Observation SO₂ and CO₂ fluxes in and around the active crater of Aso Nakadake Volcano

Volatiles of the magma such as H₂O, CO₂, SO₂, H₂S and HC1, which are the main components of volcanic explosions, discharge to the surface of the earth as volcanic gases, hot springs, and river waters. The discharge occurs not only in volcanic eruptions but also in non-erupting activities. In order to clarify the mass balance of volatiles, the hydrothermal structure and the volcanic activities, it is very important to characterize the chemical constituents and the amount of the volatiles discharged from volcanoes. It is thought that the volcanic gases reflect immediately the changes of magmatic activity because the mobility of gases is the most abundant among the discharge forms of volatiles. The diffuse degassing from soil is one of the discharge forms paid attention in recent years. As CO₂ is hardly influenced by the groundwater due to low solubility in water, the direct information on volcanic activities can be obtained by observing the CO₂ flux and its changes. Aso volcano is one of the most active volcanoes in Japan, and magmatic eruptions have occurred several times in historical times. However, the research on the amount of the CO₂ flux from the soil in and around the active craters had not been performed at Aso volcano. In this study, we estimated the CO₂ fluxes from the Nakadake crater and the circumference soil at Aso volcano and compared the results with the CO₂ fluxes at Kusatsu-Shirane volcano. At Aso volcano, the amount of the total CO₂ flux from soil was estimated to be about 0.12 ton d^-1. While the amount of CO₂ flux from the Nakadake First crater was estimated to be 597 ± 95 ton d^-1, and thus the CO₂ flux from the crater was much larger than that from the soil at Aso volcano. On the contrary, the CO₂ flux from the soil was comparable to that from the craters (fumaroles) at Kusatsu-Shirane volcano.