The Rock Magnetism Research Of The Topsoil In Northwest China

The magnetic susceptibility variation of loess-paleosol sequences in Chinese Loess Plateau (CLP) coincides with paleoclimatic changes and hence is widely used for climate reconstruction. Previous studies indicate the existence of a linear relationship between precipitation and magnetic susceptibility of topsoil, which is, however, speculated more complicated.Here we study topsoil (170samples and together with other61published) including Gobi and CLP in northwest of China, spanning96°E-110°E,33°N to43°N, with precipitation50-700mm/a. Low field magnetic susceptibility χ1f and frequency dependent susceptibility (xfd and χfd%.) have been obtained using Bartington MS2. The precipitation, ranging from1971-2000, at each sample site is simulated and determined with GIS methods.The correlation between magnetic susceptibility and precipitation presented in this study indicates that:linear relationship between susceptibility and precipitation only occurs to a high precipitation larger than200mm/a and300mm/a. Below this threshold, the relationship is unintelligible. This relation boundary likely attributes to different pedogenic degree over this area. Superparamagnetic (SP) particles bearing very high susceptibility form during the strong pedogenesis in humid area; while in arid area due to weak pedogenic process, the contribution of SP particles of pedogenic origin is limited.Based on systematical analysis on rock magnetism, this paper studies the topsoil from the edge of Tengger Desert in the northwestern China. The measured magnetic parameters indicate that main magnetic minerals of those topsoil is magnetite, the other may have maghemite and hematite. The grain size of magnetic minerals is coarse, mostly within range of multi-domain. The magnetic susceptibility has little relation with the pedogenesis but closely relates with landform and parent materials. The topsoil from such arid area or aeolian dust, show similar magnetic mineralogy between them, and do not undergo clearly pedogenesis. Their enhancement mechnism of magnetic susceptibility differs from that in the Loess Plateau. The magnetic susceptibility in such dried region indicates information of grain size or wind-vigor, instead of pedogenesis.