Granet,M., Chabaux,F., Stille,P., France-Lanord,C. and Pelt,E.(2007): Time-scales of sedimentary transfer and weathering processes from U-series nuclides: Clues from the Himalayan rivers. Earth and Planetary Science Letters, 261, 389-406.

『ウラン系列核種からみた堆積輸送と風化過程の時間スケール：ヒマラヤの河川からの手がかり』

『Abstract
　In order to define time-scales of erosion and sedimentary transfer in the Himalaya, 238U-234U-230Th disequilibria have been analysed in river bank sediments and in bedloads collected along the Kali Gandaki river, one of the main Nepalese rivers, and in the Ghaghara and Gandak rivers, two major plain tributaries of the Ganges.
　The Th activity ratios and U/Th ratios in river sediments of the two Ganges tributaries constantly decrease from upstream to downstream. This is related to the maturation of sediments by weathering during their transfer to the plain. The U-series data allow to calculate a transfer time for the sediments in the alluvial Gangetic plain from the chain front to the confluence with the Ganges of about 10 kyr for both rivers. The Kali Gandaki river sediment data highlight a decrease of both the Th isotopic and U/Th ratios which is explained by a mixing between two sources with similar U/Th ratios but having suffered a different U-Th fractionation history. Interpretation of the U-series data in the frame of this scenario gives long time-scales of weathering of several 100's kyr for the Himalayan terranes. The results imply that Himalayan bedrocks are submitted to a long in situ stage of weathering before their erosion and transfer into the rivers. In addition, occurrence of similar U-Sr signatures in dissolved (i.e.＜0.1-0.2μm) and sediment phases of the Kali Gandaki river suggests that “dissolved” uranium could be carried by colloids constituted by sedimentary microparticles. This precludes the use of U-series disequilibria in this river to calculate weathering budgets and to assess whether the erosion is working at steady-state or not.

Keywords: weathering; sedimentary transfer; 238U-234U-230Th disequilibria; time-scale; Himalaya』

1. Introduction
2. Geological and sedimentrological context
3. Sampling location and analytical procedures
4. Results
　4.1. Water samples
　4.2. Sediment samples
5. Discussion
　5.1. Origin and time implications of U-series disequilibria in river sediments
　　5.1.1. The case of the Ghaghara and Gandak rivers
　　5.1.2. The case of the Kali Gandaki river in the LH-HHC
　5.2. Signification of the time-scales of weathering and transfer
　5.3. Origin of the dissolved U in the Himalayan rivers
6. Conclusion
Acknowledgments
Appendix A
References

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