Study On Environmental Magnetism Of The Climatic Changes In The Northwestern Qaidam Basin Since The Mid-Pleistocene

Qaidam Basin, a large inland basin, is located at the north part of the Tibetan Plateau. The widely seen Quaternary lacustrine sediments in the basin particularly document the uplift processes of the Tibetan Plateau and the cycles of climatic change thanks to the unique location of the basin. This thesis focuses on the lacustrine core from Lake Gas Hure, in the northwest margin of the Qaidam Basin in attempt to provide more regional climatic information since the mid-Pleistocene based on paleomagnetic analysis and environmental magnetism. A brief summary of our research is as follows:1. The study of lake sediments of GSK222 core shows that changes in the MS (mass magnetic susceptibility) and the color proxy data are well consistent with the nineteen climatic stages registered in the ODP677 oxygen isotope record since the past 0.78 Ma. Higher values of MS and b* and a* are correspond to the interglacials indicated in the ODP677 oxygen isotope record;Lower values of MS and b* and a* correspond to glacials indicated in the ODP677 oxygen-isotope curve. This correlation shows that climate change of the study area may have global significances since the mid-Pleistocene.2. Magnetic susceptibility is a good proxy that records the climatic variations since the mid-Pleistocene in the study area. Magnetic mineral experiments show that magnetite and maghemite with a low-coercivity and hematite and goethite with a high-coercivity are the dominant magnetic minerals in the sediments. Hematite and goethite significantly increase during glacial ages, whereas magnetite and maghemite dominate during interglacials. Mass magnetic susceptibility and frequency-dependent magnetic susceptibility which have no clear correlation indicating that higher magnetic susceptibility signal is due primarily to magnetic minerals with low-coercivity, rather than to the super-paramagnetic particles.3. Six major stages of environmental changes in the study area have been identified on the basis of magnetic susceptibility since the past 0.78Ma. There is a general trend towards a colder and dryer climate throuth several fluctuations since the mid-Pleistocene. Our research also demonstrated that ten strong and obvious climatic events may have occurred since the mid-Pleistocene, six drying events of which closely correlate with the intense uplifts of the Tibetan Plateau at 620 ka, 480 ka, 420 ka, 250 ka, 140 ka and 30 ka ago respectively, reflecting the accelerating trend ofaridification in study area.4. By analyzing and comparing the characteristics of the climatic and environmental changes indicated by magnetic susceptibility of the GSK222 core in the northwestern Qaidam Basin since the mid-Pleistocene, we find that climatic changes exhibit periodicities of orbital (20-100 ka) timescale, and climate variabilities on ten-thousand-year and millennial timescales as well. Climatic changes in Qaidam Basin may result from a number of factors, reflecting both the signature of global climatic changes and obvious regional characteristics. The dominant external factor may be solar insolation. Among the interior factors, the middle and high latitudes, the Tibetan Plateau, the marginal seas and the tropic oceans may be the most important ones that drive climate changes in the Qaidam Basin.