Cold seep deposits of Beauvoisin (Oxfordian; southeastern France) and Marmorito (Miocene; northern Italy): microbially induced authigenic carbonates

The relation of two well-known ancient carbonate deposits to hydrocarbon seepage was confirmed by this study. Archaea are found to be associated with the formation of Oxfordian seep carbonates from Beauvoisin and with a Miocene limestone from Marmorito ("tube-worm limestone"). Carbonates formed due to a mediation by archaea exhibit extremely positive or extremely negative δ¹³C_carbonate values, respectively. Highly positive values (+15‰) reflect the use of ¹³C-enriched CO₂ produced by methanogenesis. Low δ¹³C values of the Marmorito carbonates (–30‰) indicate the oxidation of seepage-derived hydrocarbons. Likewise, the δ¹³C content of specific tail-to-tail linked isoprenoids, biomarkers for archaea, was found to be strikingly depleted in these samples (as low as –115‰). The isotopic signatures corroborate that archaea were involved in the cycling of seepage-derived organic carbon at the ancient localities. Another Miocene limestone ("Marmorito limestone") shows a strong imprint of methanotrophic bacteria as indicated by δ¹³C values of carbonate as low as –40‰ and biomarker evidence. Epifluorescence microscopy and field-emission scanning electron microscopy revealed that bacterial biofilms were involved in carbonate aggregation. In addition to lucinid bivalves previously reported from both localities, we infer that sponges from Beauvoisin and tube worms from Marmorito depended on chemosynthesis as well. Low δ¹³C values of nodules related to sponge taphonomy (–27‰) indicate that sponges might have been linked to an enhanced hydrocarbon oxidation. Tube worm fossils from Marmorito closely resemble chemosynthetic pogonophoran tube worms from Recent cold seeps and are embedded in isotopically light carbonate (δ¹³C –30‰). Received: 13 October 1998 / Accepted: 5 February 1999     

Lipid Biomarkers and Their Stable Carbon Isotopes inAncient Seep Carbonates from SW Taiwan,China

Four massive brecciated, chimney-like, and slender pipe network carbonate samples(JA-4, JA-5, JX-8 and BG-12) were collected from southwestern Taiwan, which were suggested to have formed as a result of anaerobic oxidization of methane(AOM). Considering that the environmental conditions of the carbonates precipitation and the sources of carbon and organic matter need to be further declared, molecular fossils and compound-specific carbon isotopic investigations of the carbonates were conducted in this study. According to lipid biomarkers of 2,6,10,15,19-pentamethyleicosane(PMI) and squalane diagnostic to methanotrophic archaea, as well as the extremely low δ¹³C values(as low as -1¹³.4‰) detected in samples JA-4, JA-5 and JX-8, these carbonates were revealed to be a result of AOM. Based on the varied δ¹³C values of characteristic archaea biomarkers in specific samples, biogenic methane was proposed to be responsible for the formation of samples JA-4 and JA-5, whereas a mixed carbon source of ¹³C-depleted methane and ¹³C-enriched residual CO_2 from methanogenesis was suggested for the carbonate of JX-8 due to the co-occurrence of a highly positive δ¹³ C_carb value(+8‰) and a moderate ¹³C depletion of PMI. The low content of AOM-related biomarkers and the absence of indicators for ANME-2 suggested that these carbonates were formed in weak seep settings. By comparison, no typical lipid biomarkers for methanotrophic archaea was detected in carbonate BG-12. The short-chain and long-chain n-alkanes accounted for 30% and 45% of all hydrocarbons, respectively, with a CPI value of 1.2, suggesting that the n-alkanes were derived from both marine organisms and terrestrial inputs. A low thermal maturity could be revealed by the incomplete equilibrium value of the C³¹αβ 22S/(22S+22R) ratio(0.5), and the carbonate BG-12 was probably deposited in a suboxic condition indicated by a value of Pr/Ph ratio(2.5).     

A comprehensive approach to the study of methane-seep deposits from the Lincoln Creek Formation, western Washington State, USA

ABSTRACT A comprehensive approach using palaeontology, petrography, stable isotope geochemistry and biomarker analyses was applied to the study of seven small methane‐seep carbonate deposits. These deposits are in the Oligocene part of the Lincoln Creek Formation, exposed along the Canyon and Satsop Rivers in western Washington. Each deposit preserves invertebrate fossils, many representing typical seep biota. Authigenic carbonates with δ¹³C values as low as ?51‰ PDB reveal that the carbon is predominately methane derived. Carbonates contain the irregular isoprenoid hydrocarbons 2,6,11,15‐tetramethylhexadecane (crocetane) and 2,6,10,15,19‐pentamethylicosane (PMI), lipid biomarkers diagnostic for archaea. These lipids are strongly depleted in ¹³C (δ¹³C values as low as ?120‰ PDB), indicating that archaea were involved in the anaerobic oxidation of methane. Small filaments preserved in the carbonate may represent methanotrophic archaea. Archaeal methanogenesis induced the formation of a late diagenetic phase, brownish calcite, consisting of dumbbell‐shaped crystal aggregates that exhibit δ¹³C values as high as +7‰ PDB. Clotted microfabrics of primary origin point to microbial mediation of carbonate precipitation. Downward‐directed carbonate aggregation in the seeps produced inverted stromatactoid cavities. Large filaments, interpreted as green algae based on their size, shape, arrangement and biomarkers, imply that deposition occurred, in places, in water no deeper than 210 m.     

