Jörn Ludwig Peckmann

Molecular signals for anaerobic methane oxidation in Black Sea seep carbonates and a microbial mat

Linked to gas seeps on the Ukrainian shelf (northwestern Black Sea), massive authigenic carbonates form as a result of anaerobic methane oxidation. Lipid distributions in these ‘cold seep’ carbonates and an associated microbial mat were investigated for process markers reflecting the presence and metabolic activity of distinctive methane-related biota. The samples contain free, irregular isoprenoid hydrocarbons, namely the tail-to-tail linked acyclic C20-isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane), its C25-homologue 2,6,10,15,19-pentamethylicosane (PMI), and several unsaturated derivatives thereof. Furthermore, specific acyclic and cyclic C40-isoprenoids were released upon ether cleavage of the polar fraction from the carbonate. The abundance of these compounds indicates a pronounced role of particular Archaea in the biogeochemical cycling of carbon at methane seeps. Stable carbon isotopic analyses of these lipids reveal extraordinary depletions in 13C corresponding to δ-values in the range of −100±30‰ PDB, whereas other compounds show isotopic compositions normally observed for marine lipids (around −30‰ PDB). The isotope data imply that the biosynthesis of the archaeal isoprenoids occurred in situ and involved the utilization of isotopically depleted, i.e. methane-derived, carbon. Apart from archaeal markers, the carbonate and the mat contain authigenic, framboidal pyrite and isotopically depleted fatty acids, namely iso-, and anteiso-branched compounds most likely derived from sulphate-reducing bacteria (SRB). The indications for a tight association of these normally competitive organisms support a model invoking a syntrophic relationship of SRB with Archaea responsible for the anaerobic oxidation of methane. The biomarker patterns obtained from the Black Sea samples were further compared to those from a Oligocene seep carbonate (Lincoln Creek Formation, WA, USA) in order to evaluate their biomarker potential for ancient settings. The prominent occurrence of isotopically light crocetane (−112‰) and PMI (−120‰) meets the findings for the contemporary materials. Thus, isotopically depleted isoprenoids provide diagenetically stable fingerprints for the reconstruction of carbon cycling in both, modern and ancient methane seep systems.

Highly isotopically depleted isoprenoids: molecular markers for ancient methane venting

Abstract We propose that organic compounds found in a Miocene limestone from Marmorito (Northern Italy) are source markers for organic matter present in ancient methane vent systems (cold seeps). The limestone contains high concentrations of the tail-to-tail linked, acyclic C20 isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane), a C25 homolog 2,6,10,15,19-pentamethylicosane (PME), and a distinctive glycerol ether lipid containing 3,7,11,15-tetramethylhexadecyl (phytanyl-) moieties. The chemical structures of these biomarkers indicate a common origin from archaea. Their extremely 13C-depleted isotope compositions (δ13C ≈ −108 to −115.6‰ PDB) suggest that the respective archaea have directly or indirectly introduced isotopically depleted, methane-derived carbon into their biomass. We postulate that a second major cluster of biomarkers showing heavier isotope values (δ13C ≈ −88‰) is derived from sulfate-reducing bacteria (SRB). The observed biomarkers sustain the idea that methanogenic bacteria, in a syntrophic community with SRB, are responsible for the anaerobic oxidation of methane in marine sediments. Marmorito may thus represent a conceivable ancient scenario for methane consumption performed by a defined, two-membered bacterial consortium: (1) archaea that perform reversed methanogenesis by oxidizing methane and producing CO2 and H2; and (2) SRB that consume the resulting H2. Furthermore, the respective organic molecules are, unlike other compounds, tightly bound to the crystalline carbonate phase. The Marmorito carbonates can thus be regarded as “cold seep microbialites” rather than mere “authigenic” carbonates.

Signatures of hydrocarbon venting in a Middle Devonian Carbonate Mound (Hollard Mound) at the Hamar Laghdad (Antiatlas, Morocco)

