Erosivity controls sediment–solute transport ratios in rivers

Abstract The positive feedback between physical erosion and chemical weathering represents the mechanism through which tectonics indirectly regulates Earth's climate over geological timescales by both releasing and sequestering atmospheric CO 2 . Improving our understanding of this coupling is therefore crucial. Here, we investigate the mechanisms controlling the global pattern observed in the relationship between the products of weathering and erosion—sediment ( SSY ) and solute ( SY ) river loads. Previous work has shown that in rivers with high values of both fluxes—high‐denudation environments— SSY can exceed SY by up to 60 times because weathering is limited by kinetics and thermodynamics, whereas erosion continues even as increasingly resistant fresh rock is exposed. By contrast, it remains unclear why in rivers with low values of both fluxes—low‐denudation environments— SY can exceed SSY by up to 10‐fold. A multiple regression of ratios from dozens of Andean rivers spanning much of the global variability in SSY and SY indicates that SY dominates in low‐denudation environments because dissolved load is not limited by transport thresholds, whereas sediment export is constrained by reduced stream power and limited hillslope–channel connectivity. Thus, the changing ratio from low‐ to high‐denudation settings can be explained primarily in terms of basin erosivity. Where < 1, this ratio can serve as a proxy for the fraction of sediment retained within basins.