A dual mechanism drives the enrichment of pedogenic magnetic particles derived from red beds

Iron oxides and associated magnetic properties are considered good indicators of the pedogenic environment and regional climate. However, these indicators are under debate for soils developed from sediments. The accumulation of magnetic particles derived from sediments depends not only on the formation of iron oxides but also on the existence and transformation of detrital iron oxides. To explore the formation mechanism of magnetic particles accompanied by pedogenetic processes, a toposequence consisting of six profiles derived from red beds with contrasting drainage conditions under a subtropical monsoon climate is examined. We generally observed upward magnetic enhancement but inconsistent correlations between magnetism and iron oxide phases. The magnetic enhancement in the well drained profiles was dominated by the reverse transformation of detrital hematite to fine maghemite particles, whereas in the poorly drained profiles, coarse magnetite particles formed with soil organic matter enrichment in an anaerobic way. The lower magnetism in the transition profiles is attributed to the hydration of iron oxides into goethite before the pedogenic environment became anaerobic. The different magnetic enrichment mechanisms explain the nonlinear responses of magnetism and color to climate across temporal and spatial scales. These findings also suggest that soil magnetism is promising for tracing soil moisture and ecological evolution, especially in red soils and sediments across the surfaces of the Earth.