Pedogenic magnetic enhancement in a Latossolo Vermelho (Oxisol) formed on slate in the Brazilian Cerrado: an integrated record of repeated wetting–drying cycles

Bianca Soares Vale, Elder Yokoyama, Uidemar Morais Barral, Janine Araujo do Carmo, Arthur Macedo, Adriana Horbe, Alexandre Silva Santos, Sebastião William da Silva

Seasonal wetting–drying cycles in tropical soils can drive redox reactions that control the formation and transformation of ferrimagnetic iron oxides. In the profile-scale case study, we investigated a well-drained, clayey Latossolo Vermelho (Oxisol/ Ferralsol) developed on weakly magnetic slate in the Brazilian Cerrado. The profile was sampled to 190 cm depth in the dry season (July 2023) and at the end of the wet season (April 2024). Magnetic susceptibility, thermomagnetic curves, hysteresis loops, first-order reversal curves, isothermal and anhysteretic remanent magnetization, and coercivity spectrum unmixing were combined with soil properties, X-ray diffraction, and Raman and ATR-FTIR spectroscopy. Low-field susceptibility on the upper horizons (up to ~233 × 10⁻⁸ m³ kg⁻¹) was more than 30 times higher than that of the parent slate (~6 × 10⁻⁸ m³ kg⁻¹), and the soil is dominated by low-coercivity ferrimagnets, whereas the slate is dominated by high-coercivity hematite. High frequency-dependent susceptibility (~12%) and median ARM acquisition fields of ~12–13 mT indicate a fine-grained (superparamagnetic to single-domain) magnetite–maghemite assemblage, probably spanning a range of oxidation states, coexisting with hematite. The magnetic enhancement is therefore predominantly pedogenic. Despite contrasting hydrological conditions, seasonal differences were subtle, indicating that the magnetic signature reflects an integrated memory of repeated wetting–drying cycles rather than a rapidly reversible response to a single season. The mineral assemblage and pedoclimatic setting are consistent with a long-term mechanism, inferred rather than directly observed, involving magnetite neoformation in transiently anoxic microsites, possibly through microbial Fe (III) reduction, followed by partial oxidation to maghemite.