Use of terrestrial cosmogenic 10Be to quantify anthropogenic sediment yield from mountainous watersheds: Application in reconstructing environmental change in the Tanakami Mountains, central Japan
Use of terrestrial cosmogenic 10Be to quantify anthropogenic sediment yield from mountainous watersheds: Application in reconstructing environmental change in the Tanakami Mountains, central Japan
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DOI：
10.1016/j.geomorph.2022.108201
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发表时间：
2022-05
期刊：
影响因子：
3.9
通讯作者：
R. Ohta;Y. Matsushi;H. Matsuzaki
中科院分区：
文献类型：
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作者：
R. Ohta;Y. Matsushi;H. Matsuzaki
这项研究提出了一种方法，利用陆地宇宙核素来量化人类对土壤覆盖的山坡的影响程度，并通过山区流域的人为沉积物产量重建从保存状态到破坏状态的环境转变历史。该方法的可行性在日本中部的田上山脉进行了测试。在目标区域，由于森林资源的长期消耗和随后山坡侵蚀的加速，保存完好的丘陵流域和遭到破坏的丘陵流域彼此相邻分布。 10Be 浓度是在现代河流沙中测量的，这些沙子来自对比流域和在受破坏最严重的流域附近的陆地汇中回收的增生遗留沉积物的核心。山坡的人为质量损失是根据从保存完好的流域和遭到破坏的流域收集的河沙中 10Be 浓度之间的差异计算的。根据沉积岩芯的 10Be 分析和 14C 测年确定了受破坏流域中严重人为侵蚀的时间和持续时间。为了比较来自不同流域和遗留沉积物的数据集，我们通过核素产生速率标准化了 10Be 浓度，从而代表了沙粒穿过土壤和浅基岩区域的停留时间。受保护的流域和遭到破坏的流域的平均表观停留时间分别为 10.5 ± 1.8 和 5.4 ± 1.4 kyr，反映了对流域的人为影响的对比。 10Be 和 14C 档案表明，遗留沉积物中的 10Be 浓度已在较浅的深度被稀释，并在过去 300 年中表现出明显的波动。被破坏流域的总质量损失为 5.3 × 105 至 2.9 × 106 g m−2，通过假设地下物质的密度为 1.6 × 106 g m−3，可以转换为去除 0.3–1.8 m 的厚度。这一结果与该地区被毁坏山坡的实际状况相符，其特点是土壤覆盖物完全消失，随后裸露基岩受到积极侵蚀。岩心中 10Be 浓度的波动可能是由于流域内保留区和毁坏区的沉积物颗粒混合造成的，反映了随着毁坏前沿的传播而发生的人为环境转变的性质。流域破坏的时间和持续时间与历史记录中记录的大规模森林过度采伐时期相吻合。
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 between10Be 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 on10Be profiling and14C dating of the sediment cores. To compare the datasets from the different watersheds and the legacy sediments, we normalized the10Be 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. The10Be and14C archives indicate the10Be 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 × 105to 2.9 × 106g 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 × 106g 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 in10Be 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.