5:15 PM - 6:30 PM
[MIS26-P03] Total organic carbon and its carbon isotope ratio in subsurface sediments off Hokkaido area
Keywords:TOC, gas hydrate, methane
For five years we collected hydrate-bearing and gas-rich sediments at gas plume sites off Hokkaido to understand the formation process of natural gas hydrates. The research target of this study is an organic matter deposited on the sea floor, because it is an ultimate origin of hydrate-bound hydrocarbon and its total organic carbon relates to formation of natural gas hydrates. Carbon isotopes of the organic matter provides information of their origin. Here, we report total organic carbon and its δ13C in subsurface sediment retrieved off Hokkaido area (the Sea of Okhotsk, Japan Sea, and the Pacific Ocean).
Sediment samples were obtained off Abashiri (C032, 1KY17, HKS17, HKS18, and HKS20), off Monbetsu (GA18 and GA19), off Esashi (HKY17), off Rebun Is. (HKY19), off Hidaka (K2-2020), and off Tokachi (C046, C061, C080, and C095). In total 446 sediment samples were dried, homogenized, and about 5 mg of samples were put into silver foil capsules. They were decarbonized with 12N hydrochloric acid for 24 h, and then dried and wrapped in tin capsules. These samples were analyzed their TOC with a Flash 2000 Elemental Analyzer connected to a Delta V isotope ratio mass spectrometer. The system was calibrated using a standard of JCAC-01 (L-alanine, -23.43‰ in V-PDB).
The median values of TOC and its δ13C for all data were 0.9% and -24.2‰, respectively. Sediment cores collected at the gas seepage sites were mainly gas-rich; however, several normal cores with little gas were also included in these data. The TOC and its δ13C values show distinct regional characteristics. In particular, certain areas off Tokachi and Hidaka had higher TOC (1.5% to 2.0%) and larger δ13C values (-21‰). As for hydrate-bearing sediment cores, TOC-rich (from 1% to 2%) cores had δ13C around -25‰, which was similar to other sediment cores without gas hydrates. On the other hand, in the C095-2002 core retrieved off Tokachi, TOC was small (0.6-0.8%) and its δ13C was extremely small (from -46‰ to -30‰). Hydrate-bearing cores of 1KY17 obtained by ROV survey off Abashiri showed such small values in TOC δ13C (below -30‰). The δ13C of TOC was also relatively small in the case that sediments attached to gas hydrate crystals. These results suggest that the organic matter in the sediments close to gas hydrates may be affected by oxidized methane. Although little is known about the formation age of gas hydrate and the exchange processes of gas hydrate crystals with the surrounding gas (dissolved and gas bubble), some exchange of carbon between seepage gas, gas hydrate crystals, and organic matter might occur in the subsurface sediments.
Sediment samples were obtained off Abashiri (C032, 1KY17, HKS17, HKS18, and HKS20), off Monbetsu (GA18 and GA19), off Esashi (HKY17), off Rebun Is. (HKY19), off Hidaka (K2-2020), and off Tokachi (C046, C061, C080, and C095). In total 446 sediment samples were dried, homogenized, and about 5 mg of samples were put into silver foil capsules. They were decarbonized with 12N hydrochloric acid for 24 h, and then dried and wrapped in tin capsules. These samples were analyzed their TOC with a Flash 2000 Elemental Analyzer connected to a Delta V isotope ratio mass spectrometer. The system was calibrated using a standard of JCAC-01 (L-alanine, -23.43‰ in V-PDB).
The median values of TOC and its δ13C for all data were 0.9% and -24.2‰, respectively. Sediment cores collected at the gas seepage sites were mainly gas-rich; however, several normal cores with little gas were also included in these data. The TOC and its δ13C values show distinct regional characteristics. In particular, certain areas off Tokachi and Hidaka had higher TOC (1.5% to 2.0%) and larger δ13C values (-21‰). As for hydrate-bearing sediment cores, TOC-rich (from 1% to 2%) cores had δ13C around -25‰, which was similar to other sediment cores without gas hydrates. On the other hand, in the C095-2002 core retrieved off Tokachi, TOC was small (0.6-0.8%) and its δ13C was extremely small (from -46‰ to -30‰). Hydrate-bearing cores of 1KY17 obtained by ROV survey off Abashiri showed such small values in TOC δ13C (below -30‰). The δ13C of TOC was also relatively small in the case that sediments attached to gas hydrate crystals. These results suggest that the organic matter in the sediments close to gas hydrates may be affected by oxidized methane. Although little is known about the formation age of gas hydrate and the exchange processes of gas hydrate crystals with the surrounding gas (dissolved and gas bubble), some exchange of carbon between seepage gas, gas hydrate crystals, and organic matter might occur in the subsurface sediments.