Distinguishing subalpine soil types using extractible Al and Fe fractions and REE geochemistry

Abstract

Only a few paleoecological studies have attempted to use the geochemistry of lacustrine sediments to reconstruct the history of soil genesis. The geochemical signatures of modern soils must firstly be determined before using them to investigate lake sediments. Our study explores the geochemical variability between eight subalpine soil profiles from the internal zone of the Alps to find relevant sediment tracers for present-day soils. Based on the concentrations of secondary Al- and Fe-bearing phases, as well as major element and Rare Earth Element (REE) geochemistry, different subalpine soil types can be clearly differentiated in the study area. Although formed in similar geomorphological settings, the soil typologies are diverse: Podzol group soils are developed under old-growing coniferous forest, whereas Cambisols occur under present-day old-meadows. By applying PCA including pedological and geochemical parameters, we can clearly discriminate between Podzol and Cambisol samples, revealing differences that reflect the intensity of chemical weathering. In contrast, deep and surface samples can be distinguished using the degree of physical weathering. The proportions of organic-bound Al and Fe are significantly higher in Podzols, thus providing a sediment tracer to reconstruct the intensity and degree of podzolisation. The increase of Light REE fractionation from the C-horizons to the topsoil horizons is explained in terms of soil processes. The high-field-strength element Nb is used to estimate enrichment or depletion relative to the unweathered parent material. The resulting normalized REE patterns indicate that Podzol profiles are more depleted than Cambisols. Moreover, normalized REE patterns of A- and B w-horizons are similar in the Cambisol profiles, whereas the E- and B s-horizons of Podzols exhibit large variations. Normalized REE patterns may thus provide a precise tracer of the degree of weathering of materials. Hence, we suggest that REE behaviour and soil weathering are clearly related. This study also highlights the effects of millennial-scale agricultural practices on soils, e.g. the eventual depodzolization related to deforestation.