Pedogenic information from fine sand: A study in Mediterranean soils

Abstract

The fine sand fraction (50–250 μm) of Mediterranean soils from southern Spain provides valuable information on soil genesis and the origin of their parent materials. This study considers the whole fine sand and heavy fine sand (ρ > 2.82 g cm−3) of Luvisols, Calcisols and Fluvisols, which form a chronosequence (ages from 600 to 0.3 ka) of the River Guadalquivir terrace system. Advanced techniques (X-ray diffraction, inductively coupled plasma mass spectrometry, inductively coupled plasma atomic emission spectroscopy, variable pressure scanning electron microscope with an energy dispersive X-ray spectroscopy system and Raman analysis) were employed. Inheritance is the principal pedogenic process. The whole fine sand consisted of carbonates (calcite and dolomite), tectosilicates (quartz, K-feldspar and plagioclases), phyllosilicates (illite/moscovite, biotite, Na-mica, chlorite, kaolinite, interstratified vermiculite-chlorite, vermiculite-illite and smectite-illite) and iron oxides (goethite and haematite). The minor minerals (rutile, anatase, ilmenite, zircon, staurolite, monazite, barite, apatite, andalusite, garnet and titanite) are concentrated, also through inheritance, in the heavy fine sand. However, there is also substantiated evidence for neoformation of rutile in these soils, never reported previously. In addition, we report that (a) the geochemical indices calculated in fine sand (SiO2/CaO, Chemical Index of Weathering (CIW), Weathering Index of Parker (WIP), Weathering Index (WI), Base Depletion Index (BDI), Weathering Ratio (WR) and Sr/Zr) are closely related to soil age, allowing chronofunctions to be established, and (b) geochemical indices provide information on the origin of soils and permit the establishment of a “critical point” corresponding to “time zero;” that is, the start of pedogenic alteration of the parent material.