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由布火山北麓に分布する腐植質土壌層の炭素14年代学(第16回名古屋大学タンデトロン加速器質量分析計シンポジウム平成15(2003)年度報告)

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Abstract

Yufu volcano is a stratovolcano located in northeastern Kyushu, Japan. The last eruption is the effusion of Ikeshiro and Summit lavas, block-and ash-flows (Yf-N, Yf-S pfl) and related volcanic ash (Yf-1). To evaluate radiocarbon dates of soil samples based on a stratigraphic relation with tephra layers, we collected and dated two charcoal fragments and five soil samples from an outcrop at the northern foot of the volcano. The stratigraphy at this site is black soil (thickness=15 cm), Yf-N pfl (46 cm), black soil (19 cm), Kuju-Danbaru scoria (Kj-DS) (scattered), black soil (15 cm), Kikai-Akahoya tephra (K-Ah) (12 cm), and black soil (43 cm) in descending order. After AAA treatment, graphite targets were produced. We measured radiocarbon dates using the GIC accelerator mass spectrometer at Nagoya University, and they were corrected for carbon isotopic fractionation, and calibrated to calendar year. Obtained dates are almost consistent with stratigraphy. The calibrated dates of soils just below and above the K-Ah are about 7.3 cal ka BP and 5.7 cal ka BP, respectively. The time gap of 1600 years indicates the difference of closing time of each carbon cycle of layers; the former indicates the deposition age of the K-Ah, and the latter, the accumulation age of soil above the K-Ah. Our new data on soil sample just below the Kj-DS is 3780±80 BP. On the other hand, Kamata and Kobayashi (1997) reported radiocarbon dates of soils just below and above the Kj-DS at the southeast foot of Kuju volcano, to be 4300±270 BP and 3570±110 BP, respectively. This suggests that a system for carbon cycle did not close by the fallout of the Kj-DS. Two radiocarbon dates of charcoal fragments (2090±80 BP and 2010±100 BP) from the Yf-N pfl show a good agreement, a date of soil sample just below the Yf-N pfl (2560±110 BP) is older than them by 500 ^<14>C yr. The accumulation rate of black soil is estimates to be 6.8 cm/ky based on the surface and calibrated dates of three tephras. This slow accumulation rate suggests that the addition and renewal of soil organic matter were inadequate. Using this accumulation rate, the age of basal part of black soil at this site is estimated to be 13.5 cal ka BP. It is 1900 years older than its calibrated date (11.6 cal ka BP). This time lag can be interpreted as the difference of closing system.

第16回名古屋大学タンデトロン加速器質量分析計シンポジウム(平成15年(2003年度)報告 Proceedings of the 16th Symposium on Researches Using the Tandetron AMS System at Nagoya University in 2003 日時:平成16(2004)年1月22日(木)、23日(金) 会場:名古屋大学シンポジオン Date:January 22nd and 23rd,2004 Place:Nagoya University Symposion Hall

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