Effect of climate on the magnetic susceptibility of soils

THE magnetic susceptibility of soils derived from sedimentary rocks is normally significantly higher than that of the parent rock. Le Borgne^1,2 has suggested that this enhanced susceptibility of the soil is due to the in situ conversion of the iron oxides from an antiferromagnetic form such as haematite (α Fe₂O₃) or goethite (α FeOOH) to the ferrimagnetic form, maghaemite (γ Fe₂O₃). He also proposed two possible mechanisms involving a reduction process followed by reoxidation to maghaemite. In the first (fermentation mechanism), the reduction occurs as a result of the decay of organic matter in the soil in anaerobic conditions achieved during wet periods, and reoxidation to maghaemite occurs in the aerobic conditions prevailing during subsequent dry periods. In the second (heating mechanism) the burning of organic material produces the temperature increase and reducing atmosphere necessary for the reduction to magnetite (Fe₃O₄) in a thin layer of soil underlying the fire and reoxidation occurs during the cooling down of the fires when air enters the system.

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