HR: 1340h
AN: OS23A-1044    [Abstracts]
TI: Dissolved Methane in seawater and seafloor mud around the gas plumes, in off Naoetsu, eastern margin of Japan Sea
AU: * Ishizaki, O
EM: ishizakios@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, Hongo7-3-1, Bunkyo-ku, Tokyo, 113-0033, Japan
AU: Matsumoto, R
EM: ryo@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, Hongo7-3-1, Bunkyo-ku, Tokyo, 113-0033, Japan
AU: Hiruta, A
EM: hiruta@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, Hongo7-3-1, Bunkyo-ku, Tokyo, 113-0033, Japan
AU: Tomaru, H
EM: tomaru@mail.kitami-it.ac.jp
AF: Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido, 090-8507, Japan
AU: Igeta, Y
EM: igeta@s.kaiyodai.ac.jp
AF: Tokyo University of Marine Science and Technology, Konan 4-5-7, Minato-ku, Tokyo, 108- 8477, Japan
AU: Yoshida, N
AF: Tokyo Institute of Technology, 4259 Nagatsuka, Midori-ku, Yokohama, 226-8502, Japan
AU: Yamada, K
AF: Tokyo Institute of Technology, 4259 Nagatsuka, Midori-ku, Yokohama, 226-8502, Japan
AU: Inoue, K
AF: Tokyo Institute of Technology, 4259 Nagatsuka, Midori-ku, Yokohama, 226-8502, Japan
AB: The giant gas plumes were observed by acoustic echo sounder in off Naoetsu, eastern margin of Japan Sea. Methane hydrates have been collected by piston corers during UT04 cruise in 2004, and hydrate coated methane bubble during NT06-19 cruise in 2006 have been observed. In order to evaluate the impact of methane seepage on the ocean and atmosphere systems, the concentration and isotopic compositon of methane in the seawater columns and interstitial waters were measured. Methane concentrations of the seawater above methane plumes were extermely high as much as 100nmol/L.Furthermore, the concentration was reached to 15umol/L at just above the gas plume. The seawater are still undersaturated, but they are anomoulously high compared to background seawater (5nmol/L). Hence observations suggests that the plumes are responsible for high concentrations of methane in this area. High metahne concentration in seawater column were also obserbved in area far from the plume. The anomalies are explained as the result of lateral migration due to currents which were clearly detected by ADCP (Acoustic Doppler Current Plofiler). Vertical profiles of methane concentrations in seawater column showed characteristic features.Shallow high concentration anomalies wereobserved at 200m to 300m below sea level (mbsl). This depth is related to hydrostatic pressure boundary of methane hydrate stability as well as the boundary of the water mass called Japan Sea Proper Water (JSPW). We concluded that the methane hydrate particles floated up to the depths, and were dissociated and dissolved around this depths. Dissolved methane laterally migrated along the boundary of the water masses. Another higher concentration zones are at 100m to 200m above the seafloor regardless of the methane plume sites. Such high anomalies are thought to be caused by back-ground diffusion of methane emitted from the seafloor. The depth of SMI of the area indicate high methane fluxes. Carbon isotopic compositions of methane dissolved in sediment interstitial water showed regional features. Heavier values of delta carbon 13(-29 to -55permil PDB) were found around the gas plumes. Heaviest delta carbon 13 values were seen in the pockmarks and gas hydrate bearing samples. While those obtained far from the plumes were lighter (-70 to -90permil PDB). Methane of the plume sites are thought to be composed of thermogenic decomposition of organic matters. Methane in sediments are most depleted in C-13 at around the SMI depth, and tends to increases upward and downward. Fractionations observed above SMI were explained as the result of microbial methane oxidation, whereas the profile below the SMI is likely to reflect increasing amount of thermogenic methane and/or differential upward diffusion-advection of isotopically light methane.
DE: 3004 Gas and hydrate systems
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting