Magnetic minerals in Calcic Luvisols (Chromic) developed in a warm Mediterranean region of Spain: Origin and paleoenvironmental significance

Soil formation under aerobic conditions usually enhances magnetic signals, mainly through neoformation of sub-micron magnetite and/or maghemite. In order to better understand the nature and magnitude of the enhancement, in this work we characterized the iron (Fe) oxides and magnetic properties of two Calcic Luvisols (Chromic) developed on calcareous orthoquartzites in a Spanish region with a warm Mediterranean climate. Weathering of the Fe-bearing minerals in the parent material (mostly Fe-rich smectite) had resulted in the neoformation of hematite and maghemite rather than goethite, thus causing a substantial increase in magnetic susceptibility, χ, saturation magnetization, Ms, and other magnetic signals. The absolute frequency-dependent magnetic susceptibility, χFD, which is a typical proxy for the concentration of sub-micron superparamagnetic pedogenic maghemite, was strongly correlated with the concentration of hematite (Hm). The estimated percent frequency-dependent magnetic susceptibility, χFD%, was 15.7%. This and various other magnetic indicators indicate that the grain size distribution (GSD) of the pedogenic maghemite is narrower around the superparamagnetic/single domain threshold than is that of maghemite in soils and paleosols developed under other temperate climates (e.g., the Chinese Loesss Plateau, where χFD% < 13%). Our results are consistent with predictions based on the ferrihydrite → maghemite → hematite transformation model. Also, they suggest that a wise combination of the Hm/χFD ratio, χFD%, the hematite/goethite ratio, and several indicators of the degree of weathering provides a better tool for paleoclimatic reconstructions than does the current use of χ alone.