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Ding Zhongli\ Sun Jimin (Institute of Geology,Chinese Academy of Sciences,Beijing\ 100029) Zhu Rixiang\ Guo Bin (Institute of Geophysics,Chinese Academy of Sciences,Beijing\ 100101)  
n the Loess Plateau,most of the complete loess soil sequences are directly underlain by a set of reddish clay silt deposits named informally as the Red Clay formation.The contact between loess and the Red Clay has been paleomagnetically dated in several sections to be about 2 6 Ma B.P.(around the Matsuyama/Gauss reversal). Recently we have made careful field observations on several Red Clay sections in different parts of the Loess Platear,and analyzed 1 800 particle size and 30 geochemical samples taken from the Jiaxian section situated in the northern part of the Plateeau.Field and laboratory studies show the following interesting features.(1)The pedostratigraphic structure in different Red Clay sections can be farily well correlated although they are located over far distances.(2)The mass accumulation rate is almost identical in most of the Red Clay sections.(3)The particle size of the Red Clay samples is predominantly below 65μm that is thought to be the upper limit for long distance transport of eolian dust by winds.(4)The particle size distribution partern is similar to that of the Pleistocene loess.(5)The concentrations and distribution patterns of the rear earth elements in the Red Clay are almost the same as those in loess.(6)The texture of the Red Clay shows obvious homogeneity in differnet places.These characteristics greatly support the ideas that the Red Clay also wind blown in origin,and loess deposition in the Quaternary can be regarded as the continumation of Pliocene eolian dust accumulation in northern China. Our studies have also proided new insights into the evolution of the East Asia summer monsoon during the Pliocene that can be summarized as follows.(1)In general the summer monsoon during the Pliocene was much stronger than that during the Pleistocene.The paleosols in the Red Clay appear to have been subject to strong weathering,indicated by B t horizons with thick,reddish clay films and many dark Fe Mn films.They are interpreted as having been formed under steppe forest coditions.By comparison,the paleosols formed in the Pleistocene are characterized by massive structure and A and /or B w horizons,implying lower monsoonal precipitation during the interglacial periods of the Quaternary.(2)According to the pedological features,the Red Clay deposits can be divided into three parts,each of which is characterized by different intensity of pedogenic weathering.The lower part of it formed in the early Gibert Chorn compreses many reddish paleosols with B t horizons.They appear to be better developed than the soils within the middle part of the late Gilbert Chorn.The soils in this part are characterized by B w horizon.There fore the summer monsoon intensity in the early Gilbert was infered to be much stronger than that in the late Gilbert.The paleosols within the Gauss Chorn are best weathered in the entire Pliocene Red Clay.It is hence interpreted that the strongest summer monsoon occurred over the time interval of ca.3 6~2 6Ma B.P.in the Plio Pleistocene period.(3)In the Red Clay deposits,some yellowish horizons with varying thickness can be occasionally seen.These horizons appear to be similar to the weak soils in the Pleistocene loess sections,and could be formed under dry steppe environments with relative weak summer monsoon.When we compare the pedogenetical characteristics of different horizons within the Red Clay,the conclusion can be drawn that dramatic summer monsoon changes also occurred during the Pliocene.
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