Quantitative geochemical approach to pedogenesis: importance of parent material reduction, volumetric expansion, and eolian influx in lateritization

Abstract

Using mass balance techniques we test the prevalent view that laterite genesis is dominated by in situ residual enrichment during chemical weathering of bedrock. Through calculation of net mass fluxes through the laterite soils in Mali, West Africa, we show that residual enrichment by removal of mobile elements with a corresponding increase in bulk porosity and decrease in bulk density, contributes only a very minor fraction of the enrichment of Al, Fe, Si and Au. Instead, we demonstrate that the abundance of these elements is due to the influx and accumulation by selective retention of chemically mature detritus of local and foreign origin clearly evident in micromorphological infilling features. At the same sample depths that accumulation reaches extreme values, we show that volumetric expansion in excess of 200% has occurred locally. We infer that these spatially coincidental zones of mass influx of rock-forming metals Fe, Al, Si, and also Au with dilational hyperstrains result from a mutually reinforcing, mechanical interaction between material influx and the effects of subsurface deformational processes such as shrink-swell cycles and root growth and decay. We propose that with progressive infilling of available connected voids by illuvial microsedimentary deposits of insoluble resistate and neoformed minerals, the capacity for the combined skeleton and plasma to remain isovulumetric is exceeded. We speculate that the resultant space problem is relieved by upwards expansion towards the overlying free surface. Continued translocation and void infilling occur and are limited to the depth where the size of translocational particles is smaller than that of the connected voids. Consequently, progressive hyperstrains accumulate above this critical depth by the long-term influence of a proposed translocational wedge of chemically-resistant minerals against which numerous generations of plant roots have exerted stresses. Eclectic surficial contaminants involved are continuously derived from above leaving no indication of a relict source region within the present soil profile from which they might have been extracted. Instead, the source region is largely the existing regolith column itself which releases local material and in addition, is supplemented by deposition of colluvial detritus shed nearby by escarpment retreat