干旱区生态环境知识资源中心

Relative age indicators using geomorphologic and pedologic parameters are important tools for palaeoenvironmental reconstruction. Here we present results from two study areas in arid south-western North America (Mojave Desert, California, USA, and Laguna Salada, northern Baja California, Mexico), which advise using relative age indicators with care because we found no direct relationship between surface properties, soil development and age. On middle Pleistocene basalt flows with stone pavement cover in the Mojave Desert, soil properties vary largely within a small lateral distance without accompanying variation in surface properties. In Laguna Salada soils of similar maturity prevail on abandoned alluvial fan terraces of different surface characteristics. In both cases, chronosequence approaches via surficial relative age indicators explain variations in soil evolution insufficiently. Vice versa it is inappropriate to deduce surface ages from soil properties. Rather, we conclude soil and surface features represent proxies from uncoupled, independent archives. Geomorphologic processes, triggered by e.g. climatic change, have acted even on abandoned surfaces, leading to a small-scale variability of sediments and soils which do not necessarily have the same age throughout a single surface. Regarding stone pavements as dynamic surface constituents can explain diverging results of the two archives. However, a dynamic pavement has to be integrated in surface dating approaches. Using profile development indices for comparing soils from different regions has limitations stemming from inadequate standard soil profiles and regional specific climate history.