| There are a series of major geological, climatic events occurring on the Earth in the Cenozoic, which have been leading to the tremendous change of the global environment and climate.One of the most important scientific issues is that the significant stage cooling in the Cenozoic, two ice sheets formation and development, and whether which are due to the weathering erosion caused by the uplift of the Tibetan Plateau enhanced to absorb more atmospheric CO2 which ultimately led global cooling; or vice versa, the global cooling has led to the weakening of the continental weathering. Study the scientific issues is crucial for our understanding on the mechanisms and simulation and prediction of today’s climate changes in the future. The global cooling event occurring in the late Middle Miocene or Middle Miocene transition MMCT is a great way to study the scientific issues.The event in just about 0.5Ma period (14.2Ma-13.7Ma), while the global average temperature from the Miocene Climatic Optimum period MCO (17Ma to 14Ma) suddenly plunged by about 3℃～4℃. Accurately determine the mainland weathering strength and rate of change, and uplifts recorded of the Tibetan and global CO2 and the temperature recorded comparison which can reveal more detailed phase relationship between them, which are providing a mechanism driver information, can determine whether the uplift of the plateau tectonic or the global climate change driving continental weathering changes.The Qaidam Basin is a giant basin of the northern margin of the Tibetan Plateau, which have developed a continuous complete over 12,000 m thick Cenozoic strata. In the paper, we select the Naoge section (18.4Ma-8.1Ma), which have deposition continuous and accurate stratigraphic control in the NE Qaidam Basin. Trying to analysis the type of clay minerals, content, degree of crystallization, Illite Chemistry Index and their ratio, to reveal the characteristics of the weathering intensity and rate of change during the MCO and MMCT, then we combine with sedimentary facies, lithology, elements, other indicators of pollen and structure analysis in the Qaidam Basin. Exploring the magnitude of the area weathering and rate of change in the regional and global climate change or the Tibetan Plateau uplifting and possibley coupling mechanism since the Miocene. Finally wo have the following main conclusions:(1)Having extracted, separated and done X-ray Diffraction (XRD) the clay minerals of the Naoge sediments in the NE Qaidam Basin of the northern margin of the Tibetan Plateau,then try to do the XRD qualitative, quantitative analysis to obtain the characterized high-resolution sediments changes sequence of relative content of the clay mineral, degree of crystallization, chemical index, and clay minerals content ratio during 18.4 Ma～8.1Ma.(2)Naoge section clay minerals indicators change sequences reveal that the change of clay minerals in the region one of the biggest characteristics is that the content of Smectite drastically reduced, suddenly plunged from a maximum of about 98% to about 10%, and Illite content just the opposite, jumped from as low as about 1% to about 80%, the Illite crystallinity corresponding plummeted from about 0.22 to less than 0.1,while Kaolinite and Chlorite claymineral change is not very obvious in the thickness of about 800 meters (about 14Ma). Secondly, since about 14 Ma broadly in line with the thickness of about 1,500 meters (about 12Ma) and 2100 meters (about 10Ma) is bounded can also be divided into three sub-phases, Chlorite content of from about 12Ma significantly increased from the pre-phase (14Ma-12Ma) average about 5% to 20%, the Illite crystallinity its variability has been reduced, CI decreased from early (14Ma-12Ma), an average of about 0.25 to 0.1, while other clay minerals and indicators change are not obvious. Since approximately 10Ma, in addition to a slight increase in the average content of clay minerals, the most important characteristic is the variability of all clay minerals and indicators are significantly increased.(3)The characteristics of climate changes which are recorded in Clay minerals of the Naoge section can be divided into four stages:18.4Ma～14Ma relatively warm and humid climatic characteristics; 14Ma～12Ma relatively dry and cold climate; 12Ma-10Ma relatively drier; during 10Ma～8.1Ma the climate is arid,cold and warm,humid alternating fluctuations in phase, and about 8Ma have drought trend. Combined with the results of previous studies, we speculate that the pre-two stages (18Ma～12Ma) is likely mainly influenced by global climate change control, the latter two phases (12Ma～8.1Ma) may be affected by regional drought or tectonic uplift of the Tibetan Plateau impact,or together action.(4)About 14Ma, the content of Smectite drastically reduced, while Illite sharp increase and its crystallinity value the corresponding dips, which reveals sediment source area weathering intensity sharply reduced,that may be reposed to the Middle Miocene climate transition MMCT Event. While clay minerals variability increases significantly on regional climate later phase, which is due to significantly region drier and the source region significantly uplift, that can lead to erosion strength and source wide fluctuations in frequent small-scale. Compared to the more stable global Middle Miocene climatic optimum period (MCO) warmer climate, the change of frequency and amplitude of the sediment source area weathering intensity which are revealed by clay minerals in the Naoge section of the Qaidam Basin, which is far greater than the volatility of the global climate. That may show the characteristics of high frequency and large fluctuations,which may reflect the impact of the dual role of terrestrial climate variability and Tibetan Plateau tectonic uplift in the Mid-Miocene. |
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