Out-of-Phase Susceptibility and Viscous Magnetization: Effective Tools for Magnetic Granulometry of Loess and Paleosols

Certain geological and environmental processes, such as pedogenesis, generate ultra-fine magnetic mineral neo-formation. The variation in content of these newly formed minerals has been routinely investigated using frequency-dependent magnetic susceptibility (kFD). Although this technique is well-established, the calculation of the frequency-dependent coefficients requires repeated measurements at least in two different operating frequencies which significantly prolongs measurement time and may be prone to operator mistakes and specimen mishandlings. Moreover, the kFD reaches its limits when applied to very weakly magnetic (low-susceptibility) materials.

To overcome these drawbacks, a refined technique is proposed which utilizes the phase angle between in-phase (ipMS) and out-of-phase (opMS) susceptibilities. The phase angle is directly related to the kFD via the pi/2 law provided that the opMS is solely due to the viscous phenomena and not due to electrical eddy currents or weak field hysteresis. Using MFK1/2 Kappabridges (AGICO, Inc.), phase angle is directly measured, although very rarely considered and processed, together with the routine ipMS measurements.

Alternatively, especially in case of low-susceptibility materials, the relative content of ultra-fine particles can be assessed by quantification of time-decay of viscous remanent magnetization. For that purpose we employed a LDA5/PAM1 Pulse Magnetizer coupled with a JR-6A Spinner Magnetometer (AGICO, Inc.). Both instruments are controlled as such that they work in the same time frame. Magnetic remanence is measured repeatedly as a function of time and exponential decay curves are fitted on the acquired data and the relative ratio of viscous and non-viscous particles is estimated.

Our novel approach is demonstrated on a set of specimens from the Dejvice loess-paleosol sequence (Prague, Czech Republic) and compared to other Czech loess/paleosol sections and the pilot specimens from the Xifeng classical loess-paleosol section located in the central part of the Chinese Loess Plateau. The relative concentration of the ultra-fine magnetic grains assessed by both phase angle and viscous magnetization decay methods correlates very well with the kFD.