Carbon and oxygen stable isotopes in the middle-upper miocene and lower pliocene carbonates of the Eastern Paratethys (Kerch-Taman Region): Palaeoenvironments and post-sedimentation changes

C and O isotope composition of Middle-Upper Miocene and Lower Pliocene carbonates from Kerch-Taman Region (Eastern Paratethys) have been studied in order to reconstruct palaeoenvironmental variability and post-sedimentation changes. The δ¹³C and δ¹⁸О values of the Upper Sarmatian to Lower Pliocene organogenic carbonates reflect the desalinization of paleobasins, global Late Miocene Cooling, and increase in seasonal temperature fluctuations. Isotopic composition of the Middle Sarmatian organogenic carbonates was strongly influenced by evaporation processes, high bioproductivity, and local submarine methane emissions. Warm climate and low bioproductivity together with unstable hydrological regime during the Late Chokrakian and the Karaganian times influenced the isotope composition of primary carbonates. Calcite shell of Spiratella sp. (δ¹³C =–0.4‰ and δ¹⁸О =–0.4‰) from Tarkhanian sediments was formed in warm marine environment. Dolomitization prevails over other secondary mineralization in the studied carbonate rocks. Two groups of secondary dolomites that are characterized by negative and positive δ¹³C values have been recognized. Lowe δ¹³C values (up to–31.4‰) in dolomites indicate the influence of both dissolved inorganic carbon (DIC) from oxidized organic matter (С_org) and methane. Dolomites with positive δ¹³C values (7.0 and 7.8‰) associat with migration of CO₂- and CH₄-containing saline groundwater.     

Geochemical comparison between gas in fluid inclusions and gas produced from the Upper Triassic Xujiahe Formation,Sichuan Basin,SW China

Natural gas in the Xujiahe Formation of the Sichuan Basin is dominated by hydrocarbon (HC) gas, with 78–79% methane and 2–19% C₂₊ HC. Its dryness coefficient (C₁/C_1–5) is mostly < 0.95. The gas in fluid inclusions, which has low contents of CH₄ and heavy hydrocarbons (C₂₊) and higher contents of non-hydrocarbons (e.g. CO₂), is a typical wet gas produced by thermal degradation of kerogen. Gas produced from the Upper Triassic Xujiahe Formation (here denoted field gas) has light carbon isotope values for methane (δ¹³C₁: −45‰ to −36‰) and heavier values for ethane (δ¹³C₂: −30‰ to −25‰). The case is similar for gas in fluid inclusions, but δ¹³C₁ = −36‰ to −45‰ and δ¹³C₂ = −24.8‰ to −28.1‰, suggesting that the gas experienced weak isotopic fractionation due to migration and water washing. The field gas has δ¹³C_CO2 values of −15.6‰ to −5.6‰, while the gas in fluid inclusions has δ¹³C_CO2 values of −16.6‰ to −9‰, indicating its organic origin. Geochemical comparison shows that CO₂ captured in fluid inclusions mainly originated from source rock organic matter, with little contribution from abiogenic CO₂. Fluid inclusions originate in a relatively closed system without fluid exchange with the outside following the gas capture process, so that there is no isotopic fractionation. They thus present the original state of gas generated from the source rocks. These research results can provide a theoretical basis for gas generation, evolution, migration and accumulation in the basin.     

Determining the origin of carbon dioxide and methane in the gaseous emissions of the San Vittorino plain (Central Italy) by means of stable isotopes and noble gas analysis

