[M-IS21] Chemosynthetic community and mud volcano: biology, geochemistry, and geophysics of fluid discharge

[MIS21-01] Characteristic and Origin of erupted sediments in submarine mud volcanoes, off Tanegashima Island

*Ryoma Setoguchi¹, Akira Ijiri², Takeyasu Yamagata³, Hiroyuki Matsuzaki³, Kyoko Hagino¹, Juichiro Ashi⁴, Masafumi MURAYAMA¹ (1.Kochi University, 2.Japan Agency for Marine-Earth Science and Technology, 3.The University Museum, The University of Tokyo, 4.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:Mud Volcano, Submarine Mud Volcano, Methane Gas, Clay mineral

Mud volcanoes are remarkable geological features formed by the vertical intrusion of low density and deformable sediments from deep subsurface to the surface, and are distributed both on land and seafloor at the continental margins. The mud volcanoes are an important source of methane to the hydrosphere and the atmosphere. In the sea around Japan, it is widely distributed in the Kumano Basin off the Kii Peninsula and off Tanegashima Island. The mud volcanoes in the Kumano Basin are well studied compared to those off Tanegashima, and the mechanism of the eruption has been inferred from fluid and sediment analyses. So far, 15 mud volcanoes have been identified in topographic surveys off Tanegashima Island and numbered from MV#1 to #15. During the KH-15 and KH-19 cruises in 2015 and 2019 by R/V Hakuho-Maru, we retrieved piston cores from MV#1 (30^o53´N, 131^o46´E; water depth: 1540 m), #2 (30^o55´N, 131^o50´E; water depth: 1430 m) , #3 (31^o03´N, 131^o41´E; water depth: 1200 m) , and #14 (30^o11´N, 131^o23´E; water depth: 1700 m) . We investigated the origin of sediments and fluids based on the chemical analyses of them. For the sediment cores, the internal structure was observed by X-ray CT, and the sediment age was estimated by ¹⁰Be and calcareous nannofossils. Bulk mineral composition and clay mineral compositions were estimated by XRD analysis, and then the temperature history of sediments was estimated from the clay mineral composition. The origin of methane were investigated by the methane to ethane concentration ratio (C₁/C₂), and the carbon isotopic composition (δ¹³C) of methane. The sediments of MVs # 1, # 2, and #3 contain a large amount of mud breccia, and the modern hemi-pelagic sediments covered on the erupted sediments were not observed, suggesting that those mud volcanoes are active. On the other hand, nannofossils observations suggest that MV # 14 is dormant mud volcano in which the hemi-pelagic Quaternary sediments have covered on the erupted sediments. The nannofossil ages and the ¹⁰Be ages of MVs#2 and #3 consistently show the Middle to Late Miocene. These ages are younger than the Eocence Shimanto belt distributed around Tanegashima (Ujiie et al., 2000), suggesting that the mud-volcano sediments are provided from the sedimentary layers above the Shimanto belt. The mud-volcano sediments were generally dominated in quartz, illite, and plagioclase, and there is no particular mineralogical difference among the mud volcanoes. Clay minerals were mostly dominated in illite, suggesting the experience 60–160 ^oC where smectite to illite transformation occurs. There were some differences in clay mineral composition among the mud volcanoes. The illite/smectite mixed-layer (I/S) was detected in MV # 1, but not detected in MVs #2, #3, and #14. In MV #1, the illite content in the I/S mixed-layer, estimated from the diffraction pattern, is 42 to 45%. The content indicates the early stage of illitization, suggesting that the experienced temperature is 60 to 100 ^oC. On the other hand, the MVs#2, #3 and #14 sediments, in which the mixed-layer was not detected, might be experienced higher temperatures than MV#1, due to the progress of smectite to illite transformation. At the active MVs #1, # 2, and #3, generally low C₁/C₂ ratio (30–50) and δ¹³C of methane (-57‰ – -42‰) indicate that the hydrocarbon gases are derived from thermal decompositions of organic matter in deep sediments where the in situ temperature is >80^oC. At the inactive MV#14, the C₁/C₂ ratios were high as 700–4000, and δ¹³C of methane were -75‰ and -77‰, respectively. The data indicate the biogenic origin. Considering the geothermal gradient (25–50 ^oC/km) observed in the area, the conditions for thermogenic gas production (> 80 ^oC), and the temperature history of the sediments, the sediments and hydrocarbon gas would be provided from deep sedimentary realm deeper than 1.5 km below seafloor.