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)×10⁵tonsyear^-1, and the long-term rates of CO₂ consumption by alkaline earth silicates were (0.03-0.5)×10⁵molkm^-2year^-1, commensurate with the average global CO₂ consumption rate (0.25×10⁵molkm^-2year^-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 0.6mmyear^-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 10-150ky 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.