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Magnetic susceptibility changes in relation to pedogenesis in a Xeralf chronosequence in northwestern Spain

Pedogenesis under aerobic conditions is usually associated with an increase in magnetic susceptibility (χ, mass‐specific), which results from the formation of nano‐sized magnetite and/or maghaemite. In this study, we systematically investigated the χ trend in Xeralfs from a chronosequence of 13 river terraces in an area with a Mediterranean climate in Spain. The soils in the younger terraces are well drained and reddened, but those of the older terraces exhibit redoximorphic features and reduced χ values, because of the presence of a perched water table. In the well‐drained members of the chronosequence, the frequency dependent magnetic susceptibility, χFD, which is a proxy for the concentration of nano‐sized pedogenic ferrimagnets such as maghaemite, was highly correlated with the concentration of haematite (Hm), but not with that of goethite (Gt). Also, the peak χFD increased with increasing age in the younger freely drained soils, indicating that no steady state had been reached, and decreased with increasing depth. The formation of maghaemite can be explained by either or both of two formation models, namely: (i) ferrihydrite → maghaemite → haematite solid state transformations; and (ii) the reaction of bacterially produced Fe(II) with an Fe hydroxide phase to form magnetite, which is later oxidized to maghaemite. Comparison between these soils and those in other climatic regions indicates that the Hm/χFD ratio increases with increasing soil temperature and with the degree of weathering. In the horizons exhibiting redoximorphic features on the higher terraces, there was significantly less Hm and χFD in clay than in the well‐drained younger soils, possibly because of reductive dissolution of haematite and maghaemite. Laboratory experiments showed that susceptibility to reductive dissolution increased in the sequence goethite < maghaemite < haematite, which is consistent with the relative abundance of these minerals in the redoximorphic horizons.