Causes and consequences of savanna catenary dynamics: 200ky of landscape chromatography

Stream catchment erosion rates (¹⁰Be) across a climate gradient (MAP 500-800 mm, MAT 20-22 C) on the granitic western half of Kruger National Park average <5 m/My, suggesting soil residence times >200ky. Along multiple catenas, upper backslope soils show losses of all base cations and total iron, aluminum, and silicon while lower backslope soils show enrichment (~200%) of iron, with all changes relative to parent material. Chemical depletion fractions (CDF) calculated with soil and parent material zirconium values for pedons in the watershed are high, averaging 0.82 (SE 0.01), implying total denudation of this landscape is primarily chemical. Upslope of seeplines is "sourveld" or nutrient-poor vegetation, with chemical anti-herbivory defenses; downslope of seeplines but above the barren saline-sodic zones, "sweetveld" or nutrient-rich vegetation including Acacia spp. dominates, with physical protection (e.g., thorns). Our interpretation of these patterns is that long soil residence times allow differentiation of soil properties along our 150-m catena (total relief of <15 m). The removal of colloids and base cations from crest soils creates a coarse-textured zone that promotes bioturbation; the downslope transfers of clays and salts leads to clay-rich lower backslope soils and saline-sodic footslope soils, respectively. The accretion of clays inhibits bioturbation and the accumulation of salts and clays leads to soil dispersion and crusting. As a landscape unit, accretionary duplex soils (sandy surface horizons, clayey subsurface horizons) should move upslope until upslope contributing volume falls below some threshold. The ecotone between sourveld and sweetveld has significant consequences for hydrologic flowpaths: subsurface flow is diverted to the surface where sandy soils are juxtaposed with clayey soils. These duplex soils are also subject to physical erosion by headcut propagation downslope of seeplines, suggesting that redistribution of material over long periods of time can alter the balance of chemical and mechanical landscape denudation.