Earth Planets Space, Vol. 54 (No. 3), pp. 239-247, 2002
Y. Shimoike1, K. Kazahaya2, and H. Shinohara2
1Research Institute of Materials and Resources, Akita University, 1-1 Tegata Gakuen-cho, Akita 010-8502, Japan
2Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba 305-8567, Japan
(Received January 15, 2001; Revised November 28, 2001; Accepted December 29, 2001)
Abstract: Soil gas surveys were carried out in November 1999 and October 2000 at Satsuma-Iwojma volcano, southwest Japan. The chemical composition of the soil gas was a mixture of CO2 and air components, with CO2 concentrations ranging from 0.03 to 59 vol%. The origin of the soil CO2 was evaluated on the basis of the variation of d13CCO2 and CO2 concentration. Although most of the CO2 is of biogenic origin, large volcanic contributions of greater than 50% were found close to the caldera rim, where soil temperature anomalies were observed. Emission rate of volcanic CO2 was estimated by assuming simple mixing among volcanic, atmospheric, and biogenic CO2. High rates of diffuse emission of volcanic CO2 were also observed at the caldera rim and close to some hot springs, indicating that part of the volcanic-gas discharge ascends through fissures along the caldera rim. The flux of the volcanic CO2 through the soil is estimated to be 20 t day-1; this is 25% of the total CO2 flux through soil and 20% of the CO2 flux from volcanic fumaroles in the main crater of Iwodake.