【作者】 杨勋林；

【导师】 陈发虎； 张平中；

【作者基本信息】 兰州大学 ， 自然地理学， 2007， 博士

【摘要】 过去2000年（a）气候环境的变化是“过去全球变化”（Past Global Changes，PAGES）和“气候变率与可预测性”（Climate Variability and Predictability，CLIVAR）两大国际研究计划的重要研究内容之一，其中近1000a来的气候变化更是现代器测资料与过去代用指标序列衔接的关键时段，在这一时期存在着一些重要的气候事件，例如中世纪暖期（MWP，900～1300AD）、小冰期（LIA，1300～1850AD）和二十世纪变暖（1900AD至今），同时也是人类活动影响不断加剧的时期，因此成为各国科学家关注的热点。洞穴石笋生长时间跨度大、储存信息完整并适合于精确定年和具有独立的绝对年代标尺，是大陆气候代用指标一个独特数据源，成为当前国际研究领域的重要发展方向之一。黄龙洞和万象洞处于典型季风系统相互作用的青藏高原东部边缘地带，洞穴内部现代碳酸盐沉积物非常发育，石笋氧同位素组成变化对季风的变化十分敏感，是研究石笋沉积和季风变化关系的理想位置。本文基于²¹⁰pb和²³⁰Th两种定年方法，并结合黄龙洞和万象洞石笋δ¹⁸O测试数据建立了青藏高原东部近1200a以来平均分辨率达到1年的亚洲季风变化序列，分析了近1200a来亚洲季风演变的规律，探讨了在短时间尺度上亚洲季风的驱动机制，主要得出以下结论：1．通过对黄龙洞和万象洞石笋碳酸盐氧同位素—水体系的研究，显示现代洞穴滴水与洞穴周围大气降水的氧同位素具有一致性，洞穴滴水主要来自于周围大气降水；石笋方解石与洞穴滴水是在同位素平衡分馏状态下沉积的，表明洞穴上覆岩层或土壤表面的蒸发过程没有对洞穴滴水产生重要的影响，洞穴滴水的同位素组成反映了洞穴上部大气降水的同位素组成特征，这两个洞穴的石笋氧同位素记录可以作为反映气候和环境变化的指标，指示当时降水的同位素组成和洞穴温度的变化。2．精确定年的石笋δ¹⁸O记录与现代器测气候数据、季风指数和同时期的石笋记录进行对比分析，结果显示黄龙洞和万象洞石笋δ¹⁸O的轻重主要受季风带来的降水量效应降水量越多，反之亦然。3．黄龙洞石笋HL021和万象洞石笋WX42δ¹⁸O记录在距今1200a内显示出相似的变化趋势，反映了高海拔的黄龙洞地区和低海拔的万象洞地区西南季风具有相似的变化规律：900～1550AD，西南季风由强盛阶段开始逐渐减弱，降水呈现出逐渐减少的趋势；1550～1800AD，季风由衰弱时期开始逐渐加强，降水也逐渐增多；1800～2002AD，这个时期石笋HL021的δ¹⁸O值整体上由负的峰值逐渐偏正，显示出西南季风再次由强盛时期开始逐渐减弱。两个洞穴的石笋记录都显示了在1200a内气候变化和季风强度都不稳定，存在一系列10a～100a等短时间尺度的季风突变事件。石笋记录都显示了中世纪暖期间季风强盛，降水充沛；小冰期期间季风处于相对弱的时期。4．亚洲季风区的洞穴石笋δ¹⁸O值随着纬度的升高而逐渐降低（变轻），反映了亚洲季风降水δ¹⁸O值变化的纬度效应特征，也间接说明亚洲季风区石笋同位素组成变化继承了大气降水的主要特征，季风区石笋同位素组成具有广泛的气候意义。在最近1200a里，亚洲季风区的石笋δ¹⁸O记录显示了相似的变化趋势，都经历了石笋δ¹⁸O值由轻加重—由重变轻—再次由轻加重三个阶段，而且在中世纪暖期石笋δ¹⁸O值比较轻，小冰期石笋δ¹⁸O值相对重，表明亚洲季风在最近1200a里具有一致的变化趋势。5．石笋记录显示的亚洲季风变化与重建的气温序列指示的温度变化存在着密切的关系，在800～1900AD期间，季风强度变化与温度变化呈正相关，季风增强（减弱），温度升高（降低），是一种暖湿（干冷）的气候特征；尤其是在中世纪暖期，石笋记录显示季风比较强盛，表明当时为暖湿的气候；在小冰期，黄龙洞和万象洞石笋记录显示季风减弱，显示在此期间是干冷的气候，气候寒冷、干燥。在1900～2000AD，温度升高，季风减弱，则为暖干的气候类型，说明在二十世纪升温的背景下，亚洲季风变化与温度变化之间的关系发生了根本性的变化，都显示出温度和降水之间具有反相关关系．6．石笋HL021δ¹⁸O值的变化与利用历史时期气候资料重建的近1000a来长江中下游旱涝灾害等级之间存在着密切的联系，石笋HL021δ¹⁸O值加重时，季风减弱，降水减少，旱涝等级降低，表明洪涝减少，干旱在逐渐加重；反之亦然。同时，黄龙洞石笋HL021δ¹⁸O记录与利用历史文献资料重建的中国东部季风区江南地区最近1200a内旱涝指数变化也有类似的关系，石笋HL021δ¹⁸O值指示的季风加强时期与旱涝指数显示的气候湿润、洪涝灾害加重、降水增多时段相一致；季风减弱阶段与气候干燥、旱灾加重和降水减少相对应，更进一步的表明黄龙洞石笋HL021δ¹⁸O记录准确的反映了我国季风区最近1200a季风强度变化和降水变化历史。7．黄龙洞和万象洞石笋δ¹⁸O变化曲线的功率谱分析结果显示，西南季风的变化具有明显的太阳活动周期和ENSO振荡周期，石笋记录与大气△¹⁴C和宇宙核素产率变化序列具有相似的变化趋势，显示出西南季风变化与太阳活动和能量输出密切相关，说明太阳活动和输出能量的变化是10～100a短时间尺度上驱动亚洲季风变化的主要因素，同时也显示西南季风演变与ENSO之间具有密切联系，说明西南季风可能还受到低纬热带海洋—大气耦合系统自山振荡的ENSO影响。石笋记录与北半球温度变化基本一致，与格陵兰冰芯显示的温度变化遥相关，并且季风强度的变化与ITCZ位置的南北摆动密切联系，支持了高纬地区是激发亚洲季风变化主要区域的观点。更多还原

【Abstract】 Climate change During the Past 2000 years is one of the important research for "Past Global Changes" （PAGES） and the "Climate variability and predictability" （CLIVAR） two major international research project. Especially, Climatic Changes of the Past 1000 Years is an important hinge period which link up instrumental climatic data and reconstructed climatic records, during this period Human activities increasingly impact on climate and environmental impacts, and there were some major climatic events, such as the Medieval Warm Period （MWP. 900～1300AD） , the Little Ice Age （LIA. 1300～1900 AD） and the 20th century warming （1900AD to the present） , so more and more people research climatic and environmental Changes of the Past 1000 Years. Stalagmite record has a much longer time span and reserves integrated information, so it is a valuable supplement archive to records of ice core, loess, lake sediment and peat core, etc. Furthermore, stalagmite is an ideal material for precise U-series dating and yields independent time scales. Therefore, stalagmite has become a major data source for continent paleo-climate reconstruction. Among multi-proxy records in stalagmite including stable carbon/oxygen isotope, trace