青森県上北平野で掘削された中期更新世後半以降のテフラ-土壌累積層の植物珪酸体群集に基づく環境変遷と段丘との対比

摘要

The phytolith assemblage of a tephra-soil succession after the latter half of the middle Pleistocene drilled at the Kamikita Plain, NE Japan, shows a glacial-interglacial cycle. The phytolith assemblage was quantified for tephra-soil successions in two boring cores, which were drilled at the northern and southern Kamikita Plain. Based on fluctuations of appearance ratios of the Bambusoideae phytolith assemblage, assemblage zones A-H were identified upward in the phytolith assemblage of both cores. Assemblage zones A, C, E, and H are dominant in phytoliths of Pleioblastus sect. Nipponocalamus and Nezasa, and not dominant in phytoliths of Sasa sect. Sasa and Crassinodi. These assemblage zones show relatively warm climate periods. Assemblage zones B, D, F, and G are dominant in phytoliths of sect. Sasa and Crassinodi, and not dominant in phytolith of sect. Nipponocalamus and Nezasa. These assemblage zones show relatively cool climate periods. As assemblage zones A-H were applied to marine oxygen isotope stages (MISs 1, 2...), using the Toya tephra 112-115 ka as a time-control horizon, they were correlated to MISs 1-9. Eight marine and fluvial terraces were tephrostra-tigraphically correlated to assemblage zones A-H in the Kamikita Plain. Based on the correlation between assemblage zones A-H and marine oxygen isotope stages, these terraces were correlated to the stages after MIS 9.%青森県上北平野で掘削された中期更新世後半以降のテフラ-土壌累積層の植物珪酸体群集は，rn氷期-間氷期サイクルに対応する．ボーリングコア2本のテフラ-土壌累積層から得られた植物珪rn酸体群集に対して，タケ亜科珪酸体群集の出現率の変動に注目して，下位より群集帯A～Hを設rn定した．群集帯A，C，E，Hは，メダケ属珪酸体が優勢もしくはササ属珪酸体が劣勢であり，相対rn的な温暖期を示す．群集帯B，D，F，G は，メダケ属珪酸体が劣勢もしくはササ属珪酸体が優勢でrnあり，相対的な寒冷期を示す．群集帯A～Hは，112～115kaの洞爺テフラの層準を基準にして海rn洋酸素同位体ステージ（MIS 1，2…...）に当てほめると，MIS 1～9の時代に相当する．上北平野rnの七百～三本木段丘は，テフラとの層位関係から群集帯A～Hと対応する．群集帯A～Hと海洋rn酸素同位体ステージとの対応に基づくと，MIS 9以降の段丘群と考えられる．