11:00 AM - 1:00 PM
[MIS26-P09] Factors that change the concentration of dissolved methane in the waters off Tottori Prefecture, Sea of Japan
Keywords:Methane, Gas hydrate, Sea of Japan
Gas chimneys have developed on the western slopes of the Oki Trough off the coast of Tottori Prefecture, and large amounts of methane are continuously released from the seafloor into the ocean. Previous studies have revealed that the surface of methane released in the form of gas from the seafloor is hydrated and rises to shallow areas, but the dynamics and concentration fluctuations of methane itself in seawater have been clarified. The cause of this was not fully discussed. Therefore, in this study, we used the temporal and spatial fluctuations of methane concentration in seawater collected off the coast of Tottori Prefecture, the characteristics of seawater such as salt content, dissolved oxygen content, and water temperature, the characteristics of seafloor sediments, and the gas chimney under the seafloor. A multifaceted study was conducted based on environmental conditions such as the presence or absence of a structure, the formation and decomposition of gas hydrate, and the effects of ocean currents.
The methane concentration directly above the gas chimney was almost constant in the deep layer below 400 m, and it was clarified that the ejected methane was covered with hydrate and the reaction with seawater was hindered. At a water depth of 400 to 300 m, which is the upper limit of the gas hydrate stable region, the methane concentration increased due to the methane released from the decomposed hydrate. In the surface layer shallower than 300 m, the methane concentration tended to decrease in response to the increase in the dissolved oxygen content. It is thought that this is because methane in seawater was oxidized by bacteria on the surface layer and diffused by ocean currents. In the vicinity of the sea surface, the equilibrium with the atmosphere was reached, and the dissolved methane became the minimum.
The methane concentration directly above the gas chimney was almost constant in the deep layer below 400 m, and it was clarified that the ejected methane was covered with hydrate and the reaction with seawater was hindered. At a water depth of 400 to 300 m, which is the upper limit of the gas hydrate stable region, the methane concentration increased due to the methane released from the decomposed hydrate. In the surface layer shallower than 300 m, the methane concentration tended to decrease in response to the increase in the dissolved oxygen content. It is thought that this is because methane in seawater was oxidized by bacteria on the surface layer and diffused by ocean currents. In the vicinity of the sea surface, the equilibrium with the atmosphere was reached, and the dissolved methane became the minimum.