Environmental magnetic study of a Xeralf chronosequence in northwestern Spain: Indications for pedogenesis

Magnetic enhancement of A and B horizons during soil development is a common phenomenon. To better understand the exact mechanism for the magnetic enhancement, especially the mineral transformation pathways, systematic rock magnetic studies were conducted on a Xeralf chronosequence in northwestern Spain. A positive correlation was found between the average grain size and the concentration of pedogenic maghemite particles with the exception of some A horizons, in which multiple factors seem to influence the nature and concentration of neoformed ferrimagnets. It is argued that the interaction between the positively charged iron oxides and the anionic ligands present in the solution of these acidic soils plays an important role in this respect. The time trends of the average grain size of pedogenic ferrimagnets and the strong correlation between the concentrations of hematite and maghemite in the magnetically enhanced horizons (A and B) are consistent with the hypothesis of a gradual formation of maghemite (later converted into hematite) via a precursor (most likely ferrihydrite). Therefore, this study provides strong evidence for the dynamic evolution of magnetic minerals upon pedogenesis.