Pedogenic processes in a chronosequence of very deeply weathered soils in southeastern Brazil

Abstract

Brazil has some of the oldest soils of the world. Landscapes in southeastern Brazil are very old and stable. Some of the soils formed therein have suffered many pedogenetic cycles and hence are difficult to study thoroughly due to their depth and age. In the southeastern State of Minas Gerais, Brazil, we studied four soils developed from gneiss formed on these old surfaces (one sampled to a depth of 26 m) that comprised a chronosequence. For most soils of the world, the major change in properties is at the C/B horizon interface, but in these soils the major change is at the R/C interface. In each soil, a certain suite of clay minerals (kaolinite, gibbsite, goethite, and hematite) had weathered from the primary minerals of gneiss. The composition of the suite differed among the four soils, but in an individual soil it was relatively constant with depth, including through the C/B boundary. In the C horizon, many of the minerals were in the form of aggregates in the silt fraction. Across the C/B boundary there was little change in chemistry and mineralogy, but there the aggregates broke down to clay particles that were incorporated into soil structure. Overall, the chronosequence showed that soil properties changed greatly from the youngest, lowest part of the landscape, while between the older soils, higher in the landscape, the change was small. Soil properties appeared influenced by the different zones of the weathered mantle, hypothesized to be gibbsitic on the very top, kaolinitic in the most part, and bissialitic (2:1 clay minerals) closer to the fresh rock.