Ancient Methane Seeps Research Articles

Discharge timing and origin of fluids at methane seeps in the late Cretaceous subduction zone of Hokkaido, Japan: Coupling U–Pb calcite dating with Sr–Nd isotope analysis

Seepage of methane-rich fluids is common in subduction zones throughout Earth's history. However, the causes and hydrogeological regimes of methane seepage in the geological past are commonly elusive. To provide insights into the hydrogeological regime and structural evolution of the Cretaceous subduction zone of Hokkaido, Japan, this study investigates the timing of discharge and migration pathways of fluids emitted at five ancient methane seeps using U–Pb dating and coupled Sr, Nd, C, and O isotope analyses of the authigenic seep carbonates. The carbonates were collected from forearc-basin sediments of the Yezo Group, which are underlain by a mafic igneous basement (lower Sorachi ophiolite) detached from the subducting plate. The subduction zone is known to have experienced intermittent accretionary and non-accretionary stages, and the U–Pb dates determined for seep carbonates document two fluid discharge events, ∼100 and ∼80 Ma, falling within the accretionary periods. The 87Sr/86Sr ratios (0.70682–0.70781) and εNd values (−5.9 to −0.1) of the carbonates show significant variability, indicating the seeping fluids experienced interactions with the mafic rocks of the lower Sorachi ophiolite and with siliciclastic sediments of the Yezo Group. The δ13C values of the carbonates, as low as −49‰, suggest a primarily microbial origin of the seeping methane, which makes methane migration from the underplating materials at high temperatures (>100 °C) unlikely. These results demonstrate that the episodic expulsion of the methane-rich fluids, originating from within the forearc basin rather than from the underplating materials, occurred during the accretionary stages and played a major role in methane seepage in the Cretaceous subduction system.

Archaeal lipid biomarker as a tool to constrain the origin of methane at ancient methane seeps: Insight into subsurface fluid flow in the geological past

The origin of methane discharged at ancient methane seeps is a key to understand subsurface fluid-flow processes in sedimentary basins. The carbon isotopic composition of methane-seep carbonates can be used to estimate the origin of methane, but mixing of multiple carbon sources results in ambiguous interpretation. We here constrained the origin of methane at ancient seeps by using the carbon isotopic composition and the isotope fractionation of lipid biomarkers of anaerobic methane-oxidizing archaea. We estimated the carbon-isotope fractionation between methane and biomarkers as the offset between their δ13C values at modern seep sites in the Japan Sea and other regions of the world. The biomarker pentamethylicosane (PMI) extracted from a modern seep carbonate of the Japan Sea shows a 44‰ offset from the seep methane. On the basis of literature data, we calculated the carbon-isotope offset as mostly ~20–60‰ for PMI at modern seeps across the world. We also assessed the relationship of the δ13C values between methane and biomarkers by regression analysis on the data taken at the modern seeps, obtaining regression formulas to estimate the δ13C values of methane from those of biomarkers. For ancient seeps, we examined Miocene to Pleistocene methane-seep carbonate rocks collected from Japan Sea sediments. PMI extracted from these ancient carbonates have δ13C values ranging from −137‰ to −93‰. Using the δ13C values of PMI and the carbon-isotope offset and regression formulas, we estimated that the methane at the ancient seeps was mainly of biogenic origin and migrated from the shallow subsurface.

Ancient Methane Seeps Tell Tale of Sudden Warming

Newly discovered rock mounds left by ancient methane seeps give scientists clues that methane on ancient ocean floor was released by ancient global warming.

