History of Climate Change Recorded by Grain Size at the Zhaosu Loess Section in the Central Asia since the Last Glacial Period

Abstract

The Ili region is located in the northeastern part of Central Asian region,dominated by the westerlies all the year round,loess sediments are widely distributed in the piedmonts of Tianshan Mountains and river terraces,which enables the Ili region to be one of the ideal places for the studies on Asian interior aridification,dust sources of the Northern Hemisphere,past atmospheric circulation,and past global climate change.In this paper,we reconstruct the history of climate change since the last glaciation by analyzing the grain size of Zhaosu section loess.The result shows that the content of grain size63 μm can be well compared with the records of Greenland ice core that the climate fluctuations of the global significance such as Heinrich events and the Younger Dryas cold event as well as D-O cycles,reveals the Central Asian's climate changes with the global features of millennial scale and high frequency oscillation since the last glaciation. The analysis of grain size indicates that the loess grain size distribution of the Ili region is similar to that of the Chinese Loess Plateau,dominated mainly by silt,but the contents of sand in the Ili loess is lower and fine clay higher,which is different from those of the Chinese Loess Plateau.However,compared with the Chinese Loess Plateau,the overall integrated condition of water and heat in the Ili region is poorer than that of the Loess Plateau,which basically can be concluded that4 μm grain size fraction in the Ili loess may be mainly derived from the Central Desert region,only very small part of the clay results from the role of weathering and pedogenesis or debris.Therefore,given the regional topography and circulation conditions,the dust sources may be mainly from its western desert region of Central Asia,the annually prevalent westerlies and distal dust may play an important role in the formation of the Ili loess. According to the variations of the content of the loess grain size,the Ili region's climate change can be divided into four stages since the last glaciation:(74 ~ 60.2 ka) cold period,(60.2 ~ 30.4 ka) warm period,(30.4 ~ 11 ka) cold period and 11 ka warm and dry period.The abrupt climate events reflected from changes in atmospheric circulation revealed by loess grain size at the Zhaosu section loess can be well compared with the Heinrich events and the Younger Dryas cold event as well as D-O cycles revealed by the oxygen isotope records from GISP2 since the last glaciation,suggesting that the Ili region dominated westerlies is likely to be a simultaneous change with the high latitude climate of the Northern Hemisphere since the last glacial period.