Pedological Characteristics Of The Tianshui-Qin'an Miocene Deposit And Their Paleoenvironmental Signification

TianShui-Qin'an area, which is located among east Asian summer monsoon region, northwest arid region of china and paramos region of Tibetan Plateau , is the joined zone between Tibetan Plateau and Loess Plateau. It is very sensitive to climatic changes. Its Cenozoic deposits can provide a good continental record of paleoclimatic changes of northeastern Tibetan Plateau. Paleosol is a good carrier of the paleoenvironmental and paleoclimatic informations, by which we can deduce and rebuild the paleoenvironment of forming period of paleosol. Because morphological character is very steady, morphological methods are used efficiently in the study of paleosol.In this article, Xiashan profile is studied mainly with soil morphology, chroma and CaCO₃ methods. The results, compared with Oxygen isotope curve of Foraminifora, indicate that the changing processes of paleoclimate and paleoenvironment are as following: warm and dry - being cold and wet - cold and wet - being warm and dry-warm and dry - cold and wet - cold and dry. The preliminary conclusions are as following:1. The interval from 17 to 6.2Ma is tentatively divided into seven parts based on the soil characteristics. The soil type and paleoenvironment of each part are: (1) 17-16Ma: warm-dry climate, Aridisoils, mainly temperate and semiarid steppe environment; (2) 16-13Ma: colder compared with previous stage, maily Ustalfs, warm-temperate and subhumid-submiarid forest or steppe-forest environment; (3) 13.3-10.9Ma: cold and wet climate, mainly Udalfs, occasionally Ustalfs, warm -temperate and humid-subhumid forest or steppe-forest environment; (4) 10.9-9.8Ma: warmer compared with previous stage, maily Ustalfs, temperate and subhumid forest or steppe-forest environment; (5) 9.8-8.3Ma: warm and dry climate, mainly Aridisols, temperate and semiarid steppe environment; (6) 8.3-7.2Ma: cold and wet climate, mainly Udalfs and Ustalfs, warm-temperate and subhumid-semiarid forest or steppe-forest enbironment; (7) 7.2-6.2Ma: cold and dry climate, mainly Aridisols, temperate and semiarid steppe environment.2. During 17-6.2Ma the evolution of East Asian summer monsoon could be tentatively divided into five phases based on the paleosol characters of Xiashan profile. There were tow stronger periods and three weaker periods, the stronger ones was from 13.3 to 10.9 Ma and from 8.3 to 7.2 Ma, which were individually corresponding with two obvious humid periods, and three weaker periods are 17-16, 9.8-8.3 and 7.2-6.2 Ma, That the latter two weaker periods are corresponding with two obvious arid periods. Compared with the studying results of adjacent areas, the author suggests that aridification in northern China might begin near 7.2Ma.3. In Xishan profile, most of the paleosoils experienced the gleying and hydromorphic processes. There were many little ferromanganese nodules, coatings , rust spot and stains. It may because of that the paleosols had experienced large-period soak within water. The pedogenesis of calcareous mudstone isn't obvious. The average CaCO₃ content of xiashan profile is 15.3%, and the coarse particle (>10μm) is spare in the microscope. Meanwhile, calcium carbonate is micrite calcite of normal deposit. These characteristics show that calcareous mudstone is obvious different from aeolian loess. Maybe it was the deposit of low energy environment of floodplain.