Vegetation change in the main island of Okinawa, southern Japan from late Pliocene to early Pleistocene

摘要

Samples were taken from five outcrops that formed from late Pliocene to early Pleistocene on the main island of Okinawa. The five sites are located in Sonan, Yabu, Oyakawa, Akakimata and Goga. The age of Sonan and Yabu is ca. 1.0-0.8 Ma, that of Oyakawa is ca. 1.5 Ma, that of Akakimata is ca. 1.7 Ma and that of Goga predates ca. 1.9 Ma. Pollen analysis of these sediments revealed the following types occur together: the coniferous trees of Pinus subgen. Haploxylon and Cryptomeria, the deciduous broadleaved trees of Ulmus, Zelkova, Liquidamber and Lagerstroemia, and the evergreen broadleaved trees of Castanopsis and Symplocos. This pollen profile is similar to that of the last interglacial period in the Japanese mainland. The coniferous trees of Abies, Tsuga, Pinus subgen. Haploxylon, Keteleeria and Cryptomeria appear until the late Pliocene. Presently these taxa are not present on the Ryukyu Islands. Abies, Tsuga and Keteleeria pollen disappeared by the end of the Pliocene (1.78 Ma). The climate became cold rapidly and many Tertiary plants disappeared from late Pliocene to early Pleistocene in the Japanese main islands, as well as on the main island of Okinawa. At ca. 1.7 Ma, Lagerstroemia pollen suddenly increased. Tectonic activity was rapid and the other vegetation species could not adapt. Subsequently, ca. 1.5 Ma, Pinus subgen. Haploxylon, Ulmus, Zelkova and Cryptomeria pollen increased. This period is associated with regression and the climate was cool and wet. Pinus subgen. Diploxylon and evergreen broadleaved tree pollen grains increased, ca. 1.0-0.8 Ma, and the climate was warm. Subsequently, the climate became similar to the present climate in the main island of Okinawa. The formation of the modern evergreen broadleaved forest, dominated by Castanopsis sieboldii, dated from ca. 0.8 Ma.