Magnetostratigraphy And Paleoclimate Records Of Late Cenozoic Eolian Sequence In The Loess Plateau Of China

On the systematic geological survey, 5 sections of the Late Cenozoic eolian sequence or Late Tertiary Red Clay sequence were chose for investigation on Stratigraphy, paleomagnetism and paleoclimatology. These section include Lintai, Bajiazui, Zhaojiachuan in the central part, Duanjiapo in the southern part and Fugu section in the northern part of the Loess Plateau. Thermal demagnetizer and cryogenic magnetometer were used to systematically measure the magnetic remanence from room temperature up to 500 or 600oC with 50oC step in order to dertimime magnetostratigraphy of the five sections. Meanwhile, magnetic susceptibility was measured for all sections to get paleoclimatic proxy index. Grain size of Lintai and Bajiazui section were measured using a laser grain sizer, which was interpreted as a proxy indices of atmospheric circulation. Magnetic polarity boundary ages are used as the controlling points, while the grain size of eolian sediment is used as an indicator of eolian accumulation rate for building up the time scale of the eolian accumulation sequence. Consequently the bottom age of eolian sequence is estimated. East Asian monsoon history of Late Cenozoic is reconstructed by using grain size and susceptibility are as the monsoon index respectively. After all, paleoclimatic mechinism is discussed, which has been running in the development of eolian pedogenic accumulation sequence of the late Cenozoic. The dominate role of Tibet Plateau on the onset and the evolution of the East Asia monsoon as well as eolian sequence formation on Loess Plateau is manifested in eolian paleoclimatic records. By comparison between paleoclimate record of East Asia monsoon variation and deep-sea records, the important role of global ice volume on eolian accumulation of China is discussed. Sebsequently, signal of ice volume variation is distinguished from paleoclimate record of eolian sequence, and a kind of possible connection between Tibet uplift and global ice volume variation is proposed. On the systemic analysis to these data. The following points mark the main conclusions of this thesis.? On a systematic thermal demagnetization and remanence measurement using a cryogenic magnetometer, Magnetostratigraphy of four eolian sequence profile were produced for the Late Cenozoic strata or the Late Tertiary Red Clay strata, which show a consistent polarity structure. Magnetostratigraphy of Red Clay sequence consists of normal polarity Chron of Gauss reversed Chron of Gilbert and Chron3. Chron Gauss is characterized by a long normal polarity with two reversed event at lower part of the Chron. Chron Gilbert can be determined by its polarity structure of two reversed sub-Chron which are separated by 4 small normal events. All complete eolian sequence profile consistently show the bottom of Red Clay is within Chron 3B, and bottom age of eolian sequence is about 7.2Ma which marks the initiation of eolian accumulation on Chinese Loess Plateau and also marks the beginning of drying process of internal Asia continent. On the assumption that this eolian sequence is closely correlated to the East Asia monsoon development and to the uplift of Tibet Plateau, the initiation of eolian sequence at 7.2Ma probably imply that Tibet Plateau had reached some critical elevation and the relation Asia monsoon system had develop by 7.2Ma.? Red Clay sequence consists of 7 paleosol complex interbeded with 7 reddish pedogenic loess complex. Summer monsoon variation reflected by susceptibility and pedogenisis of eolian Red Clay sequence show stepwise strengthening in the Late Cenozoic and show evident 1Ma variation mag-cycle in the Late Tertiary which reveals the dominate role of stepwise uplift of Tibet Plateau on East Asia summer monsoon.? Over the Late Cenozoic paleoclimate history, they're two important boundaries. 7.2Ma marks the drying beginning of Asia continent and onset of the Asia monsoon system. 2.6Ma of Q/T boundary is an important transition of paleoclimate variation of East Asia monsoon. After this transition, internal Asia continent became further dry, and cold. Meanwhile, winter monsoon circulation strengthens. Monsoonal variation amplitude also increases in Quaternary.? The important role of global ice volume on eolian accumulation and East Asia monsoon variation is proven on the orbital cycle variation of East Asia monsoon and some of the paleoclimatic boundary in eolian sequence, such as eolian dust appeared at 7.2Ma and eolian accumulation rate further increase at 3.2-3.4Ma. Some of long wave variation character in the susceptibility and grain size curves are also found in the deep-sea record. This similarity probably implies a possible connection between uplift of Tibet Plateau and the global ice volume changes.