Elemental Geochemical Characters And Palaeoclimatic Changes Of Lacustrine Sediments Of 14-4.4Ma In Linxia Basin

Tibetan Plateau, which has often been called the "Roof of the World", forms a barrier in the stratosphere. Tibetan Plateau Uplift is believed to have made great affects on the environment of its vicinity. Linxia basin, which is a Cenozoic foreland basin, borders on Northeast of Tibetan Plateau with a fault zone. It is not only in the cross area of three main geographic zone but also in monsoon triangle zone. Strata series should be a good record of the history of region tectonic, climate changing and arid environment development and Linxia basin would be an ideal area to study Tibetan Plateau Uplift and evolution of Cenozoic climate-environment.Linxia basin has deposited, steady, continuous lacustrine sediments in 14-4.4Ma by analyzing the sedimentary environment and facies, so it perfectly recorded the location environmental changes. With the high-resolution sampling and elements analyzing, upbuild a high-resolution variety sequence of steady lake sedimentary elements. Comparing to the deep sea oxygen isotope curve, we draw a conclusion that changes of the CaO/Sr ratio can sensitively reflect the palaeoclimate change, and act as a proxy of palaeoclimate change.High resolution analyzing results of the elements contents in long time scale of stable lake, accompanied with the results of pollen spore, calcium carbonate, chlorine ion, and aeolian sands etc. reveal that there are grant climate transform events at 8Ma and 6.2Ma respectively. Of them, climate transform event at 8Ma may represent the start of the desertification of asian interior and the one at 6.2Ma indicates the enhance of the desertification.The results of study on the lacustrine sedimentary chemical elements in Linxia basin between 14 and 4.4Ma, show that the environmental change can be indicated by the elements Ti, Al, Fe, Si, P, Rb, Ca, Na. The stable lacustrine sedimentary elements, TiO₂,Al₂O₃,TFeO,SiO₂,P₂O₅ and Rb, their contents positive correlating to the humid climate, CaO,Na₂O,CaO/MgO,and CaO/Sr direct proportion to the aridity climate. CaO/Sr ratio can act as a proxy of cool and aridity climate, and it directly proportions cool and aridity climate.The results of study on the change of CaO/Sr ratio which is staggered coupling with the north atlantic deep sea oxygen isotope curve, and the sedimentary records in Linxia basin in 14-4.4Ma, we consider that the asian winter monsoon was formed before 12Ma. And its'development experienced three phases: 12-8.0Ma as the forming phase of ancient winter monsoon, 8.0-6.2Ma as the phase of ancient winter monsoon, 6.2-4.4Ma was the time during which ancient winter monsoon evolved to modern winter monsoon. Through the contrast CaO/Sr curve with the north atlantic deep sea oxygen isotope curve, we can find the phenomena that the oxygen isotope changes always lag the changes of CaO/Sr, which may reveal that the Tibetan Plateau Uplift leads to augment of the arctic ice sheet. The results of study on the change of Rb/Sr ratio which is staggered coupling with the south ocean oxygen isotope curve, and the sedimentary records in Linxia basin in 14-4.4Ma, we consider that ancient summer monsoon experienced : 14-12.6Ma as the stable form stage of ancient summer monsoon , 12.6-10Ma as the prevailing stage of ancient summer monsoon, 10-7.8Ma as the declining stage of ancient summer monsoon, ancient summer monsoon transiting into modern summer monsoon in 7.8-6.2Ma,and 6.2-4.4Ma as the evolution stage of modern summer monsoon.The coupling relationship between Tibetan Plateau Uplift course, the desertification of Asian interior and the course of the East Asian monsoon development reveals that Plateau Uplift in late Cenozoic may force the palaeoclimate changes and the formation and development of Asian winter monsoon in Linxia basin between 14 and 4.4Ma.