Methane Seep Sites Research Articles

メタン湧水域における海底自然ガンマ線異常―日本海東縁・台湾南西沖の計測結果―

メタン湧水域における海底自然ガンマ線異常―日本海東縁・台湾南西沖の計測結果―

Biogenic fabrics in seep carbonates from an active gas vent site in Green Canyon Block 238, Gulf of Mexico

Carbonate crusts collected from an active methane seep site in the offshore Louisiana of Gulf of Mexico have δ C PDB 13 = - 49.16 ‰ to - 52.29 ‰ and a group of unsaturated 2,6,10,15,19-pentamethylicosane (PMI Δ ) compounds produced by methane oxidizing archaea (MOA) with δ C PDB 13 of PMI Δ compounds from - 107.2 ‰ to - 115.5 ‰ , indicating that the crusts formed as the result of bacterially catalyzed methane oxidation and sulfate reduction. The fabric of the two flat sides of the crust is distinctly different. The “top” side is composed entirely of microcrystalline calcite while the bottom is composed entirely of “wormy” carbonate cement that could be a random, low fidelity replacement of bacteria. In the interior of the ∼ 1 cm thick crust microcrystalline calcite is coated by “wormy” carbonate cement, and dispersed pyrite framboids appear to be collaborating colonies of MOA and sulfate reducing bacteria (SRB). We propose that the “wormy” carbonate texture, particularly with microcrystalline calcite and pyrite framboids present, may be a useful indicator of biologically controlled fabrics produced during methane oxidation and sulfate reduction by MOA and SRB.

Fusion of 3D seismic exploration and seafloor geochemical survey for methane hydrate exploration

The MH21 Research Consortium has conducted a high-resolution 3D seismic survey and a seafloor geochemical survey, to explore methane hydrate reservoirs in the eastern Nankai Trough, offshore Japan. Excellent geological information about shallow formations was obtained from the high-resolution 3D seismic survey, which was designed to image the shallow formations where methane hydrates exist. The information is useful in constructing a geological and geochemical model, and especially to understand the complex geology of seafloor, including geochemical manifestations and the structure of migration conduits for methane gas or methane-bearing fluid.By comparing methane seep sites observed by submersibles with seismic sections, some significant relationships between methane hydrate reservoirs, free gas accumulations below the seafloor, and seafloor manifestations are recognised. Bathymetric charts and seafloor reflection amplitude maps, constructed from seismic reflections from the seafloor, are also useful in understanding the relationships over a vast area. A new geochemical seafloor survey targeted by these maps is required.The relationships between methane hydrate reservoirs and seafloor manifestations are becoming clearer from interpretation of high-resolution 3D seismic data. The MH21 Research Consortium will continue to conduct seafloor geochemical surveys based on the geological and geochemical model constructed from high-resolution 3D seismic data analysis.In this paper, we introduce a basis for exploration of methane hydrate reservoirs in Japan by fusion of 3D seismic exploration and seafloor geochemical surveys.

Molecular and isotopic analysis of anaerobic methane-oxidizing communities in marine sediments

Convergent lines of molecular, carbon-isotopic, and phylogenetic evidence have previously indicated (Hinrichs, K.-U., Hayes, J.M., Sylva, S.P., Brewer, P.G., DeLong, E.F., 1999. Methane-consuming archaebacteria in marine sediments. Nature 398, 802–805.) that archaea are involved in the anaerobic oxidation of methane in sediments from the Eel River Basin, offshore northern California. Now, further studies of those same sediments and of sediments from a methane seep in the Santa Barbara Basin have confirmed and extended those results. Mass spectrometric and chromatographic analyses of an authentic standard of sn-2-hydroxyarchaeol (hydroxylated at C-3 in the sn-2 phytanyl moiety) have confirmed our previous, tentative identification of this compound but shown that the previously examined product was the mono-TMS, rather than di-TMS, derivative. Further analyses of 13C-depleted lipids, appreciably more abundant in samples from the Santa Barbara Basin, have shown that the archaeal lipids are accompanied by two sets of products that are only slightly less depleted in 13C. These are additional glycerol ethers and fatty acids. The alkyl substituents in the ethers (mostly monoethers, with some diethers) are non-isoprenoidal. The carbon-number distributions and isotopic compositions of the alkyl substituents and of the fatty acids are similar, suggesting strongly that they are produced by the same organisms. Their structures, n-alkyl and methyl-branched n-alkyl, require a bacterial rather than archaeal source. The non-isoprenoidal glycerol ethers are novel constituents in marine sediments but have been previously reported in thermophilic, sulfate- and nitrate-reducing organisms which lie near the base of the rRNA-based phylogenetic tree. Based on previous observations that the anaerobic oxidation of methane involves a net transfer of electrons from methane to sulfate, it appears likely that the non-archaeal, 13C-depleted lipids are products of one or more previously unknown sulfate-reducing bacteria which grow syntrophically with the methane-utilizing archaea. Their products account for 50% of the fatty acids in the sample from the Santa Barbara Basin. At all methane-seep sites examined, the preservation of aquatic products is apparently enhanced because the methane-oxidizing consortium utilizes much of the sulfate that would otherwise be available for remineralization of materials from the water column.

Mass spectra of sn‐2‐hydroxyarchaeol, a polar lipid biomarker for anaerobic methanotrophy

Extremely 13C‐depleted sn‐2‐hydroxyarchaeol is widely observed at marine methane seep sites. It is the first biosynthetic product that provides information about a previously unknown microorganism involved in anaerobic methanotrophy, a process that occurs widely in marine sediments. The mono‐ and di‐TMS derivatives of this compound yield sharply differing and distinctive mass spectra. Depending on reaction conditions, either of these products can be formed in the course of conventional analyses.

New observations on the biology ofSiboglinum poseidoniFlügel & Langhof (Pogonophora) from the Skagerrak

Abstract Specimens of Siboglinum poseidon were collected by box coring and studied together with their undisturbed habitat. Occupied tubes occur in depse aggregations beneath the sediment surface directly at methane seep sites. Fertilized eggs develop into small larvae within the anterior transparent region of the tube. The oldest larvae observed in tubes measure c. 370 µm in length. It is concluded that released larvae develop in the sediment in the oxygenated top layer. Three tube types have been found: 1, the larval envelope; 2, the juvenile, transparent tube and 3, the ringed, adult tube. The smallest specimens recovered from the sediment measure c. 2 mm. They are complete with a tentacle, pyriform glands, bacteriocytes and juvenile opisthosoma. Some larvae were reared under laboratory conditions for 10 weeks. At that stage red blood becomes visible and a small tentacular bud is formed dorsally.