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

Abstract

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.

Weak monsoon event at 4.2 ka recorded in sediment from Lake Rara, Himalayas

“…Some studies report spatial variations in soil carbon along ridge top to hillslope (Harden et al, 2002;Yoo et al, 2006). Considering the difference in erosion rates within a watershed (Heimsath et al, 1999(Heimsath et al, , 2001Nakamura et al, 2014a), monsoon-induced fluctuations in the spatial pattern of soil erosion could change the oldcarbon offset. Consequently, further research into understanding the mechanism of the old-carbon offset is not only important for sediment geochronology, but also strongly related to vegetation and soil carbon dynamics.…”

Transient Response and Adjustment Timescales of Channel Width and Angle of Valley-Side Slopes to Accelerated Incision

“…Contributions of slow and fast muons to the total 10 Be production at the surface were assumed to be 1.2% and 0.65%, respectively (Braucher et al, 2003). We assumed a bulk density of 1.63 g/cm 3 for the shallow subsurface materials on hillslopes (Nakamura et al, 2014). Although it sometimes snows in Iwaki, we did not consider the effect of snow shielding on 10 Be production because the snow cover was mostly less than several centimeters deep during 1960-2008 (Japan Meteorological Agency, 2021)．…”

“…To verify these estimates, we calculated erodibility coefficients for similar granitic rocks in the Abukuma massif (Kubo & Yamamoto, 1990) north of the study area using the same DEM and procedure we used in Iwaki (Figure S4; Tables S3, S4). We relied on 10 Be concentrations of fluvial sand reported by Regalla et al (2013), Nakamura et al (2014), and Matsushi et al (2014) to recalculate basin-averaged erosion rates using the same method used in Iwaki. The resulting erodibility coefficients were similar to those obtained in Iwaki: 1.58×10 −5 (𝑛 = 2/3), 4.52×10 −6 (𝑛 = 1), and 3.90×10 −7 (𝑛 = 5/3) m 0.1 /yr.…”

Surface rupture and characteristics of a fault associated with the 2011 and 2016 earthquakes in the southern Abukuma Mountains, northeastern Japan, triggered by the Tohoku-Oki earthquake

“…Fujiwara et al (2001) reported average basin-scale denudation rates of ~ 0.132 mm/ year in the Abukuma Mountains based on the relationship between sediment delivery rates to reservoirs and the dispersion of elevation. Nakamura et al (2014) estimated denudation rates in the Abukuma Mountains of 0.114-0.180 mm/year based on the concentration of cosmogenic nuclides in fluvial sediments. Although we cannot discuss the slip rate of the Mochiyama fault directly due to lack of cumulative offsets on fluvial terrace or alluvium, these previous studies suggest that the slip rate of the Mochiyama fault is no greater than ~ 0.1 mm/ year.…”