天山黄土区与黄土高原表土磁性特征对比及环境意义

Based on systematic analysis on rock magnetism and grain size, this paper studies the topsoils of the loess region of Tianshan Mountains and the Loess Plateau in northwestern China. Their magnetic characteristics indicate that the main magnetic minerals of these topsoils are magnetite, maghemite, hematite and lepidocrocite or goethite. The grain size of magnetic minerals in Tianshan topsoils is generally much coarser; mostly within range of multi-domain (MD). Hard magnetic minerals in Tianshan topsoils are higher than that of Loess Plateau. The magnetic susceptibility of topsoils from Loess Plateau shows a good positive relation with frequency dependent susceptibility. Climate condition, such as temperature and precipitation, especially the latter one, is considered as the major factor effecting susceptibility enhancement in the Loess Plateau. Tianshan topsoil contains a very small amount of ultrafine magnetic grains produced during pedogenesis, which has very limited contribution to the susceptibility enhancement. Tianshan topsoils generally show a characteristic of less weathering or weak pedogenesis, aeolian coarser grains, therefore play an important role in mechanism of magnetic susceptibility enhancement. Frequency dependent susceptibility shows a close relation with mean annual precipitation (MAP) in all studied areas. However magnetic susceptibility demonstrates difference with MAP in various regions; showing positive correlation in the central Loess Plateau (studied area); but negative relation with MAP in Tianshan area. Therefore, great care should be taken when explaining environments using magnetic susceptibility values from different regions.