The role of weathering and aeolian inputs on clay minerals and geochemistry in Podzols on ophiolitic materials

The process of podzolization in soils developed from ultramafic materials remains enigmatic, despite a few isolated cases described in subalpine areas under particularly acidifying vegetation and wet microclimatic conditions. The aim of this study was to elucidate the pedogenic processes operating in Podzols developed under humid subalpine vegetation on ultramafic and mafic parent materials in the northwestern Alps. We used clay mineralogical and geochemical approaches, also accounting for the possible contribution of aeolian inputs. The results indicate that weathering was particularly intense under these subalpine conditions, driven by strong soil acidification and resulting in extensive dissolution of serpentine minerals and in the transformation of primary Mg-chlorite into Al- and Fe-rich smectites and vermiculites, via intermediate Fe-rich soil chlorites and hydroxy-interlayered minerals. Mass balance calculations and chemical weathering indices consistently indicate a high degree of weathering and substantial elemental losses associated with podzolization in these subalpine soils. During pedogenesis, up to ∼ 80% of Mg and Si, ∼85% of Ni, and ∼ 65% of Cr appear to have been removed from the soil profiles. However, weathering of ophiolitic parent rocks alone cannot fully account for the observed soil geochemistry. Aeolian inputs—primarily Saharan dust and possibly minor contributions from late-glacial loess—are required to explain the elevated contents of relatively immobile elements, particularly Al, Fe, Zr, and Ti, and these exogenous materials likely also influenced the mineralogical composition of the fine soil fractions. These results highlight the combined role of intense chemical weathering and external material inputs in the development of Podzols on ultramafic substrates under favorable climatic and ecological conditions.