[M-IS22] Interactions of Geosphere-Hydrosphere-Biosphere and Deep-sea Methane Environments

[MIS22-P03] Measurement of dissolved methane concentrations in the hydrate area off Jyoetsu by methane sensors

*Hideyoshi Yoshioka¹, Shinsuke Aoki², Miho Asada¹, Mikio Satoh¹ (1.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 2.Kagawa University)

Keywords:methane, methane hydrate, methane sensor, ROV

Concentration of dissolved methane in seawater in methane hydrate-bearing areas is important to evaluate environmental effect on the surface area and elucidate the formation process of methane hydrate, because it would be related to the methane flux from the subseafloor and the distribution of methane hydrate in the subsurface. However, distribution of dissolved methane concentration near the seafloor in methane hydrate areas is not known in detail. In this study, we measured dissolved methane concentrations near the seafloor in the methane hydrate-bearing area off Joetsu using methane sensors installed on the ROV Kaiyo 3000 (kaiyo Engineering Co., LTD.) during a high-resolution seafloor 3D image mapping survey (conducted in June 2022). We used three types of methane sensor: the METS sensor (Franatech GmbH), the Methane sensor type LMS (Franatech GmbH), and the Suboceanic MILS (A2 Photonic Sensors). Since the METS sensor uses dissolved oxygen concentration to calculate dissolved methane concentration, dissolved oxygen sensor ARO-USB (JFE Advantech Co., LTD.) was also used. The ROV surveyed the seafloor surface along the pre-planned lines, maintaining at an altitude of about 4-5 m above the seafloor and a speed of about 0.2-0.5 knots. Niskin water bottles were installed on the ROV to collect seawater samples during the survey to compare methane concentrations with those by the methane sensors. After the survey, seawater was collected from the water bottle into glass vials, and benzalkonium chloride solution was added for sterilization and kept refrigerated for measurement of dissolved methane concentrations.
Methane concentrations were detected in the range of 0 to 2 μmol/L, showing comm areas of high methane concentration by different types of the methane sensor. Peak values of methane concentration differed among the sensors, with the Suboceanic having the highest value. The other two sensors tended to show a gradual decrease in values after the peak, suggesting that the response time of the detectors was longer than that of the suboceanic MILS.
This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).