開析谷の遺跡とそれをとりまく古環境復元--関東平野中央部の川口市赤山陣屋跡遺跡における完新世の古環境 ("陸の古環境復元"特集号)

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概要

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• Paleoenvironmental reconstruction of the Holocene since 8, 000 years ago, based on detailed archaeological excavation and geological and paleobotanical studies, including fossil pollen assemblage, plant macrofossil assemblage, and fossil wood assemblage, has been performed on the sediments in the dissected valley at the Akayama Site (35°50′55″N, 139°45′10″E), about 15km north of Tokyo, in the southeastern part of the Ohmiya Upland, central Kanto Plain. From the results, 5 epoch-making events (E-1 to E-5) and 6 paleoenvironmental stages (I to VI, divided based on their events) were identified. An event is characterized by an erosion and a subsequent drastic change of the sedimentary environment in the valley.Both Stage I(ca 8, 000-6, 500y.B.P.) and Stage II (ca 6, 500-5, 300y.B.P.), corresponding to the Earliest and Early Jomon Ages in archaeology, are characterized by a landslide or collapse of the precipice along the marginal upland ridge, and the reclamation by them or their secondary sediments in a valley under a marine or brackish environment. The sea reached a level 3m above its present level in late Stage I. During both stages, a warm-temperate deciduous broad-leaved forest composed of mainly Quercus subgen. Lepidobalanus stood on the upland to scarp, and Polygonum thunbergii, Chrisoplenium, and Carex communities covered the ridge at the edge of the water, in the valley.Stage III(ca 5, 300-4, 000y.B.P.) corresponds to the Middle Jomon Age, and is distinguished by a standing of Alnus japonica forest in the valley and an invasion of Aphananthe and Celtis in the upland to scarp forest. The deposition of clayey soil containing many Castanea-Castanoposis and Mallotus plant fossils is evidence of the destruction of the forest and the cultivation or semicultivation of Castanea on the scarp to upland.Stage IV (ca 4, 000-2, 200y.B.P.) corresponding to the Late and Later Jomon Ages is defined as an invasion of Fraxinus mandshurica, Aesculus turbinata and Acer to the Alnus forest in the valley, and the beginning of a wide expansion of Cryptomeria, Chamaecyparis group and Quercus subgen. Cyclobalanopsis on the upland. These facts testify to a cooler and wetter climate than that in previous stages. During this stage, part of the bottom forest was destroyed by man, and then manufacturing of Aesculus products was begun.Stage V (ca 2, 200-500y.B.P.) and Stage VI (since 500y.B.P.), later than the Yayoi Age in terms of archaeology, were characterized by a strong impact of man on vegetation and by a swampy valley bottom in which Cyperaceae and Gramineae communities stand. Almost all of the scarp and bottom forests have disappeared mainly as a result of human activities.

著者

• 辻 誠一郎

  大阪市立大学 理学部

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