Formation and migration of magnetic particles associated with iron oxide transformation at a hillslope scale

Abstract

Iron oxides are widespread on the surface of the Earth as a weathering product of iron-bearing primary minerals, which often accompany the enrichment of magnetic particles under aerobic conditions. The magnetic properties determined by the content and ratio of magnetic particles have been widely employed in paleoclimate reconstructions and erosion simulations. However, water redistribution at a slope scale could cause a phase transformation of iron oxides and a corresponding change in magnetic properties. In this study, we investigated a catena with a distinct soil colour change from red to yellow, which commonly occurs in tropical and subtropical hilly and mountainous regions. From upslope to downslope, the free iron oxide content decreases while the amorphous iron oxide content increases. The hematite diminishes significantly while the goethite shows little variation. The ferrimagnetic particles decrease in phase with hematite except for the finest superparamagnetic particles. However, the concentration of finer ferrimagnetic particles relative to coarser particles and the concentration of ferrimagnetic particles relative to antiferromagnetic particles both increase downslope. This increase validates the formation path of ferrimagnetic particles as an intermediate product of hematite but reveals more significant particle growth and transformation into hematite with decreasing water activity in granitic soils with low iron content. In addition, these processes could be enhanced by the preferential migration of the finest ferrimagnetic particles or their transformation from amorphous iron oxides enriched downslope. The slope-scale redistribution of iron oxides and related magnetic particles increases their uncertainty in paleoclimate reconstruction and erosion simulation but brings promising prospects to trace geomorphic evolution and hydrologic change over the long term.