The chemistry and isotope ratios of He, C (δ¹³C) and H (δD) of free gases collected in the San Vittorino plain, an intramontane depression of tectonic origin, were determined to shed light on mantle degassing in central Italy. The C isotopic composition of CO₂ (δ¹³C–CO₂ −2.0‰ to −3.8‰) and He isotope ratios (R/R_A 0.12–0.27) were used to calculate the fraction of CO₂ originating from mantle degassing vs. sedimentary sources. The results show that CO₂ predominantly (average of 75%) derives from the thermo-metamorphic reaction of limestone. Between 6% and 22% of the CO₂ in the samples derives from organic-rich sedimentary sources. The mantle source accounts for 0–6% of the total CO₂; however, in two samples, located in proximity to the most important faults of the plain, the mantle accounts for 24% and 42%. The presence of faults and fractures allows upward gas migration from a deep source to the Earth’s surface, not only in the peri-Tyrrhenian sector, as generally reported by studies on natural gas emissions in central Italy, but also in the pre-Apennine and Apennine belts. Isotope ratios of CH₄ (δ¹³C–CH₄ −6.1‰ to −22.7‰; δD–CH₄ −9‰ to −129‰) show that CH₄ does not appear to be related to mantle or magma degassing, but it is the product of thermal degradation of organic matter (i.e. thermogenic origin) and/or the reduction of CO₂ (i.e. geothermal origin). Most of the samples appear to be affected by secondary microbial oxidation processes.     

Formation of iron sulfide nodules during anaerobic oxidation of methane

The biomarker compositions of iron sulfide nodules (ISNs; upper Pliocene Valle Ricca section near Rome, Italy) that contain the ferrimagnetic mineral greigite (Fe₃S₄) were examined. In addition to the presence of specific terrestrial and marine biomarkers, consistent with formation in coastal marine sediments, these ISNs contain compounds thought to originate from sulfate reducing bacteria (SRB). These compounds include a variety of low-molecular-weight and branched alkanols and several non-isoprenoidal dialkyl glycerol diethers (DGDs). In addition, archaeal biomarkers, including archaeol, macrocyclic isoprenoidal DGDs and isoprenoidal glycerol dialkyl glycerol tetraethers are also present. Both SRB and archaeal lipid δ¹³C values are depleted in ¹³C (δ¹³C values are typically less than −50‰), which suggests that the SRB and archaea consumed ¹³C depleted methane. These biomarker and isotopic signatures are similar to those found in cold seeps and marine sediments where anaerobic oxidation of methane (AOM) occurs with sulfate serving as the terminal electron acceptor. Association of AOM with formation of greigite-containing ISNs could provide an explanation for documented remagnetization of the Valle Ricca sediments. Upward migration of methane, subsequent AOM and associated authigenic greigite formation are widespread processes in the geological record that have considerable potential to compromise paleomagnetic records.     

Carbon, oxygen, strontium, and sulfur isotopic compositions in late Precambrian rocks of the Patom Complex, central Siberia: Communication 2. Nature of carbonates with ultralow and ultrahigh δ13C values

The Patom Complex is characterized by a unique association of carbonate rocks with ultralow (≤8‰) and ultrahigh (>6‰) δ¹³C values. The thickness, stable isotopic composition along the strike, and lithological and geochemical parameters suggest that these rocks could not form as a result of short-term local events or epigenetic processes. Ultralow δ¹³C values (less than ?8‰) in carbonate rocks of the Zhuya Group, which substantially exceed all the known negative C isotope anomalies in thickness (up to 1000 m) and amplitude (δ¹³C = ?10 ± 2‰), point to sedimentation under conditions of extreme “contamination” of water column by oxidized isotopically light organic (hereafter, light) carbon. The decisive role in this contamination belonged to melting and oxidation of huge volumes of methane hydrates accumulated in sediments during the powerful and prolonged Early Vendian glacial epoch. The accumulation of δ¹³C-depleted carbonates was preceded by the deposition of carbonates with anomalously high δ¹³C values. These carbonates formed at high rates of the burial of organic matter and methane in sediments during periods when the sedimentation basin consumed carbon dioxide from the atmosphere and organic carbon was conserved in sediments.     

Carbon and sulfur isotopic fluctuations associated with the end-Guadalupian mass extinction in South China

Concentrations of total organic matter (TOC), carbon isotopic compositions of carbonate and organic matter (δ¹³C_carb, δ¹³C_org), and sulfur isotopic compositions of carbonate associated sulfate (δ³⁴S_sulfate) across the Guadalupian–Lopingian (G–L) boundary were analyzed from identical samples of Tieqiao section, Laibin, Guangxi province, South China. The δ¹³C_carb values show a positive excursion from − 0.45‰ to the peak of 3.80‰ in the Laibin limestone member of the Maokou Formation, followed by a drastic drop to − 2.60‰ in the lowest Heshan formation, then returned to about 1.58‰. Similar to the trends of the δ¹³C_carb values, Δ¹³C_carb–org values also show a positive excursion followed by a sharp negative shift. The onset of a major negative carbon isotope excursion postdates the end Guadalupian extinction that indicates subsequent severe disturbance of the ocean–atmosphere carbon cycle. The first biostratigraphic δ³⁴S_sulfate values during the G–L transition exhibit a remarkable fluctuation: a dramatic negative shift followed by a rapid positive shift, ranging from 36.88‰ to − 37.41‰. These sulfate isotopic records suggest that the ocean during the G–L transition was strongly stratified, forming an unstable chemocline separating oxic shallow water from anoxic/euxinic deep water. Chemocline excursions, together with subsequent rapid transgression and oceanic anoxia, were likely responsible for the massive diversity decline of the G–L biotic crisis.     

