ABSTRACT

Hydrothermal plumes above the HAKUREI and JADE sites, two high-temperature hydrothermal vent sites in the Izena Cauldron at the mid-Okinawa Trough, were investigated in order to gain a preliminary understanding of gas geochemical characteristics at underlying hydrothermal vent sites. Three geochemical tracers, H₂, CH₄ and δ³He, covary with each other above the HAKUREI site but only CH₄ and δ³He are correlated above the JADE site. The carbon isotope ratio of methane within the Izena Cauldron can be accounted by a combination of the fluid dilution by ambient seawater and microbial consumption with the kinetic carbon isotope effect (KIE) of 1.007. An estimated endmember δ¹³C value of -32‰ in the HAKUREI fluid was obtained. Both the plumes above the HAKUREI and JADE sites showed C₁/C₂ ratios between 10³∼10⁴. Only the bottom water around the HAKUREI site showed significant N₂O excess with isotopically light δ¹⁵N and δ¹⁸O, suggesting N₂O input from microbial activity in the sediment. A linear correlation between H₂ and CH₄ in the HAKUREI plume gives a H₂/CH₄ ratio of the HAKUREI fluid of more than 0.022. The estimated H₂/CH₄ ratio in the HAKUREI fluid is significantly higher than that of the JADE fluid, comparable with those of fluids venting at other sediment-related hydrothermal systems, and also comparable with those of thermogenic gases produced by hydrothermal sediment experiments. These facts suggest that fluid-sediment interaction during fluid upwelling appears to modify gas geochemical characteristics at the HAKUREI site but have little influence at the JADE site. This study demonstrates the availability of the Izena Cauldron hydrothermal field and the HAKUREI and JADE sites as a natural laboratory for investigating the fluid-sediment interaction during fluid upwelling.