『Abstract
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.
Keywords: U-series disequilibria; Iceland; basalt; erosion; weathering
timescale; steady-state』
1. Introduction
2. Sampling area
3. Analytical procedure
3.1. Major and trace elements
3.2. U-series nuclides
4. Results
4.1. Major and trace elements
4.2. U-series nuclides
4.3. Erosion rates
5. Discussion
5.1. The steady-state nature of erosion processes
5.2. Weathering timescale and rates
5.3. Assessing links between erosion rates and U-series
6. Conclusions
Acknowledgements
Appendix A
Appendix B
References