Enhancing weak magnetic fabrics using field-impressed anisotropy: application to the Chinese loess

Abstract

SUMMARY The anisotropy of the low-field magnetic susceptibility (AMS), alternating-field-treated AMS (AF-AMS), and anisotropy of anhysteretic remanent magnetization (AARM) have been systematically examined for a sandwich sequence of loess/palaeosols crossing the upper loess unit L2 (Marine Oxygen Isotope Stage 6, MIS6) to the last interglacial maximum (palaeosol unit S1S3, MIS5e) from the Yuanbao section, northwestern China. Results show that a weak, but detectable, magnetic lineation is controlled by coarse-grained aeolian Fe3O4 (magnetite). Because the long axes of the coarse-grained magnetites are statistically parallel to the palaeowind directions, the declination of the maximum susceptibility principal axis of AMS can be used as an indicator of palaeowind direction. In contrast, fine-grained pedogenic magnetic particles are responsible for the magnetic foliation. We found that AF-AMS can indicate the domain state of the magnetic particles, which is consistent with Potter & Stephenson's earlier model. We also found that AF demagnetization can significantly alter the AMS. In conclusion, the angular dependence of AF-AMS can enhance the weak AMS fabrics.