渭南黄土剖面十五万年以来的主要地层界线和气候事件──年代学方面的证据

本文在渭南黄土剖面所测得的年龄结果和时间标尺研究的基础上，结合粒度、磁化率等气候曲线的分析，对该剖面十五万年以来一些主要地层界线和气候事件的年龄进行了初步讨论，结果如下：１)Ｓ１／Ｌ２界线位于剖面１１８０ｃｍ处，从时间标尺上可知其年龄为１２８８００ａＢ．ｐ．，同末次间冰期起始的年龄相一致；２)Ｌ１／Ｓ１的地层界线年龄为７４２２０ａＢ．Ｐ．，同ＳＰＥＣＭＡＰ曲线中末次间冰期的结束年龄相吻合；３)渭南剖面所指示的末次盛冰期约在２００００～１８０００ａＢ．Ｐ．之间；４)在２００００～１８０００ａＢ．Ｐ．之间的末次盛冰期，有一段不足１０００ａ的快速堆积期，其堆积速率是中国黄土平均堆积速率的１０倍以上。

THE MAJOR STRATIGRAPHIC BOUNDARIES AND CLIMATIC EVENTS IN WEINAN LOESS SECTION SINCE 0. 15 Ma B. P.: BASED ON CHRONOLOGICAL EVIDENCES

By systematically using the AMS I4C and TL methods to date the samples from Weinan loess section since 0.15.MaB.P., the preliminary high resolution time scale of this section has been tuned. Considering detailed analysis on this time scale and other climatic proxies such as susceptibility curve, etc., we summarized several important age results regarding some important stratigraphic boundaries since 0.15 MaB.P.:(1) S1 /L2 boundaryTwo samples located at 1 216cm and 1 240 cm below S1 were dated by TL method.The results were respectively 134 500±17 700 aB. P. and 135400±23 900 aB. P., the apparent hasal age of S1 was suggested 134 500 aB.P. at 1 216cm in depth.In previous study, S1 was considered to be consistent of stage 5 in the deep sea oxygen isotope curve, the later has a hasal age of 128000 aB.P. that present the initial time of the last interglacial period. Here, we can find a difference between the initial age of the last interglacial period and the age of bottom of S1 Why they had this difference? Based on the pedogenesis study in Weinan section, Guo Zhengtang et al. considered that the original S1 /L2 boundary should be located in 1 180cm but not in 1 216 cm4], which means the layer between 1 180 cm to 1 216cm was L2 loess in Origin and was weathered by the later pedogenesis processes (that is, the last interglacial epoch). So, the real S1 /L2 boundary should be located at 1 180cm depth that has an age of 128 800 aB. P. determined by the time scale5], which was very consistent of SPECMAP curve. In the past dating on the boundaries of S1 /L2 in other sections,most results were distributed from 0.130～0.145 Ma B.P., different from the age of 128 800aB.P. showed in SPECMAP curve. Through our dating works, it is thoughtthat the weather processes of the original loess layer by the later paleosol genesis must be treated as an important factor when dating the boundaries from cold stage to warm stage, i.e., the age of stratigraphic boundary in the field must be distinghshed from the age of paleoclimatic event. So based on Weinan section, we can get that the initial age of the last interglacial stage was 128 800aB.P.(2) L1 /S1 boundary A sample at 785 cm was dated an age of 68100±3 780 aB.P., another sample at 870cm had an age of 75 600±10 500 aB.P. The boundary of L1 /S1 at 854cm depth may be suggested an age of 74220aB.P. from the time scale. Considering the age' s error, this age was corresponded to the age of 72000 aB.P. at Luochuan section and the age of about 73 000aB.P. in the SPECMAP curve. It can be demonstrated that the age of stratigraphic boundary in the field from warm stage to cold stage could agree with the age of paleoclimatic event in that time. So it is shown by Weinan section that the ending age of the last interglacial stage was about 73 000～74000aB.P.(3) The maximum of the last glacial stage The age of the maximum of the last glacial stage renected in Chinese loess section have not been strictly dated before. It is known that the maximum of the last glacial stage occurred in about 18000aB.P. determined by SPECMAP curve, so this event reflected in Chinese loess section was in generally considered to be in 18000 aB.P. In this paper, by using AMS 14C dating method, we found a different fact thangeneral before.In this paper, the age of the maximum of the last glacial stage was 20000 aB.P.in grain size curve, and 18 700aB.P. in susceptibility curve. Both ages were not the same. In fact, we found different climatic curves could show different ages of this event. As we known, the deposits of Chinese loess were controlled by monsoon as well as westlies, and the grain size of loess was directly controlled by the intensity of wind, but the susceptibility of loess must be fixed through a series of chemical processes after loess deposit. So the grain size of loess was more sensitive to the climatic changes than susceptibility. When the climatic changed, the intensity of wind was firstly changed, which made the grain size of dust changed immediately. As a conclusion, based on the AMS 14C dating of Weinan loe