Application of laser ablation ICP-MS and traditional micromorphological techniques to the study of an Alfisol (Sardinia, Italy) in thin sections: Insights into trace element distribution

Abstract

An innovative methodological approach is proposed in this study, based on the chemical analysis of different portions of soil horizons (illuvial pedofeatures, pedogenic matrix and skeletal parent rock fragments) by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), associated with traditional micromorphological (optical and scanning electron microscopy) techniques. Validation of the LA-ICP-MS technique provides in situ accurate and reproducible (RSD 13–18%) analysis of low concentration trace elements and rare earth elements (REE) in soil samples (0.001–0.1 ppm). Analyses were carried out on thin sections prepared from undisturbed soil samples, collected from three different Bt (argillic, i.e. clay-illuviated) horizons in an Alfisol from the Muravera area (southeastern Sardinia, Italy). The main aims were (i) to develop and test the reliability of a LA-ICP-MS analytical method on soil thin sections and (ii) to explore the idea that concentrations of trace and rare earth elements in the different portions of a soil profile can be used to investigate their distribution, as a response to pedogenetic processes. The three horizons show similar features, particularly in terms of element concentrations and patterns. By comparing the concentration of REE and other trace elements in the clay coatings, the soil matrix and the skeletal rock fragments in the same horizon, a general enrichment trend is observed from the parent rock to the pedogenic matrix up to the illuvial pedofeatures, suggesting a prominent role of pedogenetic processes in element fractionation and distribution during weathering. In particular, the highest concentration of trace elements (and specifically REE) in clay coatings, which represent the most mobile solid phase in the soil profile, indicates that the former can be used as a reliable indicator of soil weathering if a preliminary assessment of the presence of clay coatings is achieved. In addition, the discontinuous trend of some trace elements among soil features of different horizons is consistent with field evidence of slope dynamics, marked by an erosive and depositional discontinuity along the soil profile.