关键词: 东亚冬季风, 极端冷期, 千年尺度变化, 黄土粒度

Abstract: Two typical loess-paleosol profiles in Chinese Loess Plateau were investigated in order to obtain high resolution records of East Asian monsoon variations during climatic extremes in north China."Upper sand" and "Lower sand" units of loess-paleosol sequences at Luochuan and Xifeng,were sampled at 5~6 cm intervals.More than 800 samples were measured for grain size distribution and magnetic susceptibility,and,an independent and detail time scale was achieved for the stratigraphic units.The results prove that East Asia winter monsoon changes on a millennial scale in climatic extremes in North China.The changes presented high frequency,high amplitude pattern in the "cold " periods.The intensified winter monsoon events last at an average of 1.8~2.4 ka,but the winter monsoon was relatively stable at the "warm" period.Millennial scale variations of the winter monsoon may be caused by instability of westerly jet,which is determined by temperature differences between polar and equatorial regions.While the magnetic susceptibility signals are practically constant in the two coldest periods,doesn't like its couple of grain-size which fluctuates wildly.This may be ex plained when the climate was extremely dry,no magnetite/maghemite at all has been formed in the soil,and a precipitation threshold has to be crossed before the magnetic susceptibility starts to register the summer monsoon changes.

Key words: East Asian winter monsoon, quaternary climatic extremes, millennial time-scale, grain-size distribution