Determination of magnetic carriers of the characteristic remanent magnetization of Chinese loess by low-temperature demagnetization

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. (C) 2003 Elsevier B.V. All rights reserved.