千葉県八千代市新川低地における４５００年前以降の古環境変遷

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  • Paleo-environmental Transitions in the Shinkawa Lowland in Yachiyo City, Chiba Prefecture, Central Japan, since ca. 4,500 yrs BP
  • チバケン ヤチヨシ シンカワ テイチ ニ オケル 4 500ネンマエ イコウ ノ コ カンキョウ ヘンセン

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Plaeo-environmental transitions since ca. 4, 500yrs BP were clarified through pollen and diatom analyses in and around a lowland along the Shinkawa River (Shinkawa Lowland) in Yachiyo City, Chiba Prefecture.<br>From pollen analysis, we recognized three pollen zones for regional vegetation changes of the hill area around the lowland. The vegetation of KaP-I pollen zone (ca. 4, 500 to 3, 400yrs BP) shows a deciduous broadleaved forest mainly of Quercus (Lepidobalanus). The KaP-II pollen zone (ca. 3, 400 to <1, 400yrs BP) is characterized by the increase of Cryptomeria, Abies and Tsuga in the deciduous broadleaved forest. The KaP-III zone (<1, 400yrs BP to the present) occurred with the establishment of the Pinus secondary forest.<br>Environmental transitions in the Shinkawa lowland are shown as follows.<br>KaD-ib diatom zone and KaP-IA pollen zone: At ca. 4, 500yrs BP, sedimentation took place under terrestrial environments. The vegetation was an open grassland composed of Compositae, Gramineae, and Cyperaceae plants.<br>KaD-ic diatom zone and KaP-IB pollen zone: These grassland turned to shallow swamp, and aquatic Cyperaceae and Gramineae plants lived around it.<br>KaD-iia diatom zone and KaP-IC pollen zone: Swamp forest of Fraxinus and Alnus expanded, probably due to the lowering of the groundwater level.<br>KaD-iib diatom zone and KaP-IIA, C pollen zone: Terrestrial conditions occurred again in this diatom zone. The vegetation was characterized by the expansion of a Ulmus-Zelkova forest.<br>KaD-iiia diatom zone and KaP-IID pollen zone: The occurrence of planktonic diatoms indicates swamp or lacustrine conditions. Emerging communities of Cyperaceae, Gramineae, and Typha expanded.<br>KaD-iiia, b diatom zone, and KaP-III pollen zone: The zone is characterized by the establishment of modern paddy fields in the low-land.<br>In the KaP-IIB pollen subzone (ca. 3, 000yrs BP), Pinus and Gramineae pollens occurred in high percentage abundances, accompanied by Fagopyrum, Sagittaria, Alisma, Monochoria, Xanthium, and Aneilema. Plant opals of Oryza sativa were also found in the subzone. These facts suggest the short-term expansion of the Pinus secondary forest and the existence of paddy rice fields. These vegetational changes may have been caused by cultural activities in and around the lowland during the late to latest Jomon Periods.

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