SummaryThe Middle Devonian Hollard Mud Mound is situated in the eastern Hamar Laghdad, which is a small mountain range in the Tafilalt in SE Morocco. In contrast to the well known Lower Devonian Kess-Kess mounds, the Hollard Mound is of Middle Devonian age. The facies in the core of this mud mound differs from that of the other parts of the mound, and exhibits signatures of ancient hydrocarbon venting. The carbonate phases of the core facies are derived from the oxidation of vent fluids and consist of clotted micrite, a cryptocrystalline carbonate associated with spheres of uncertain origin, and a calcitic rim cement (rim cement B). These vent carbonates show δ13C values in the range of −11 to −20% PDB indicating that some of their carbon is derived from isotopically light hydrocarbons. Fossiliferous micrite has been affected by hydrocarbon venting in the proximity of the vent site, which is indicated by intermediate δ13C values between vent carbonates and not affected sediments. Bivalves occur in dense populations within the core facies. They form autochthonous shell accumulations and are almost exclusively articulated. it is likely that these bivalves were dependent on chemosynthesis similar to their counterparts at modern vents. The vent deposits also exhibit an unusual prasinophyte assemblage, which might have been linked to the specific nutrient availability at the vent site.The ancient vent site is characterized by an enhanced carbonate precipitation and rapid lithification. The latter is corroborated by the three-dimensional preservation of phytoplankton (prasinophytes and acritarchs) and the occurrence of stromatactoid pores. An early phase of carbonate corrosion predating the formation of vent carbonates affected the fossiliferous micrite of the core facies and is thought to be related to a phase of H2S-rich venting.

Molecular fossils reveal fluid composition and flow intensity at a Cretaceous seep

An isolated Hauterivian marine limestone from the Crimean Peninsula containing masses of articulated specimens of the dimerelloid brachiopod Peregrinella has previously been interpreted to represent a hydrocarbon-seep deposit. In order to constrain the intensity of seepage and the composition of fl uids, we investigated the lipid biomarker inventory of this seep limestone. The dominant biomarkers are (13)C-depleted isoprenoids including tail-to-tail linked pentamethylicosane (delta(13)C value: -108 parts per thousand), representing molecular fossils of methanotrophic archaea. This observation reveals that the seepage fl uids contained methane. Because the seep carbonates have been found to be only moderately (13)C-depleted (delta(13)C values as low as -14 parts per thousand), a signifi cant contribution from a less (13)C-depleted carbon source than methane, probably marine carbonate, is apparent. Such a degree of admixture of marine carbonate is typical for seep limestones resulting from low fl ow rates. The observed biomarker pattern with the prominent occurrence of biphytanes, but lacking crocetane, reveals that the methanotrophic archaea at the Hauterivian seep site were similar to archaea of the ANME-1 cluster. Archaea of this cluster are known to be able to cope with lower methane concentrations than ANME-2 archaea; therefore ANME-1 archaea are better adapted to low seepage rates and diffusive fl ow. The Peregrinella limestone contains only a small amount of early diagenetic cement. Based on a comparison with biomarker patterns of other ancient seep deposits, it is apparent that diffusive seepage typically results in limestones with little cement, whereas advective, more intense seepage appears to favor cement precipitation. If applied with caution, this supposed relationship can be used as a fi rst approximation of seepage intensity.

A new constraint on the antiquity of anaerobic oxidation of methane: Late Pennsylvanian seep limestones from southern Namibia

Late Pennsylvanian seep limestones (ca. 300 Ma) enclosed in the Ganigobis shales in southern Namibia formed by microbial activity. The process that induced carbonate precipitation was the anaerobic oxidation of methane. The presence of 13 C-depleted pentamethylicosane (PMI) (−113‰) and a mixture of crocetane and phytane (−112‰) in concert with similarly 13 C-depleted pseudohomologous series of regular isoprenoids reveals that methanotrophic archaea oxidized methane anaerobically at the ancient seep site. Biphytane and a C 39 pseudohomologue are other archaeal molecular fossils with δ 13 C values of −99‰ and −97‰, respectively. The former presence of sulfate-reducing bacteria as the syntrophic partners of methanotrophic archaea in the anaerobic oxidation of methane is indicated by isotopically depleted iso - and anteiso -alkanes. These compounds most probably derive from non-isoprenoidal monoethers and diethers, synthates of sulfate-reducing bacteria. These findings show that anaerobic oxidation of methane is at least 300 m.y. old, extending the record of this process for ~140 m.y. As the molecular fossils of archaea and bacteria are preserved in a product of their own metabolic activity (i.e., methane-derived carbonates with δ 13 C values as low as −51‰), the syngenicity of molecular fossils and enclosing deposits is unambiguous. This reveals that microbially formed rocks can represent excellent archives for studying past biogeochemical processes.

