地形
Online ISSN : 2759-2529
Print ISSN : 0389-1755
35 巻, 2 号
選択された号の論文の6件中1~6を表示しています
特集:岩石中の宇宙線生成核種を用いた地表面の編年・削剥速度決定
  • 松四 雄騎, 松崎 浩之
    原稿種別: その他
    2014 年 35 巻 2 号 p. 113-114
    発行日: 2014/04/25
    公開日: 2024/12/25
    ジャーナル フリー
  • 松崎 浩之
    原稿種別: 研究論文
    2014 年 35 巻 2 号 p. 115-130
    発行日: 2014/04/25
    公開日: 2024/12/25
    ジャーナル フリー

    In situ CRNs (Cosmogenic RadioNuclides) play a decisive role in the long term quantifiation of the geomorphological transformation rate, especially the rate of erosion. Among many kinds of CRNs, 10Be and 26Al are most often used for this application. Both nuclides have long half lives (1.36×106 yr, 7.05×105 yr, respectively) and are extremely rarely abundant in the environment. To quantify these nuclides by mass spectrometry, isobar interferences (e.g. 10B against 10Be, 26Mg against 26Al) must be effectively eliminated. Among many methods, only AMS (Accelerator Mass Spectrometry) can achieve this. The fundamental principles for isobar supression in AMS are 1) selectivity in the negative ion generation and 2) isobar identification by the information of energy loss in media. The first characteristic derives from the basic AMS configuration, i.e., the combination of a negative ion source and a tandem type electrostatic accelerator. At the negative ion source, ion species of “negative electron affinity” cannot be ionized. For this reason, 26Mg ions are eliminated in the 26Al-AMS. The second feature can be realized by ion energies in the order of MeV/nucleon. In AMS, ions are accelerated to energies of several MeVs. In this energy range, the amount of energy loss in matter can be measured by a gas ionization chamber or a semiconductor detector. Isobars (like 10Be and 10B) can be identified by the difference of their energy losses. The overall efficiency for 10Be detection in AMS is the order of 10-3. Considering typical production rates of 10Be in the quartz of the crust (3-30 atoms/g-quartz/yr) and a practical time scale of the application (> 1000 yr), ca. 100 g quartz is needed for the AMS measurement within several per cent analytical error. Similarly, in the case of 26Al, the overall detection efficiency (10-4 order), typical production rates (20-200 atoms/g-quartz/yr), and practical exposure age (> 1000 yr) also result in ca. 100 g quartz needed for the detection of 26Al within several per cent analytical error.

  • 渡壁 卓磨, 松四 雄騎, 小玉 芳敬, 進木 美穂, 松崎 浩之
    原稿種別: 研究論文
    2014 年 35 巻 2 号 p. 131-146
    発行日: 2014/04/25
    公開日: 2024/12/25
    ジャーナル フリー

    Reconstructing the incision history of bedrock rivers can provide evidence of formation processes in knickzones on longitudinal river profiles. In this study, we determined incision rates at the upstream end of a knickzone in the Oshika Gorge, Tottori prefecture, southwest Japan, by exposure dating of a series of granitic strath terraces using terrestrial cosmogenic 10Be. The measured 10Be concentrations increased with the height of the sampling surface above the present-day riverbed. Exposure ages were determined as 50.2 ± 3.9 kyr BP (Before Present) for the highest strath terrace, which was 11.0 m above the present riverbed, and 5.3 ± 0.7 kyr BP for the lowest at 1.6 m. Regression analysis of all data gave an average incision rate of 0.24 mm/yr, which is comparable to the uplift rate in this region. Results from the multiple sampling points suggest that the incision rate may have changed in a stepwise manner over the past 50 kyr BP, as follows: 0.05 mm/yr between 50.2 and 32.4 kyr BP, 0.53 mm/yr between 32.4 and 19.9 kyr BP, and 0.15 mm/yr between 19.9 kyr BP and the present. This pattern of an increasing and then decreasing incision rate is difficult to explain in the context of the retreating knickzone that is located just downstream from the sample site. The timing of the increase in incision rate corresponds to the beginning of decreasing precipitation and temperature toward the Last Glacial Maximum. Although the present study was unable to determine the reason for this accelerated incision, it possibly reflects the response of the channel to enhanced sediment supply from the surrounding hills during the glacial period.

