Grain growth and transformation of pedogenic magnetic particles in red Ferralsols

Abstract

Soil magnetism depends on the mineral composition and grain size distribution of magnetic particles. The accumulation of magnetic particles with different grain sizes has been found in the laboratory to precede the formation of hematite (Hm), which competes with goethite (Gt) in natural systems. To explore changes in the correlation between magnetic particles and Hm during pedogenesis, we focus on three representative red Ferrasol profiles with comparable total concentrations of iron oxides but different Hm/(Hm+Gt) ratios, from 0.14 to 0.74. Superparamagnetic particles and single-domain particles commonly become enriched as the concentration of Hm increases in the profiles. However, the accumulation rates of the particles decrease with Hm/(Hm+Gt) along the profiles. We attributed these changes to the simultaneous grain growth of pedogenic magnetic particles and their partial transformation into Hm once a large amount of pedogenic magnetic particles have accumulated in hematitic soils.