Lucinid bivalves from ancient methane seeps

Twelve species of lucinid bivalves are reported from late Jurassic to late Miocene methane-seep deposits worldwide. Among them, eight species and two genera are new. Amanocina n. gen. includes Nipponothracia yezoensis from the Cenomanian of Japan as type species, Cryptolucina kuhnpassetensis Kelly, 2000 from the Berriasian of Greenland, A. raukumara n. sp. from the Albian of New Zealand and A. colombiana n. sp. from the Oligocene of Colombia. Tehamatea n. gen. includes Lucina ovalis Stanton, 1895 and Lucina colusaensis Stanton, 1895 from the late Jurassic to early Cretaceous of California, T. vocontiana n. sp. from the Hauterivian of southern Europe and T. agirrezabalai n. sp. from the Albian of northern Spain. The new species are: Cubatea awanuiensis from the Albian and Cenomanian of New Zealand, Nymphalucina panochensis from the early Palaeocene of California, Elliptiolucina washingtonia from the late Oligocene of Washington State, USA, and Elongatolucina peckmanni from the Oligocene of Colombia. New combinations are provided for Nipponothracia lomitensis (Olsson, 1931) from the Oligocene of Peru and Elliptiolucina hetzeli (Martin, 1933) from the late Miocene of Indonesia. The anterior adductor muscle scar of Beauvoisina carinata is documented for the first time. The lucinids found at Jurassic and Cretaceous deep-water methane seeps belong to the subfamily Myrteinae and within this to genera that are restricted to the seep environment (Beauvoisina, Tehamatea, Amanocina and Cubatea); shallow-water seeps were inhabited by the codakiine genus Nymphalucina, which is not seep-restricted. Amanocina, Cubatea and Nymphalucina survived into the Cenozoic. Genera that newly colonized deepwater seeps during the Cenozoic include members of both Myrteinae (Elongatolucina, Nipponothracia and Elliptiolucina) and Codakiinae (Epilucina and Lucinoma); the latter are clearly not seep-restricted.

Aerobic methanotrophy at ancient marine methane seeps: A synthesis

The molecular fingerprints of the chemosynthesis based microbial communities at methane seeps tend to be extremely well preserved in authigenic carbonates. The key process at seeps is the anaerobic oxidation of methane (AOM), which is performed by consortia of methanotrophic archaea and sulphate reducing bacteria. Besides the occurrence of 13C depleted isoprenoids and n-alkyl chains derived from methanotrophic archaea and sulphate reducing bacteria, respectively, 13C depleted triterpenoids have been reported from a number of seep deposits. In order to evaluate the significance of these apparently non-AOM related molecular fossils, the biomarker inventories of one Campanian and two Miocene methane seep limestones are compared. These examples provide strong evidence that methane was not solely oxidized by an anaerobic process. Structural and carbon isotope data reveal that aerobic methanotrophy was common at some ancient methane seeps as well. The Miocene Marmorito limestone contains abundant 3β-methylated hopanoids (δ 13C: −100‰). Most likely, 3β-methylated hopanepolyols, prevailing in aerobic methanotrophs, were the precursor lipids of these compounds. A series of isotopically depleted 4-methylated steranes (lanostanes; δ 13C: −80‰ to −70‰) and similarly isotopically depleted 17β(H),21β(H)-32-hopanoic acid in the Miocene Pietralunga seep limestone also are derived probably from aerobic methanotrophs. Lanosterol, which is known to be produced by aerobic methanotrophs, is the most likely precursor of 4-methylated steranes. Less obvious is the origin of 8,14-secohexahydrobenzohopanes (δ 13C: −110‰ to −107‰) in Late Cretaceous seep limestones. These hopanoids probably reflect early degradational products of precursor lipids locally produced by seep endemic aerobic methanotrophs.

Stratigraphy, Sedimentary Structures, and Textures of the Late Neoproterozoic Doushantuo Cap Carbonate in South China

Abstract The 3- to 5-m-thick Doushantuo cap carbonate in south China overlies the glaciogenic Nantuo Formation (ca. 635 Ma) and consists of laterally persistent, thinly laminated and normally graded dolomite and limestone indicative of relatively deep-water deposition, most likely below storm wave base. The basal portion of this carbonate contains a distinctive suite of closely associated tepee-like structures, stromatactis-like cavities, layer-parallel sheet cracks, and cemented breccias. The cores of tepees are composed of stacked cavities lined by cements and brecciated host dolomicrite. Onlap by laminated sediment indicates synsedimentary disruption of bedding that resulted in a positive seafloor expression. Cavities and sheet cracks contain internal sediments, and they are lined by originally aragonitic isopachous botryoidal cements with acicular radiating needles, now replaced by dolomite and silica. Pyrite and barite are common, and calcite is locally retained as a primary mineral. These features share morphological and petrographic attributes with modern and ancient methane seeps in which methane gas and fluids provide both a force for physical disruption from buoyancy and a source of alkalinity for significant cementation. The presence of δ13C values as low as −41‰ in well preserved limestone crusts and cements within and immediately above the tepee-like structures provides unequivocal evidence for methane influence, and the widespread distribution of identical sedimentary structures and paragenetic cement sequences across the entire basin at the same basal cap carbonate level is consistent with gas hydrate destabilization and the development of methane seeps as a result of postglacial warming of the ocean. Considering the broad distribution of similar features at the same stratigraphic level in other cap carbonates globally, we suggest that the late Neoproterozoic postglacial methane release may have influenced the oceanic oxygen level as well as contributed to postglacial warming via the greenhouse effects of methane.

