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[MIS11-P13] Mechanism for maintaining wetland diversity in large-scale landslides considering development process of Naganuma in Hachimantai, northeastern Japan
Keywords:lacustrine origin wetland, geomorphic settings, drainage, large-scale landslide, quaternary volcano
In this study, we drilled cores from the Naganuma wetland, formed within a large-scale landslide located northwest of Mt Hachimantai to elucidate the development process of Naganuma and to consider the distinctions in the development process from Oyachi, a mire within the same landslide. Sediment analysis and radiocarbon dating indicated that Naganuma formed as a lake before 7000 calBP, and terrestrialization proceeded from the south. Around 2000 calBP, the wetland landscape became as it is today, with open water remaining in the northern area. The presence of emergent plant pollen throughout a wide depth range of the organic silt layer suggests that the wetland had been in shallow water for an extended period. We attribute the prolonged retention of water bodies in Naganuma to the presence of spring waters and the stability of the surrounding topography. Naganuma is situated on the upper part of the landslide body, which is relatively stable, and the dissection front has not yet reached it.
In contrast, Oyachi high moor, located on the edge of the landslide body 800 m southwest of Naganuma, formed as a lake in a landslide depression before 8600 calBP. During the transition from wetland to forest, the lake formed due to river damming approximately 5500 years ago. Subsequently, the lake drained to form a mire of about 3300 calBP.
Wetlands formed within large-scale landslides develop at varying rates and through diverse processes depending on surrounding geomorphic conditions. In the Komonomori landslide, after the formation of wetlands caused by landslide activity, wetlands like Naganuma, surrounded by stable topography, can persist as water bodies for extended periods. Conversely, in cases such as Oyachi, wetlands formed in unstable topography, susceptible to local topographical changes, including river damming due to collapse and incision, experience rapid progression in wetland development and sometimes reversal of the developmental process, such as a return from mires. Therefore, large-scale landslide sites are valuable for fostering diverse wetland environments, accommodating wetlands in various stages of development.
In contrast, Oyachi high moor, located on the edge of the landslide body 800 m southwest of Naganuma, formed as a lake in a landslide depression before 8600 calBP. During the transition from wetland to forest, the lake formed due to river damming approximately 5500 years ago. Subsequently, the lake drained to form a mire of about 3300 calBP.
Wetlands formed within large-scale landslides develop at varying rates and through diverse processes depending on surrounding geomorphic conditions. In the Komonomori landslide, after the formation of wetlands caused by landslide activity, wetlands like Naganuma, surrounded by stable topography, can persist as water bodies for extended periods. Conversely, in cases such as Oyachi, wetlands formed in unstable topography, susceptible to local topographical changes, including river damming due to collapse and incision, experience rapid progression in wetland development and sometimes reversal of the developmental process, such as a return from mires. Therefore, large-scale landslide sites are valuable for fostering diverse wetland environments, accommodating wetlands in various stages of development.