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

Detailed rock magnetic studies show that susceptibility (mass-specific chi) and anhysteretic remanent magnetization (ARM) of the Chinese loess/paleosol sequences are carried by almost identical magnetic carriers. Therefore the ratio Deltachi/chi(ARM) (or equivalently chi(ARM)/Deltachi, where Deltachi is defined as chi-chi(0), and chi(0) is the intercept susceptibility of the plot of chi versus ARM, and chi ARM is field-normalized ARM) can be used to quantify the grain size of chi 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 Deltachi/chi(ARM) against Deltachi samples can be divided into four regions (I, II, III, and IV). Region I corresponds to the least pedogenically altered primary loess samples, with Deltachi/chi(ARM) 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 chi values identical to those in region I but have lower Deltachi/chi(ARM) of 0.09-0.165. With increasing susceptibility in zone III, Deltachi/chi(ARM) is positively correlated with Deltachi, indicating the gradually increasing influence of SP particles. Finally, in zone IV with Deltachi higher than similar to6.5 x 10(-7) m(3) kg(-1), Deltachi/chi(ARM) is independent of the variations in Deltachi, suggesting that Deltachi/chi(ARM) 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.