Mineral magnetism of the Jingbian loess-paleosol sequence and its paleoclimatic significance

Chinese loess plays an important role in the study of Quaternary climate and environmental changes in East Asia. The Jingbian loess section is located on the northern margin of the Loess Plateau and is sensitive to the East Asian monsoon. The main magnetic minerals in Jingbian loess are magnetite, maghemite, and hematite. The low-temperature oxidation of detrital magnetite in loess has formed a structure in which the maghemite rim encapsulates the magnetite core, resulting in a significant increase in coercivity. Magnetic susceptibility versus temperature curves (chi -T) show that content of fine-grained single domain pedogenic ferrimagnetic minerals (magnetite and maghemite) decreased after at least L15 (approximately 1. 25 Ma), indicating a periodic weakening of the East Asian summer monsoon intensity. The ratio of magnetic susceptibility to anhysteretic susceptibility (chi/ zARM) in the loess layers is related to the content of coarse-grained magnetic minerals from the source area, which increases with the increasing East Asian winter monsoon (EAWM) strength. The chi/chi(ARM) of the loess layers in the Jingbian section increases overall after L14, and gradually increases since L6, indicating two episodes of stepwise enhancements of EAWM. The corrected ratio of magnetic susceptibility to anhysteretic susceptibility ((chi-chi(o))/chi(ARm)) directly represents the magnetic mineral content from the source area. Since 2. 6 Ma, the (chi -chi,)chi(%ARM) gradually increased in the Jingbian section, and the increase of magnetic mineral content from the source area reveals a long-term strengthening of EAWM. The ratio of median destructive field of anhysteretic remanent magnetization to saturation isothermal remanent magnetization (MDFARm/MDFsmm) is related to the microcoercivity of the sample. The coarser magnetic minerals have high coercivity because of a weaker magnitude low-temperature oxidation, and they always have high MDFARm/ MDFsnim. The MDFAxm/MDFsnim of the Jingbian section increased after L14 and L6, respectively, indicating that the particle size of the magnetic mineral became coarse, representing the two episodes of stepwise southward expansion of the desert. The above multiple magnetic parameters consistently show that the Jingbian loess section effectively records the evolution of the East Asian monsoon, which is of great significance for studying the evolution of the arid environment in the Asian inland.