element and lamina, the stalagmiteδ¹⁸O is the most widely used proxies at present. There are multi-answers in explaining oxygen isotopic compositions of stalagmites from different regions because the shifts of the stalagmiteδ¹⁸O values are controlled by many factors, such asδ¹⁸O in precipitation, depositing process of calcite, cave temperature during depositing period, especially in the regions strongly influenced by summer monsoon. Therefore, it is necessary to study oxygen isotopic composition and its implications of modern stalagmite systematically. Although, great progress in history, amplitude and driving factors of the Asian monsoon variation has been made by the stalagmite records on glacial/interglacial and millennial time scales currently, much research work is still needed on modern stalagmite depositing process and the changes of oxygen isotope composition in responses to the climate change on short time scales. Based on ²³⁰Th/²¹⁰Pb dating and stable oxygen isotope analysis data, we have reconstructed precisely and high-resolution Asian monsoon records with an average 1-year-resolution monsoon record over the 1200 years by modern stalagmites from Huanglong Cave and Wanxiang Cave in the Eastern Qinghai-Tibetan Plateau, and drawn the following conclusions:1. The research on modern condition of stalagmite depositing and oxygen isotopic system in the depositing process suggests there is a remarkable consistency of oxygen isotopic composition between cave drip-water and meteoric precipitation around the cave, and the modern stalagmite is precipitating under isotopic equilibrium with the modern drip water.Therefore, we interpret stalagmiteδ¹⁸O values mainly depending on temperature andδ¹⁸O of meteoric precipitation.2. Based on the stalagmite oxygen isotope geochemistry theory and compared with local instrumental meteorological data, the stalagmiteδ¹⁸O records of Huanglong Cave andWanxiang Cave largely reflected the changes in theδ¹⁸O of precipitation conveyed through Southwest monsoon, while temperature effects are relatively small. The stalagmiteδ¹⁸O records in monsoon region of China reflect variations of Asian monsoon and represent the information of the oxygen isotopes of monsoonal precipitation not only on glacial/interglacial scales, but also on short time scales, and the stalagmiteδ¹⁸O more negative values, the greater the summer monsoon, the monsoon precipitation more, and vice versa.3. There is a similar changing trend betweenδ¹⁸O records of Huanglong Cave and Wanxiang Cave over the past 1200 years, indicating that the Asian monsoon is synchronous in the high-elevation areas and low-elevation areas during the past 1200 years : 900-1550AD, two stalagmiteδ¹⁸O values has generally increasing trend showing Southwest monsoon intensity was gradually weakening during this period; 1550～1800AD, stalagmiteδ¹⁸O values became step-wisely lighter （negative shift） indicating the Southwest monsoon intensity strengthened gradually and the summer monsoon precipitation gradually increased during this period; 1750～2002AD, Stalagmitesδ¹⁸O values begun to become gradually heavier demonstrating strong Southwest monsoon intensity has gradually weakened during this period. The two cave stalagmite records show that Climate Change and the Asian monsoon intensity is not stable and there were a number of short-time-scale （10a～100a） the monsoon events.4. There is a close relationship between the Asian monsoon variations constructed by stalagmiteδ¹⁸O records and reconstructed the Northern Hemisphere temperature record during the past 1200 years. Correlation analyses suggested