Soil genesis, morphodynamic processes and chronological implications in two soil transects of SE Sardinia, Italy: Traditional pedological study coupled with laser ablation ICP-MS and radionuclide analyses

Soils are characterized by an intrinsic variability through time and space at different scales of observation and are often affected by morphodynamic processes strictly interacting with pedogenesis, under changing (paleo)environmental/climatic conditions. We focus on exploring this interplay in modern soils and paleosols of southeast Sardinia (Italy), integrating two innovative techniques to a traditional pedological study: (i) laser ablation ICP-MS spot analyses applied to detect microscale distribution of trace elements in discrete soil features of organic-mineral (A) and argillic (Bt) horizons in thin sections, and (ii) radioactive disequilibria as tracers and chronometers of pedogenetic alteration and geomorphic dynamics. The Pleistocene paleosols developed mainly during interglacial periods, with important clay illuviation and rubification. The other soils are essentially characterized by organic matter incorporation into the profiles. Field discontinuities described within soil profiles, related to phases of sediment aggradation and/or surface erosion, are often supported by laboratory results, mainly micromorphological and geochemical data. LA-ICP-MS data show an overall trend of increase of most trace elements (REEs included) from skeletal rock fragments to both clayey and organic-rich soil matrix to clay coatings. This pathway is clearly controlled by the degree of weathering of primary minerals and by pedogenetic processes, where element adsorption onto reactive sites of organic matter and clay particles and clay illuviation play prominent roles. Recent trends of the studied soils/paleosols in terms of geomorphic stability (equilibrium) or morphodynamic processes (erosion/accumulation rates) are estimated using radionuclide disequilibria in the order of 1 cm/a or lower.

Research highlights
► Clay films/rubification and humus characterize paleosols and soils in SE Sardinia.
► In situ LA-ICP-MS analyses of soil thin sections highlight pedogenic processes.
► Trace elements increase from rock fragments to pedogenic matrix to clay coatings.
► Radioactive disequilibria are tracers of soil alteration and geomorphic dynamics.
► Erosion/accumulation rates ≤ 1 cm/a are estimated using radionuclide disequilibria.