Abstract
Two soil gas surveys were carried out in May and September 1998 at Miyakejima volcano, in the Izu Mariana arc, Japan. CO 2 flux values for May and September surveys ranged from 0.1 to 18,150 g m −2 day −1 and from 0.1 to 9685 g m −2 day −1, respectively. Statistical graphical analysis showed three overlapping populations. The spatial distribution of these emissions correlated quite closely with the geothermal and geological characteristics of the studied area. The structure releasing higher CO 2 is the summit cone Oyama and surrounding areas, where the most obvious geothermal features occur. A total output about 100–150 t day −1 is estimated from this area. A good correlation was observed between soil CO 2 flux and soil temperature at the summit caldera indicating extensive condensation of fumarolic steam within the upper part of Miyakejima. Carbon isotopic analysis of selected samples inside the summit caldera ( δ 13C–CO 2=−0.90‰ to −5.70‰) suggests a mixing of carbon derived from marine limestone and magmatic CO 2 while a clear biogenic origin ( δ 13C–CO 2=−14.76‰ to −25.52‰) is observed for the diffuse degassing of CO 2 outside summit caldera.
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