Holocene Aeolian Loess and Its Pedogenic Modification in the Upper Hanjiang River Valley,China

Loess-like sediments over the first level river terraces of the Upper Hanjiang River Valley between Yunxian and Yunxi were surveyed in detail and then sampled systematically. Pedogenic modification characteristics and the structure of soil profiles were observed in the field survey.Particle-size distribution,mineral assemblage,major elements and trace elements,magnetic susceptibility,CaCO 3 ,micromorphology and OSL age were analyzed in laboratory and compared with Holocene loess-paleosol sequences in the Weihe River Valley. Results are shown as follows.(1)The loess-like sediments have aeolian features due to continuous accumulation of dust storms since the Late Pleistocene,with the profile structure TS-L 0 -S 0 -Lt-L 1 -AD.They are the same as the Holocene loess-paleosol sequences in the Weihe River Valley in these aspects,including stratigraphic structure,color,physical and chemical properties.(2)The dust storm sediments went through different degrees of pedogenic modification in different periods,namely weak pedogenic modification in the late Pleistocene, very strong pedogenic modification during the mid-Holocene and weaker pedogenic modification during the late Holocene.(3)It was cold and wet during the late Pleistocene (11500 a BP)and frequent dust storms led to the formation of Malan loess layer L 1 .It is a climate shift from cold-wet to warm-humid during the early Holocene(between 11500-8500 a BP)and dust storm activity gradually decreased to form a transitional loess layer Lt.It was warm and humid during the mid-Holocene(between 8500-3000 a BP),and dust storm sediments were strongly modified to form paleosol S 0 .It tended to be cooler and drier during the late Holocene(since 3000 a BP),and dust storms were obviously inrensified to form modern loess L 0 .