[M-IS18] Gas hydrates in environmental-resource sciences

[MIS18-P02] Deployment of long-term monitoring instruments at active cold seep sites in Joetsu Basin, Japan

*Shusaku Goto¹, Shinsuke Aoki¹, Naoki Saito¹, Masayuki Nagao¹ (1.National Institute of Advanced Industrial Science and Technology (AIST))

Keywords:Long-term monitoring, Seafloor environment, Cold seep activity, Shallow gas hydrate, Joetsu Basin

Joetsu Basin, located in the eastern margin of the Japan Sea, is one of the intensively studying areas of marine shallow gas hydrate around Japan. Joetsu Basin has two characteristic topographic features: one is an isolated knoll, named “Joetsu Knoll”, and the other is topographic high stick-out from continental shelf, called “Umitaka Spur.” On the summit areas of these topographic features, several mounds and pockmarks are discovered (Matsumoto et al. 2009; Saeki et al. 2009; Freire et al. 2011). Seismic profiles across Joetsu Knoll and Umitaka Spur reveal acoustically blanking zone, which suggest existences of upward fluid migration, occupying below mounds and pockmarks (Matsumoto et al. 2009; Saeki et al. 2009; Freire et al. 2011). Gas hydrates were discovered in mud-dominant sediments at relatively shallow depth below the seafloor of mounds and pockmarks by sediment core sampling and drilling (Matsumoto et al., 2009; Kakuwa et al., 2013a, 2013b). Visual observation using a remotely operated vehicle (ROV) revealed that carbonate rocks of various sizes were widely distributed on the seafloor of mounds of Joetsu Knoll and Umitaka Spur. Many bacterial mat areas were also observed during the ROV observations (Matsumoto et al., 2009; Machiyama et al., 2009). Furthermore, exposures of massive gas hydrate were observed on several mounds of Joetsu Knoll and Umitaka Spur during ROV observations.
To monitor in-situ seafloor environments and cold seep activities in cold seep sites in Joetsu Basin, we deployed various types of instruments in and around active cold seep sites discovered on Joetsu Knoll and Umitaka Spur in June 2022. The instruments we deployed are acoustic Doppler current profilers (ADCPs), CTD meters, turbidity meters, dissolved oxygen meters, sediment traps, osmotically pumped fluid samplers, seepage meters, bottom-water temperature meters, sediment temperature meters, and in-situ thermal property meter. For reference observation, we deployed ADCP, CTD meter, turbidity meters, dissolved oxygen meter, bottom-water temperature meter, and sediment temperature meter in an area without active cold seep in Umitaka Spur. These instruments will be recovered in the first of second quarter of 2023 JFY.
This study was conducted as part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).