Quantitative understanding of variability in weathering fluxes on the modern Earth is limited because little is known about where the most important weathering reactions take place. This is partly because the locus of weathering is difficult to measure empirically. Inverse analysis of a parametric model presented here provides first-order constraints on variability in the thickness of the zone of active weathering. Results suggest that the effective thickness of the weathering zone varies relatively little across several orders of magnitude of denudation rate. At low to moderate denudation rates, reactions in soils may dominate weathering fluxes at the catchment scale, but the contribution from soil weathering decreases at higher denudation rates. Consequently, increased erosion leads to higher weathering fluxes, sustained by progressively greater contributions from weathering in bedrock. The effect of climate (temperature and runoff) on weathering fluxes is apparently weaker at low denudation rates than at high denudation rates, such that erosion, and potentially associated bedrock weathering, may be important for maintaining climate-stabilizing feedbacks in Earth’s carbon cycle.