The Soil-forming Mode And Paleoclimatic Changes Of Pengyang Loess Section

Along with the warming and drying of modem climate, the evolution of arid and semiarid regions has become the focus in the research on climatic changes around the world. Studies of climatic variability in the past are essential for evaluating its evolution. Knowledge of the timing and extent of the growth and retreat of deserts is important for the prediction of future global changes and for understanding their mechanisms. Thick sequences of interstratified loess and paleosols from the Chinese Loess Plateau provide one of the most complete and most sensitive terrestrial records of environmental change for the past 2.5million years.In the past climatic change research, the extraction and establishment of indexes are extremely important. In this paper, an attempt is made to integrate various physical and chemical proxies into a representative paleoclimate index, and verify mutually information from different proxies. The Pengyang section of the northwest Chinese Loess Plateau has been studied and a soil development index has been set up. Correspondingly, the paleoclimatic change patterns of this arid-semiarid region spanning the past 800,000 years have been developed.The soil-forming mode of loess is mainly composed of the particularity of parent materials, synchronization of soil-forming and sediment, bi-directional weatherings, uniform and ambivalent properties and so on.The defined B/M boundary is located in loess L8, in good agreement with that of the central loess plateau. Taking B/M boundary at 780 ka B. P., the average sedimentary rate in the Brunhes Chron is 13.21 cm.ka^-1, therewith, the age of the section bottom is extrapolated to～828 ka B. P.Compared with other loess section, the particle-size distribution of Pengyang section was coarser than that of luochuan, Xian, Lanzhou and Xinjiang Dongwan section. This is not surprising, given the closer proximity of Pengyang to the Asian inland dust sources; The average content of calcium carbonateis of Pengyang section is not only higher than that of central loess plateau, but also higher the west of Chinese Loess Plateau, which reflected Pengyang loess had been deposited by near Liupanshan's substances that contained more calcium carbonateis; The content of diffluent element of 10m depth in the section increased fast, which indicated that the modern eluviation and the leaching depth of loess may be stronger than our guess; The soil growth index can reflect the intensity of soil forming and the climatic changes; The "abrupt chang" of glacial and interglacial period was caused by strong soil-forming processes that enlarged the discrepancy of granularity between loess and paleosol. However, the magnetic susceptibility nearly cannot reflect the existence of S₈ and the magnetic susceptibility of S₇ is highest in the whole section, which is inconsistent with other studies that S₅ has the highest magnetic susceptibility.Thin-section examination shows that the micro-texture of the soil layers is divided into both cementing texture in S₇, S₅ and S₁, and granular texture in all others. These paleosols exhibit both a Bt horizon and an argillic horizon with aped structure defined by slickensided clay skins and deeply penetrating sand-filled cracks and are similar to the most ancient known soil of well-drained forest ecosystems and S₅ (subtropical paleosol) of the Xian sequences. Because aeolian grains contain much calcium, pedogenesis must have been prolonged. Based on these facts together with the difference in particlesize distribution within loess, we prefer to consider the S₇, S₅ and S₁ (720-667ka, 513-412ka and 114-73 ka B. P.) as a kind of semitropical spectacle. Certainly, this needs be further validated.The integrative analysis revealed that Qinling Mountains and Liupan Mountains were all in subtropics climate in 500,000 year before present, the soil growth index, red degree, the shape and the content of calcium carbonateis reflected that the climate of Liupan region had the dry tendency. The soil growth index has particular effects on reflecting climatic changes due to its generational mechanism. The paleoclimatic change patterns of arid-semiarid region spanning the past 800,000 years reveal that the desert expansion and approach in China happened in about 796-792, 766-756, 730-722, 510-504, 350-336, 100-88ka B. P..