The Desert Formation And Paleoenvironmental Evolution Of The Tengger Desert From A Drill Core In Its Interior

Asian drying is associated with the global cooling, Tethys extinction, stepwise uplift of the Tibetan Plateau and evolution of Asian monsoon during the Cenozoic. These tectonic-enviromental events are closely interacted with each other in northwestern China, where the major arid region in central Asian and big deserts located. Tengger Desert, one of these big deserts, located to the northeastern Tibetan Plateau and the northweastern Loess Plateau, is a key region interplayed by monsoon and westerly, and a transition area from semiarid to arid area and from the higher plateau to the lower basin. Owing to overlapping of sand dunes and lacking of natural outcrops in the desert region, a drilling program was conducted in the central desert in order to establish the stata of the desert center and to further recognize the drying history of the desert.The drill core (WEDP01), located in the center of the Tengger Desert, is of276m in depth with core recovery rate of62%(78%in0-147m,50%in147-276m). After determining the main magnetic carriers of the remanent magnetization magnetic hysteresis loops and thermomagnetic curves analysis from typical samples,remanance of paleomagnetic samples with an interval of20cm was measured using the superconductive magnetometer after systematic thermal demagnetization. Electron Spin Resonance (ESR) dating method was also applied to compare with the paleomagnetic measurement results for cross check in order to thoroughly establish the chronological framework of the drill core. Scanning Electron Microscope and grain-size component analsis were employed for sedimentary facies analysis; and grain-size, high and low frequency magnetic susceptibility and color index were all measured with an interval of10cm for paleoevironmental reconstruction. In addition, element was measured using the X-ray fluorescence spectrometer with an interval of1m. The paleoenvironment in the Tengger Desert center was reconstructed according to the series of redness, aeolian components and chemical weathering index. Based on the comparison of paleoenvironmtal indexes in Tengger Desert, Loess Plateau and deep ocean, the driving factors affecting the desert formation and evolution are further discussed. The following are the main conclusions of this thesis. 1) On a systematic thermal demagnetization and remanence measurement, magneto stratigraphy of the drill core was produced including Brunhes Normal Polarity Chron, Matuyama Reversed Polarity Chron and nearly overall Gauss Normal Polarity Chron. The bottom of the drill core is in C2An.3n, thus the basal age (～3.55Ma) was calculated by extrapolation; and the B/M and M/G boundary is at33.17m and144.02m, respectively.2) The results of sedimentary facies analysis and paleoenvironmental records showed that the five stages of depositional environment changed in Tengger Desert after late Pliocene:3.55-3.1Ma, relatively hot and humid alluvial\diluvial-fluvial depositional environment, with little dust;3.1-2.6Ma, relatively warn and humid fluvial dominated fluvial-lacustrine depositional environment with low dust content;2.6-1.22Ma, relatively warm and alterlating humid and arid lacustrine depositional environment with high dust content;1.22-0.9Ma, relatively cold and arid lacustrine depositional environment with high dust content;0.9-OMa, cold and arid desert-lacustrine depositional environment. The continuous aeolian sand appeared afer0.9Ma, revealing the origin of Tengger Desert; and the the typical aeolian sand like present after0.68Ma suggesting the formation of Tengger Desert. Meanwhile, the weakening chemical weathering and abruptly increase of dust content suggest that significant drying event occurred in the Tengger Desert during2.85-2.6Ma; and the redness, an indication of temperature, also significantly decreased at2.85,0.9and0.68Ma, which is consistant with the drying processes, suggesting a drying-cooling trend in the Tengger desert area.3) The high sedimentary rate of the drill core from3.55-2.6Ma, revealed the uplift of Qilian and Helan mountains near the desert, consistant with the uplift of the Tibetan Plateau. The origin of Shiyang River in the Tengger Desert northwestern to the core after～0.9Ma, maybe suggest a further uplift of Qilian Mountains. The coupling of the tectonic movement with the drying events indicates that the stepwise uplifted Tibetan Plateau plays an important role in driving the desert formation and expansion in north China. This demonstrates the previous viewpoint about the desert evolution in northwestern China that the uplift of the Tibetan Plateau directly drove the desert formation. The uplift not only blocks the moisture from ocean resulting in the arid climate in the north China, but also supplies massive fluvial-lacustrine and alluvial-pluvial clastic sediments for the desert formation and expansion. Based on these, we concluded and suggested this formation modal of large desert in northwestern China as "Tectonic uplift-River and lake development-Desert formation". In addition, the North Hemisphere ice volume evolution also has a similar trend with the drying event in the Tengger Desert, indicating a close connection between them.However, the specific driving and feedback mechanism is still not clear and needs further investigations.4) Based on the previous study, the process of the desert formation and expansion in northwestern China are summarized from the late Pliocene, which includes four stepwise expansion processes from westerm distal deserts to eastern proximal deserts. Since～3.4Ma, the Taklimkan Desert and Guerbantunggut Desert first formed and further expansion occurred by～2.8-2.6Ma, while the desert environment did not develop in the Lop Nor area. Form0.9to0.6Ma, the Taklimakan Desert and Guerbantunggut Desert enlarged again and the Qaidam Desert, the Badain Jaran Desert and the Tengger Desert nearly simultaneously formed, suggesting the desert landscape emerging in the northweatern China; by the0.15Ma, these deserts expanded again and some deserts in the eastern and northeastern China also successively formed. Along with the deserts expansion, the dust loading increased and the distance between the source and deposition area also shortened, which resulted in the coherently rasing dust accumulation rate in the Loess Plateau and Pacific Ocean at3.4Ma,2.8Ma and0.9Ma.