[M-IS20] Interactions of Geosphere-Hydrosphere-Biosphere and Deep-sea Methane Environments

[MIS20-P01] New sediment samplers for sediment-intrinsic megabenthos sampling of surface-type methane hydrates development

*Hiroshi Ishida¹, Eriko Seo¹, Shinya Suzuki², Tohru Fujiki³, Yousuke Ohnishi⁴, Hiroki Kise⁵, Yusuke Miyajima⁵, Ayumi Tsukasaki ⁵, Masahiro Suzumura⁵, Atsushi Suzuki⁵ (1.The Marine Ecology Research Institute , 2.Marine Biological Research Institute of Japan Co., Ltd., 3.Marine Works Japan, Ltd., 4.KANSO TECHNOS Co., Ltd., 5.National Institute of Advanced Industrial Science and Technology)

Keywords:surface-type methane hydrate, ROV, sediment sampler, megabenthos, live deep-sea bivalve

An ocean environmental baseline survey of surface-type methane hydrate development was conducted as part of an environmental impact assessment. In the study area, sites of dead deep-sea bivalve accumulation were observed on the seafloor surface. Several shells thought to be Conchocele sp. (shell length of approximately 80 mm) were collected. Previous studies in the same area have reported shells of dead Conchocele sp., but there is no information on any living individuals. The presumed Conchocele sp. shell is not fossilized, and the periostracum covering suggests the presence of a living species. However, no live Conchocele sp. have been collected to date, possibly because Conchocele sp. dwell deep within the sediment, have low population density, and proper sediment sampling has not been performed. For the sampling of megabenthos and macrobenthos living in the sediment in this project, we used an Ekman barge-type grab sampler modified for operation by the manipulator of an ROV (hereafter, ROV-Ekman). However, with this sediment sampler, only one sample can be taken per operation, and a sample may not be collected if there is a sampler operation miss.
Therefore, in this study, we designed and fabricated several new sediment samplers that can be used for repeated sampling using the manipulator of an ROV and that can also be used to collect a large amount of sediment. The following four types of new sediment sampler were developed based on the concept of using an existing sediment sampler or utilizing other technologies such as fishing equipment:
(1) Using existing sediment samplers
a. ROV-Ekman, b. Smith-McIntyre grab sampler
(2) Utilizing other technologies such as fishing gear
a. Fishing gear-made metal cage (“Joren” in Japanese) for gathering shellfish in shallow water such as tidal flats, b. Application of existing benthic chamber technology.
When these sediment samplers were mounted on the ROV and trials were conducted, all samplers were able to perform sediment sampling repeatedly. The ROV-Ekman modified sediment sampler sampled more than 20 times during a single dive, and one live bivalve with a shell length exceeding 80 mm and several live bivalves with a shell length of approximately 10 mm were collected from the sediment samples.
This technique enables the collection of sediment samples in large amounts and quantitatively (as area) and is expected to advance understanding of benthic species and biomass living in sediment. This technique could be useful for environmental impact assessments for future deep-sea resource development.
This study was conducted as a part of the methane hydrate research project funded by the Ministry of Economy, Trade, and Industry of Japan.