7 research outputs found
Evaluating Earth degassing in subduction zones by measuring helium fluxes from the ocean floor
No full textExtremely High Methane Concentration in Bottom Water and Cored Sediments from Offshore Southwestern Taiwan
No full textEvaluating Earth degassing in subduction zones by measuring helium fluxes from the ocean floor
No full textOrigin of methane-rich natural gas at the West Pacific convergent plate boundary
Full text linkInternational audienceMethane emission from the geosphere is generally characterized by a radiocarbon-free signature and might preserve information on the deep carbon cycle on Earth. Here we report a clear relationship between the origin of methane-rich natural gases and the geodynamic setting of the West Pacific convergent plate boundary. Natural gases in the frontal arc basin (South Kanto gas fields, Northeast Japan) show a typical microbial signature with light carbon isotopes, high CH 4 /C 2 H 6 and CH 4 / 3 He ratios. In the Akita-Niigata region-which corresponds to the slope stretching from the volcanic-arc to the back-arc-a thermogenic signature characterize the gases, with prevalence of heavy carbon isotopes, low CH 4 /C 2 H 6 and CH 4 / 3 He ratios. Natural gases from mud volcanoes in South Taiwan at the collision zone show heavy carbon isotopes, middle CH 4 /C 2 H 6 ratios and low CH 4 / 3 He ratios. On the other hand, those from the Tokara Islands situated on the volcanic front of Southwest Japan show the heaviest carbon isotopes, middle CH 4 /C 2 H 6 ratios and the lowest CH 4 / 3 He ratios. The observed geochemical signatures of natural gases are clearly explained by a mixing of microbial, thermogenic and abiotic methane. An increasing contribution of abiotic methane towards more tectonically active regions of the plate boundary is suggested. Volatile elements are transported from the Earth's interior to the hydrosphere and the atmosphere through volcanic and hydrothermal systems in addition to micro-and macro-seepages from active tectonic areas 1,2. If the geological carbon cycle is well documented for its oxide forms, such as CO
Extremely High Methane Concentration in Bottom Water and Cored Sediments from Offshore Southwestern Taiwan
Full text linkIt has been found that Bottom Simulating Reflections (BSRs), which infer the existence of potential gas hydrates underneath seafloor sediments, are widely distributed in offshore southwestern Taiwan. Fluids and gases derived from dissociation of gas hydrates, which are typically methane enriched, affect the composition of seawater and sediments near venting areas. Hence, methane concentration of seawater and sediments become useful proxies for exploration of potential gas hydrates in a given area. We systematically collected bottom waters and sedimentary core samples for dissolved and pore-space gas analyses through five cruises: ORI-697, ORI-718, ORII-1207, ORII-1230, and ORI-732 from 2003 to 2005 in this study. Some sites with extremely high methane concentrations have been found in offshore southwestern Taiwan, e.g., sites G23 of ORI-697, N8 of ORI-718, and G96 of ORI-732. The methane concentrations of cored sediments display an increasing trend with depth. Furthermore, the down-core profiles of methane and sulfate reveal very shallow depths of sulfate methane interface (SMI) at some sites in this study. It implies sulfate reduction being mainly driven by the process of anaerobic methane oxidation (AMO) in sediments; thus indicating that there is a methane-enriched venting source, which may be the product of dissociation of gas hydrates in this area
Origin of methane-rich natural gas at the West Pacific convergent plate boundary
Full text linkMethane emission from the geosphere is generally characterized by a radiocarbon-free signature and might preserve information on the deep carbon cycle on Earth. Here we report a clear relationship between the origin of methane-rich natural gases and the geodynamic setting of the West Pacific convergent plate boundary. Natural gases in the frontal arc basin (South Kanto gas fields, Northeast Japan) show a typical microbial signature with light carbon isotopes, high CH4/C2H6 and CH4/3He ratios. In the Akita-Niigata region – which corresponds to the slope stretching from the volcanic-arc to the back-arc –a thermogenic signature characterize the gases, with prevalence of heavy carbon isotopes, low CH4/C2H6 and CH4/3He ratios. Natural gases from mud volcanoes in South Taiwan at the collision zone show heavy carbon isotopes, middle CH4/C2H6 ratios and low CH4/3He ratios. On the other hand, those from the Tokara Islands situated on the volcanic front of Southwest Japan show the heaviest carbon isotopes, middle CH4/C2H6 ratios and the lowest CH4/3He ratios. The observed geochemical signatures of natural gases are clearly explained by a mixing of microbial, thermogenic and abiotic methane. An increasing contribution of abiotic methane towards more tectonically active regions of the plate boundary is suggested.論
