地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
花粉分析からみた琉球列島の植生変遷と古気候
黒田 登美雄小澤 智生
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1996 年 105 巻 3 号 p. 328-342

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Paleoclimatic and vegetational changes during the Pleistocene and Holocene in the Ryukyu Islands were inferred from pollen assemblages. Pollen analyses of some early Pleistocene to Holocene sections in the Ryukyu Islands show several drastic vegetational changes with climatically meaningful variations.
A mixed pollen assemblage containing the warm and cool temperate trees such as Cryptomeria, Abies and Tsuga, and the tropical-subtropical trees including Dacrydium, Liquidambar, , Lagerstroemia and Sapium occurred in the upper Pliocene to lower Pleistocene Shinzato Formation and the basal part of the lower Pleistocene Kunigami gravel formation. These pollen assemblages strongly support that there were high mountains over 1, 000 to 1, 500 m above the sea-level in the Ryukyu Islands in the late Pliocene to early Pleistocene. The warm and cool temperate forests in the mountainous areas were disappeared possibly by the subsidence of the basement rocks of the Islands in relation with the opening of the Okinawa Trough. Invasion of evergreen broad-leaved trees to the Ryukyu Islands in the early Pleistocene was indicated by the first occurrence of evergreen broad-leaved tree pollen in the lower Pleistocene Kunigami Gravel Formation.
The pollen assemblages in the last Glacial period (ca. 22, 000 y.B.P) are dominated by the coniferous trees such as Pinus and Podocarpus, indicating that an arid climate was prevalent in the Ryukyu Islands. This evidence is consistent with the wide development of ancient dunes and paleosoil of aeolian sand origin in the islands during the last Glacial period. The pollen sequence of the uppermost Pleistocene to Holocene reveals the change of vegetation and climate from the pine forests of the late Glacial period to the climax forests of evergreen broad-leaved trees of the Climatic Optimum. It is suggested that the climatic condition in subtropical Ryukyu Islands around 20, 000 years B.P. was not so much cool but arid than the climate of the present day.

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