Carbon cycling at ancient methane–seeps

The oldest known seep deposits with macroinvertebrate taxa are Devonian in age. Although more and more examples continue to be discovered, the Palaeozoic and Mesozoic record of methane–seeps is still fragmentary. The relationship of ancient carbonate deposits to methane seepage is established by (i) the geological setting and structural features, (ii) the occurrence of low diversity but high abundance faunal assemblages, (iii) carbonate fabrics, (iv) low δ 13C values of early diagenetic carbonate phases, and (v) characteristic 13C-depleted biomarkers. In this review, special emphasis is placed on biogeochemical processes at ancient methane–seeps. Specific carbonate fabrics and phases include, in order of decreasing significance for seep-dominated environments, (i) inverted stromatactoid cavities, (ii) upside-down stromatolites, (iii) globular fabrics, (iv) botryoidal aragonite, (v) micritic nodules, (vi) fractures, (vii) clotted micrites, and (viii) constructive seams representing fossilised biofilms. The carbon of the early diagenetic carbonate phases was derived predominantly from the oxidation of methane which resulted in low δ 13C values. Pyrites enclosed in the seep limestones generally show low δ 34S values and a significant variability of their isotopic composition on a small scale, both indicating bacterial sulphate reduction as the sulphide-supplying process. 13C-depleted lipid biomarkers characterise the microbial populations involved in the cycling of carbon at ancient methane–seeps. Typical compounds include isoprene-based lipids from archaea, and linear and monomethyl-branched carbon skeletons assigned to sulphate-reducing bacteria (SRB), consistent with the syntrophic relationship of these microbes in the anaerobic oxidation of methane (AOM).

The role of bacteria in the formation of cold seep carbonates: geological evidence from Monferrato (Tertiary, NW Italy)

Abstract Methane-derived carbonate rocks (Lucina limestone and Marmorito limestone) crop out in Monferrato (NW Italy) and represent one of the first described examples of rocks produced at fossil cold seeps. These rocks, of Miocene age, consist of strongly carbonate-cemented siliciclastic sediments ranging in grain size from mud to coarse sand. The methane-related origin of Monferrato carbonates is based on: (a) outcrop-scale evidence: patchiness of cementation, chemosymbiotic fossil communities, presence of a network of polyphase carbonate-filled veins not related to tectonics; (b) isotope geochemistry: very depleted δ13C values, as low as −50‰ PDB; (c) peculiar petrographic features. Diverse microbial communities have been observed in present-day cold seeps. These communities include sulphate-reducing, sulphur-oxidizing and methane-oxidizing bacteria. The present work is focused on the identification and description of fossil evidence of such microbial activity in the Monferrato carbonates. Examples of fossilization of microbial structures are probably represented by pyritic rods and dolomite tubes referable to sulphur-oxidizing and to unspecified bacteria, respectively. Less direct but more abundant evidence has been found through petrographic and SEM studies of seep carbonates. Many features point to the presence of organic clumps or mats capable of trapping sediment and promoting carbonate precipitation: microcrystalline calcite peloids; dolomite crystals with irregular hollow cores; dolomite spheroids with dumbbell-shaped cores; laminated internal sediments lining cavities completely. All these features are interpreted to result from bacterially mediated, sedimentary and diagenetic processes and can therefore be considered as an additional evidence of ancient methane seeps.