Shun-ichiro Igari

Hydrogen and carbon isotopic composition of methane as evidence for biogenic origin of natural gases from the Green Tuff Basin, Japan

Abstract Hydrogen and carbon isotopic analyses of methane were carried out to investigate the origin of natural gases commercially produced from volcanic rocks in the Green Tuff Basin of northeast Japan, whose high 3He/4He ratios have been interpreted as an indication of their abiogenic origin. Methane isotopic signatures (δ13C ≈ −32 to −67‰, δD ≈ −163 to −202‰, respectively) suggest that these are thermogenic gases or mixtures of thermogenic gases with bacterial gases. The correlations of methane δD vs. δ13C for the gases from specific fields are consistent with a mixing model between bacterial and thermogenic end-members, which was previously proposed on the basis of 13C contents of C1 to C4 hydrocarbons (Sakata, 1991; Berner and Faber, 1996). The Green Tuff Basin gases are isotopically distinct from well-known abiogenic methanes, e.g. geothermal methane from the East Pacific Rise. A significant contribution of abiogenic methane would, therefore, be unlikely for these gases.

A distinct freshwater‐adapted subgroup of ANME‐1 dominates active archaeal communities in terrestrial subsurfaces in Japan

Anaerobic methane-oxidizing archaea (ANME) are known to play an important role in methane flux, especially in marine sediments. The 16S rRNA genes of ANME have been detected in terrestrial freshwater subsurfaces. However, it is unclear whether ANME are actively involved in methane oxidation in these environments. To address this issue, Holocene sediments in the subsurface of the Kanto Plain in Japan were collected for biogeochemical and molecular analysis. The potential activity of the anaerobic oxidation of methane (AOM) (0.38-3.54 nmol cm⁻³ day⁻¹) was detected in sediment slurry incubation experiments with a (13) CH(4) tracer. Higher AOM activity was observed in low-salinity treatment compared with high-salinity condition (20‰), which supports the adaptation of ANME in freshwater habitats. The 16S rRNA sequence analysis clearly revealed the presence of a distinct subgroup of ANME-1, designated ANME-1a-FW. Phylogenetic analysis of the mcrA genes also implied the presence of the distinct subgroup in ANME-1. ANME-1a-FW was found to be the most dominant active group in the archaeal communities on the basis of 16S rRNA analysis (75.0-93.8% of total archaeal 16S rRNA clones). Sulfate-reducing bacteria previously known as the syntrophic bacterial partners of ANME-1 was not detected. Our results showed that ANME-1a-FW is adapted to freshwater habitats and is responsible for AOM in terrestrial freshwater subsurface environments.

Origin of light hydrocarbons from volcanic rocks in the “Green Tuff” region of northeast Japan: Biogenic versus magmatic

Abstract Geochemical analyses were carried out to assess the origin of light hydrocarbons in natural gases derived from volcanic rocks in the “Green Tuff” region of northeast Japan, whose high He isotope ratios have suggested a possible magma gas component. Biogenic steroid hydrocarbons were detected unambiguously in the oils associated with the natural gases, and they showed similar molecular distributions from C 27 to C 29 as those of the oils accumulated in the adjacent sedimentary rocks. Correlation of C 1 ( C 2 + C 3 ) vs. δ 13 C CH 4 for the light hydrocarbons indicated that they are thermocatalytic products from sedimentary organic matter. On the assumption that the magmatic CH 4 3 He ratio is 10 6 , no more than 0.1% of the total methane can be attributed to a magmatic origin. The remaining methane is likely to have migrated from the adjacent sedimentary layers.

Organic geochemistry of petroleum from the Green Tuff basin, Japan

Abstract Stable carbon isotopic composition and distribution of saturate hydrocarbons were analyzed for oils accumulated in volcanic (VR) and sedimentary (SR) rocks from the Green Tuff basin of Northeast Japan in order to investigate their origin. The average δ 13 C values relative to PDB of the whole phase and saturate hydrocarbon fraction are identical for both the VR and SR oils within experimental error. They lie within the range of δ 13 C of kerogen in the sediments from the same basin. The distributions of saturate hydrocarbons of the VR and SR oils are similar to each other, and not distinguished from that of a normal marine oil. They include n -alkanes as major components, and acyclic isoprenoid alkanes such as pristane and phytane as the next abundant components. These data indicate that the VR oils are biogenic, i.e. pyrolysis products of kerogen just like the SR oils. The abiogenic scenario assuming that the VR oils alone were derived from mantle carbon by the Fischer-Tropsch reaction is not plausible.