TY - JOUR

T1 - Isotopic composition of gas hydrates in subsurface sediments from offshore Sakhalin Island, Sea of Okhotsk

AU - Hachikubo, Akihiro

AU - Krylov, Alexey

AU - Sakagami, Hirotoshi

AU - Minami, Hirotsugu

AU - Nunokawa, Yutaka

AU - Shoji, Hitoshi

AU - Matveeva, Tatiana

AU - Jin, Young K.

AU - Obzhirov, Anatoly

N1 - Funding Information: Acknowledgements We greatly appreciate the support of the shipboard crews of R/V Akademik M.A. Lavrentyev during the expeditions of LV32 (CHAOS1), LV36 (CHAOS2), and LV39 (CHAOS3). The CF-IRMS measurements were supported by Tomoko Kosaka, Takahiro Ozeki, and Yukihiro Saito. We also acknowledge the two reviewers for their helpful comments. This work was supported by funding agencies in Japan (Japan Society for the Promotion of Science KAKENHI 18206099 and 19550077, Ministry of Education, Culture, Sports, Science and Technology KAKENHI 19740323), the Korean Ministry of Land, Transport and Maritime Affairs Grant PM08030, the Grant RFFI 06-08-96928, and the Grant Goskontract 02.515.11.5017. Copyright: Copyright 2010 Elsevier B.V., All rights reserved.

PY - 2010

Y1 - 2010

N2 - Hydrate-bearing sediment cores were retrieved from recently discovered seepage sites located offshore Sakhalin Island in the Sea of Okhotsk. We obtained samples of natural gas hydrates and dissolved gas in pore water using a headspace gas method for determining their molecular and isotopic compositions. Molecular composition ratios C1/C2+ from all the seepage sites were in the range of 1,500-50,000, while δ13C and δD values of methane ranged from -66.0 to -63.2‰ VPDB and -204.6 to -196.7‰ VSMOW, respectively. These results indicate that the methane was produced by microbial reduction of CO2. δ13C values of ethane and propane (i.e., -40.8 to -27.4‰ VPDB and -41.3 to -30.6‰ VPDB, respectively) showed that small amounts of thermogenic gas were mixed with microbial methane. We also analyzed the isotopic difference between hydrate-bound and dissolved gases, and discovered that the magnitude by which the δD hydrate gas was smaller than that of dissolved gas was in the range 4.3-16.6‰, while there were no differences in δ13C values. Based on isotopic fractionation of guest gas during the formation of gas hydrate, we conclude that the current gas in the pore water is the source of the gas hydrate at the VNIIOkeangeologia and Giselle Flare sites, but not the source of the gas hydrate at the Hieroglyph and KOPRI sites.

AB - Hydrate-bearing sediment cores were retrieved from recently discovered seepage sites located offshore Sakhalin Island in the Sea of Okhotsk. We obtained samples of natural gas hydrates and dissolved gas in pore water using a headspace gas method for determining their molecular and isotopic compositions. Molecular composition ratios C1/C2+ from all the seepage sites were in the range of 1,500-50,000, while δ13C and δD values of methane ranged from -66.0 to -63.2‰ VPDB and -204.6 to -196.7‰ VSMOW, respectively. These results indicate that the methane was produced by microbial reduction of CO2. δ13C values of ethane and propane (i.e., -40.8 to -27.4‰ VPDB and -41.3 to -30.6‰ VPDB, respectively) showed that small amounts of thermogenic gas were mixed with microbial methane. We also analyzed the isotopic difference between hydrate-bound and dissolved gases, and discovered that the magnitude by which the δD hydrate gas was smaller than that of dissolved gas was in the range 4.3-16.6‰, while there were no differences in δ13C values. Based on isotopic fractionation of guest gas during the formation of gas hydrate, we conclude that the current gas in the pore water is the source of the gas hydrate at the VNIIOkeangeologia and Giselle Flare sites, but not the source of the gas hydrate at the Hieroglyph and KOPRI sites.

UR - http://www.scopus.com/inward/record.url?scp=77952952372&partnerID=8YFLogxK

U2 - 10.1007/s00367-009-0178-y

DO - 10.1007/s00367-009-0178-y

M3 - Article

VL - 30

SP - 313

EP - 319

JO - Geo-Marine Letters

JF - Geo-Marine Letters

SN - 0276-0460

IS - 3-4

ER -