現地性宇宙線生成同位体を用いた露出年代法による日本における新ドリアス期の氷河前進の認定(<特集>環境同位体の利用と地形学) Younger Dryas Glacial Advances in Japan Dated with in situ Produced Cosmogenic Radionuclides(<Featured Article>Environmental Isotopes and Geomorphology)

本研究では飛騨山脈の3つの圏谷(水晶D圏谷,黒部五郎渓谷,秩父平渓谷)において,カール底末端部に位置する端堆石堤を構成する岩屑を対象に,現地性宇宙線生成同位体のひとつであるベリリウム10を用いた露出年代法を適用し,1万1000年〜1万年前の数値年代を得た.これは晩氷期の新ドリアス期に相当し,本邦における同期の氷河前進に関する 最初の数値年代測定結果となった.岩屑の表面に蓄積された^<10>Be, ^<14>C, ^<27>Alなどの現地性宇宙線生成同位体の量は,岩石を構成する石英の宇宙線に対する被曝期間,すなわち「露出年代」によって説明される.本年代法は放射性炭素年代法と異なり有機物を含む堆積物の有無に依存しないため,氷河地形をはじめさまざまな堆積地形・浸食地形の形成年代測定に有効である.

The Glacial advance during the Younger Dryas (YD) event in the Hida Mountains of Japan was confirmed by the exposure dating method using beryllium 10 produced in the surface minerals by cosmic ray bombardment, hereafter called cosmongenic beryllium 10. The formation dates of terminal terminal moraines located on cirque bottoms of Mount Suisho, Mount Kurobegoro, and Mount Yumiore were about 11 ka. In situ cosmogenic radionuclides (in situ CRNs ; c.g., ^<10>Be, ^<14>C, and ^<27>Al) are produced by the bombardment of cosmic rays against quarts (SiO_2) on exposed rock surfaces. Therefore, the amount of accumulated in situ CRNs in the quartz is determined by the duration of the exposure period of the rock surface. This method can be applied to both depositional and erosional surfaces. Consequently, exposure dating is a suitable dating method for not only glacial landforms but also various other kinds of landforms.