Diffuse emission of CO2 from Miyakejima volcano, Japan

Two soil gas surveys were carried out in May and September 1998 at Miyakeji ma volcano, in the Izu Mariana are, Japan. CO2 flux values for May and Sept ember surveys ranged from 0.1 to 18,150 g(-2) m(-1) day and from 0.1 to 968 5 g m(-2) day(-1), respectively. Statistical graphical analysis showed thre e overlapping populations. The spatial distribution of these emissions corr elated quite closely with the geothermal and geological characteristics of the studied area. The structure releasing higher CO2 is thr summit cone Oya ma 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 CO2 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 (delta C-13-CO2 = -0.90 parts per thousand to -5 .70 parts per thousand) suggests a mixing of carbon derived from marine lim estone and magmatic CO2 while a clear biogenic origin (delta C-13-CO2 = - 1 4.76 parts per thousand to - 25.52 parts per thousand) is observed for the diffuse degassing of CO2 outside summit caldera. (C) 2001 Elsevier Science B.V. All rights reserved.