Signature of magnetic enhancement in a loessic soil in Nebraska, United States of America

Abstract

Our study of a loessic soil profile from east-central Nebraska shows that the A horizons of the modern soil are characterized by higher concentrations of fine-grained (<0.1 μm) magnetic minerals. This pedogenic magnetic component leads to higher values of concentration-dependent parameters, such as magnetic susceptibility ( χ), isothermal remanent magnetization (IRM) and anhysteretic remanent magnetization (ARM), combined with increases in frequency-dependent susceptibility ( χ fd) and ARM/IRM ratios. Hysteresis properties are relatively insensitive towards the presence of this pedogenic magnetic component. The magnetic properties of the soil profile are dominated by ferrimagnetic magnetite or maghemite. Analyses of “soft” IRM (sIRM) and “hard” IRM (hIRM), however, do show that approximately 80–90% of the remanence carrying magnetic component exists in the form of hematite or goethite and that the magnetically enhanced horizons are enriched in both ferri- and antiferromagnetic minerals. The pedogenic magnetic component is most likely caused by the conversion of paramagnetic, iron-bearing minerals to ferri- and antiferromagnetic minerals. Soil compaction, lessivage or decalcification cannot explain the observed magnetic soil properties. Magnetic analyses of loess-paleosol sequences from the midwestern United States may yield valuable information about regional variability of paleoclimate. Based on the fine-grained nature of the pedogenic magnetic component, we expect grain-size-dependent proxies (ARM, ARM/IRM, χ fd) to yield better paleoclimatic information than low-field magnetic susceptibility used in previous analyses.