HR: 0800h
AN: V41B-1392    [Abstracts]
TI: Methane Distribution In Plumes Of The South Mariana Back-arc Spreading Center
AU: * Toki, T
EM: toki@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AU: * Toki, T
EM: toki@ori.u-tokyo.ac.jp
AF: Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo, 060-0810 Japan
AU: Hirota, A
EM: hirota-akinari@aist.go.jp
AF: Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo, 060-0810 Japan
AU: Hirota, A
EM: hirota-akinari@aist.go.jp
AF: National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Tsunogai, U
EM: urumu@ep.sci.hokudai.ac.jp
AF: Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo, 060-0810 Japan
AU: Gamo, T
EM: gamo@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AU: Gamo, T
EM: gamo@ori.u-tokyo.ac.jp
AF: Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo, 060-0810 Japan
AU: Nakamura, K
EM: koichi.nakamura@aist.go.jp
AF: National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Noguchi, T
EM: k048570@sci.u-ryukyu.ac.jp
AF: Faculty of Science, University of the Ryukyus, 1, Chihara, Nishihara-cho, Chuto-gun, Okinawa, 903-0213 Japan
AU: Taira, N
EM: k038316@eve.u-ryukyu.ac.jp
AF: Faculty of Science, University of the Ryukyus, 1, Chihara, Nishihara-cho, Chuto-gun, Okinawa, 903-0213 Japan
AU: Oomori, T
EM: oomori@sci.u-ryukyu.ac.jp
AF: Faculty of Science, University of the Ryukyus, 1, Chihara, Nishihara-cho, Chuto-gun, Okinawa, 903-0213 Japan
AU: Ishibashi, J
EM: ishi@geo.kyushu-u.ac.jp
AF: Faculty of Science, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
AU: Utsumi, M
EM: utsumi@bsys.tsukuba.ac.jp
AF: Institute of Agricultural and Forest Engineering, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, 305-8572 Japan
AB: In the South Mariana Back-arc Spreading Center, two methane plumes were observed in water column based on analysis of methane in seawater samples collected during the R/V Thompson expeditions in 2003 around water depth of 2,700 m over the Fryer site on the ridge-axis seamount (12\deg57.22N, 143\deg37.16E, depth: 2,850 m). The estimated end-member isotopic compositions of methane in the two plumes are \delta$^{13}C$_{CH4}$ = -5\permil PDB and -50\permil PDB. These values indicated that the two plumes were originated from the different sources. During YK03-09 cruise using the submersible {\it Shinkai 6500} from October to November in 2003, detailed seafloor observation discovered sulfide chimneys emitting black and clear hydrothermal fluid on the off-axis seamount at Pika site (12\deg55.15N, 143\deg36.96E, depth: 2,773 m). The result of analysis of isotopic composition of methane in the hydrothermal fluids recovered from the off-axis hydrothermal vents using WHATS (Water and Hydrothermal {\it Atsuryoku} Tight Sampler) was averaged value of -4\permil PDB (standard deviation = 1\permil PDB, n = 3). Hydrothermal fluids from the Fryer site were also sampled and were measured: average value = -6.7\permil PDB, standard deviation = 0.3\permil PDB, n = 3. During the R/V Thompson expeditions in March 2004 using ROV ROPOS, 11 ROPOS dives and CTD-RMS plume surveys were conducted, and newly discovered a huge hydrothermal structure with active fluid venting at Achaean site on the ridge skirt (12\deg56.37N, 143\deg37.92E, depth: 2,990 m). The \delta$^{13}$C$_{CH4}$ value of the fluid sample from the site using ROCS (Rotary Clean Seawater sampler) was -14.7\permil PDB. Analysis of isotopic composition of methane in the plume samples collected using the CTD-hydrocast at water depth of 2,500 m over the Archaean site showed -45\permil PDB. Source of methane (\delta$^{13}$C$_{CH4}$ = -50\permil PDB), however, in the two plumes of the South Mariana Back-arc Spreading Center has been missing. The \delta$^{13}$C of methane cannot be considered in sediment-starved seafloor hydrothermal fluids as the results from an abiogenic reaction in magma. Alternative explanation would be the secondary stimulated plume of methane that is formed in invertebrate guts of zooplankton swarmed about microbes in the plume, as proposed about a subsurface CH$_{4}$ maximum in the upper oceanic water column. The secondary methane plume may be associated with methane plume without a corresponding enrichment in $^{3}He, observed in the Mariana Trough Back-arc basin at 14\deg N.
DE: 8424 Hydrothermal systems (8135)
DE: 4815 Ecosystems, structure and dynamics
DE: 4832 Hydrothermal systems
DE: 4803 Bacteria
DE: 3045 Seafloor morphology and bottom photography
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting