Abstract
Hydrate-bearing sediment cores were retrieved from the Joetsu Basin (off Joetsu city, Niigata Prefecture) at the eastern margin of the Japan Sea during the MD179 gas hydrates cruise onboard R/V Marion Dufresne in June 2010. We measured molecular and stable isotope compositions of volatiles bound in the gas hydrates and headspace gases obtained from sediments to clarify how the minor components of hydrocarbons affects to gas hydrate crystals. The hydrate-bound hydrocarbons at Umitaka Spur (southwestern Joetsu Basin) primarily consisted of thermogenic methane, whereas those at Joetsu Knoll (northwestern Joetsu Basin, about 15 km from Umitaka Spur) contained both thermogenic methane and a mixture of thermogenic and microbial methane. The depth concentration profiles of methane, ethane, propane, CO2, and H2S in the sediments from the Joetsu Basin area showed shallow sulfate–methane interface (SMI) and high microbial methane production beneath the SMI depth. Relatively high concentrations of propane and neopentane (2,2-dimethylpropane) were detected in the headspace gases of the hydrate-bearing sediment cores obtained at Umitaka Spur and Joetsu Knoll. Propane and neopentane cannot be encaged in the structure I hydrate; therefore, they were probably excluded from the hydrate crystals during the structure I formation process and thus remained in the sediment and/or released from the small amounts of structure II hydrate that can host such large gas molecules. The lower concentrations of ethane and propane in the sediment, high δ13C of propane and isobutane, and below-detection normal butane and normal pentane at Umitaka Spur and Joetsu Knoll suggest biodegradation in the sediment layers.
Highlights
Gas hydrates are crystalline clathrate compounds consisting of water and gas molecules that form at low temperatures and high pressures [1]
We investigate the molecular and stable isotope compositions of hydrate-bound gas and gas in sediment cores retrieved from Umitaka Spur and Joetsu Knoll and focus on C2, C3, and higher hydrocarbons, those may change crystallographic structure of the shallow gas hydrates in these areas
Molecular and stable carbon and hydrogen isotopic compositions of hydrate-bound and pore-water gas were reported for samples retrieved from Umitaka Spur and Joetsu Knoll at the eastern margin of the Japan Sea
Summary
Gas hydrates are crystalline clathrate compounds consisting of water and gas molecules that form at low temperatures and high pressures [1]. Natural gas hydrates are found worldwide in continental margin sediments [2,3,4] and in near-surface sediments associated with active gas plume that vent from the seafloor [5,6,7]. Crystallographic structures of natural gas hydrate are usually either structure I (sI), which is composed of two 12-hedra and six 14-hedra with space group Pm3n, or structure II (sII), which is composed of sixteen 12-hedra and eight 16-hedra with space group Fd3m [1]. The concentrations of C2 and C3 in hydrate-bound gas from the Gulf of Mexico represent 3%–5% and >15% of the total gas, respectively [13]. Gas hydrate from the Sea of Marmara contained high concentrations of C3 (18.8%)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
