Determining Long-term Sediment Yield from Mountainous Watersheds by Terrestrial Cosmogenic Nuclides

Abstract

 This paper introduces basic principles for use of terrestrial cosmogenic nuclides in quartz, and demonstrates applications for determining watershed denudation in a granitic region in the Northern Japanese Alps. Samples of fluvial sediment were collected at outlets of 15 watersheds in the Ashima and Takase Rivers for accelerator mass spectrometry of ¹⁰Be and ²⁶Al. Denudation rate of the watersheds ranges from 2 ×10² to 7 ×10³mm kyr^－1,which increases with increasing mean basin slope until 40° but declines for steepest watersheds with > 40° mean slope. This may indicate transition of hillslope denudation regimes from transport-limited to detachment- and/or weathering-limited conditions. The rate of sediment yield deduced from cosmogenic nuclides provides a clue to understand sediment dynamics in a long timescale and to assess mountain hazard in active orogens.