that Asian monsoon intensity is positive correlation with temperature changes for the past 1200 years, i.e. the monsoon enhanced stronger, the temperature get warmer, and it is synchronous for Asian and temperature. Especially in the Medieval Warm Period, stalagmiteδ¹⁸O values became light, indicating relatively strong monsoon and more summer rainfall with a warmer climate; In the Little Ice Age, stalagmiteδ¹⁸O values got heavy suggesting the Asian monsoon weakened andthe summer rainfall decreased with a dry and cold weather. In 1900～2000AD, Temperature changes and stalagmite records take on negative correlation which suggesting the Asian monsoon became weakened, but climate is warming.5. There is a close link between stalagmite HL021δ¹⁸O record and drought-flood time series by using the historical climate data reconstruction period nearly 1000 within middle and lower reaches of Yangtze River. When stalagmiteδ¹⁸O values became heavy, indicating monsoon weakened and precipitation decreased, whereas, drought increased and flood reduced, and vice versa. Meanwhile, there is a similar relationship between stalagmite HL021δ¹⁸O record and dry-humid index time series by the use of historical documents and data inthe China monsoon region over the past 1200 years. Stalagmite HL021δ¹⁸O v‘alues got light and the Asian monsoon got stronger with more precipitations, whereas, dry-humid index indicates rainfall increased and flood disaster increases. Monsoon weakening is phase with dry climate, drought increased and precipitation reduction correspondingly. Huanglong Cave stalagmiteδ¹⁸O record further reflects accurately the variations of Asian monsoon precipitation intensity over the past 1200 years.6.The cave stalagmiteδ¹⁸O values became gradually lower （lighter） with increasing latitude in the Asian monsoon region reflecting the effect latitude of Asian summer monsoon precipitation and stalagmite isotopic composition changes from Asian monsoon region has a broader climatic significance. In the recent 1200, the Asian monsoon regionδ¹⁸O stalagmite records show a similar trend which experienced similar three stage stalagmite: stalagmiteδ¹⁸O values got heavier, and then lighter, finally heavier again, which demonstrated Asian monsoon variations has three stage changes : 900～1550AD monsoon weakened andl 550～1800AD monsoon strengthened, at the beginning of 1800 AD the Asian monsoon weakened finally again.7. The power spectrum analysis result of stalagmitesδ¹⁸O records from Huanglong Cave and Wanxiang Cave showed that the Southwest monsoon variation has solar activities cycles and the El Nino-Southern Oscillation （ENSO） cycles, and the variations of stalagmite records is consistent with the vitiations of atmospheric ¹⁴C and cosmonuclide production changes, which indicating that the Southwest monsoon variations is solar activity and solar energy output is main forcing factor for the Asian monsoon changes on from the short time scale （ 10～100 years） . And there is close tie between the Southwest monsoon variations and ENSO cycles suggesting that an atmospheric-oceanic couple over the tropical Indian Ocean plays an important role in rapid changes of the Southwest monsoon variations over the 1200 years. The variability of Asian southwest monsoon, which is remotely relevant migration of inter-tropical convergence zone （ITCZ） , is synchronous with the North Hemisphere temperature change and the 5 0 records of Greenland ice core, which supports high-latitude forcing mechanism that high-latitude region is key regions for the Asian monsoon variations over the past 1200 years.更多还原