带来的降水量信息,这与亚洲季风区其它地区石笋δ¹⁸O受雨量效应影响的研究结果一致。3.在评价史料可靠性的基础上,利用旱涝史料和现代器测气象数据建立了青藏高原东部较高海拔地区近500多年的旱涝指数变化序列,该序列与我国东部和全国旱涝灾害等级（或干湿指数）相似度很高,显示出最近500多年内存在一系列的10～100 a等短时间尺度的降水波动,而且这些事件在发生和持续的时间上具有很好的一致性,表明靠近夏季风降水区边缘的武都地区旱涝指数序列同样能够代表我国西部季风边缘区的旱涝变化。4.利用万象洞石笋WX42-B顶部的氧同位素记录重建的武都地区近100多年来的亚洲季风降水序列显示,近100多年来亚洲季风的变化历史可分为季风降水增强期、季风降水减弱期和季风降水再次增强期三个气候段,这三个季风变化阶段与通过历史文献记载建立的旱涝指数变化相似。同时,利用²³⁰Th方法得到的测年结果与历史文献记载的饥荒事件的时间在定年误差范围内一致,表明通过石笋δ¹⁸O值恢复历史时期气候变化具有可靠性和准确性,利用高分辨率的石笋氧同位素来延长器测资料并重建本研究区季风降水是可行的。5.利用石笋δ¹⁸O重建了近500多年来亚洲季风降水演化历史,在1470～1875AD小冰期气候阶段内,亚洲季风降水在经历高频波动后稳定在季风降水相对较强的水平上,1875AD进入当代暖期后,季风降水开始增强,而且降水的变化幅度明显增大。近500多年亚洲季风降水与早涝指数变化相似,重建降水量指示的季风降水加强/减弱与旱涝指数指示的洪涝/干旱相位比较一致,近500多年来季风降水演化历史的三个阶段与旱涝指数序列具有同步性,季风降水减弱造成的饥荒对人类文明产生了不容忽视的影响。对亚洲季风降水的变化机制初步分析表明,短时间尺度上亚洲季风降水变化与太阳活动有关,并且与高纬度地区温度变化遥相关,功率谱分析显示,亚洲季风降水变化周期与太阳活动周期相近。更多还原

【Abstract】 As one of the most heated topics in the research of global changes, the reconstruction of climate changes in historical periods has been paid special interest on recently. In this kind of research, the discussion on the mechanism and influence of climate changes on short timescale has become the most active topic worldwide. The characteristics of chemical sediment, the composition of inner isotope and trace element and robust age framework make the stalagmite as one of the most vitally important archives for paleoclimate study. For the moment, stalagmite has been widely implemented in the study of past climate changes in monsoonal area which result in some crucial achievements for academic world. In this study, an ideal site, Wangxiang Cave, has been deliberately selected in order to improve our understanding on past monsoonal changes as much as possible. The Wanxiang Cave is located on the transitional area of eastern edge of Qinghai-Tibet Plateau and southwest Loess Plateau where near the northern limit of modern Asian Summer Monsoon （ASM） , therefore, it can provide information of the growth and decline of ASM system. Moreover, the Wudu area, where Wanxiang Cave situated, is famous for its civilization of long stand where plenty of historical documents are kept well to benefit our study.In this paper, high-precise dating results of ²³⁰Th and oxygen isotope data of stalagmite calcite from WX42-B are used to quantitatively reconstruct a high-resolution monsoonal precipitation sequence over the past 500 years. Meanwhile, based on the evaluation and judgment of the Chinese historical documents, the wet/drought index sequence over the past 500 years has been established as well. By comparison and analysis of these two sequences, the following results can be found:1. The study of modern cave carbonate-water isotope system indicates the dripwater of Wangxiang Cave is mainly from local precipitation, thus the oxygen isotope of the dripwater can reflect the variation in the oxygen isotope of meteorological precipitation, in other word, the isotope equilibrium between secondary carbonate and dripwater has been reached. Furthermore, the test of past deposition record of oxygen isotope in Wangxiang Cave can also suggest that no obvious dynamic fractionation effect occurred during the deposition process of stalagmite. On the basis of the principal of Hendy isotope equilibrium test, the result of the analysis on modern depositional process and the past equilibrium test indicates that the oxygen isotope formed under the equilibrated fractionation can reflect the isotopic composition of local precipitation and variation of temperature. Therefore, the oxygen isotope of stalagmite can be used to reconstruct the past climate changes.2. In comparison among modern instrumental data, ASM index and oxygen isotope value of seasonal drip water from Wangxiang Cave, the monsoonal precipitation is the most influential factor to induce the changes of oxygen isotope value. Thus, the oxygen isotope data can sensitively reflect the changes of monsoonal precipitation and strength variation of ASM, which consist with the results of corresponding studies on other monsoonal areas.3. Through the evaluation on the reliability of historical documents, the sequence of wet/drought index in higher altitudes of the eastern Qinghai-Tibet Plateau was established by the historical documents and modern meteorological data. This sequence is quite similar to the grade of flood /drought in the eastern China and the whole China. A series of precipitation fluctuations within 10～100yr timescales occurred over the recent 500 years, and these events are consistent with one another for the time of onset and duration. It shows the variations of drought/wet in the Wudu area near the northern limit of ASM correspond with those in the ASM domains and even in the whole China.4. The precipitation sequence of ASM in the recent 100 years established on the oxygen isotope form the bottom of stalagmite （WX42-B） in the Wangxiang cove shows three climate periods in the recent 100 years can be divided, namely, the enhanced monsoonal precipitation period, the weakened monsoonal precipitation period and the reinforced monsoonal precipitation period. These three periods are similar to the periods according to the grade data of flood/drought developed by the historical documents. In the meanwhile, the dating results of the ²³⁰Th of the famine event fall into the range of time documented by historical archives. Therefore, the accuracy and reliability of oxygen isotope to reflect the climate changes can be ascertain, and it is feasible to extend and reconstruct the monsoonal precipitation by the high-resolution stalagmite oxygen isotope data.5. In the use of the negative correlation betweenδ¹⁸O of stalagmite and ASM precipitation of the second half year, the evolved history of monsoon precipitation of Asia over the past 500 years has been reconstructed quantitatively. In the Little Ice Age from AD 1470 to AD 1875, after periods of high-frequency fluctuation, the monsoon precipitation of Asia kept stable on relatively stronger level. After 1875AD when entering the contemporary warm period, the monsoonal precipitation began to strengthen and the variation of precipitation obviously increased. The changes of monsoon precipitation index and those of wet/drought index were similar over the past 500 years. Furthermore, three evolutionary periods of monsoonal precipitation consist with the stages partitioned based on the wet/drought index. Famine events due to reduced monsoonal precipitation exerted great negative influence on the human civilization. The preliminary study of mechanism of Asian monsoonal precipitation indicated that, the variation of monsoonal precipitation was relevant to solar activity on the short time scale and correspondent with temperature changes in the high latitudes. The analysis of power spectrum showed that the variation cycles of Asian monsoon precipitation were close to those of solar activities.更多还原