Chemical and isotopic compositions of submarine gas discharged from shallow coastal waters of Himeshima volcano

Free CO₂ gas of deep (mantle + subducting slab) origin has been discharging accompanied by cold spring water of Hyoshimizu Hot Spring located at the eastern coast of Himeshima island, which is a Pleistocene volcanic island located northeast of Kyushu, Japan. On the western side of this island, it can be also observed free gas upwelling off the coast of Nishiura village. Therefore, we sampled upwelling bubble gas for the first time and report chemical and isotopic compositions of the sample gas. The major chemical component of the bubble gas from the Nishiura submarine gas discharge (Nishiura SGD) is CO₂ as in the gas associated with the Hyoshimizu Hot Spring. The He-Ar-N₂ relative composition and the ³He/⁴He-⁴He/²⁰Ne relation of the Nishiura SGD gas are common with those of the free gas of Hyoshimizu Hot Spring and indicate that the Nishiura SGD gas will be of deep origin. However, the Nishiura SGD gas is rich in CH₄ while its concentration in the free gas in Hyoshimizu Hot Spring is negligibly small. The carbon isotope composition (δ¹³C) of CH₄ in the Nishiura SGD gas suggests that it should be of abiogenic origin. Since we found gases released on the eastern and western coasts of Himeshima island are both derived from deep, it is expected that such deep-originated gas seeps out to the atmosphere from ground inland of Himeshima island. (author)