The Chinese loess/paleosol sequences can provide excellent paleomagnetic records for detailed studies on both magnetostratigraphy and secular variations of the Earth's magnetic field. However, the nature of the loess/paleosol characteristic remanent magnetization (ChRM), isolated by thermal demagnetization from the natural remanent magnetization, still remains in debate. In this study, we directly measure the thermal, alternating field, and low-temperature demagnetization (cooling/warming cycle of remanence in zero field between 300 and 50 K) spectra of ChRM of the loess samples at Touxiangdao, Xining, PR China. The results show that coarse-grained, pseudo-single-domain (PSD)/multi-domain (MD) magnetite and PSD maghemite particles are the main magnetic carriers of the loess and paleosol ChRMs, respectively. By comparing the ChRM characteristics across a loess/paleosol transition zone, we have found that during pedogenesis, the thermally separable ChRM (original detrital remanent magnetization) carried by the PSD/MD magnetite in loess is gradually overprinted by the chemical remanent magnetization carried by PSD maghemite. Even moderate pedogenesis (e.g., for the pedogenically altered loess) can strongly affect the loess ChRM. Thus more attention has to be focused on the paleosol ChRM separated solely by thermal demagnetization, especially when constructing a continuous paleomagnetic record covering both loess and paleosol units.
Bibliographical noteFunding Information:
This study was supported by NSF Grant No. EAR 0003421 and EAR/IF 9818704. The fieldwork and sampling is partly supported by NSFC Grant No. 40125001 and 40221402. All rock magnetic measurements were made at the Institute for Rock Magnetism (IRM), which is supported by the W.M. Keck Foundation, the Earth Science Division of the US National Science Foundation, and the University of Minnesota. We are grateful to C.L. Deng and B. Carter-Stiglitz for collecting samples from the Touxiangdao section, and to David Dunlop and Joe Rosenbaum for thorough and helpful reviews. This is IRM publication #0309. [KF]
- Characteristic remanent magnetization
- Chinese loess
- Low-temperature demagnetization
- Magnetic carrier