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

[MIS22-P10] Distribution of Mud volcanoes off Kikai-jima to Hyuga-nada – Preliminary result from geophysical survey during KH-23-4 cruise -

*Hironori Otsuka¹, Akira Ijiri², RYO NAKANISHI³, Rina Fukuchi⁴, Tatsuhiko Hoshino⁵, Tomohiro Toki⁶, Takuya Itaki⁷, Kazuya Kitada⁵, Miho Asada⁷, Mako Nakao⁹, Yu Suzuki⁹, Yashunari Hatano⁸, Makoto Matsushita⁸, Kanji Tatsumi², Takeru Yoshimoto², KH-23-4 shipboard scientists (1.KOBEC, Kobe University, 2.Kobe University, 3.Kyoto University, 4.Naruto university of Education, 5.JAMSTEC, 6.University of the Ryukyus, 7.AIST, 8.Kochi University, 9.MOLMEC)

Keywords:Mud Volcano, Ryukyu Trench, Subbottom profiler, bathymetry

Mud volcanoes represent topographic features formed by fluid and sediment from sub-surface due to They are mostly observed in continental margins worldwide. The detailed mechanisms and the origins of the erupted materials and fluids in individual regions still need to be fully understood. Mud volcanoes are distributed along the Nankai Trough and the northern part of the Ryukyu Trench. A high density of mud volcanoes has recently been reported from the Hyuga-nada to off-Tanegashima island. Nevertheless, detailed investigations still need to be included in the south of the off-Tangeashima regions.
During the research cruise KH-23-4 aboard the R/V Hakuhomaru in August 2023, we conducted a comprehensive acoustic mapping with multibeam echo sounder and sub-bottom profiling over the areas from off Kikai-jima Island to the Hyuga-nada. As a result, backscatter intensity distribution indicated that approximately 30 anomalies were identified in addition to previously reported mud volcanoes over the surveyed area. Many of these anomalies were found nearby topographic elevations, suggesting that the anomalies consisted of mudflows or ejecta from the highs based on their distribution patterns. Acquired sub-bottom profiles of topographic highs that are considered candidates of mud volcanoes in the survey area typically exhibit transparent bodies and weak reflection at the seafloor. These features are consistent with those of other mud volcanoes; surface deposits are thought to have erupted from deeper formations based on sediment and water samples. On the other hand, strong reflection was observed on the topographic highs in the shallow (< 200 m) area near the Kikai-jima island. From the video obtained by the deep-sea camera attached to the Multiple Corer system, it was observed that large amounts of gravel cover the summits of these mounds. The distribution of mud volcanoes is not homogeneous within the study area, with some areas having a high density of mud volcanoes and others having little distribution. Consequently, our findings strongly imply a broader and dense distribution of mud volcanoes on the landward side of the Ryukyu Trench than previously considered. Subsequent detailed individual investigations of these candidate mud volcanoes are expected to provide insights into why numerous mud volcanoes develop in this region, their origins, and variations within the region.