【作者】 赵侃；

【导师】 汪永进；

【作者基本信息】 南京师范大学 ， 第四纪地质学， 2011， 博士

【摘要】 过去2000年气候环境的变化是“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。其中近1000年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。北半球温度重建显示这一时期存在着重要的气候变化特征,例如中世纪暖期、小冰期和二十世纪变暖,是当前气候/气象学家所关注的焦点。然而,对于这一时期季风变化事件的起止时间,内部结构和转换速率等特征的认识却受到缺乏同时具备高精度年龄和高分辨率气候记录的制约。亚洲季风影响着世界上近半数人口的生活和生产,季风降水变化是关系到季风区国家社会发展和稳定的重要问题。同时,亚洲季风是快速而敏感物理系统,是联系海-气-冰和南北半球气候的重要载体,并且通过西风带、低纬越赤道气流传输高低纬、南北半球气候事件信号。因此,亚洲季风变化的研究有利于理解南北半球气候的耦合机制,加深低纬季风变化机制的理解。太阳活动作为地球外部能量的唯一来源,往往被作为气候变化的重要驱动因子,然而代用指标和数据分析显示太阳活动和气候之间存在复杂因果联系(提前、滞后或耦合关系不明确),其真实的相位关系缺乏精确时标年分辨率气候记录的支持。同时,微小的太阳辐射量变化(1～2mW/m2)通过海—气系统的何种机制得以放大,从而来影响大尺度的气候变化(如季风)也尚未认识清楚。洞穴石笋生长时间跨度大、储存信息完整并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,成为当前全球变化研究领域的重要发展方向之一。本文基于贵州省董哥洞两支石笋20个230Th年龄、1903条年纹层、1746个氧同位素分析数据,建立了年分辨率的最近1026年以来的亚洲季风区气候变化序列。董哥洞两支年纹层石笋δ18O曲线具有很好的重现性,表明石笋很大程度上生长于同位素平衡分馏条件下,记录的气候信号没有受到噪音的干扰。通过现代器测数据的校正,石笋δ18O变化主要反映了洞穴滴水δ18O值的变化。对比亚洲季风指数重建和其他气候重建资料,本文认为年代际一百年尺度上石笋δ18O在很大程度上受控于夏季降水氧同位素变化,反映了亚洲夏季风强度变化。基于年纹层控制的年分辨率石笋氧同位素序列精确标定了季风小冰期事件。该事件开始于约公元1321年,结束于公元1633年,持续时间为312年。期间季风变化显示出显著的两谷夹一峰特征,并且呈现多次快速的季风强弱转换(<10年)。与历史资料对比发现：夏季风强弱对于中国古代农业社会具有重要影响。弱夏季风阶段往往容易产生饥荒和动乱,甚至于直接影响到中国朝代的更替。董哥洞石笋记录亚洲夏季风强度和秘鲁石笋记录南美洲夏季风强度对比发现,两者存在这反相位关系,表明近1000年来南北半球季风降水在百年尺度上仍然维持着"see-saw"模式。太阳辐射量变化导致南北半球海表温梯度的变化,从而影响ITCZ位置的南北摆动,可能是"see-saw"模式形成的机制。来自亚洲,非洲和美洲大陆和热带海洋、湖泊的古气候记录也证实近千年ITCZ位置始终持续的南北摆动,尤其在小冰期时期长期维持在南半球。采用小波分析方法进行时间序列的分析能够很好的表达信号的局域频谱特征。本文通过时间序列的子波分析,分解亚洲夏季风、太阳辐射强度和北半球温度序列在全时间域上的多时间尺度周期,得到其主要的周期性波动,再用具有波动形式的余弦函数进行全序列拟和,使拟和的偏差平方和达到最小。本文主要考察近1000年来亚洲夏季风、北半球温度和太阳活动在200年准周期上的相关关系,对比结果显示：在准200年周期成分上亚洲季风与太阳辐射强度和北半球温度存在反相位关系。交叉功率普分析也证实在准200年周期上,亚洲季风和太阳辐射量存在反相位关系。本文推测大洋的热储库效应或者热惰性有可能造成了这一反相位关系。更多还原

