HIGH RESOLUTION ROCK MAGNETIC RECORDS OF CENOZOIC SEDIMENTS IN THE LINXIA BASIN AND THEIR IMPLICATIONS ON DRYING OF ASIAN INLAND
Fang Xiaomin①② Xu Xianhai② Song Chunhui② Han Wenxia②Meng Qingquan② Masayuki Torii③(①Center for Basin Resource and Environment, Institute of Tibetan Plateau Research Chinese Academy of Sciences, Beijing 100085; ②Key Laboratory of Western China's Environmental Systems (Ministry of Education), College of Earth and Environment Sciences, Lanzhou University, Lanzhou 730000; ③Department of Earth and Planetary Sciences, Faculty of Sciences, Kyoto University, Kyoto, Japan)
Some important environmental events occurred in the Cenozoic that are closely related to our present earth system are global cooling, Asian monsoon, and drying of Asian inland. Among them, the aridification of Asian inland and other parts of the world is one of the most important scientific questions for its severe influence on wide areas with large population. The present state of the aridity in NW China is not only a result of long-term natural environmental evolution but also affected by severe human activities. In order to predict and reduce the aridity progress in NW China, we must answer firstly questions including: when did this aridity begin? how was it evolved? how does it run? what is its driving mechanism? and what are its relationships with the uplift of the Tibetan Plateau and global cooling? For answering these questions, the most urgent thing at present is to obtain a series of Cenozoic continuous arid climatic environment records in semiarid and arid areas. Fortunately, there are many basins in NW China filled with thick Cenozoic sediments which archive great information on drying of NW China. One of these basins is the Linxia Basin located at the joint part of the southwestern Chinese Loess Plateau and the northeastern margin of the Tibetan Plateau in Linxia City, Gansu Province, an ideal place to carry the study above due to its almost continuous persistence of mostly fine lacustrine sediments in arid environment.The Linxia Basin yields one of the most abundant fossil mammal sites in China. Detailed paleontological, stratigraphical and paleomagnetic studies have divided the stratigraphy in seven formations upperwards as the Tala Fm.(ca. 29.0～21.4Ma), Zhongzhuang Fm.(21.40～14.68Ma), Shangzhuang Fm.(14.68～13.07Ma), Dongxiang Fm.(13.07～7.80Ma), Liushu Fm.(7.80～6.25Ma), and Hewangjia Fm.(6.25～4.34Ma). These formations reflect five sedimentary cycles manifested as alluvial-flood plain-shallow lake(29.0～21.4Ma), braided river-shallow lake(21.40～13.07Ma), delta-lake(13.07～6.25Ma), and river-flood plain(6.25～4.34Ma).Acquisition and back-field measurement of isotherm remanent magnetization(IRM), thermal demagnetization of natural remanent magnetization(NRM), and three-axis components of IRM show that hematite, maghemite, and magnetite are primary magnetic minerals in the Linxia sediments; but the relative proportions(percentage occurrence in accounting samples)of these magnetic minerals changed significantly since ca. 8Ma, before which hematite, magnetite, and maghemite are respectively of 67.8%,32.2%, and 45.8%, while after at 50.0%, 62.3%, and 69.3%, similar to those of eolian red clay and loess. High-resolution magnetic susceptibility, ARM, and grain size time series records show that they run at relative stable fluctuations along their means between 29.0Ma and 8.6Ma. Exceptions are only for some short intervals at 21.5～19.0Ma, 13.9～4.7Ma, 13.0Ma, 11.5Ma, and 9.8～8.2Ma(in Fig.1), when sedimentary facies changed obviously from persistent fine lake sediments to coarse sediments. The values of all records began to increase slowly but persistently since 8.6Ma, but the speed fast up since 7.4～6.4Ma and again since 5.3Ma, with their characteristics(magnetic mineral and properties and grain size)comparable with those of typical eolian red clay and loess. We interpret that the Linxia paleoenvironment was relatively stable and had no significant response to some important ocean-revealed global climatic events between 29.0Ma and 8.6Ma. This may imply that the Linxia area was long controlled by planetary winds during that time. After 8.6Ma, the eolian dusts were gradually transported into the Linxia Basin. The transportation was enhanced since 7.4～6.4Ma and 5.3Ma respectively, suggesting that the drying of Asian inland began at 8.6Ma and was strengthened rapidly since 7.4～6.4Ma and 5.3Ma. Comparing our records with the Tibetan uplift and global climatic records, we propose that the uplift of the Tibetan Plateau at ca. 9～8Ma and the subsequent rapid uplifts may be responsible for the initial drying of Asian inland and sine about 7.4～6.4Ma, global cooling, especially the onset of Arctic ice sheet may have added force to drive the fast aridification of Asian inland.
【Fund】： 国家重点基础研究发展规划项目(批准号:2005CB422001);; 中国科学院知识创新工程重要方向项目(批准号:KZCX2-YW-104);; 国家自然科学基金项目(批准号:90211013)资助
【CateGory Index】： P318.41
【CateGory Index】： P318.41