Study on Loess-Weathering Paleosol Sequence and Paleoclmate Evolution in QFC Profile in Yunxian,China

Qinling Mountains as a natural boundary of South-North of China geography,climate,natural environment,and as the Loess Plateau of South barrier. The upper reaches of Hanjiang River valley area is located in the south of Qinling Mountains,belonging to the northern tropical climate. Through the upper reaches of Hanjiang QFC profile susceptibility,loss on the ignition,particle size,Rb /Sr and other physicochemical properties were studied. The results showed that:( 1) since the last glacial dust storms may well crossed the Qinling Mountains in the south of accumulation,Loess-Paleosol sequence stratigraphic sequence from top to bottom: MS-L0-S0-Lt-L1-AD; these sediments records recorded a complete terraces of the late Pleistocene climate change information,stratigraphic units by various periods of different periods of different degree of intensity of soil information.( 2) Physical and chemical properties were significantly at different stratigraphic units of QFC profile. Such as magnetic susceptibility,LOI,high value of Rb / Sr index average content appear in paleosols,the lower value in the loess layer,and the change of the content of Zr /Rb is just the opposite; these physicochemical properties suggest that the development of paleosol period,hydrothermal conditions into the most suitable period,sediment weathering pedogenesis significantly; in the stage of loess accumulation,cold and dry climate,mainly by the dust deposition,sediment weathering pedogenesis weaker.( 3)Qinling Mountains South North Asia tropical upper reaches of Hanjiang River Front terrace profile of chemical weathering intensity change reveals the loess paleosol climate changes: since the last glacial period( about 18 ~ 11.5 ka B.P.),climate is dry,frequent sandstorms,sediment in the loess accumulation,pedogenesis weak,forming a typical loess( L1); the early Holocene( about 11.5 ~ 8.5 ka B.P.),climate change from dry and cold to warm and humid direction,but mainly in the dry and cold,to form a transition layer( Lt); in the Holocene warm period( 8.5 ~ 3.1 Ka B.P.),climate conditions to achieve the optimal stage,with good heat water,biological activity active,pedogenesis is very significant,development of ancient soil S0; Since the late Holocene( 3.1 Ka B.P.),climate change from warm humid to cold direction, pedogenesis significantly weakened,sand storms appear more frequently,formed in loess deposits of Holocene loess based( L0). Modern surface layer( MS) in( 1.5 Ka B.P.),since the climate warming,and our long-term agricultural tillage disturbance,formed in the Holocene Loess L0 top stack.