【Abstract】 Climate changes over the past 2000 years are important part of the research for the "Past Global Changes"(PAGES) and the "Climate variability and predictability"(CLIVAR), two major international research projects. Especially, climatic changes of the past 1000 years are an important hinge period which links up instrumental data and proxy records. Northern Hemisphere temperature reconstructions reveal significant fluctuations during this period, such as the Medieval Warm Period, the Little Ice Age and the unprecedented 20th century warming, which are focused by the climatologists/meteorologists worldwide, however, little is known about the characters in monsoon. The Asian monsoon system affects more than half of humanity worldwide and its changes appear to have played a major role in shaping significant regional societal changes, yet the dynamical processes that govern its complex spatiotemporal variability are not sufficiently understood to model and predict its behavior, due in part to inadequate annually-resolved long-term climate observations. Furthermore, as changes in Asian monsoon link directly to high-northern latitude climates and also to Southern climates, via across-equatorial air flows, it is possible to decipher the precise relationship between the two hemisphere climates and to understand mechanisms of the monsoon variability. As the only energy supplier for the earth, the sun is reasonably assumed to be the important forcing of the globe climate change. However, the causality relationship between monsoon changes and solar irradiance remains uncertain. Stalagmite record has the precise and independent chronology and reserves integrated information. Therefore, stalagmite has become a major data source for continent paleoclimate reconstruction.In this dissertation, annually resolved oxygen isotope records of two stalagmites from Dongge Cave in southeast Guizhou province, based on 20 high-precision U/Th dates,1903 annual layers and 1746 oxygen data, provide a continuous history of the Asian monsoon over the past 1026 years. Uranium-series age dating and counts of annual growth bands enable an excellent age calibration. The new calciteδ18O records, including the previously reported Dongge records(DA and D15), are very similar in timing, shape, and amplitude on inter-decadal to centennial scale, indicating that the speleothems most likely grew under isotopic equilibrium conditions and sample specific noise did not blur the climatic signal. Calibrated with the local instrumental meteorological records, shifts in stalagmiteδ18O largely reflect changes inδ18O values of cave drip water. Furthermore, the Donggeδ18O records exhibit a close similarity to the amount of summer monsoon rainfall and various types of monsoon index, supporting the ideal that theδ18O record is a proxy for Asian monsoon intensity.The oxygen isotope profile clearly shows the transition at～1321 AD from a generally stronger monsoon Medieval Warm Period to a weaker monsoon Little Ice Age that lasted from approximately AD 1321-1633. The weak monsoon Little Ice Age is characterized by two weaker monsoon events and a relative strong monsoon event. The former weak monsoon event is 17 years, the latter is 77 years. The gradually decrease in monsoon rainfall since the 1960s is also obvious in stalagmiteδ18O records from the Middle East to northern China, indicating that changes in speleothemδ18O largely represent variations of precipitation isotopic composition associated with large-scale summer monsoon circulations. In our research, most of the weak monsoon periods were characterized by popular unrest, suggesting that climate played an important role on the demise of Chinese Dynasties.Comparisons of the Dongge cave records with the precisely dated contemporaneous records from Cascayunga Cave in Peru, show that minima inδ18O (wet periods, intense Asian monsoon) at our site are synchronous with maxima inδ18O (dry periods, weak South American Monsoon) in Northeast Peru (within precise dating errors) and vice versa. This anti-phased precipitation relationship between two low-latitude locations may be inter-hemispheric in extent, based on comparison with records from other sites. Monsoon precipitation anti-phasing may be related to north-south shifts in the mean position of the intertropical convergence zone (ITCZ). The variations of the solar energy maybe cause the shifts of the ITCZ.Time series of Asian monsoon intensity, solar radiation and Northern Hemisphere surface temperature were used to analyze their causality relationship with～200 periodic oscillations by wavelet transform. The results indicate that the Asian monsoon intensity is temporally and inversely correlated to the solar radiation and the Northern Hemisphere surface temperature. This is also supported by the pattern matching as well as the cross-spectral analysis of monsoon data and solar activity records. It is suggested that a complex causality relationship between climate and solar activity. The dynamics of the Southern Ocean, especially the heat reservoir effect, may play an important role to modulate this relationship.更多还原