Vigier et al.(2006)による〔『The relationship between riverine U-series disequilibria and erosion rates in a basaltic terrain』(258p)から〕

『玄武岩地域における河川のウラン系列非平衡と浸食との関連』


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


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