Grain sizes of susceptibility and anhysteretic remanent magnetization carriers in Chinese loess/paleosol sequences.

Liu, Qingsong S. and Banerjee, Subir K. and Jackson, Michael J. and Maher, Barbara A. and Pan, Yongxin X. and Zhu, Rixiang X. and Deng, Chenglong L. and Chen, Fahu H. (2004) Grain sizes of susceptibility and anhysteretic remanent magnetization carriers in Chinese loess/paleosol sequences. Journal of Geophysical Research: Solid Earth, 109 (B3 (B0).

Abstract

Detailed rock magnetic studies show that susceptibility (mass-specific χ) and anhysteretic remanent magnetization (ARM) of the Chinese loess/paleosol sequences are carried by almost identical magnetic carriers. Therefore the ratio Δχ/χARM (or equivalently χARM/Δχ, where Δχ is defined as χ − χ0, and χ0 is the intercept susceptibility of the plot of χ versus ARM, and χARM is field-normalized ARM) can be used to quantify the grain size of χ and ARM carriers. By determining this ratio for three Chinese loess/paleosol profiles (Jiuzhoutai, Yuanbao, and Yichuan) characterized by different degrees of environmentally controlled pedogenesis and sedimentation rates, we show that the lower grain-size limit of aeolian magnetic particles in the less pedogenically altered loess units is about 100–300 nm, in the finer-grained pseudosingle domain (PSD) grain-size range. In contrast, the grain sizes of pedogenically produced magnetic particles for mature paleosols dominantly cover both the superparamagnetic (SP) and single-domain (SD) ranges. On the basis of plots of Δχ/χARM against Δχ, samples can be divided into four regions (I, II, III, and IV). Region I corresponds to the least pedogenically altered primary loess samples, with Δχ/χARM of 0.165–0.24. Samples in region II, a transition zone between the least altered loess and the onset of development of paleosols, have χ values identical to those in region I but have lower Δχ/χARM of 0.09–0.165. With increasing susceptibility in zone III, Δχ/χARM is positively correlated with Δχ, indicating the gradually increasing influence of SP particles. Finally, in zone IV with Δχ higher than ∼6.5 × 10−7 m3 kg−1, Δχ/χARM is independent of the variations in Δχ, suggesting that Δχ/χARM is totally controlled by the pedogenic finest-grained particles and the size distribution of these particles remains almost constant. The development of soils in the Chinese loess revealed by these three profiles from three sites can be clearly explained by a continuous process of pedogenesis, increasing from zone I to zone IV. The definition of the pedogenic zones can help to improve our understanding of the underlying mechanisms and variability of pedogenesis and thus could enable more successful and accurate separation of the authentic pedogenic signals from the background signal of the aeolian inputs at different loess sites worldwide.