In late Holocene swamp deposits near and around Lake Biwa, central Japan, a characteristic volcanic ash layer which lies above the Akahoya volcanic ash layer (K-Ah; ca. 6, 300yrs BP) was reported by several previous studies. In order to confirm their correlation, we collected three volcanic ash layers in Sonenuma, Kojorogaike, and Torihama Archaeological Site by coring, and analyzed them for the eight major elements and refractive indices of their volcanic glass shards. As a result of these analyses, these volcanic ash layers are safely correlatable. On the other hand, chemical composition and refractive index were measured in the volcanic glass shards of Kawagotaira pyroclastic flow deposit derived from Kawagotaira Volcano, Izu Peninsula, central Japan. On the basis of the similarities of the chemical and optical properties of their glass shards, Kawagotaira Volcano is identified as the source of the above described volcanic ash layers.
In the mid-latitude area of the northern hemisphere, westerlies prevail throughout the year. However, the Kawagotaira volcanic ash layer was found on the western side of the source volcano, indicating that an easterly wind must have prevailed at the time of the Kawagotaira eruption. Three types of pressure distribution result in a prevailing easterly wind: the Baiu frontal type in early summer, the Akisame frontal type in autumn, and the type associated with a huge typhoon. One of them was probably the pressure distribution condition at the time of the Kawagotaira eruption.

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The insoluble residues of Quaternary and Paleozoic limestones have been regarded as the most important parent material of the Dark Red soils developed on these limestones in the Nansei Syoto islands. Oxygen and carbon isotopic studies of the Red and Yellow Soil Group, Quaternary and Paleozoic limestones, Tertiary sandstone, and Paleozoic phyllite in the Nansei Syoto and piston cores in the Okinawa Trough, the Nansei Syoto Trench, and the Amami Plateau, which were located between 24°N and 31°N, were carried out. The oxygen isotopic ratios (δ¹⁸O) of quartz (1-10μm) isolated from 43 soils were +17.2±0.6‰. These ratios were similar to those of quartz from the 10 piston cores (+18.1±1.2‰) but slightly higher than those of quartz from loess and loess-derived soils in China, Japan, and Korea, which were located between 31°N and 45°N. The soils belonging to the Red and Yellow Soil Group in the Nansei Syoto were strongly influenced by the East China Sea sediments predominantly transported from the Yangtze River in China and dried during the marine regression in the glacial ages, in which quartz yielded approximately 1 to 2‰ higher δ¹⁸O ratios than those in loess and loess-derived soils and the Yellow Sea sediments dominantly transported from the Yellow River. Oxygen and carbon isotopic ratios of quartz, muscovite and calcite isolated from Quaternary limestones and sparites indicated that quartz and muscovite were not authigenic minerals, but eolian deposits transported on coral reefs. Based on these data, the authors conclude that most of the soils belonging to the Red and Yellow Soil Group and developed on limestones and other base rocks in the Nansei Syoto were not residual soils from these rocks, but eolian deposits transported long distances from the arid and semi-arid regions of interior China and East China Sea sediments dried during the marine regression in the glacial ages.

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A middle Pleistocene shallow marine sediment, the Toriyamahama Formation of southwest Kyushu, was investigated from the viewpoint of sedimentary features and molluscan associations. This formation, less than 36m thick, consists of unconsolidated gravel, sand, and silt, and fills both the topographic depressions in the Miyazaki Group and its surrounding submerged river channels. Upward fining structures in the sediments are recognized throughout the succession as well as within each single bed set. In addition, it is presumed that the lower part of the formation corresponds to estuary, the middle part to beach, and the upper part to tidal mud flat or bay-bottom. Therefore, the Toriyamahama Formation, as a whole, can be regraded as the sedimentary product of a transgression during the middle Pleistocene.
The formation contains 74 identified species of molluscan fossils belonging to 61 genera which are mostly extant. Among them, four ecological associations are differentiated on the basis of autochthonous occurrence: Corbicula, Crassostrea-Cerithidea, Protothaca, and Turritella-Dosinia associations. The Corbicula association is composed of Corbicula japonica monospecifically, and is found in sandy mud beds which are rich in plant remains. This association corresponds to a molluscan community of the fluvial environment, which is referable to an initial stage of transgression. The Crassostrea-Cerithidea association in the lower part of the mud beds of the formation is characterized by standing Crassostrea gigas colonies in a muddy habitat, accompanied by the muddy surface dweller Cerithidea djadjariensis and sessile benthic Anomia chinensis, Barbatia virescens, and Trapezium lilatum. It indicates that the nature of molluscan association shifted to that of an uppermost infralittoral or intertidal community. The Protothaca association of the basal part of the overlying mud bed is represented Protothaca schencki, Barnea manilensis, and Saxidomus purpuratus standing directly on a small semi-hard substrate layer of the basement. The Turritella-Dosinia association is found in a massive mud bed dominated by Turritella (Kurosioia) kurosio and Dosinis angulosa, but also containg Paphia undulata, Theora fragilis, and Ringicula doliaris. This association is a shallow bottom community which represents an inbayment habitat at a later stage of transgression.
For comparison, 49 species belonging to 38 genera of ostracods in muddy sediments are examined from the view of paleoecology. The Crassostrea-Cerithidea association is accompanied by two ostracod assemblages: one predominated by Sinocytheridea sp. and Spinileberis furuyaensis and the other by Spinileberis quadriaculeata. The former two species are usually found in intertidal mud flats, and the last is an inhabitant of the uppermost infralittoral or intertidal zone in the present embayment environment. In the Turritella-Dosinia association, ostracod assemblage is predominated by Neomonoceratina microleticurata of the upper infralittoral zone. Conclusively, the result of an ecological analysis for molluscan associations in the Toriyamahama Formation is not inconsistent with the result of analysis for ostracod assemblages.

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[in Japanese]

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