A Comprehensive Research Of Asian Monsoon Precipitation Variation By High Resolution Stalagmite δ¹⁸O Record And Historical Documents In The Western Loess Plateau Over The Past 500 Years

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 Moreover, the Wudu area, where Wanxiang Cave situated, is famous for its civilization of long stand where plenty of historical documents are kept well to benefit our study.In this paper, high-precise dating results of ²³⁰Th and oxygen isotope data of stalagmite calcite from WX42-B are used to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 Meanwhile, based on the evaluation and judgment of the Chinese historical documents, the wet/drought index sequence over the past 500 years has been established as well.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.