THE GOLDILOCKS CATENAS OF SOUTH AFRICA: MAXIMUM SOIL DIFFERENTIATION ACROSS GRANITIC HILLSLOPES

Understanding the processes by which soils become differentiated along hillslopes is critical to understanding the structure and function of terrestrial ecosystems. Catena soils --if stable-- can become increasingly differentiated over time as weathering releases ions from rock and these products are translocated. Our semiarid savanna study site in Kruger National Park, South Africa, has gentle slopes (<10%) underlain by granite. Crest soils were sandy, red, and Fe-poor; midslope soils were also sandy, but white, and very Fe-poor; lower backslope soils were duplex (sandy at the surface and clayey below), dark, and Fe-enriched; toeslope soils were sparsely-vegetated sodic zones with high clay. Over very short distances near the midslope, depth-weighted soil pH jumped from 5 to 9, clay in the subsurface from <15% to >45%, and Fe remaining (relative to unweathered granite) from 10% to 130%. This midslope ecotone separates eluvial zones upslope from illuvial zones downslope; the accumulation of subsurface clay creates seeplines, as subsurface flowpaths are diverted to the surface. This ecotone also marks the boundary between broad-leaved savanna and fine-leaved savanna. This strong differentiation of soils results from 1. a climate that is neither too wet nor too dry--a wetter climate might flush weathering products from the hillslope whereas a drier climate might not catalyze weathering or transfers; 2. a lithology that is neither too susceptible nor too resistant to weathering--the skeletal, quartz-rich matrix enables chromatographic separation of ions and colloids; and 3. very long soil residence times (~200 ky). Hillslope differentiation is maximized under these conditions provided a fourth factor is present: bioturbators such as aardvarks, elephants, and termites. Their mixing of soils, primarily at crests, enhances the processes of infiltration, weathering, and transport. Where these four factors are not present, catenas are less well differentiated. For example, basalt and rhyolite hillslopes in the park with similar climate show little differentiation. These processes govern the structure and function of savannas in quiescent tectonic settings but also provide a useful framework for understanding geochemical catenas in other settings.