16:45 〜 17:00
[MIS26-10] Origin of gas hydrate at Hikurangi subduction margin, New Zealand
キーワード：国際深海科学掘削計画 第372・375次航海、ガスハイドレート、間隙水、ヨウ素129、ヒクランギ沈み込み帯、ニュージーランド
Marine organisms such as algae are enriched in iodine because iodine has a strongly biophilic character. Iodine and methane are released into interstitial water during the degradation of the organic materials and migrate together due to the similar diffusion coefficient in interstitial water. Therefore the high methane flux area, such as the gas hydrate field, is characterized by high iodine flux. Iodine has a radioactive isotope (129I) with a half-life of 15.7 million years, and 129I has been used for tracer of the origin of methane and interstitial water in the gas hydrate field. The Hikurangi subduction zone, off the east coast of the North Island of New Zealand, is formed by the subduction of the Pacific Plate beneath the Australian Plate. Pore-filling gas hydrates are formed in the accretionary complex and overlying landslide deposits. This study aims to understand the origin of gas hydrate in the Hikurangi subduction margin using 129I and other geochemical datasets.
International Ocean Discovery Program (IODP) Expeditions 372 and 375 conducted drilling off the Hikurangi subduction margin. We collected interstitial water samples from gas hydrate-bearing sediment (Site U1517, 1518, and 1519).
Gas hydrate saturation calculated by chloride concentration ranged from 2 to 68 %, and these values correlated to LWD resistivity. The depth of the methane-sulfate transition zones corresponded to 7 to 16 mbsf, and methane concentrations ranged from several hundred to 10,000 ppmv, indicating high methane flux in the entire area. The 129I/I ratios ranged from 573×10-15 to 144×10-15 at Site U1517, 220×10-15 to 155×10-15 at Site U1518, and 275×10-15 to 180×10-15 at Site U1519, indicating the ages of dissolved iodine as old as the age of backstop through the entire area/depth. Fluid enriched in methane and iodine from the backstop migrates and distributes from deep to shallow sediment homogeneously offshore Hikurangi. Some methane and water from these fluids form pore-filling gas hydrate within the sand layers.
International Ocean Discovery Program (IODP) Expeditions 372 and 375 conducted drilling off the Hikurangi subduction margin. We collected interstitial water samples from gas hydrate-bearing sediment (Site U1517, 1518, and 1519).
Gas hydrate saturation calculated by chloride concentration ranged from 2 to 68 %, and these values correlated to LWD resistivity. The depth of the methane-sulfate transition zones corresponded to 7 to 16 mbsf, and methane concentrations ranged from several hundred to 10,000 ppmv, indicating high methane flux in the entire area. The 129I/I ratios ranged from 573×10-15 to 144×10-15 at Site U1517, 220×10-15 to 155×10-15 at Site U1518, and 275×10-15 to 180×10-15 at Site U1519, indicating the ages of dissolved iodine as old as the age of backstop through the entire area/depth. Fluid enriched in methane and iodine from the backstop migrates and distributes from deep to shallow sediment homogeneously offshore Hikurangi. Some methane and water from these fluids form pore-filling gas hydrate within the sand layers.