Geochemistry of soils of a deep pedon in the Okavango Delta, NW Botswana: Implications for pedogenesis in semi-arid regions

Abstract

Pedogenic parameters and data on geochemical compositions were analysed to improve our understanding of pedogenesis and soil morphology in an ecologically and geologically unique and relatively quiescent tectonic setting of the Okavango Delta, northern Botswana. A complex interaction of pedogenic and gradational processes in the area resulted in the formation of a deep (~4 m) pedon with three distinct morphologies: a weakly structured surface horizon, modern pedogenic carbonate enriched B horizons at stage III development and slightly pedogenically altered sediment C horizons. The pH ranged from 8.1 to 10.0 implying alkaline soil environment, while electrical conductivity ranged from 2 to 55 dS/cm. Gleying, an evidence hydromorphism, was present in the wetting front (C horizon). Results of the total elemental composition show the dominance of silica followed by CaO, Al2O3, MgO and Fe2O3 as the major oxides. Trace elements had lower HFSE (e.g. Zr and Nb) than LILE (e.g. Ba, Sr, Rb, Cr and V) contents. Calcite, quartz and opal-CT are the minerals present in the fine silt and clay fractions of the soils. Geochemical indicators points to incipient weathering and pedogenesis. Evolutionary accumulation of pedogenic carbonate in the diagnostic horizons bestows Calcisols as the Reference Soil Group and not Arenosols as previously known. Other pedogenic processes including leaching losses, redox processes and salt accumulation were strongly influenced by the saline groundwater processes. By comparing these site-specific findings with examples from other settings, this study has demonstrated the applicability of morphological, geochemical and mineralogical properties of soils in assessing pedogenic processes and classification in deep calcic soils associated with alluvial and aeolian depositions.