The Environmental Changes And The Land Desertification Of Fenwei Basin Over The Past 1.2Ma

Located in the Monsoonal Triangle of China, Fenwei Basin is a very sensitive area in responding to global climatic changes. Therefore, we choose Laozhuang section which lies in the joint of the Yellow River and the Fen River as our research object and try to well understand the climatic changes and desertification process over the past 1.2 Ma.In our research, we present the results of grain-size analyses, magnetic susceptibility, paleomagnetic polarity and the carbonate content. Sediments' sedmentary facies, the grain-size composition and grain-size parameter including frequency curves, mean size, standard deviation, skewnes and kurtosis are especially focused on. Based on the data and discussions, the pattern of climatic change in the past can be concluded and also, we can sum up the development trend in the future.From our study, we conclude:(1) According to the correlations among grain size, magnetic susceptibility and paleomagnetic polarity, Laozhuang section contains most of the Aeolian stratigraphic sequences over the past 1.2Ma. And this section is characterized by the abundance of sand, and above L9 (0.9 Ma B.P.),11 sand units were well developed.(2) Grain size and quartz micro-physiognomy study indicate that the sediment in Laozhuang section is eolian sourced. Further investigations suggest that the specific geographical location, plentiful sand supply, strong wind and arid climate all contribute to the formation of the paleo-eolian sand in Laozhuang section. Our study also discusses the source of the sand which is mainly from the low water riverbed of Yellow river, Fenhe River and the paleo- Fenhe lake basin(3) Climate indexes revealed that the climate change of Fenwei Basin in the past 1.2Ma can be divided into two obvious phases: From1.2 to 0.9Ma, it was comparatively warm and wet; From 0.9Ma to present, it was relatively cold and dry. 0.9Ma is a transition period of the climate possibly due to the response to the global climatic transition event in the middle Pleistocence. (4) Total of 11 paleo-eolian sand, 14 loess layers with 8 paleosol intercalated are donated in our profile in terms of stratigraphic sediment physiognomy, which imply 11 great expansion of desertification, 14 stable expansion and 8 reverse desertification respectively in Fenwei Basin since 0.9Ma B.P. Further research proves that the processes of positive and negative desertification in Fenwei Basin are mainly determined by the temperature fluctuations during the glacial and interglacial periods. In addition, regional climate change, regional tectogenesis and the uplift of the Tibetan Plateau make contributions to the consequences.