A Microbial Mat of a Large Sulfur Bacterium Preserved in a Miocene Methane-Seep Limestone

A Miocene methane-seep limestone from the Romagna Apennine (Pietralunga, Italy) was found to contain an extraordinarily well-preserved microbial mat consisting of filamentous fossils. Individual filaments of the lithified Pietralunga mat are 50 to 80 μ m in diameter and resemble the sulfide-oxidizing bacterium Beggiatoa. Mats of sulfur bacteria are common around modern methane-seeps, but have not yet been reported from ancient seep limestones. This is thought to be related to the conditions prevailing in metabolically active mats of sulfur bacteria that do not favor carbonate formation. The preservation of the Pietralunga mat was most likely caused by a sudden change from oxidizing to anoxic conditions, leading to the rapid carbonate precipitation induced by anaerobic oxidation of methane. Lipid biomarkers specific for archaea and sulfate-reducing bacteria linked with the anaerobic oxidation of methane co-occur with compounds derived from methanotrophic bacteria and ciliates. These findings confirm a close proximity of oxic and anoxic conditions, as required for the growth of sulfide-oxidizing bacteria in the methane-based ecosystem. The lack of earlier reports on fossilized thiotrophic mats in seep limestones is most likely related to the rarity of environmental changes rapid enough to preserve the filaments rather than to a lower frequency of thiotrophic mats around methane-seeps in the geological past.

A LATE DEVONIAN HYDROCARBON-SEEP DEPOSIT DOMINATED BY DIMERELLOID BRACHIOPODS, MOROCCO

Abstract A spectacular carbonate deposit from the western Meseta of Morocco consists of microbial and brachiopod limestones. The small, meter-sized Famennian limestone blocks are embedded in Carboniferous shales, implying later displacement. Primary deposition of the limestones apparently occurred below the depth limit of phototrophic organisms. The brachiopods belong to the genus Dzieduszyckia within the rhynchonellide superfamily Dimerelloidea. They occur in fantastic abundance, in a low-diversity faunal assemblage. The microbial limestones contain fossilized threadlike microorganisms of unknown affiliation. The enormous accumulation of brachiopods on the Famennian seafloor is best explained by the former existence of a seep. Crude oil was a major component of the seepage fluids, as indicated by the occurrence of pyrobitumen (metamorphosed petroleum) and δ13Ccarbonate values as low as −12‰, akin to the isotopic composition of carbonates forming at modern oil seeps. The lowest δ13C values in the Dzieduszyckia deposit are found for a carbonate phase referred to as banded/botryoidal cement. It is a fibrous, low-Mg calcite cement in the form of isopachous rims and botryoids that precipitated directly on and within brachiopod shells. The microbial limestones predominantly consist of this phase and therefore can be classified as cement framestones. Reticulate patterns of botryoidal aggregates and an elevated Sr content agree with an aragonite precursor of the banded/botryoidal cement. The interpretation of the Dzieduszyckia deposit as seep related supports the hypothesis that Paleozoic and Mesozoic members of the rhynchonellide superfamily Dimerelloidea were a lineage long associated with seeps that survived from Late Devonian to Early Cretaceous time.

The Late Eocene 'Whiskey Creek' methane-seep deposit (western Washington State) - Part II: Petrology, stable isotopes, and biogeochemistry

