洛川黄土堆积粗颗粒变化记录与华北第四纪哺乳动物迁徙关系的初步探讨

A PRELIMINARY STUDY ON THE RELATIONSHIP OF CLIMATIC EVENTS RECORDED BY COARSE FRACTIONS IN LOESS-PALEOSOL SEQUENCE FROM LUOCHUAN AND SOUTHWARD MIGRATION OF MAMMALIAN ANIMALS IN NORTH CHINA

黄土高原的黄土是典型的风尘堆积,气候干冷时期,西北冬季风加强,黄土堆积颗粒较粗,反之较细.据此对洛川黄土-古土壤序列的非碳酸盐粗颗粒（0.10～0.0625mm）进行测试并绘制其变化曲线.结果显示第四纪时期出现过10次冬季风超级强盛事件,判定这些事件的地质年龄后,发现它们与欧洲阿尔卑斯山冰期有较好的对应关系,表明黄土堆积非碳酸盐粗颗粒值可能是极端气候变化事件的替代性指标.第四纪时期,华北哺乳动物群曾出现4次迁徙事件,它们与10次冬季风超强事件有一定的联系,事件对古人类活动也显示了某些影响,说明极端气候事件与哺乳动物进化有明显的关系,是值得今后进一步深入研究的领域.

The loess-soil sequences in the Chinese Loess Plateau （CLP） comprise high-resolution archives of regional and global climate changes. These eolian dust deposits were mainly carried by the East Asian winter monsoon （EAWM） , sourced from the Siberian High, and thus bore the information of changes in high northern latitudes climate. The particle size of loess increased with the strengthened northwesterly EAWM during arid and cold climate conditions and vice versa. The non-carbonate coarse fractions in loess, less affected by the post-depositional weathering, may provide new sight on the cold climate events in Northern Hemisphere. In this study, the classic Luochuan section in central CLP, located in Heimugou, Potou village of Luochuan county （35°45＇N, 109°25＇E） , was investigated. The 149-m thick section comprises the upper Pleistocene loess-soil sequence with a thickness of 135m and the lower Pliocene Red Clay deposit with a thickness of 14m. These two formations have a disconformable contact without clear erosion. Previous paleomagnetic study revealed the contact of the two formations is around M/G boundary with an age of 2.6Ma. The section was sampled at 0.2m interval and totally 638 bulk samples was taken. In the laboratory, 10% of HCL was added to a beaker containing ca.10g of air-dried bulk samples to remove the carbonate materials, then 0.05mol/L of （NaPO3）6 was added to disperse the samples. The dispersed samples was sieved by 140- and 230-Mesh screens, respectively, to get the weight percentage of the particles between 0.10mm and 0.0625mm. These coarse fractions were confirmed to be non-carbonate minerals, examined under microscope. The changes in percentage of coarse fractions show 10 intervals with enhanced content of coarse fractions, indicating 10 cold climate events, characterized by strengthened EAWM winds, during the past 2.6Ma. These cold climate events are： C1 （2.49-2.31Ma）, C2 （2.13-2.12Ma）, C3 （1.50=1.22Ma）, C4 （0.94- 0.84Ma）, C5 （0.68-0.59Ma）, C6 （0.48-0.39Ma）, C7 （0.36-0.33Ma）, C8 （0.31-0.25Ma）, C9 （0.23- 0.13）, CIO （0.025-0.013Ma） , according the published orbitally tuning time scale of Chinese loess. These cold climate events were found to be broadly well consistent with the glaciation history of Alps regions： C1 - C2 correspond to Biber, C3 to Donau, C4 to Gtinz, C5 to Mindle, C6-C9 to Riss, and C10 to Wtirm Glaciations. During the Pleistocene, there were four events of southward migration of mammalian fauna in North China, which were broadly consistent with occurrences of these 10 cold climate events ： the first migration correspond to C3 event, the second one to C4 event, the third to C6-C9 events, and the fourth, C10 event. This indicates that extreme cold climate events may have exerted a profound influence on the evolution of mammal. This study suggests that the coarse fraction of loess sediments over CLP regions have a great potential in reconstructing history of the extreme cold climate.