Soil gas emission of volcanic CO₂ at Satsuma-Iwojima volcano, Japan

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 δ¹³C_CO2 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.