Robust geochemical indices for redox and weathering on a granitic laterite landscape in Central Uganda

Most geochemical studies of laterite formation have been based on detailed examinations of a small number of hopefully “representative” profiles. Due to spatial variability in bedrock composition and geomorphic processes that sort materials by size, density, and other properties, experimental designs developed for the geochemical analysis of single soil profiles are of limited use in landscape investigations. In this study of a laterite landscape on a granitic rock in Uganda, we used element concentrations from neutron activation analysis (NAA) to develop a suite of robust geochemical indices for landscape-scale analysis of weathering and oxidation-reduction (redox) processes in laterite formation. To capture K-feldspar weathering the K/Hf ratio served as a good proxy for feldspar/zircon mineral ratios while K/Na ratios indexed feldspar concentration to quartz. Using principal components analysis, we derived an index of Rare-Earth Elements (REE) leaching (relative to Th), and an index of Heavy to Light REE (HREE/LREE) fractionation. We used the Fe/Sc ratio to ascertain the absolute (redox) vs. relative (weathering) Fe accumulation in laterite formation—with absolute Fe accumulation also associated with lower Fe/As ratios. Mn-Ce-Ba nodules and concretions were identified by characteristically high Ce values—with individual Ce values as high as 5000 μg g⁻¹. The relatively large amounts of K-feldspar (K_avg is 4000 μg g⁻¹) and low degree of HREE/LREE fractionation suggest that this landscape is weathered less than generally assumed. The Fe- and Mn-derived indices highlight the important role of oxidation-reduction processes in iron accumulation. Overall, our findings suggest that the current ferricrete-mantled hilltops were formerly located on a lower, poorly drained part of the landscape.