EQUILIBRIUM LANDSCAPES: WHERE SOIL PRODUCTION FUNCTIONS FAIL

Quantifying soil production processes and rates underpins interdisciplinary efforts to predict soil sustainability, landscape response to climate change, landslide hazards, and carbon sequestration across the Earth’s critical zone. Climatic and tectonic forcing drive rates at regional scales, while overlying soil thickness sets rates at hillslope scales. Local processes depend ultimately on climate, which determines water availability, biology, and the relative roles of physical and chemical weathering. The presence of a relatively thin (~ 1 m) soil, defined here as physically mobile sediment, across a landscape shaped early thinking that sediment production rates should be in equilibrium with erosion rates. The last twenty years of observations suggested, however, that the conceptualization of landscape equilibrium is complicated. Here we use a well-studied South African climosequence to tackle the problem of landscape evolution in response to changing climate through the lens of soil production and erosion. We use concentrations of cosmogenic ¹⁰Be to quantify soil production and average erosion rates and link sediment transport processes to termite burrowing. Namely, we show that termites transport ¹⁰Be-depleted sediment from depth, mixing it with the higher ¹⁰Be concentrations liberated by soil production processes in the physically mobile zone. Overland flow and biophysical transport processes deliver sediment to channel networks and we compare ¹⁰Be concentrations between channel sediments and saprolite being actively converted to soil. We also measured ¹⁰Be concentrations of stone lines from ridge crest to base to quantify their divergence from steady-state ¹⁰Be profiles and suggest a residence time index to quantify differences in sediment transport processes between the fine grained material moved by termites and coarser clasts. We combine the ¹⁰Be measurements with four deep concentration profiles of chemically immobile Zr arranged in a catena at the wet site. These profiles, combined with our measurements of ¹⁰Be in saprolites and stone lines, and the previously quantified termite distributions enable us to test the landscape equilibrium hypothesis in detail for a slowly eroding landscape under three different climate regimes.