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Yoshitaka Hase, Goshoura Cretaceous Museum (Japan)
Yasuo Miyabuchi, Forestry and Forest Products Research Institute (Japan)
Utako Uchikoshiyama, Kumamoto Prefectural Office (Japan)
Naoko Sasaki, Research Institute for Humanity and Nature (Japan)
Akiko Iwauchi, Avance Co. (Japan)
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Aso Caldera is situated at the central part of Kyushu, Japan. The Kurokawa River runs through an alluvium plane in the northern part of the caldera known as the Aso Valley. Sediments found under the alluvium plane, consisting of silt and sand with ash were deposited into a lake that existed there prior to 8,200 years BP, as well as on the fluvial bottom plane and swamps in the caldera. The environmental change since the Last Glacial Age was clarified by pollen and diatom analyses of the samples from drilling cores taken in the Uchinomaki area at the northern part of the caldera. The six major pollen assemblage zones are distinguished in ascending order as Zone U-I: Pinus-Picea-Abies-Tsuga zone, Zone U-II: Pinus-Q. subgen. Lepidobalanus -Carpinus-Betula zone, Zone U-III: Q. subgen. Lepidobalanus-Fagus Carpinus -Betula zone, Zone U-IV: Q. subgen. Lepidobalanus-Fagus-Carpinus zone, Zone U-V: Ulmus /Zelkova-Celtis-Q. subgen. Cyclobalanopsis zone and Zone U-VI: Podocarpus-Q. subgen. Cyclobalanopsis-Castanea/Castanopsis zone. It is estimated that the mixed broad-leaved and /coniferous forest, which are currently distributed in subarctic and upper cool-temperate zones, covered the Aso Caldera region during the Last Glacial Age. As a result of progressively increasing temperatures after the Last Glacial Age, a cool-temperate, deciduous broad-leaved forest replaced the existing subalpine forest. Subsequently, vegetation successively changed into the warm-temperate broad-leaved deciduous forest and warm-temperate broad-leaved evergreen forest. The diatom assemblages of the sites A and B of on the Kurokawa River side are distinguished by details from the bottom to the top of the drilling cores (drill depth of the sites A and B were 85.3m and 25.3m, respectively). Site A (Hosen Bridge site); U-a: Aulacoseira zone, U-b: Aulacoseira, Cyclotella ktzingiana zone, U-c: Aulacoseira, Stephanodiscus astraea zone, U-d: Tabelleria fenestrate, Synedra ulna, Aulacoseira zone, U-e: Aulacoseira, Synedra ulna zone, U-f: Cyclotella zone, U-g: Cyclotela ktzingiana zone, U-h: Aulacoseira zone and U-i: Synedra ulna, Navicula, Cocconeis placentula, Achnanthes zone. Site B (Kario site); K-a: Cyclotella ktzingiana zone, K-b: Aulacoseira, Synedra ulna zone, K-c: Aulacoseira, Cyclotella sp. zone and K-d: Navicula spp., Cocconeis placentula, Achnanthes lanceolata zone. These diatom assemblage changes show that an ancient lake existed at the northern part of the caldera in the final interval of the Last Glacial Age. The lake reduced in area temporally at the start of the Post Glacial Age, after which it stabilized in condition and size until about 8,200 years BP. After that time, the area has been dominated by fluvial and swampy conditions.
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