Chemical Weathering Rate of Granitic Mountain in Northern Japanese Alps: Estimation from Terrestrial Cosmogenic Nuclides and Geochemical Mass Balance Method

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  • HATTANJI Tsuyoshi
    Faculty of Life and Environmental Sciences, University of Tsukuba
  • MATSUSHI Yuki
    Disaster Prevention Research Institute, Kyoto University
  • KITAMURA Yuki
    Graduate School of Life and Environmental Sciences, University of Tsukuba
  • OGUCHI Chiaki T.
    Geosphere Research Institute, Saitama University
  • HACHINOHE Shoichi
    Research Institute, Center for Environmental Science in Saitama, Saitama Prefecture
  • MATSUZAKI Hiroyuki
    Micro Analysis Laboratory, Tandem Accelerator, Depar tment of Nuclear Engineering and Management, School of Engineering, The University of Tokyo

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  • 宇宙線生成核種と物質収支法を用いた花崗岩山地の化学的風化速度の推定－北アルプス芦間川流域の事例－
  • ウチュウセン セイセイカクシュ ト ブッシツ シュウシホウ オ モチイタ ハナオカ イワヤマチ ノ カガクテキ フウカ ソクド ノ スイテイ : キタアルプス アシ カンガワ リュウイキ ノ ジレイ

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<p>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.</p>

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