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

• Published in: Earth and Planetary Science Letters
• Main Authors: Vigier, N, Burton, K, Gislason, SR, Rogers, N, Duchene, S, Thomas, L, Hodge, E, Schaefer, B
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2016
• Subjects: Iceland
• Online Access: https://doi.org/10.1016/j.epsl.2006.07.001; https://ora.ox.ac.uk/objects/uuid:21c30d7a-6edf-4bf1-ba83-a014def96faa

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. © 2006 Elsevier B.V. All rights reserved.