末次间冰期以来沙漠－黄土边界带的环境演变

沙漠－黄土边界带地处东亚季风边缘，这一地区的风成沉积即古风成砂－黄土－古土壤序列很好地反映了东亚季风环流的演变历史。末次间冰期的沉积(Ｓ１)由三层古土壤和夹于其间的两层黄土组成，可与深海氧同位素阶段５的五个亚阶段(ａ－ｅ)则很好对比。末次冰期的沉积(Ｌ１)由三层古风成砂和夹于其间的两层黄土构成，由此反映了与上述三层古风成砂对应的阶段２、阶段４及阶段３的中期是沙漠－黄土边界带冬季风活动加剧、沙漠扩大的时期。全新世的沉积是由一层草原上(Ｓ０)和其上的流沙组成，但这层流沙是在最近两千年来不合理的人类活动和当地特定风场条件的共同作用下形成，已不具备明显的气候指示意义。

ENVIRONMENTAL CHANGES IN THE DESERT-LOESS TRANSITIONAL ZONE OF NORTH CHINA SINCE BEGINNING OF THE LAST INTERGLACIAL

Desert and its peripheral loess form a coupled system. In China the desertloess transitional zone between the Mu Us Desert and the Loess Plateau is very sensitive to climatic changes and very important in studying the past global changes (PAGES). Although desert evolution has been studied in Africa and North America, no continuous eolian deposit profiles seem to be found in the two regions. The desert-loess transitional zone provides a possibility to find such profiles. Since 1983, many Chinese scientists have investigated palaeoeolian sands in the desert-loess transitional zone. So far, however, more completely pedostratigraphic knowledge of this zone and relevant climatic curves with high resolution have not been obtained.In recent years, we have investigated the Caijiagou and Shimao profiles in this zone. The Caijiagou profile is located in Yuanyichang, 20 km away from Yulin County; the Shimao profile is 55 km south of Yulin. A typical sedimentary sequence in the desert-loess transitional zone consists of alternative sand, loess andpalaeosol layers. Generally, sand layers correspond to an extremely cold and dry climate, palaeosol layers a warm and humid climate, and loess layers an intermediate climate. Studies show that the Caijiagou profile is nearly continuous in accumulation. In this paper, therefore, we focus on studying this profile to construct paleoenvironment since the beginning of the last interglacial. The Shimao profile is studied as a reference.The Caijiagou profile includes three units: Holocene palaeosol (S0),the Malan Loess(L1) and interglacial palaeosol (S1). TL dating shows that the age of interglacial palaeosol (S1) is between 126 000-70 000 a B.P. S1 can be subdivided in three palaeosol layers (S1-1, S1-2, and S1-3,) and two loess layers. The Malan Loess (L1) consists of three sand layers and two loess beds. The top loess of L1 yields a TL age of 31 000 a B.P. and the bottom loess of L1 yields a TL age of 50 200a B.P. The Holocene palaeosol (S0) is characterized by containing a dark humus layer. These division and subdivision can be correlated with those based on marine 18O measurements.Such proxy data as magnetic susceptibility can be used to infer environmental changes. For example, the four peaks in the Caijiagou magnetic susceptibility curve correspond to four palaeosol layers. Thus it can be inferred that there were four strong stages of summer monsoon during the last glacial-interglacial cycle. In most cases, however, the relationship between magnetic susceptibility and climate is not so simple. In fact, the magnetic susceptibility can not exactly reflect the extent of pedogenesis. Our results show that the curve of organic matter can be correlated with the curve of magnetic susceptibility, suggesting that organic matter would make substantial contribution to magnetic susceptibility.The desert-loess transitional zone is within the East Asia Monsoon Zone, and so its environmental evolution has been controlled by the monsoon circulation. Dry climate and strong winter monsoon should be two key factors to constrain the development of the Mu Us desert. We believe that three eolain sand layers in the Caijiagou profile should represent three times of extension of desert which were driven by extremely strong cold-dry winter monsoon, while four palaeosol layers should represent four stages of strong summer monsoon.