Weathering dynamics reflected by the response of riverine uranium isotope disequilibrium to changes in denudation rate

The ratio between the activity of dissolved ²³⁴U and ²³⁸U in river water, (²³⁴U/²³⁸U)_Riv, shows promise for tracing weathering processes. It has been suggested that preferential ²³⁴U release generates a high (²³⁴U/²³⁸U)_Riv under low weathering intensity while congruent weathering, as expected for high weathering intensity, would release uranium with a secular equilibrium (²³⁴U/²³⁸U)_Riv of 1. However, weathering intensity depends on the combination of exposure age of weathering interfaces and dissolution rate, complicating the use of (²³⁴U/²³⁸U)_Riv as a tool in weathering studies. This work attempts to resolve the weathering dynamics reflected by (²³⁴U/²³⁸U)_Riv using catchment scale denudation rate (D) as a first order approximation for the age of weathering interfaces. A global dataset (n = 653), including 132 new measurements from Chinese catchments, indicates changing responses of (²³⁴U/²³⁸U)_Riv to D. Higher values (>1.5) and larger variability of (²³⁴U/²³⁸U)_Riv are observed in catchments both with high (> 3 mm/yr) and low D (< 0.1 mm/yr). In contrast, the lowest values (approaching secular equilibrium) and the least variable (²³⁴U/²³⁸U)_Riv are associated with catchments of medium to high D (0.3-2 mm/yr). The observed pattern can be explained by a model that involves competing uranium release from active- and inactive- weathering interfaces in response to a shifting weathering regime, from kinetically-limited to supply-limited. (²³⁴U/²³⁸U)_Riv decreases with decreasing D under a kinetically-limited weathering regime due to the increasing contribution of congruent weathering relative to the preferential dissolution of ²³⁴U from radioactively damaged sites as D decreases. In contrast, (²³⁴U/²³⁸U)_Riv increases with decreasing D under a supply-limited weathering regime because the minerals that remain under these conditions continue to release ²³⁴U to solution by recoil, while contributing little to the dissolved ²³⁸U because of their slow dissolution rate. A much deeper depth of the weathering interface is suggested for catchments with high D due to higher weathering contribution from deep fractures relative to the poorly developed surface regolith.