湖北神农架近2000年来的石笋气候记录

【作者】 况润元；

【导师】 汪永进；

【作者基本信息】 南京师范大学 ， 自然地理学， 2003， 硕士

【摘要】 近2000年来气候变化的研究，对于认识未来气候变化有重要的意义。作者从神农架犀牛洞采到两根分别长255mm（NO.SN）和212mm（NO.BFl）的石笋。基于16个²³⁰Th年龄、1187个氧碳同位素数据分析和显微岩相研究，首次建立了神农架高海拔地区近2000年气候演化序列，通过与历史记录、湖泊沉积、孢粉资料对比分析了东亚季风区不同地区气候演化的相似性和差异性。并初步探讨了气候驱动机制问题。 石笋SN通过9个²³⁰Th年龄的多项式拟合建立年代序列，利用清晰可数年层段检验表明，除底部（>196mm）外其它时段基本吻合；BF1结合²³⁰Th年龄和年层计数法定年，年层计数结果表明定年在测年误差范围之内。两根石笋氧同位素曲线交叉检验表明建立的时间标尺具有较高精度。 结合相邻地区大气降水氧同位素组成与温度的关系，分析得出石笋δ¹⁸O与温度（T）呈明显的负相关性(dδ¹⁸O/T≈-0.64‰／℃)。过去2000年中神农架温度最大变幅约为3.5℃，可细分7个百年尺度以上的冷暖旋回。其中，中世纪暖期呈现两峰夹一谷变化，小冰期划分为5个冷谷4个暖锋。与中国东亚季风区各种温度记录对比表明，东汉时期是一较暖的时期，但冷暖程度不同；中世纪暖期大致从南往北时间跨度越短，中部地区没有东部明显；神农架小冰期开始时间（1480aAD）晚于其它记录（1420aAD），都显示了持续的降温过程。 石笋纹层厚度和灰度曲线比较清晰地揭示了本区过去2000年降水演化过程，可划分7个干湿旋回。与其它地区干湿记录对比显示，长期变化趋势具有较好的对比性。1100aAD前各记录都显示高降水特征，变幅较大，1100aAD后相对较低，振幅小。由于各记录的分辨率不同，细节对比上稍有差别，也反映了区域性特征。相同时段氧同位素与纹层厚度、灰度对比表明，过去2000多年主要是暖湿、干冷的气候组合方式，但在气候转型期温度、降水并没有完全表现同步变化，体现干暖、冷湿组合。 受洞穴岩溶系统影响，两根石笋碳同位素变化不完全一致。通过与氧同位素、纹层和灰度指标对比表明，14世纪以前在气候控制下地表植被变化不大，主要以C₃型植被为主，植被变化略滞后于气候变化。15世纪初开始洞穴上覆土壤带植被可能受人类活动影响有逐步退化趋势。结合孢粉资料分析，在冷暖气候变化中本区植被类型变化不大，以落叶阔叶木本植被为主，草本植被为辅。 近2000年中17次太阳活动和1400aAD以来29次火山活动对气候变化影响的印迹在石笋氧同位素中都有明显的记录，表明气候事件与太阳活动、火山活动有较好的相关性。石笋各代用指标谱分析显示126a、100a、81a、60a、40a、22a、11a等周期成分，也进一步支持了太阳活动对季风区短尺度气候变化的驱动作用。更多还原

【Abstract】 To predict the evolution of climate in the future, it is important to explore and analyze the processes and mechanism of the climate system over the past 2000 years. Here the author uses two stalagmites (NO. SN with 255mm and BF1 with 212mm in height) from Xiniu Cave, Shennongjia to investigate a decadal-century scale climatic changes of East Asian monsoon covering the most part of the last two millenary. Based on 16 230Th dating ages and 1187 data of stable isotopic compositions as well as annual laminated sequences of the stalagmites, the author firstly presents the climate records of the past 2000 years in Shennongjia region. By comparing the stalagmite record with historical records, lake sediments and pollen data, the author analyzes comparability and differences of climate change in East Asia monsoon area and preliminarily investigates driven forces of climate change.A time scale of the sample SN has been reconstructed by a polynomial fitting with 9 dated ages and further tested by annual band counting results. In general, the fitting polynomial line is consistent with the annual band growth rate except for that the bottom part of the stalagmite (a height between 196 to 255mm from the top). However, a time scale of the sample BF1 has been established by the annual band counting chronology bracketed with 7 Th-230 dates. The two chronologies from the different age-determination for the two stalagmites agree well in anuncertain of 30 years if using the same 18O signals as tie points.A linear regression between 18O of precipitation and in situ temperature in the adjacent regions shows that a temperature-dependent 18O gradient coefficient is d18O/T=0.64%/ C. Using the gradient coefficient, the author estimated that there is the maximum amplitude of 3.5 C in temperature over the past 2000 years in shennongjia region. The 18O proxy climatic time series can be divided into seven cold/warm cycles. The 18O record displays two warm periods with a relative cold event during the Medieval Warm Period, and that is quite different from other records in the studied area. The Little Ice Age indicated by the 18O curve is divided into five cold valleys and four warm peaks. During Donghan Dynasty, the 18O record of stalagmite, the same as all of temperature records in the East Asia monsoon area, shows a significant warming up trend that is different from other records. During the Medieval Warm Period, the lasting time is shorter and shorter from the south to the north in china, and the east china is more obvious than the central china; the beginning time of the Little Ice Age in shennongjia region (1480aAD) is later than that in other regions (1420aAD), but they all display a persistent decrease process of temperature.Both the annual banding growth rate curve and grey level curve show the evolution processes of the precipitation over the past 2000 years this region and that divide into seven dry/wet cycles. By comparing precipitation proxies of the shennongjia region with the dry/wet records in other regions, the author finds that the long-term change trend of all records is approximately consistent. All records show the characteristics of high precipitation and large swing before 1100aAD, but the characteristics of low precipitation and small swing after 1100aAD. During the Medieval Warm Period there is a very dry period. During the Little Ice Age precipitation is little. Due to different resolution, every record has difference in detail and also reflects area character. By comparing I8O curve with annual growth rate and grey level curve in the stalagmite during the same period, the author brings forward that that climate in shennongjia region mostly embodies matching modes of warm/wet, cold/dry over the past 2000 years, but temperature was out of harmony with precipitation in the course of climate transforming, which shows that shennongjia climateembodies matching modes of cold/wet, warm/dry at part period of time.There are inconsistent between the two stalagmites δ13C curve vs. age due to the cave sy更多还原