5:15 PM - 7:15 PM
[BBG02-P05] Microbial methanogenesis activity in sediments in the presence of methane hydrate on the eastern margin of the Japan Sea
The accumulation of methane hydrates in marine sediments generally occurs along plate boundaries and continental margins associated with upward migration of thermogenic methane and/or in situ microbial methane production. Two types of reservoirs are typically found: deep sandy horizons widespread above the base of the hydrate stability zone and shallow sediment columns near the seabed. As methane hydrates formed at the shallow deposits are massive and technically difficult to penetrate by piston coring. Although coring of the shallow deposit can be achieved by rotary core barrel drilling, microbial contamination from drilling fluid is a technical problem that leaves microbiological investigations behind. Consequently, the formation mechanism of the shallow deposit is largely unknown.
To overcome this technical drawback, a new technique that enables feasible monitoring of drilling fluid contamination with a soluble fluorescent dye called amino G acid was previously developed. In this study, the newly developed technique was applied to investigate methanogenic activities in marine sediments associated with the shallow deposits in Oki Trough and Joetsu Basin in the eastern Japan Sea, where methane hydrates are geochemically inferred to contain biogenic and thermogenic methane, respectively. In addition, environmental factors controlling methanogenic activities such as porewater concentrations of sulfate, acetate, formic acid, alkalinity and H2 and the contents and stable isotopic compositions of C and N in solid organic matter.
Laboratory incubation of marine sediments with 14C-labelled bicarbonate and acetate and subsequent quantification of 14C-labelled methane production revealed that methane production rates via CO2 reduction (undetected or 3×10-1~6 pmol/cm3/day) were higher than those via acetate fermentation (undetected or 6×10-6~3×10-2 pmol/cm3/day). High CO2 reduction activity was found at shallow horizons (< 30 meter below seafloor: mbsf) in Oki Trough and at deep horizons (> 50 mbsf) in Joetsu Basin. Environmental factors correlated with the high CO2 reduction activity were the downward increase in porewater acetate indicative of the upward migration of deeply seated fluids in Joetsu Basin and the C/N ratio and δ13C value of solid organic matter indicative of the bioavailability of organic sources deposited in Oki Trough.
Methanogenic activity occurs in marine sediments below the sulfate-methane interface (SMI), which becomes shallower with increasing the upward flux of methane. Although the upward fluid migration and thermogenic methane were not evident in Oki Trough, the near-seafloor formation of methane hydrates is limited to sites with shallow SMI depths. In addition, the high proportion of methane hydrates in marine sediments is not possible to form only by the highest measured rate of methanogenesis (6 pmol/cm3/day) by taking generic sedimentation rates in the Japan Sea (10 cm/ky) into the near-seafloor formation model of the shallow deposit in Oki Trough. In case of Joetsu Basin, the contribution of thermogeic methane migrated upwards from deeply buried formations is significant. Taken all together, the formation of methane hydrates in the shallow deposit might require the upward methane flux to extend the shallow methanogenic zone by minimizing sulfate reduction coupled to the degradation of organic matter.
This study was conducted in 2013 to 2015 under the commission from AIST as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan). We are grateful to all shipboard and onshore supports.
To overcome this technical drawback, a new technique that enables feasible monitoring of drilling fluid contamination with a soluble fluorescent dye called amino G acid was previously developed. In this study, the newly developed technique was applied to investigate methanogenic activities in marine sediments associated with the shallow deposits in Oki Trough and Joetsu Basin in the eastern Japan Sea, where methane hydrates are geochemically inferred to contain biogenic and thermogenic methane, respectively. In addition, environmental factors controlling methanogenic activities such as porewater concentrations of sulfate, acetate, formic acid, alkalinity and H2 and the contents and stable isotopic compositions of C and N in solid organic matter.
Laboratory incubation of marine sediments with 14C-labelled bicarbonate and acetate and subsequent quantification of 14C-labelled methane production revealed that methane production rates via CO2 reduction (undetected or 3×10-1~6 pmol/cm3/day) were higher than those via acetate fermentation (undetected or 6×10-6~3×10-2 pmol/cm3/day). High CO2 reduction activity was found at shallow horizons (< 30 meter below seafloor: mbsf) in Oki Trough and at deep horizons (> 50 mbsf) in Joetsu Basin. Environmental factors correlated with the high CO2 reduction activity were the downward increase in porewater acetate indicative of the upward migration of deeply seated fluids in Joetsu Basin and the C/N ratio and δ13C value of solid organic matter indicative of the bioavailability of organic sources deposited in Oki Trough.
Methanogenic activity occurs in marine sediments below the sulfate-methane interface (SMI), which becomes shallower with increasing the upward flux of methane. Although the upward fluid migration and thermogenic methane were not evident in Oki Trough, the near-seafloor formation of methane hydrates is limited to sites with shallow SMI depths. In addition, the high proportion of methane hydrates in marine sediments is not possible to form only by the highest measured rate of methanogenesis (6 pmol/cm3/day) by taking generic sedimentation rates in the Japan Sea (10 cm/ky) into the near-seafloor formation model of the shallow deposit in Oki Trough. In case of Joetsu Basin, the contribution of thermogeic methane migrated upwards from deeply buried formations is significant. Taken all together, the formation of methane hydrates in the shallow deposit might require the upward methane flux to extend the shallow methanogenic zone by minimizing sulfate reduction coupled to the degradation of organic matter.
This study was conducted in 2013 to 2015 under the commission from AIST as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan). We are grateful to all shipboard and onshore supports.