Factors determining the spatial distribution of gas-charged sediments in the continental shelf off southeastern Korea

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• 作者：Jong-Hwa Chun^a ; Byong-Jae Ryu^a ; Chang Sik Lee^b ; Young Jun Kim^a ; Ji-Young Choi^a ; Nyeon-Keon Kang^a ; Jang-Jun Bahk^a ; Jin Ho Kim^a ; Kyu Jung Kim^b ; Dong-Geun Yoo^a ; ^{dgyoo@kigam.re.kr}
• 关键词：Acoustic blanking ; Methane and carbon dioxide isotopes ; Dissolved methane concentrations ; Gas seepages ; Korea Strait shelf mud
• 刊名：Marine Geology
• 出版年：2012
• 出版时间：1 December, 2012
• 年：2012
• 卷：332-334
• 期：Complete
• 页码：27-39
• 全文大小：2803 K

文摘

A high-resolution seismic survey, piston coring, and water column measurements were conducted on the continental shelf off southeastern Korea to examine the gas-charged sediments and gas seepage in the water column. We examined the factors controlling the spatial distribution of the acoustic blanking zone in recent Korea Strait shelf mud (KSSM) deposits along the southeastern shelf. The molecular composition, methane, hydrogen, and carbon dioxide isotope values of headspace gases, and the geochemical composition of core sediments, were measured in 11 piston cores. Dissolved methane concentrations in the water column were observed at 6 of the 11 coring sites.

Acoustic blanking of the recent KSSM deposits was limited to areas with water depths between 30 and 105 m along the continental shelf, except line L-13 (133 m depth). The flanks of the acoustic blanking zone can be classified as distinct, disturbed, and step types based on their configurations. The highest methane concentration in the headspace gases was about 2.7 % , 2-4 m below the seafloor, which matched the top of the acoustic blanking zone. No obvious methane was found in piston cores obtained from the gas-free zone. The ¦Ä¹³C_CH4 ranged from ? 56.5 to ? 106.3¡ë relative to Vienna Pee Dee Belemnite (V-PDB), and the ¦ÄD_CH4 ranged from ? 194 to ? 207¡ë relative to Vienna Standard Mean Ocean Water (V-SMOW). These isotope values show that methane is produced by the microbial reduction of CO₂. CH₄ oxidation results in a significantly heavier carbon isotope fraction (from - 91.2 to - 56.5¡ë in the 2 m below the seafloor) with constant ¦Ä¹³C_CO2 isotope fractionation. No obvious differences were seen in the total carbon (TC; 2.438-3.378 % ), total nitrogen (TN; 0.159-0.548 % ), total organic carbon (TOC; 1.139-2.078 % ), or CaCO₃ (2.825-15.258 % ) in the continental shelf, whereas the TOC was high and the CaCO₃ low distant from the Nakdong River. Distinct dissolved methane anomalies (21.87 ppm) were seen in the water column in the acoustic blanking and gas seepage zones. This level was about 15 times higher than the dissolved methane level (1.5 ppm) in the water column in the gas-free zone. These results indicate that the acoustic blanking zone accumulates biogenic methane. Methane seepage was observed in the water column throughout the acoustic blanking zone of the continental shelf off southeastern Korea.