Soil evolution indices in fluvial terrace chronosequences from central Spain (Tagus and Duero fluvial basins)

Soil development in central Spain was studied through analysis of soil chronosequences from Tagus and Duero river valley terrace systems, evolved under similar Mediterranean climatic conditions throughout the Pleistocene. The most complete soil–terrace sequence, representative for our study, developed in the Tagus valley (Aranjuez–Toledo–Talavera sector). We present soil textural data for argillic horizons developed in Late to Early Pleistocene soils from five valleys. We derive soil development indices from clay and silt content and thickness of argillic horizons. Global clay content (GCC), clay content (CC) and clay/silt ratio (C/Si ratio), numerical indices related to clay illuviation, reflect soil evolution, explored by multiple regression of soil–terrace height and soil–age. Relative terrace heights above present river thalwegs (i.e. +100 m) were transformed to ages with a “height–age transfer function” on the basis of preliminary statistical geochronological analyses for central Spain. The height–age transfer function, a 3rd order polynomial (R² = 0.90), based on 60 published numerical ages and paleomagnetic data for terrace sequences:describes the overall trend of valley downcutting for the last c. 2 Ma in central Spain. It assigns numerical ages to terrace levels at different relative elevations; and gives an estimate of standardized ages for the related soils. The explored age–soil relationships, logarithmic functions (R² > 0.80), are sensitive to parent material clay content (corresponding to geology of river catchments) for most soil indices. Only the clay/silt ratio minimizes this sensitivity, and can thus be used for regional approaches (Tagus and Duero basins; central Spain) whereas the CC index is applicable only to individual river valleys or different river valleys with similar catchment geology (i.e. Tagus Basin). The C/Si ratio illustrates the logarithmic trend of weathering of soil particles into clay over time, the generation of soil textural balances sustaining illuviation processes, and therefore provides an estimate of “soil ageing”, for the implementation of soil chronofunctions for regional analysis of river valleys evolving in similar climatic conditions and with similar base-level history.