The relationship between riverine U-series disequilibria and erosion rates in a basaltic terrain

U-series isotopes have been measured in the dissolved phase, suspended load and bedload of the main rivers draining basaltic catchments in Iceland. For the dissolved phase, (²³⁴U/²³⁸U) and (²³⁸U/²³⁰Th) range between 1.08 and 2.2, and 7.4 and 516, respectively. For the suspended load and bedload, (²³⁴U/²³⁸U) and (²³⁸U/²³⁰Th) range from 0.97 to 1.09 and from 0.93 to 1.05, respectively. Chemical erosion rates, calculated from dissolved major elements, range between 13 and 333 t km^{- 2} yr^{- 1}. Physical erosion rates have also been estimated, from existing data, and range between 21 and 4864 t/km²/yr, with an average of 519 t km^{- 2} yr^{- 1}. U-series disequilibria indicate that weathering in Iceland operates at close to steady-state conditions. A model of continuous weathering indicates a maximum weathering timescale of 10 kyr, with an average rate of uranium release into water of 1.6 · 10^{- 4} yr^{- 1}, which is significant when compared to granitic terrains located at similar latitudes and to tropical basaltic terrains. All river waters display (²³⁴U/²³⁸U) greater than secular equilibrium, consistent with the effects of alpha-recoil. The same dissolved phase (²³⁴U/²³⁸U) exhibit a negative trend with physical erosion rates, explained by the dominant effect of close-to-congruent chemical weathering of hyaloclastites in the younger basaltic terrains. Therefore, chemical erosion rate and mineral weathering susceptibility play a major role in determining ²³⁴U-²³⁸U disequilibria in basaltic river waters. Comparison of global data for river basins in which weathering was recently strongly limited indicates a negative correlation between silicate weathering rates estimated with major elements and the age of weathering estimated with U-series disequilibria. This strongly suggests a key role of time and soil thickness on the chemical erosion of silicates.