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雲仙火山周辺における土壌中の二酸化炭素に対する火山性流体の影響マッピング(第15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14(2002)年度)報告)

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Abstract

土壌ガスに含まれるCO_2に対する雲仙火山の影響が,どのくらいの範囲に及んでいるのか,マグマ起源CO_2の寄与はどの程度であるのかを,炭素同位体(δ^<14>C・ δ^<13>C)を用いて解析した.その結果,雲仙地溝の内部でマグマ起源CO_2の寄与が大きなものとなったが,その傾向は地溝の東側で顕著で,西側では特筆するほどの傾向は見られなかった.これとは別に地溝帯の北側で寄与が高いものがあった.また,同位体を用いて導き出した寄与率に土壌中でのCO_2濃度や土壌からのCO_2フラックスをかけ合わせた評価を行った.\\Unzen volcano is located on the western part of Beppu-Shimabara Graben in Kyushu Island, SW Japan. We are studying about the mapping of volcanic fluid influence for CO_2 in order to estimate flux of magmatic volatiles in Shimabara Peninsula. In the present study, the carbon isotopic approach was used for identification of magmatic contribution in soil air. A large contribution of magmatic CO_2 in a dissolved carbonate of spring water in this region has already been reported (Ohsawa et al., 2002). Use of the soil air has advantages that it is easy to measure flux from soil surface and collect sample. We have collected ca. 45 soil air samples in and around Unzen Volcano, and they are analyzed for CO_2 concentration, δ^<13>C and δ^<14>C using the portable gas detector, IR-MS and AMS. The CO_2 concentration inside Unzen Graben showed higher than that outside. This suggests that the magmatic contribution is high inside the graben. The flux of CO_2 from ground surface was high at costal area. The spatial variation of flux might cause by the difference of biogenic activity at each point. The biogenic impact might be stronger than the magmatic one for the CO_2 flux, since the almost costal region used for farm field and might contain much higher amount of organic materials. The magmatic CO_2 showed high contribution in/around Shimabara City located ca. 7km east from Unzen Volcano, which agrees well with the result from ground water study using carbon isotopes (Ohsawa et al. 2002). It is noteworthy that the high contribution, flux and concentration of CO_2 are appeared along the Akamatsudani fault zone located southeast of the volcano. Our observation across the Akamatsudani fault zone was also represented to be a remarkable peak of CO_2 concentration and δ^<13>C values. These suggest that magmatic fluid comes up along the fracture zone like as fault system of Unzen Graben.

第15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14年(2002年度)報告 Proceedings of the 15th Symposium on Researches Using the Tandetron AMS System at Nagoya University in 2002 日時:平成15 (2003)年1月30日(木)、31日(金) 会場:名古屋大学シンポジオン Date:January 30th and 31st,2003 Place:Nagoya University Symposion Hall

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