Elemental Characteristics Of Miocene Sediments In The Northeastern Qaidam Basin And Implications For Chemical Weathering In The Drainage Area

There are a series of tectonic and climatic events in Cenozoic, the most famous of which are uplift of Tibetan Platean and global cooing. With the development of these research, uplift of Tibetan Plateau assciated with weatheing and erosion and their climatic effect become the hot topic among the international scienfitic field. Based on the coincidence between uplift of the Tibetan Plateau and the onset of late Cenozoic cooling in theroy and numirical models, Raymo et al. proposed that mountain uplift causes global cooling over geological timescales (Raymo et al., 1988; Raymo and Ruddiman,1992). According to the uplift hypothesis, mountain building enhances mechanical erosion, which in turn accelerates chemical weathering reactions that reduce greenhouse warming by consuming atmospheric CO2. But the evidence supporting the uplift hypothesis almost derives from the marine records and south slope of Himalaya mountains, lacking of evidence from the core region of the Qinghai-Tibet Plateau and its northern margin.Qaidam Basin is the largest inland basin located on the northeastern margin of the Tibetan Plateau. During Cenozoic, with the strong uplift of Qinghai-Tibet Plateau, Qaidam Basin has received well and continuous sedimentary deposition. The Basin is located in the mid latitudes of the northern hemisphere and in the interactive area among Westerlies, Zndia monsoon and East Asian monsoon. The strong interactions between the circulation systems make that the Qaidam basin can well persever information of the circulation system in the sediments. This study focuses on the Naoge section in Qaidam Basin, as the section sediments are well exposed and bear many mammalian fossils. In order to avoid the bias from whole rock analysis, and to obtain accurate information of silicate rock weathering in the drainage area, we analyzed elements in the <2μm fraction of the sediments extracted the pipe method. The obtained chemical weathering records reveal the silicate weathering processes and possible climatic change in Miocene in northeastern margin of Qaidam Basin and provide the crucial geological evidence for the study of the uplift of the Tibetan Plateau and their effects on weathering and erosion. The main conclusions are as follows:(1) Analyses of the relationship between minerals and elements in the <2μm fraction show that (i) there exists positive correlations between K2O and Illite, between CaO and plagioclase, CaO and montmorillonite, between Fe2O3 and chlorite and between A12O3 and kaolinite; (ii) No correlation between Na20 and the major clay minerals has been observed, suggesting a most likely plagioclase control on Na2O variations.(2) Based on the upper continental crust normalization of elements in the<2μm fraction, we found that CaO, Na2O, MnO2 and Sr show substantial depletion, and Al2O3, K2O, MgO, Fe2O3 and Rb show enrichment to some extern, while TiO2 never shows observed gain or loss.(3) According to the variations of Rb/Sr and CIA in the <2μm fraction, Miocene chemical weathering intensity history of Naoge region in the Qaidam basin exerts a long-term decrease with three major drops. Thus, four chemical weathering stages have been divided: The first stage (18.4-17Ma) is characterized by the highest values of CIA and Rb/Sr and distinct weathering for potassium-bearing minerals (illite and potash feldspar), indicating a humid climate, heavy precipitation and strong weathering processes in the drainage area. The second stage (17-12Ma) is characterized by relative high CIA and Rb/Sr with larger variability, which represents a relatively strong weathering stage, and there is no significant weathering of potassium-bearing minerals from this stage. The third stage (12-8.8Ma) is characterized by low CIA and Rb/Sr, indicating a weak chemical weathering processes. The fourth stage (8.8-7.1 Ma) is characterized by the minimum CIA and Rb/Sr and display the weakest chemical weathering.(4) Comparision of the chemical weathering records in Naoge section with the global oxygen isotope, the CIA record of ODP1146 site in the South China Sea, the carbonate oxygen isotope of huaitoutala section in the Qaidam Basin and the sedimentary rate of Naoge section, preliminary reveals that long-term decreasing Miocene chemical weathering intensity of Naoge section in Qaidam basin is mainly controlled by the global cooling and uplift of northeastern Qinghai-Tibet Plateau.