Comparative Study On Holocene Environmental Change Of Yangtze River Delta And Caohai In Guizhou

The study of paleoenvironment is the main way of understanding of present environment, and paleoenvironment reconstruction during the warm climate phase which is similar to the present environment draws more and more attention. Holocene is the most important period of modern human evolution, and characteristics of Holocene climate is most similar with the present environment. So, the study of characteristics of Holocene climate change has become the focus of paleoenvironment researches. The Holocene climate change is characterized by the rapid warming since the last glacial, but each region has its an unique geographical environment characteristics, leading to the results of Holocene climate research with the imprints of regional differentiation. By the contrast research between the regional and another, we can uncover the influence and relationship of mutual coupling between the various physical geographical factors, such as local atmospheric circulation model, environmental changes being caused by the differences in topography and the atmospheric circulation changes of the world that was influenced by Qinghai-Tibet Plateau uplift which affect Chinese local atmospheric circulation. Thus we can reveal that subtle changes of the various physical geographical factors, and explore its internal mechanisms.The Asian monsoon is an important component of atmospheric circulation and plays a significant role in the global hydrologic and energy cycles. It directly affects the features of climate and environment in South Asia, Southeast Asia and the surrounding areas. Especially, the uplift of the Qinghai-Tibet Plateau makes the regional and global atmospheric circulation become more complex. Yangtze River Delta is located in the west coast of the Pacific Ocean and east of the Asian Continent, within the sphere of the East Asian monsoon, has a typical monsoon climate characteristic. The changes of the strength of the monsoon circulation is caused by changes of solar radiation in the northern hemisphere. Analysis of sediment characteristics of the Yangtze River Delta can be a good instruction of the cycle characteristics of the East Asian monsoon in Holocene. Depending on analyses of core P J in southern wing of the Yangtze River Delta and core TH-004in the northern area of Taihu Lake, we try to reconstruct the characteristics of Holocene climate change in this region and the cycle characteristics of the East Asian monsoon. During the Holocene, the East Asian monsoon has experienced two significant fluctuations. In the study of core PJ, there are two warm periods in the8800～5400cal yr BP,2800～1200cal yr BP, and in the study of core TH-004, two warm and humid climate phases in the8000～7000cal yr BP and2600cal yr BP～1200a A.D..The regional environment evolution of Caohai in Guizhou is mainly affected by the changes of the strength and spatial conversion of the Indian Ocean monsoon. Analysis of NT peat sediment of Caohai can be a good instruction of the characteristics of climate change in the region since11800cal yr BP, and based on it we can infer the cycle characteristics of the Asian southwest monsoon since the late glacial. Overall, there are two powerful periods of the Asian southwest monsoon since11800cal yr BP. The southwest monsoon began to strengthen since11800cal yr BP, and decreased quickly about9000cal yr BP. The second significantly powerful period of the southwest monsoon began around6500cal yr BP, the most peak is around5000cal yr BP, and began to weaken until around2000cal yr BP.In millennial-scale climate cycles, there are a number of rapid mutation climate events. These mutations cold climate events has a perfect correlation between the study of TH-004core and PJ core. In the study of NT peat sediment in Caohai Guizhou, there are a number of rapid mutation climate events around1200,2600, 4300,5800,6800,7600and8130cal yr BP. The time of occurrence of these mutations climate events have a perfect correlation with changes of oxygen isotope of stalagmite in the Dongge cave and climate change in the North Atlantic.These results suggest that the Asian southwest monsoon strengthened than the southeast monsoon in advance about3000-4000cal yr BP. This paper argue that it may be caused by the consistency of the drivers of the Asian southwest monsoon and the southeast monsoon (north-south movement of intertropical convergence zone (ITCZ)), however, the formation mechanism of the Asian southeast monsoon and southwest monsoon is different, making their asynchronous of strengthening of the monsoon. The southwest monsoon, when southeast trade winds of the southern hemisphere across the equator by the Coriolis force, turns into a southwest wind, and forms the southwest monsoon that has an influence on the climate of the southwest China. When the ITCZ move to the north, the Asian southwest monsoon enhances and be able to carry a lot of water vapor from the Indian Ocean to southwest China. The Asian southeast monsoon is due to the sea-land thermodynamic contrast. When the ITCZ move to the north, the northern hemisphere receive more solar radiation energy, the Pacific Ocean surface temperature has an increasing trend, but because of the huge heat tolerance of ocean, making it should be delayed for thousands of years. And possibly the remnant Northern Hemisphere ice sheets and the lowered concentration of carbon dioxide in the atmosphere during the last glaciation that delayed the development of the East Asian monsoon, but had less influence on the tropical Indian monsoon. On the other hand, the signficant rising of sea level during deglaciation and the progressive warming of the high Qinghai-Xizang Plateau, both are favorable for the development of the Indian Ocean summer monsoon. Due to the impact of the above factors, when the ITCZ move northward, the Asian southwest monsoon can quickly respond to it, but strengthening of the southeast monsoon lags thousands of years. On the contrary, when the ITCZ move southward, the Asian southwest monsoon and the southeast monsoon has the trend of shrinking, but southeast monsoon lags thousands of years. However, changes of the Asian southwest monsoon and the southeast monsoon are controlled by north-south moving of the ITCZ. So, Holocene climate chang of the Asian southwest monsoon region and the southeast monsoon region showed similar changes characteristics. That is, the Asian monsoon may experience two large-scale strengthening during Holocene.Mutation climate events recorded in the PJ core in the Yangtze River Delta, the TH-004core in Taihu lake, and the NT peat sediments of Caohai Guizhou are supported by eviences from the Norhte Atalntic, Indian Ocean, and West Pacific Ocean. We thus believe that these climate events can be regarded as synchronization worldwide, and that they may be caused by cyclical changes of global climate factors, rather than regional climatic factors such as monsoon changes due to north-south shift of the ITCZ.