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In the present study, the carbon isotopic approach was used for identification of magmatic contribution in soil air CO_2. Here we represent the CO_2 concentration and carbon isotopic composition in soil air in order to estimate the effect from Unzen Volcano in the Shimabara Peninsula. We took soil air through a sampling tube pricked at 30cm depth below the land surface, and measured concentration of carbon dioxide by the GASTEC tube. For the isotopic analysis, soil air was sampled in a glass reservoir (100ml : for stable carbon isotopic measurement) or an aluminized plastic bag (3 litters : for radiocarbon measurement). CO_2 was collected from the sampled soil air under the vacuum at the laboratory, and its stable carbon isotopic composition was measured by the IR-MS. Some samples measured radiocarbon concentration by the accelerator mass spectrometry at the Nagoya University. The filed observations were carried out in September 2001. We had two traverses (500-600m) across the fault near the USDP-2 site and embouchure of the Waren River. We also took bubbles in the spring water at the Waren River and the smoke air of Heiseishin-zan volcano. We found that several points had the high CO_2 concentration in the two traverses of the USDP-2 and the Waren River. Some of them showed the relatively higher value of stable carbon isotopic ratio. This indicates that the magmatic CO_2 might be brought to the land surface in the area of Unzen Graven. Moreover, radiocarbon technique tells us the stable carbon isotopic ratio of magmatic CO_2 and contributions of magmatic, biogenic and atmospheric CO_2. The value of stable carbon isotopic ratio was estimated to be -5.4per-mil. The proportions of respective contributions were 75 : 0 : 25 for smoke air, 60 : 40 : 0 in soil air nearby the Waren River spring and 0 : 30 : 70 in soil air at 500m away from the Waren River.
名古屋大学加速器質量分析計業績報告書 13 105-109, 2002-03