Quantitative determination of long-term erosion rates of weathered granitic soil surfaces in western Abukuma, Japan using cosmogenic 10Be and 26Al depth profile

Abstract

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.

First report of (U–Th)/He thermochronometric data across Northeast Japan Arc: implications for the long-term inelastic deformation

“…Mean denudation rates within the last few tens of years were calculated to be ~0.1 mm/year in the Abukuma Mountains and ~0.5 mm/year in the Asahi Mountains based on relationships between altitude dispersion in drainage basins and denudation rates deduced from sediment loads in catchments (Fujiwara et al 1999). Denudation rates over the Holocene time scale in the Abukuma Mountains were also determined to be ~0.1 mm/year or lower based on cosmogenic nuclide data (e.g., Shiroya et al 2010;Regalla et al 2013;Matsushi et al 2014). In addition, bedrock uplift rates in the last 10 5 years were estimated to be 0.2-0.6 mm/year or higher on the eastern margin of the Abukuma Mountains (e.g., Suzuki 1989), ~0.5 mm/year on the eastern margin of the Asahi Mountains (Miyauchi et al 2004), and up to 0.5 mm/year on the western margin of the Asahi Mountains (Ikura and Ota 2003) by using present altitudes and emergent ages of marine and/or fluvial terraces.…”

Progress on multi-nuclide AMS of JAEA-AMS-TONO

“…Recently, a depth profile of 10 Be has been used to estimate erosion rate of surface rocks [e.g. 24]. Measurement of 10 Be in this study requires low background levels since 10 Be concentrations in rocks at surface and deep underground are low.…”

Direct comparison of site-specific and basin-scale denudation rate estimation by in situ cosmogenic nuclides: an example from the Abukuma Mountains, Japan

“…Depth profiles of in situ cosmogenic nuclides allow determination of site-specific denudation rates in humid regions without the complex assumptions required for basin-scale studies (Siame et al 2004;Matsushi et al 2006). Topographic highs such as mountain summits, hilltops, and ridges are appropriate sites for depth profiling due to the minimized influence of along-slope mass transport (Shiroya et al 2010). Another advantage stemming from the usage of depth profiles is that information regarding nuclide equilibrium is also obtained (see the following).…”

“…We reanalyzed the denudation rates reported by Shiroya et al (2010) in the western part of the Abukuma Mountains using actual measured density. They determined denudation rates from the depth profile of a quartz vein in granitic saprolite along a mountain ridge assuming 2.2 g/cm 3 for the density of wet granitic saprolite.…”

“…The reported denudation rates were 61 + 6/-4 and 62 + 4/-3 mm/kyr based on 10 Be and 26 Al depth profiles, respectively. In the current study, we applied the actual density of the rock at this site (1.76 ± 0.03 g/cm 3 ) and parameters described in Shiroya et al (2010) for recalculation, which resulted in rates of 86 ± 2 and 89 ± 3 mm/kyr for 10 Be and 26 Al-based determinations, respectively. These values are 1.4 times larger than the initial values reported by Shiroya et al (2010), indicating that direct density measurements are indeed crucial for determining accurate denudation rates.…”