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

[MIS22-P11] Porosity of mud volcano ejecta in the Western Nankai Trough and Northern Ryukyu Trench collected from research cruise KH-23-4

*Rina Fukuchi¹, YANG ZHAO², Akira Ijiri³, Takuya Itaki⁴, Takeru Yoshimoto³, Asuka Yamaguchi², Masafumi Murayama⁵, Hironori Otsuka³, Tomohiro Toki⁶, Tatsuhiko Hoshino⁷, KH-23-4 shipboard scientists (1.Naruto university of Education, 2.Atmosphere and Ocean Research Institute, The University of Tokyo , 3.Kobe University, 4.National Institute of Advanced Industrial Science and Technology, 5.Kochi University, 6.University of the Ryukyus, 7.Japan Marine-Earth Science and Technology (JAMSTEC))

Keywords:Mud volcano, Porosity

Mud Volcanoes (MVs) are generally found at convergent plate margins and are one of the important systems that transport fluids and sediments from depths from several to a dozen kilometers below the earth’s surface. The distribution of MVs is uneven, with MVs not well distributed between off-Kumano and Hyuga-nada, whereas MVs are otherwise distributed along the Nankai Trough and Ryukyu Trench. In the case of MVs in the Kumano Basin, sediment is sampled from depths of 2.3-4.3 kilometers below the seafloor(kmbsf), while the original depth of sediment released from the MV off-Tanegashima is estimated to range between 3.2-3.4 kmbsf (Pape et al., 2014; Ijiri et al., 2023). However, there are no reported cases of mud volcanic ejecta in the Hyuga-nada, and only some MVs have been reported off-Tanegashima. In general, sediment porosity decreases with depth by compaction. Therefore, the porosity of the gravel in the erupted MVs deposits was measured, and the depth of origin of the MVs was estimated.
Samples were collected MV ejecta distributed from off-Kikaijima to the Hyuga-nada using piston cores, multiple cores, and okean grabs during the research cruise KH-23-4 by R/V Hakuhomaru in August 2023. The matrix of the samples composed of gray cohesive mud, interspersed with gravels. The gravels were mainly sedimentary rocks, with mudstones and fine-grained sandstones and pumice, and cemented melange-like rocks. The porosity of the gravels varied: sedimentary rocks had porosity ranging from 0.12 to 0.51, pumice showed porosity of 0.65, and melange had a porosity of 0.28, respectively. Due to nearby deep drilling examples such as off Kumano, when compared with IODP Site C0002, depths ranging from approximately 2-3 kmbsf correspond. This depth is consistent with the depth of origin of the sediments from off-Kumano and off-Tanegashima MVs previous results. Since the plate boundary in the Hyuga-nada area is about 10 and 18 kmbsf off-Tanegashima area, the sediments in both cases are anticipated to have originated from shallower areas than the plate boundary.