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
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.lOg of air-dried bulk samples to remove the carbonate materials, then 0.05mol/L of (NaPO_3)_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), C10 (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 Gunz, C5 to Mindle, C6 ~C9 to Riss, and C10 to Wurm 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.