Use of terrestrial cosmogenic 10Be to quantify anthropogenic sediment yield from mountainous watersheds: Application in reconstructing environmental change in the Tanakami Mountains, central Japan

Abstract

This study proposes a methodology that employs a terrestrial cosmogenic nuclide to quantify the magnitude of human impacts on soil-mantled hillslopes and reconstruct the history of the environmental transition from preserved to devastated states by the anthropogenic sediment yield in a mountainous watershed. The feasibility of the approach was tested in the Tanakami Mountains, central Japan. In the target area, preserved and devastated hilly watersheds distributes adjacent each other as a result of longstanding consumption of forest resources and subsequent acceleration of hillslope erosion. 10Be concentrations in quartz grains were measured in modern fluvial sand from the contrasting watersheds and cores of accretionary legacy sediment recovered at a terrestrial sink near to the most severely devastated watershed. The anthropogenic mass loss from hillslopes was calculated based on the difference between 10Be concentrations in the fluvial sand collected from the preserved and devastated watersheds. The timing and duration of severe human-induced erosion in the devastated watersheds were determined based on 10Be profiling and 14C dating of the sediment cores. To compare the datasets from the different watersheds and the legacy sediments, we normalized the 10Be concentration by the nuclide production rate, which thus represents the residence time of the sand particles transported through the soil and shallow bedrock zones. The average apparent residence times in the preserved and devastated watersheds were 10.5 ± 1.8 and 5.4 ± 1.4 kyr, respectively, reflecting the contrasting anthropogenic impacts on the watersheds. The 10Be and 14C archives indicate the 10Be concentration in the legacy sediment has been diluted at shallower depths and shows marked fluctuations over the last 300 yr. The total mass loss from the devastated watersheds was 5.3 × 105 to 2.9 × 106 g m−2, which can be converted to remove thickness of 0.3–1.8 m by assuming the density of subsurface materials as 1.6 × 106 g m−3. This result is consistent with the actual state of devastated hillslopes in the area characterized by the complete removal of soil cover and subsequent active erosion of exposed bedrock. The fluctuations in 10Be concentration in the cores probably resulted from the mixing of sediment particles from the preserved and devastated zones within the watershed, reflecting the nature of the anthropogenic environmental transition that progressed with propagation of the devastation front. The timing and duration of watershed devastation coincided with a period of extensive forest overharvesting documented in historical records.