1989 年 28 巻 2 号 p. 61-77
The continuing history of paleoenvironments in the Holocene was investigated in a small valley, the Sakagawa lowland (Paleo-Nagareyama Bay), situated in Nagareyama City, north of Tokyo. In this area, the seawater of the Jomon transgression invaded the valley, which had been deeply dissected during the last glaciation. 14C dating, tephra identification, grain-component analysis, diatom analysis, and pollen analysis showed that the sediments recorded some important evidence of paleogeographic changes, especially in sea-level and vegetation.
As a result of the comparison in vertical changes among some fossil assemblages, the ages 6, 500, 5, 300, 4, 500, and 3, 500y.B.P. were recognized as common and important boundaries in Holocene environmental history. Moreover, such ages correspond to the turning points in the relative sea-level change curve which was drawn based on 65 14C dates obtained in the central Kanto Plain.
The Holocene environmental history of the area is as follows:
1) 7, 500-6, 500y.B.P.: Sea-level rose rapidly and a deep inner bay formed. On terraces surrounding the bay, the warm-temperate broad-leaved deciduous forest mainly composed of Quercus (Lepidobalanus) was dominant, along with lucidophyllous tress such as Castanopsis and Quercus (Cyclobalanopsis).
2) 6, 500-5, 300y.B.P.: Sea-level was stable at +3m, the tidal flat developed, and oyster reefs were formed over a wide area.
3) 5, 300-4, 500y.B.P.: About 5, 300y.B.P., sea-level was abruptly lowered to +2-+1m, and sand layers covered the reef. Those sandy materials were transported not only from the surrounding terraces, but also from the Paleo-Okutokyo bay, the major body of water in the area. The broad-leaved deciduous forest continued to be dominant on the terraces.
4) 4, 500-3, 500y.B.P.: The sea-level was stable again, and oyster reefs were formed. Marshes accompanied by swamp forests composed of Alnus covered the sea area. Lucidophyllous forest, mainly of Quercus (Cyclobalanopsis), because dominant.
5) 3, 500y.B.P.-Present: The sea area regressed gradually from 3, 500y.B.P. to 1, 800y.B.P. due to the lowering of the sea-level to -1--2m. During the early half of this stage, lucidophyllous forest was dominant on the terraces, and Alnus swamp forest developed in the valley. Alnus swamp forest, however, disappeared suddenly at 1, 800y.B.P., when the sea-level rose to 0m. After 1, 800y.B.P., Cryptomeria and aquatic plant pollen increased dramatically.