  • 八反地 剛, 松四 雄騎, 北村 裕規, 小口 千明, 八戸 昭一, 松崎 浩之
    原稿種別: 研究論文
    2014 年 35 巻 2 号 p. 147-164
    発行日: 2014/04/25
    公開日: 2024/12/25
    ジャーナル フリー

    We have used the terrestrial cosmogenic nuclides and geochemical mass balance method to estimate the long-term chemical weathering rate at nine small catchments of the Ashima River basin in the Northern Japanese Alps, central Japan. The basin consists of steep slopes (25-40°) and has a high relief of about 1500 m (800-2300 m a.s.l.). Climate varies from humid temperate at the 800-m bottom of the basin with a mean annual temperature of 9.5°C and an annual rainfall of 1000-1500 mm to a cold environment with a mean annual temperature of 0.2°C at the 2300-m divide. In situ 26Al of stream sediment in these catchments showed that denudation rate ranges from 200 to 2000 mm ky-1, which varies with the mean slope of a catchment. We calculated an index of the chemical depletion fraction (CDF), which is defined as the ratio of the chemical weathering rate to denudation rate and using a concentration of an immobile element, titanium, in bedrock and stream sediment with 0.25-2.0 mm grains. Analysis of stream sediment for each catchment showed that CDF ranges from 0.08 to 0.6 and chemical weathering rates range from 100 to 400 mm ky-1. The fraction decreases with an increasing denudation rate and altitude, which implies that both climate and erosion are contributing factors. In the two upstream catchments at 1800 m a.s.l., the catchment with a higher denudation rate of 2000 mm ky-1 has a lower CDF (0.08), which indicates the effect of erosion on this weathering. While the rate increases with increasing denudation rate of 100-1000 mm ky-1, it tends to decline at an extremely high denudation rate (> 1000 mm ky-1). This turndown in chemical weathering rate may reflect a transition from supply-limited to kinetically-limited chemical weathering of hillslope materials in mountainous basins.

  • 松四 雄騎, 松崎 浩之, 牧野 久識
    原稿種別: 研究論文
    2014 年 35 巻 2 号 p. 165-185
    発行日: 2014/04/25
    公開日: 2024/12/25
    ジャーナル フリー

    This study aims to test models of landform evolution by analyzing the relationship between topography and the long-term denudation rate of mountainous watersheds. We measured terrestrial cosmogenic 10Be in quartz grains in fluvial sediment sampled from the outlet of 31 watersheds underlain by granite or granodiorite in Japan, to determine the millennial-scale average rate of denudation over the catchment areas. Topographic analysis of the watersheds was conducted using a digital elevation model with a grid size of 10 m. Denudation rates show a strong nonlinear increase with increasing mean slope angle of the watersheds, indicating a transition in the dominant erosion process at a critical slope gradient that yields an infinite sediment flux by persistent landsliding on hillslopes. This result suggests that the sediment yield from a watershed cannot be explained by typical models of landform evolution employing a linear formulation in relationship between soil transport and the hillslope gradient, however, it seems concordant with a nonlinear model simulating a process transition from soil creep to active landsliding, thereby forming threshold hillslopes. The datasets presented in this study also reveal the existence of steep but slowly eroding watersheds beyond the threshold slope condition, indicating the importance of interactions between erosion and weathering in controlling the denudation of mountainous watersheds.

  • 城谷 和代
    原稿種別: 研究論文
    2014 年 35 巻 2 号 p. 187-197
    発行日: 2014/04/25
    公開日: 2024/12/25
    ジャーナル フリー

    Knowledge of long-term geomorphological evolution is needed in order to assess the long-term (longer than 105 years) stability of the geological disposal system of high-level radioactive waste after closure of disposal site. Japanese islands are located in an orogenic zone under humid climate, with active uplifting and erosion, frequent earthquakes and volcanic activities. Thus, quantitative understanding of long-term erosion rates is one of the most important pieces of information to scientifically evaluate the stabilities of disposal site and neighboring region. It is needed to quantitatively understand landform evolution and its mechanism from the past to the present or predict future evolution of landform, which are controlled by complicated factors such as tectonics, climate, weathering, erosion and sedimentation. However, there is little information on erosion rates in many areas in Japan. Terrestrial Cosmogenic Nuclides (TCNs; eg., 10Be, 26Al, 36Cl) are powerful in dating of the samples from 105 to 106 years, which any other methods have not been suitable for age determination, and can measure erosion rates directly. This article proposes applications of TCN methods as a tool to evaluate long-term erosion rates for the longterm stability assessment of the geological disposal system.

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