Temperature dependence of magnetic susceptibility in an argon environment: implications for pedogenesis of Chinese loess/palaeosols

Temperature dependence of magnetic susceptibility (chi - T) has been widely used to determine changes in mineralogy of natural samples during heat treatment. We carried out integrated rock magnetic experiments to interpret the chi - T curves of the Chinese loess/palaeosols in argon. We used both raw materials and heated samples. In addition, we also investigated the magnetic properties of magnetic extracts and residues to quantify contributions from each fraction to the bulk magnetic properties. For the heating curves, the susceptibility loss (similar to 30 per cent) between similar to 300-400 degrees C is caused by the inversion from pedogenic fine-grained maghemite to haematite, suggesting that the susceptibility loss can be used as a new concentration index of the pedogenic fine-grained superparamagnetic (SP) particles in the Chinese loess/palaeosols. Unlike the warming curves, the cooling curves are dominated by newly formed fine-grained magnetites with a dominant size of similar to 35 nm. The onset for the new production of these fine-grained magnetic particles occurs at similar to 400 degrees C. It is interesting that the room-temperature magnetic susceptibility (chi(ph)) of the samples heated after a 700 degrees C run is independent of the degree of pedogenesis and saturates at approximately 33-35 x 10(-7) m(3) kg(-1), indicating that the susceptibility enhancement is controlled only by the reduction of Fe-bearing aeolian minerals during heating. It appears that the 700 degrees C thermal treatment in argon could be in some sense an analogue to the pedogenic processes. Thus, we predict that similar to 33-35 x 10(-7) m(3) kg(-1) is the maximum susceptibility that pedogenesis can generate for the last interglacial palaeosol unit (S1). In practice, chi(ph) would be useful to quantify the aeolian inputs to the Chinese Loess plateau.