Diffuse emission of CO2 from Showa-Shinzan, Hokkaido, Japan:: A sign of volcanic dome degassing

Two soil CO2 efflux surveys were carried out in September 1999 and June 2002 to study the spatial distribution of diffuse CO2 degassing and estimate the total CO2 output from Showa-Shinzan volcanic dome, Japan. Seventy-six and 81 measurements of CO2 efflux were performed in 1999 and 2002, respectively, covering most of Showa- Shinzan volcano. Soil CO2 efflux data showed a wide range of values up to 552 g m(-2) d(-1). Carbon isotope signatures of the soil CO2 ranged from -0.9 parts per thousand to -30.9 parts per thousand, suggesting a mixing between different carbon reservoirs. Most of the study area showed CO2 efflux background values during the 1999 and 2002 surveys ( B = 8.2 and 4.4 g m(-2) d(-1), respectively). The spatial distribution of CO2 efflux anomalies for both surveys showed a good correlation with the soil temperature, indicating a similar origin for the extensive soil degassing generated by condensation processes and fluids discharged by the fumarolic system of Showa-Shinzan. The total diffuse CO2 output of Showa- Shinzan was estimated to be about 14.0 - 15.6 t d(-1) of CO2 for an area of 0.53 km(2).