From rock to soil: elemental mobility during pedogenesis in a deep Ultisol profile at the Mantiqueira Mountain Range, southeastern Brazil

Abstract

A better understanding of the mobility of chemical elements during weathering and leaching processes requires the complete analysis of soil profiles down to the bedrock, especially in tropical environments, due to the great thickness of the weathering mantle. We studied a deep profile comprising solum, saprolite and rock at the Mantiqueira Mountain Range in Brazil aiming to assess the mobility of major and trace elements in an Ultisol and its parent rock (gneiss). A 63-m borehole was drilled from the topsoil down to the gneiss. Samples from the solum (A + B horizons), saprolite and gneiss were collected and analyzed for chemical, physical and mineralogical attributes and elemental contents. A mass balance equation was applied to quantify the net result of weathering considering Ti as the most stable element. Despite the steep relief, a very deep soil profile was found (24-m solum + saprolite). Alkaline and alkaline earth metals (e.g., Ca, K, Sr, Rb and Ba) and Mn were strongly depleted in the solum and saprolite. Al, Fe, Zn, P, Zr, Y and V showed residual enrichment in the saprolite, while Al and Zr revealed enrichment in the transformation from saprolite to solum. Biggest changes were observed in the rock/saprolite interface, not in saprolite/solum. The efficient drainage of this soil might be related to the weathering of the saprolite. Variations of elemental contents from the soil surface down to the parent rock helped to elucidate the changes of elemental contents caused by the weathering and leaching processes.