Quantifying ultrafine pedogenic magnetic particles in Chinese loess by monitoring viscous decay of superparamagnetism

We propose a new approach for quantifying pedogenically produced ultrafine (nanometer‐scale) magnetic particles in Chinese loess. We have recorded the viscous decay (Δt = 100 s) of imposed isothermal remanent magnetization (IRM) assumed to be selectively carried by superparamagnetic particles (Jv). Application of viscosity measurements (relaxometry) to loess‐paleosol sequences from the SE extremity of the Chinese Loess Plateau reveals that Jv aptly reflects contributions of viscous superparamagnetic (SP) particles to the total remanence carried by an assemblage of magnetic particles. Time decay of Jv can be determined with a higher resolution than the conventionally used frequency‐dependent susceptibility (χfd%) for characterizing the relative abundance of SP particles. The nearly logarithmic time decay of IRM implies a rather broad grain size distribution of pedogenic particles in Chinese loess that may range from a lower size limit of ∼10 nm to an upper limit of ∼40 nm with a prevalent grain size maximum of ∼21–25 nm. Numerical calculations of the time dependence of IRM decay demonstrate that the actual contribution of fine‐grained pedogenic magnetite/maghemite to the total remanence is at least twice as large as that estimated by both χfd% and our proposed Jv% parameter. We therefore propose that determination of superparamagnetic contributions by the time dependence of IRM viscosity is a useful technique for tracking pedogenic processes and hence paleoclimatic conditions of Chinese loess.