![]() The Ryukyu Arc, a typical trench-arc system, is formed by the Philippine Sea plate subducting northwestward beneath the Eurasian plate, with variable convergent rates in the range of 4 to 7 cm/yr 14. Their fluxes from the solid Earth to the atmosphere may provide useful information on the geochemical cycle and the evolution of the atmosphere and ocean 13. ![]() The ratios of the elemental abundance of carbon and helium (C/ 3He) together with carbon isotope ratios (δ 13C) may provide key information on the origin of the carbon 12. The stable carbon isotopic composition of methane can be used to understand the global carbon cycle and to characterize geological and microbial consumption processes 10, 11. The concentrations of methane in hydrothermal fluids are often 10 4–10 7 times higher than those in ambient ocean water 8, 9. Methane is one of the most common and abundant gas species in submarine hydrothermal fluids. Major emissions of methane into the atmosphere originate from the biosphere (e.g., wetlands, rice paddies and animals), the geosphere (e.g., hydrocarbon basins and geothermal areas) and anthropogenic activity (e.g., natural gas production and distribution and coal mining) 6, 7. In addition, methane plays an important role in the greenhouse effect in the atmosphere and in the chemistry of ozone reduction. Helium-3, in particular, is considered the most important volatile tracer of mantle-derived materials 4, 5. The isotope ratio 3He/ 4He shows a relationship with global geotectonic settings. Helium is a chemically inert noble gas, and this characteristic is an advantage in deciphering the gas source. These dramatic events highlight the necessity and importance of studying submarine volcanic activity in the form of time series. The Myojinsho volcano erupted in September 1952 and caused damage to a research vessel, which killed all 31 passengers 2, 3 in Japan. In 1944, during the eruption of the Kick ‘em Jenny volcano offshore of the northern coast of Grenada, a passenger vessel sank while crossing the degassing region, resulting in the deaths of 60 people 2. In addition, shallow volcanic eruptions pose a major threat to nearby ships and boats, as a zone of lower water density causes a loss of buoyancy and increases the risk of sinking. Therefore, submarine hydrothermal plumes represent a means to observe and understand regional tectonic settings and fluid circulation. Shallow hydrothermal fluids emitted from seafloor vents and/or volcanoes generally form plumes in which this information is preserved. Hydrothermal activity provides geochemical information from the interior of the Earth (e.g., the mantle) at its surface (e.g., the atmosphere or the ocean) 1. ![]() This suggests that the Tokara Islands submarine volcanoes are a key feature of the transition zone between the volcanic front and the spreading back-arc basin. ![]() The helium isotopes suggest the presence of subduction-type mantle helium at the Wakamiko Crater, while a larger crustal component is found close to the Tokara Islands. Methane at the Wakamiko Crater is of abiotic origin but affected by isotopic fractionation through rapid microbial oxidation. The methane in the region of the Tokara Islands is a mix between abiotic methane similar to that found in the East Pacific Rise and thermogenic one. The 3He flux and methane flux in the investigated area are estimated to be (0.99–2.6) × 10 4 atoms/cm 2/sec and 6–60 t/yr, respectively. These surveys were performed during the research cruise KS-14-10 of the R/V Shinsei Maru in a region stretching from the Wakamiko Crater to the Tokara Islands. Here, we report for the first time the volatile geochemistry of shallow hydrothermal plumes, which were sampled using a CTD-RMS system after analyzing water column images collected by multi-beam echo sounder surveys. Shallow submarine volcanoes have been newly discovered near the Tokara Islands, which are situated at the volcanic front of the northern Ryukyu Arc in southern Japan. ![]()
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