第四紀研究
Online ISSN : 1881-8129
Print ISSN : 0418-2642
ISSN-L : 0418-2642
泥炭層の14C年代および13C/12C比と植生の関係
木越 邦彦鈴木 信子白木 真理
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ジャーナル フリー

1978 年 17 巻 3 号 p. 117-124

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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.

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