SummaryThe Late Eocene ‘Whiskey Creek’ deposit (Pysht Formation, Olympic Penisula, Washington State) formed at a methane-seep. Early diagenetic micrites and aragonite cement have δ13C values as low as −36‰ indicating that the seepage fluids contained methane. With respect to micrite samples, low δ13C values correlate with relatively high δ13O values andvice versa. Ongoing micrite formation after the cessation of the seepage during increased burial might have altered the isotopic composition of the microcrystalline carbonates toward lower δ13O values and higher δ13C values. Alternatively, the trend in isotope values may reflect a change in the composition of seepage fluids. The principal difference between these scenarios is the duration of seepage with respect to micrite formation. Two petrographically similar varieties of blocky calcite spar are related to different carbonate sources. The δ13C values range from −32 to −29‰ for one type of blocky spar and are either the result of methane oxidation or indicate thermal decarboxylation of organic matter. Low δ18O values are in favour of the latter. For the other type of spar, δ13C values as high as +6‰ indicate carbonate formation within the zone of methanogensis.The ‘Whiskey Creek’ limestone exhibits a chaotic fabric produced by a variety of processes, including bioturbation, concretionary carbonate formation, earlyin situ brecciation, carbonate corrosion, and late fracturing of the rock. Two varieties of micrite aggregates are responsible for the nodular fabric of the limestone. Smoothly-shaped pyritiferous micrite nodules are of diagenetic origin and formed in a manner similar to that which produces carbonate concretions. Apart from being induced by anaerobic oxidation of methane, their formation is proposed to be linked to iron reduction and sulphide formation. The second, dominant variety is represented by irregularly-shaped, nodular to angular micrite aggregates surrounded by massive rims of pyrite, resulting from carbonate corrosion. A pure, fluorescent seam-micrite, constructive in origin, lines cavities or surrounds micritic aggregates.Among authigenic non-carbonate minerals, large quartz crystals and quartz replacing aragonite cement are common constituents of the limestone. Gypsum and an unidentified prismatic calcium sulphate mineral formed in the limestone matrix. The prismatic mineral is tentatively identified as bassanite, the Ca-sulphate hemihydrate. Gypsum is the potential precursor of bassanite, the formation of which could have been induced by elevated temperatures, highly saline solutions, or both.The wide range of δ34S values obtained from ‘Whiskey Creek’ pyrite and the strong depletion in34S (S values as low as −15‰) reveal that bacterial sulphate reduction was the sulphide generating process. Comparative analyses of pyrite from other seep deposits yielded even lower values (as low as −28‰) possibly indicating the involvement of microbial disproportionation of sulphur.

Lipid Biomarker Patterns of Phosphogenic Sediments from Upwelling Regions

Sediments of upwelling regions off Namibia, Peru, and Chile contain dense populations of large nitrate-storing sulfide-oxidizing bacteria, Thiomargarita, Beggiatoa, and Thioploca. Increased contents of monounsaturated C16 and C18 fatty acids have been found at all stations studied, especially when a high density of sulfide oxidizers in the sediments was observed. The distribution of lipid biomarkers attributed to sulfate reducers (10MeC16:0 fatty acid, ai-C15:0 fatty acid, and mono-O-alkyl glycerol ethers) compared to the distribution of sulfide oxidizers indicate a close association between these bacteria. As a consequence, the distributions of sulfate reducers in sediments of Namibia, Peru, and Chile are closely related to differences in the motility of the various sulfide oxidizers at the three study sites. Depth profiles of mono-O-alkyl glycerol ethers have been found to correlate best with the occurrence of large sulfide-oxidizing bacteria. This suggests a particularly close link between mono-O-alkyl glycerol ether-synthesizing sulfate reducers and sulfide oxidizers. The interaction between sulfide-oxidizing bacteria and sulfate-reducing bacteria reveals intense sulfur cycling and degradation of organic matter in different sediment depths.

BACTERIALLY MEDIATED FORMATION OF DIAGENETIC ARAGONITE AND NATIVE SULFUR IN ZECHSTEIN CARBONATES (UPPER PERMIAN, CENTRAL GERMANY)

Abstract Neoformed diagenetic aragonite associated with organic-rich aggregations and native sulfur occur within cavities of the Ca2 and Ca3 Zechstein carbonates in Central Germany. The cavities were formerly filled with gypsum. Bacterial sulfate reduction favored the precipitation of the carbonate phase. This can be attributed to an increase in alkalinity accompanied with sulfate reduction. The high Sr concentrations of the neoformed aragonites compared to the low concentrations in the carbonate host rocks point to sulfate dissolution as the cation supplying process for the precipitation of aragonite. Low δ13C values (−10‰ PDB) of the aragonite indicate that some of its carbon is derived from organic matter that has been oxidized by bacterial sulfate reduction. Aragonite inclusions bear rhomb-shaped crystals of calcite, replacing former dolomite. Elevated Mg/Ca ratios due to this dedolomitization may have promoted the precipitation of aragonite instead of calcite. The aragonite precipitated in the near-surface meteoric–vadose zone in recent times. Aragonite crystals display a platy habit. SEM analyses show that two types of micro-rods are associated with these plates. The mineralized micro-rods are interpreted to be fossilized bacteria. Aragonite inclusions, most of which contain organic-rich aggregations, yield a distinctive biomarker pattern. High concentrations of specific unsaturated fatty acids are clearly indicative of newly produced organic matter and reflect the presence of a discrete microbial community being associated with the formation of the aragonite. At one of the studied localities the aragonite is accompanied by native sulfur. The formation of sulfur was mediated by H2S-oxidizing bacteria. This is corroborated by the presence of densely packed curved rods representing permineralized bacterial cells on and within the sulfur.