青森県八戸市, 天狗岱のテフラ-土壌累積層の植物珪酸体群集に記録された氷期-間氷期サイクル The Glacial-Interglacial Cycle Recorded in Phytolith Assemblage of Tengutai Tephra-Soil Section near Hachinohe, Northeastern Japan
Phytolith analysis of soils interstratified with tephras in Tengutai section near Hachinohe, northeastern Japan, revealed a record of climate fluctuations since the Last Interglacial. The Holocene soils have phytolith assemblages in which Panicoid dominates over Festucoid, suggesting a warm climate period. In addition, they are characterized by dominant non-Bambusoideae phytoliths, suggesting the existence of a grassland composed of non-Bambusoideae taxa. Such a grassland was thought to be the result of human impact through deforestation. The Last Glacial soils have phytolith assemblages which are characterized by dominant Pooideae, suggesting a cold climate period. In soils between Hachinohe Pumice (HP) and Biscuit-1 Pumice (BP-1), corresponding to δ<sup>18</sup>O stage 2, coniferous phytoliths generally occurred with relatively high frequency, indicating that boreal coniferous forest spread throughout this region. Soils between Biscuit-1 Pumice and Kibidango Pumice (KbP), corresponding to δ<sup>18</sup>O stage 3 to 4, were characterized by relatively high frequency of Bambusoideae phytoliths. The Last Interglacial soils generally have phytolith assemblages characterized by dominant Bambusoideae phytoliths. In the lowest layers of the soils over Madara Pumice (MP), which correlates to δ<sup>18</sup>O stage 5e, Bambusoideae phytoliths from <i>Pleioblastus</i> were dominant. This assemblage indicates a warmer climate condition than the present because <i>Pleioblastus</i> is a dominant taxon in warmer climate zones than the study region. In the uppermost layers of the soils, which correlate to δ<sup>18</sup>O stage 5a, Bambusoideae phytoliths from <i>Sasa</i> were dominant, indicating a warm to cool climate condition similar to the present. The lower part of these soils, which are interbedded with Okoshi Pumice (OP), had a relatively higher frequency of a broadleaved deciduous tree phytolith such as elliptical multifaceted phytolith from <i>Magnolia</i>, suggesting the existence of a broadleaved deciduous forest. The soils interbedded with Aosuji Pumice (AP), which correlate to δ<sup>18</sup>O stage 5b, had assemblages characterized by relatively higher frequency of Pooideae phytoliths, suggesting a cold period in the Last Interglacial. The soils which correlate to δ<sup>18</sup>O stage 5d, probably with an assemblage characterized by a relatively higher frequency of Pooideae phytolith, are yet to be recognized.
第四紀研究 38(5), 353-364, 1999-10-01
Japan Association for Quaternary Research