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.
The old burial mound“Komatsu Kofun, ”which has been buried under the River Tone alluvial soil, was discovered at Hanyu City in Saitama Prefecture. The bottom of the chamber is situated 3 meters below the surface. It is considered that the mound was sunk by the Kanto Basin-forming movement, and has been buried by river sediments. In this study, the location of Kofun ground was defined by pedological analyses of soil just under the chamber. The following conclusions can be drawn by making a comparison between these data and the characteristics of nearby Kanto loam terraces. 1) The basement of the burial mound is Tachikawa loam formation (tephra layer), which was formed in the Late Pleistocene, and Kofun was constructed on Kanto loam terrace. The formation is tephra in situ, not banked. 2) As there is no soil horizon A (Kuroboku soil) at the bottom of the chamber, it is estimated that the chamber was set on Tachikawa loam formation, which acquires bearing capacity by stripping coarse structural Kuroboku soil. 3) Tachikawa loam formation of the Kofun ground is air-borne and air-laid tephra, and non-tephra particles are also included in it. This mixed-particle tephra is not so extensive as to change the pedological characteristics of the Tachikawa loam formation.
The fissure sediments at Site 5 of Yage Quarry are composed mainly of brown mud, and yield abundant mammalian remains. Bones of large mammals from the sediments are dated 18, 040±990y. B. P. by the 14C method. The assemblage comprises at least 15 taxa of mammals. Three of them (Anourosorex japonicus, Microtus cf. brandtioides, and Ursus cf. arctos) are not currently present in Honshu. However, the other taxa are referable to the elements of the lowlands and low mountains of the present Honshu. The geological age of the assemblage approximately corresponds to the coldest phase of the Würm glacial time. However, it lacks the elements of high mountains and colder areas which are found in the contemporary assemblages from F4 of Kumaishi-do Cave and Horizon N of Kannondo Cave Site. These facts seem to indicate that the climatic and vegetational conditions of those days were considerably different in Yage Quarry and the above two localities. Taking the data for other fossil localities into consideration, it is inferred that the above-mentioned three taxa vanished from Honshu between 18, 000 and 14, 000y. B. P.
Three pollen diagrams of Komado-Ohyachi Moor in Fukushima Prefecture were compared. Data for the same period (from the climatic optimum in the Holocene to the present) were available from the three sites, which indicated that Fagus crenata forest was predominant over the whole Komado area, and at the same time one site (K12) was dominated by a Quercus mongolica var. grosseserrata stand and the other (JIMBO and SHIODA, 1980) was dominated by a Pinus parviflora var. pentaphylla stand. The former was referred to as“regional”vegetation and the latter two as“local”vegetations. Pollen analysis showed that Site K12 began as a community dominated by Cyperaceae in the latest Pleistocene and changed into a Moliniopsis japonica-Sphagnum community, a Fraxinus mandshurica var. japonica forest, a Osmundastrumcinnamomeum var. fokiense community, a Ilex crenata scrub, a Moliniopsis japonica-Osmundastrum cinnamomeum var. fokiense community and a Moliniopsis japonica-Cyperaceae community, in that order.