CLIMATE-DRIVEN SOIL DILATION AND COLLAPSE CONTROLS HILLSLOPE MORPHOLOGY IN TECTONICALLY QUIESCENT REGIONS

Element leaching leads to soil-profile collapse whereas addition of atmospherically derived carbon and salts expand profiles. We use immobile element concentrations to quantify these effects at the profile scale, but do collapse/dilation have measureable impact at the landscape scale? For most geographies, the answer is probably no, at least for hillslopes, because physical erosion removes soil material faster than chemical erosion or accumulation. Here we show that element transfers along catenas can play an integral role in determining landscape morphology in tectonically quiescent areas of southern Africa. The catenas formed in gently rolling terrain underlain by granite and their soils exhibit highly differentiated chemical and mineralogical properties along 3 to 8% slopes that extend 200 to 2000 m from crest to stream corridor. We sampled soil profiles along catenas at sites receiving 470, 550, and 730 mm annual rainfall, and measured soil production and volume change. Crests in the dry region showed dilation whereas the other two showed considerable collapse. Soil profiles deepened with increasing rainfall: 0.6 m, 1.1 m and 3 m, respectively. Crests showed progressively slower soil production with increasing rainfall: 7.8, 1.2, 0.2 m my^-1. Chemical depletion factor was 0.01, 0.52, and 0.75 with increasing annual rainfall. Hence physical erosion as a percent of soil production declined from 99 to 48 to 25. Long soil residence times along the catenas enhanced hillslope differentiation driven by solutional and colloidal transfers from the highly leached, collapsed crests to dilated or less collapsed lower slope depositional areas. Collapse in the upper parts of landscapes can be as much as 3 m and dilation in the lower parts as much as 1 m in the 730 mm rainfall region. Given that the landscape is predominated by relatively short, gentle hillslopes with overall relief of 10 to 30 m, volume change can produce significant changes in overall landscape morphology as indexed by morphometric analyses using high-resolution airborne LiDAR. Arid catenas have short, concave slopes and high drainage density, whereas subhumid slopes have broad crests where collapse dominates, and convex upward lower slopes where dilation dominates. They also have lower drainage density than the arid sites.