临夏盆地新生代沉积物高分辨率岩石磁学记录与亚洲内陆干旱化过程及原因

HIGH RESOLUTION ROCK MAGNETIC RECORDS OF CENOZOIC SEDIMENTS IN THE LINXIA BASIN AND THEIR IMPLICATIONS ON DRYING OF ASIAN INLAND

亚洲内陆干旱化是全球新生代大陆环境变化中最引人瞩目的重大事件，与新生代全球变冷和青藏高原隆升密切相关。文章通过对甘肃临夏盆地几乎连续的新生代沉积物系统岩石磁学性质研究，获取了高分辨率磁化率和非磁滞剩磁记录，揭示在29．0～8．6Ma的漫长的以湖相粉砂岩和泥岩为主的渐新世晚期和中新世早、中期没有明显的长期变化，从8．6Ma开始持续增加，尤其从6．4Ma和5．3Ma开始表现出两次快速持续增加。同时，以8．6Ma为界磁性矿物相对含量发生明显变化，此前以赤铁矿为主，磁铁矿和磁赤铁矿次之，此后以磁铁矿和磁赤铁矿为主，赤铁矿次之，磁性矿物类型和性质类似于风成红粘土和黄土。因此，我们将这种沉积物中磁性矿物组合、性质和含量的长期变化解释为流域外风成物质的加入，指示我国西北内陆现代干旱气候可能从8．6Ma开始，7．4～6．4Ma后急剧加速变干，5．3Ma后再次加速变干，并最终形成今天的干旱区的过程。9～8Ma开始的青藏高原阶段性快速隆升和随后的全球变冷可能是驱动亚洲内陆干旱化进程的动力。

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 hematite, magnetite, and maghemite are respectively of 67.8%, 32. 62.3%, and 69.3%, similar to those of eolian red clay and loess. significantly since ca. 8Ma, before which 2%, and 45.8%, while after at 50.0%, 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.