SEM image analysis in the study of a soil chronosequence on fluvial terraces of the middle Guadalquivir (southern Spain)

A Quaternary fluvial chronosequence (Guadalquivir River, southern Spain), consisting of five soil profiles with estimated ages of 300 years (Haplic Fluvisol), 7000 years (Haplic Calcisol), 70 000 years (Cutanic Luvisol), 300 000 years (Lixic Calcisol) and 600 000 years (Cutanic Luvisol), was studied. Increasing soil age was associated with increases in: reddening, development of structure, clay content, dithionite-extractable iron (Fe-d) and aluminium (Al-d) and strengthening of X-ray diffraction (XRD) peaks for phyllosilicates and iron oxides; there were also decreases in pH and percentage of carbonates in the fine earth and lower XRD peaks for calcite and dolomite. These changes indicate that the principal pedogenic processes were weathering, clay illuviation, rubefaction and the weathering and leaching of carbonates. We have further characterized the pedogenetic chronosequence by quantification of ultramicrofabrics of ped interiors using image analysis (IA) techniques on images obtained with scanning electron microscopy (SEM). We have estimated morphometric ultramicrofabric parameters for particle clusters, skeleton grains and pore space. These are closely related to analytical, mineralogical and macromorphological properties. In the principal component analysis, the first two principal components of the combined morphological, analytical and mineralogical data accounted for 78% of the total variance. The first component (48%) is loaded by variables associated with clay illuviation, relative accumulation of iron and aluminium sesquioxides and the weathering and leaching of carbonates. The components are related to ultramicrofabric development trends. We tested several chronofunctions derived from analytical and morphometric attributes. The logarithmic model fitted best, and we interpret this as indicating pedogenetic processes that are converging towards a steady state.