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 (Δ t = 100 s) of imposed isothermal remanent magnetization (IRM) assumed to be selectively carried by superparamagnetic particles ( J v ). Application of viscosity measurements (relaxometry) to loess‐paleosol sequences from the SE extremity of the Chinese Loess Plateau reveals that J v aptly reflects contributions of viscous superparamagnetic (SP) particles to the total remanence carried by an assemblage of magnetic particles. Time decay of J v can be determined with a higher resolution than the conventionally used frequency‐dependent susceptibility ( χ fd %) 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 χ fd % and our proposed J v % 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.