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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 fluids, we investigated the lipid biomarker inventory of this seep limestone. The dominant biomarkers are [sup.13]C-depleted isoprenoids including tail-to-tail linked pentamethylicosane ([delta][sup.13]C value: -108[per thousand]), representing molecular fossils of methanotrophic archaea. This observation reveals that the seepage fluids contained methane. Because the seep carbonates have been found to be only moderately [sup.13]C-depleted ([delta][sup.13]C values as low as -14[per thousand]), a significant contribution from a less [sup.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 flow 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 flow. 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 first approximation of seepage intensity.

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 [sup.13]C-depleted pentamethylicosane (PMI)(-113[per thousand]) and a mixture of crocetane and phytane (-112[per thousand]) in concert with similarly [sup.13]C-depleted pseudohomologous series of regular isoprenoids reveals that methanotrophic archaea oxidized methane anaerobically at the ancient seep site. Biphytane and a [C.sub.39] pseudohomologue are other archaeal molecular fossils with [delta][sup.13]C values of -99[per thousand] and -97[per thousand], 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 [delta][sup.13]C values as low as -51[per thousand]), 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. Keywords: anaerobic oxidation, methane, archaea, sulfate-reducing bacteria, seeps, isoprenoids, limestones.

A new genus Ibergirhynchia, a member of the rhynchonellide superfamily Dimerelloidea, is described for the species Terebratula contraria Roemer, 1850, from Early Carboniferous deposits of the Harz Mountains, Germany. Ibergirhynchia contraria is from a monospecific brachiopod limestone that formed on top of the drowned Devonian Iberg Reef which persisted as a seamount during Famennian and Early Carboniferous times. Ibergirhynchia contraria is considered a cold seep-related brachiopod based on this locality. Such seep associations have been observed for Mesozoic representatives of the rhynchonellide superfamily Dimerelloidea. Ibergirhynchia is considered the first Paleozoic representative of the family Rhynchonellinidae. Ibergirhynchia resembles Dzieduszyckia externally and may be derived from this dimerelloid.

Early Carboniferous (latest Visean) seep deposits occur on top of the drowned Middle Devonian-Late Devonian Iberg atoll reef, Harz Mountains, Germany. These deposits include limestone with a low-diversity but high-abundance fauna of rhynchonellid brachiopods and rare solemyid bivalves, as well as microbial limestone. Rhynchonellids form dense, autochthonous shell accumulations and are generally articulated. They are closely associated with hydrocarbon-derived carbonates. The carbonates exhibit [[delta].sup.13]C values as low as -32 [per thousand], relative to the Peedee belemnite standard, revealing that they are predominantly hydrocarbon derived. The fauna, carbonate fabrics, and isotope signatures provide unequivocal evidence for a seep origin of the Visean deposit. The occurrence of solemyid bivalves supports this interpretation as members of this family (1) are well known for their relationship with chemoautotrophic bacteria and (2) have been reported from ancient and modern seeps. Possible hydrocarbon sources are thermogenic methane derived from the volcanic base of the Iberg reef or methane from a petroleum reservoir. Keywords: seeps, hydrocarbons, carbonates, chemosynthesis, stable isotopes, Carboniferous.

Massive microbial mats covering up to &meter-high carbonate buildups prosper at methane seeps in anoxic waters of the northwestern Black Sea shelf. Strong [13.sup.C] depletions indicate an incorporation of methane carbon into carbonates, bulk biomass, and specific lipids. The mats mainly consist of densely aggregated archaea (phylogenetic ANME-1 duster) and sulfate-reducing bacteria (Desulfosarcina/Desulfococcus group). If incubated in vitro, these mats perform anaerobic oxidation of methane coupled to sulfate reduction. Obviously, anaerobic microbial consortia can generate both carbonate precipitation and substantial biomass accumulation, which has implications for our understanding of carbon cycling during earlier periods of Earth's history., Until recently, it was believed that only aerobic bacteria, depending on oxygen as an electron acceptor, build up substantial biomass from methane carbon in natural habitats (1). Because biogenic methane [...]

Tubes suspected to be those of vestimentiferan worms are abundant in carbonate boulders at one locality in the lower Oligocene part of the Lincoln Creek Formation along the Canyon River, Grays Harbor County, Washington. The largest tubes exhibit the same general orientation and are arranged in clusters. The tube walls are preserved as aragonite that is, in some cases, replaced by silica. The original tube walls either had a high carbonate content or were indurated very early by aragonite mineralization of the organic wall. The carbonate cements around, on, and inside of the tubes were precipitated due to the microbial oxidation of hydrocarbons at a cold-seep. After lithification, the carbonate fragmented as it slid or slumped, along with other sedimentary debris, downslope into deeper waters. This is one of the few reports of an ancient cold-seep chemosynthetic community dominated by tube worms, and the third report of an allochthonous cold-seep carbonate within a deep-water depositional setting.

Many chemosynthesis-based communities prospering in deep-sea environments rely on the metabolic activity of sulfide-oxidizing bacteria. This is the case for vestimentiferan siboglinid tubeworms, whose demand for nutrition is satisfied predominantly by their en-dosymbiotic bacteria harbored in a specialized organ called the trophosome. Such chemo-symbiosis leads to a significantly lower nitrogen isotope composition of the trophosome than in other types of soft tissue. However, the specific process of nitrogen utilization by sibo-glinids remains unclear. As a key element in the relevant enzymes (nitrogenase and nitrate reductase), molybdenum (Mo) is indispensable in the biogeochemical cycling of nitrogen. The Mo isotope composition (δ98Mo) of siboglinids is thus a potential proxy for decoding the processes involved in nitrogen metabolism. In this study, we found δ98Mo values along the chitinous tube of the vestimentiferan siboglinid Paraescarpia echinospica from the Haima seeps of the South China Sea as negative as -4.59%c (-1.13%c ± 1.75%o, 1SD, n = 19)-the lowest δ98Mo value ever reported for any kind of natural material. It is suggested that this extremely negative Mo isotope composition is caused by preferential utilization of isotopically light Mo by the tubeworm's endosymbionts or epibionts during nitrate reduction. Such Mo isotope signature could provide a means for identifying siboglinid tubeworms, a group of annelids that has previously escaped unambiguous identification due to the lack of mineralized skeleton, in the rock record. [ABSTRACT FROM AUTHOR]