Geochemical Journal, Vol. 44 (No. 6), pp. 507-518, 2010
Shinsuke Kawagucci,1 Kotaro Shirai,1 Tefang Faith Lan,1,2 Naoto Takahata,1 Urumu Tsunogai,3 Yuji Sano1 and Toshitaka Gamo1
1Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo 164-8639, Japan
2Department of Geosciences, National Taiwan University, P.O. Box 13-318, Taipei 106, Taiwan
3Faculty of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo 060-0810, Japan
(Received July 14, 2009; Accepted June 4, 2010)
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.
Key words: hydrothermal plume, gas geochemical characteristics, Okinawa Trough, H2/CH4 ratio, fluid-sediment interaction