Chemical and physical weathering in south Patagonian rivers: A combined Sr–U–Be isotope approach

We investigated physical and chemical weathering in south Patagonia, encompassing both the tectonically active Andes with alpine glaciers and the quiescent seaboard plain with arid climate. Chemical denudation rates determined from riverine dissolved major elements were (0.07–5) × 105 tons year−1, and the long-term rates of CO2 consumption by alkaline earth silicates were (0.03–0.5) × 105 mol km−2 year−1, commensurate with the average global CO2 consumption rate (0.25 × 105 mol km−2 year−1). Unradiogenic strontium isotope ratios indicated that the source of silicate weathering was volcanic sedimentary cover. Basin average total denudation rates based on 10Be measured in active streambed sediments ranged from 0.009 to 0.6 mm year−1. Uranium series disequilibria suggested that there is significant redistribution of nuclides between the dissolved and suspended material. When applying the simultaneous gain and loss model to the U-series data of the suspended load, sediment residence times of 10–150 ky were obtained. Comparison of the dissolved load-based chemical denudation rate and 10Be-based total denudation rate revealed that some basins are dominated by chemical and some by physical denudation.