The purpose of this investigation is to make clear the informations given by the isotopic composition of the carbon in the peat sample taken from the late Quaternary deposits. The radiocarbon concentration in the peat or the peaty soil may give the time of sedimentation when the carbon in this layer is free from the contamination of young organic materials. Many of the samples of peat or soil taken from the deposits just above the nonpermeable layer contain appreciable amount of organic materials transported from the upper layer after the sedimentation of the deposit. The contamination of the organic materials in the sample after the deposition yields a younger 14C date than the date of sedimentation of the sample. To find a criterion of the presence or absence of contamination of younger carbon in the peat sample, we examined the radiocarbon concentration in the three different organic fractions taken from the same sample. Table 1 shows the procedure of the extraction of these fractions. The results of 14C measurements for these fractions prepared from a series of peat samples which have the ages apparently older than 30, 000y. B.P. are shown in Fig. 1. The 14C dates of the three fractions taken from many kinds of peat samples are shown in Fig. 3. The sample which gave almost same 14C ages for these fractions may be regarded as free from the contamination of younger organic materials. When all three 14C dates for the three fractions taken from a sample of peat or soil do not agree with each other, most of these dates obey the following rules. 1) The date given by HA1 roughly agrees with that of HM. 2) The difference between the dates of HA1 and HA2 is appreciable when the peat or the soil sample is underlain by a nonpermeable layer. In many cases, a peaty soil sample gave a younger age of HA2 than that of HA1, suggesting that the secondary absorbed younger organic materials are bonded with the clay minerals surface stronger than originally existed organic matter. This may be interpreted by a plausible hypothesis that the organic materials transported from the upper layer absorbed on mineral surfaces which have been created continuously by weathering after the deposition of soil. A difference between the ages given by HA1 and HA2 is useful for the estimation of the amount of younger carbon contaminants in the peaty soil sample. The δ13C values of the carbon in the late Quaternary deposit suggest the vegetation at the time of deposition. Fig. 5 shows an example of change of δ13C values with depth in the deposits at the southeast foot of Mt. Fuji. The site is now a bush but the high δ13C values in about 1000y. B.P. and 4000y. B.P. indicate the dominant flora at these periods in plant of HS cycle, perhaps weeds such as Japanese pampas grass. The low values of δ13C in about 3000y. B.P. shows the growth of tree at that period. These inferences accord with generally accepted climatic changes in these periods.
Small flecks of charcoal are often found in the soil of Preceramic period sites, and their presence has been noted in numerous site reports since the first Preceramic excavation at Iwajuku in 1949. Dot maps of the distribution of these charcoal flecks were first made during excavations at the Nakazanya site in Tokyo in 1974, and later at the nearby Maehara, Takaido Higashi and Suzuki sites. The entire surface of one Preceramic layer was dot-mapped at Nakazanya; a number of clusters of charcoal flecks were apparent in the finished drawings. More refined data, including a cross-sectional plan of one of the clusters, were collected for several cultural layers across the whole 3, 000m2 excavated at Maehara in 1975. Similar work was done at Suzuki from 1974, but the most detailed study was made at the Takaido Higashi site in 1976. Some of the charcoal concentrates around heaps of burned gravel, while some is found with other kinds of artifacts. Many clusters of charcoal are found apart from any human habitation remains, and sometimes most of the clusters are in strata with only a very few artifacts. Several possible explanations can be offered for these patterns of distribution. The charred plant fragments might represent natural agents such as forest fires-all of the fragments identified so far have been wood. However, charcoal has been found only near human settlement sites: an excavation at the ICU campus in Tokyo in 1977, at a location some distance from the nearest known site, produced neither charcoal flecks nor artifacts. On the other hand, if this charcoal results from human behavior, there is not yet any evidence to suggest the kind of activity involved; the charcoal could reflect anything from a cooking fire to an accidental brush fire in the camp. The natural science approach to the study of large carbonized remains-floral typing and radiocarbon dating-has yielded much valuable information about prehistoric sites. An archaeological approach to the study of small carbonized remains-dot-mapping their distribution-might eventually advance the anthropological understanding of the human past. The study of micro charred remains warrants as much attention as any other single aspect of archaeological research.
A Holocene volcanic ash layer comprising abundant glass shards occurs as near-surface, soil-forming parent materials in south to north Kyushu and in Shikoku. This layer has been given several local names such as “Akahoya”, “Imogo”, “Onji”, etc. by farmers and pedologists. Its remarkable characteristics as a parent material of soil stimulated the interest of many pedologists to study its source, pedological features, distribution, etc. However, opinions on its source and proper identification varied considerably from one author to another. Detailed petrographic observation and accurate determinations of the refractive indices of the glass and several phenocryst phases in the tephra, together with extensive field work, have led to the conclusion that the Akahoya ash is the product of a single major eruption of the Kikai caldera. The ash is dacitic in composition and contains abundant bubble-walled glass shards and plagioclase, hypersthene, augite and opaque minerals as phenocrysts. The refractive index of the glass ranges from 1.505 to 1.514, and that of the hypersthene, from 1.705 to 1.714. The thickness contour of the ash layer and its grain-size distribution clearly indicate that this ash represents ejecta from the Kikai caldera, which is one of the largest calderas in Japan with an approximate diameter of 20km and largely submerged beneath the sea. The formation associated with this widespread tephra consists of three members; (1) a pumice-fall deposit as the earliest stage, (2) pyroclastic-flow deposits as the middle to the latest stages, and (3) an ash-fall deposit approximately contemporaneous with the pyroclastic flow. The 3rd member is assigned to the Akahoya ash and has the most extensive lobe with an axis length of over 1, 000km, covering most of southwest to central Japan and northwest Pacific Ocean. The volumes of the Akahoya ash-fall deposits must be greater than those of the pyroclastic flows. More than twenty-seven radiocarbon dates of the ash have been obtained so far, ranging rather widely from ca. 3, 000y.B.P. to ca. 9, 000y.B.P. However, the average value of the carbonated woods and peaty materials containing in the layer and the stratigraphical relationships with human remains give a probable age of the ash between 6, 000y.B.P. and 6, 500y.B.P. This marker-tephra is thus extremely significant for studies of Holocene climatic changes and sea levels, as well as for the correlation of archaeological sites.
The Nanmatsu Formation was newly described from the western part of the Tsugeno district, Nara Prefecture. It consists-of sands, silts and peaty muds with a characteristic volcanic ash layer. Abundant macro and micro-fossils of plants revealed were described. The radiocarbon dating of the formation using a piece of wood identified as Picea indicates the age of 28, 100±800 y. B. P. The plant fossil assemblage is characterized by an abundant occurrence of leaves, twigs, wood and pollen of such coniferous trees as Abies, Picea, Tsuga and Pinus including subgen. Haploxylon. Macro and micro fossils of deciduous broad-leaved trees are scarcely occurred. There is an evidence that the Nanmatsu Formation deposited during the Würm Glacial Age, and it appears to state that a coniferous forest mainly composed of Abies veitchii and/or A. homolepis, Picea polita, Tsuga diversifolia and Pinus subgen. Haploxylon pentaphylla might be occupied around the area of the Tsugeno district.