This study is concerned with the patterns of exploitation of aquatic resources of the prehistoric Jomon hunter-gatherers in Japan. The study has two objectives. First, it will examine the habits of the shell-midden people when collecting aquatic resources. Second, the analysis will be concerned with the identification of the aquatic species from the shell midden, which is the most fundamental data in the preparation of the shell-midden analysis. The material used for this study was collected from the Jomon shell midden at Kamitakatsu in Ibaraki Prefecture of the Kanto district. The midden was excavated several times, revealing the late Jomon deposit of ca. 4, 000 to 3, 000 B.P. The kinds and relative quantities of the aquatic species found at the midden deposit provided that the brackish-water environments were formed in the site territory during the Jomon period. The majority of the aquatic faunal remains were identified as species that the brackish-water environments of tidal to sublittoral zones in bays. In addition, some oceanic species were identified. Occurrence of these species, although the frequencies are very low, may give certain evidences that they might be caught incidentally on long trading journeys and so on, if the open sea lay beyond a reasonable walking or canoeing distance from the site.
Three fission-track ages of the volcanic ashes in the Plio-Pleistocene Tokai Group, were obtained by grain-by-grain method. 1) Ichinohara Volcanic Ash at Kono-minamidani..............2.8±0.2MY. 2) Ichinohara Volcanic Ash at the River Tashida................2.9±0.2MY. 3) Rokkoku Volcanic Ash at Rokkoku..........................1.4±0.2MY. These ages were compared with the paleomagnetic time scale and the result of NRM measurement on the Tokai Group in Fig. 3. All the data are concordant with each other. According to these results, it is sure that the grain-by-grain method of the fission-track age determination is the best technic in the case using the zircon crystals in the volcanic ash layer.
Natural remanent magnetization of the baked earth collected from the three dwelling pits (DW-2, 22, 34) found in the Ebigasaku shell-midden site was measured. For checking the reliability of the magnetization of the samples for archeomagnetic purpose, a thermomagnetic analysis was made on magnetic minerals of the baked earth. 1. Thermomagnetic curves of the specimens collected from the site DW-22 are observed to be irreversible, while they are reversible for samples from the site DW-34. 2. The country soil of the baked earth of the site DW-22 is considered to be weathered volcanic ash (“loam”), and is correlated to the upper Tachikawa Loam Formation on the basis of similarity of both thermomagnetic curves and heavy mineral assemblages. 3. The results of thermomagnetic and also microscopic analysis suggest that the ferromagnetic minerals of the samples of the site DW-34 are high-temperature oxidized titanomagnetites. The high-temperature oxidation of titanomagnetite may have been resulted from the common use of this hearth in those days. Meanwhile, in case of the samples from the site DW-22, ferromagnetic minerals are non-oxidized titanomagnetites. 4. The presence of the oxidized titanomagnetites in such samples as from the site DW-34 may be employed in checking the reliability of samples for archeomagnetic purpose.
Okierabu-jima of the Ryukyu Islands, which is a poly-terraced Pleistocene raised coral reef island, doesn't have a Holocene raised coral reef, but coastal erosional features showing higher sea levels in Holocene. The authors obtained some data indicating the period of one of the Holocene higher sea levels. All radiocarbon dates concerning Okierabu-jima's Holocene sea-level changes are plotted on the date-height coordinates (Fig. 2). The paleo sea level between 5000 and 2000 y. B. P. lies above the broken line drawn from 6m below to 2.18m above the present sea level. The period of the highest sea level in Holocene seems to be about 3000 to 2000 y. B. P. in this island. Its height is presumably 2.4m a. s. l. derived on an average from heights of stacks and coastal benches in the almost all coasts of the island (Koba, 1974). Beach rocks were already formed at the landward extremity of the reef flat corresponding to the almost present sea level about 1300 y. B. P.