Accurate determination of denudation rates is important in understanding Earth surface system dynamics. In situ produced cosmogenic nuclides, such as 10 Be and 26 Al, provide a valuable insight but are not entirely free from complications. In this study, denudation rates are determined for the Abukuma Mountains, Japan, using both site-specific and basin-scale methods. Considered with density measured in the field, distinct and systematic differences between the two methods are identified. Site-specific rates calculated from depth profiles of cosmogenic nuclides (10 Be and 26 Al) at topographic highs indicate a rate of 67 to 85 mm/kyr, whereas basin-scale averaged denudation rates derived from the concentration of cosmogenic nuclides in fluvial sediments show 114 to 180 mm/kyr. This is the first comparison of these two commonly used methods in the same region in Japan, where the entire study area is characterized by well-developed saprolite. These results indicate that differential denudation rates between topographic highs and valleys reflect increasing local topographic relief of the study area. Comparison between rates derived from depth profiles and those applicable to the entire basin is important for understanding landscape development.
We present erosion rates of granitic soil surfaces in the western Abukuma upland, Japan using depth profiles of in-situ produced cosmogenic 10 Be and 26 Al based on physical parameters for both neutron and muon interactions. Samples were obtained below the severely weathered zone, from 30 to 190 cm depth below surface (bs). We confirmed that, in this environment, deeper layers from at least 80 cm bs must be analyzed to achieve highly accurate measurement of erosion rate because near-surface layers are potentially influenced by pedogenic processes. The depth profiles obtained suggest a surface-lowering rate of 49-74 m/Myr for a mountain ridge composed of granitic soil. This newly obtained erosion rate is much higher than previously reported granitic or metamorphic bedrock, as well as quartz vein, erosion rates from several climatic environments including humid regions, suggesting that granitic soil surface is more susceptible to erosion.
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