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 (δ¹³C: −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; δ¹³C: −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 (δ¹³C: −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.     

Isotope and chemical composition of gases from mud volcanoes in the Taman Peninsula and problem of their genesis

Variations in the carbon isotope composition in gases and waters of mud volcanoes in the Taman Peninsula are studied. The δ¹³C values in CH₄ and CO₂ vary from ?59.5 to ?44.0‰ (δ¹³C_av = ?52.4 ± 5.4‰) and from ?17.8 to +22.8‰ (δ¹³C_av = +6.9 ± 9.3‰), respectively. In waters from most mud volcanoes of the peninsula, this parameter ranges from +3.3 to +33.1‰, although locally lower values are also recorded (up to ?12‰. Fractionation of carbon isotopes in the CO₂-HCO₃ system corresponds to the isotope equilibrium under Earth’s surface temperatures. The growth of carbon dioxide concentration in the gaseous phase and increase in the HCO₃ ion concentration in their water phase is accompanied by the enrichment of the latter with the heavy ¹³C isotope. The δ¹³C_TDIC value in the water-soluble carbon depends on the occurrence time of water on the Earth’s surface (exchange with atmospheric CO₂, methane oxidation, precipitation of carbonates, and other processes), in addition to its primary composition. In this connection, fluctuations in δ¹³C_TDIC values in mud volcanoes with stagnant waters may amount to 10–20‰. In the clayey pulp, concentrations of carbonate matter recalculated to CaCO₃ varies from 1–4 to 36–50 wt %. The δ¹³C value in the latter ranges from ?3.6 to +8.4‰. Carbonate matter of the clayey pulp represents a mixture of sedimentogenic and authigenic carbonates. Therefore, it is usually unbalanced in terms of the carbon isotope composition with the water-soluble CO₂ forms.     

Carbon isotope fractionation by the marine ammonia-oxidizing archaeon Nitrosopumilus maritimus

Ammonia-oxidizing archaea (AOA) are abundant and widely distributed microorganisms in aquatic and terrestrial habitats. By catalyzing the first and rate limiting step in nitrification, these chemolithoautotrophs play a significant role in the global nitrogen cycle and contribute to primary production. Here, the carbon isotopic fractionation relative to inorganic carbon source was determined for bulk biomass, biphytanes and polar lipid bound sugars of a marine AOA pure culture. Bulk biomass and biphytanes from Nitrosopumilus maritimus showed identical carbon isotope fractionation (ε_DIC/bulk and ε_{DIC/byphytanes}) of ca. −20‰. The glycoside head groups were mainly glucose, mannose and inositol, and exhibited different carbon isotopic composition. In general, these monosaccharides were enriched in ¹³C (ε −6.1‰ to −13.8‰) relative to bulk biomass and biphytanes. The fact that the carbon isotope composition of the biphytanes reflected that of the bulk biomass of N. maritimus suggests that the depletion of ¹³C in both biomass and biphytanes resulted mainly from the carbon isotope discrimination by the bicarbonate-fixing enzyme in the autotrophic hydroxypropionate/hydroxybutyrate cycle. Our results further revealed that lipid compounds represent suitable biomarkers for determining δ¹³C values of archaeal ammonia oxidizers without biosynthetic correction.     

Crassulacean acid metabolism influences D/H ratio of leaf wax in succulent plants

This study sought to characterize hydrogen isotopic fractionation during biosynthesis of leaf wax n-alkanes in succulent plants capable of crassulacean acid metabolism (CAM). The metabolic and physiological features of CAM represent crucial strategies for survival in hot and dry climates and have been hypothesized to impact hydrogen isotope fractionation. We measured the stable carbon and hydrogen isotopic compositions (δ¹³C and δD, respectively) of individual n-alkanes in 20 species of succulent plants from a global collection of the Huntington Botanical Gardens, San Marino, California. Greenhouse conditions and irrigation with water of constant δD value enabled determination of interspecies differences in net D/H fractionation between source water and leaf wax products. Carbon isotope ratios provide constraints on the extent of CAM vs. C₃ photosynthesis and indicate a wide range of CAM use, with δ¹³C values ranging from −33.01‰ to −18.54‰ (C₂₇–C₃₃ n-alkanes) and −26.66‰ to −17.64‰ (bulk tissue). Despite the controlled growth environment, we observed ca. 90‰ interspecies range in δD values from −193‰ to −107‰. A positive correlation between δ¹³C_bulk and δD_C31 values with R² = 0.60 (δ¹³C_C31 and δD_C31 values with R² = 0.41) implicates a metabolic isotope effect as the dominant cause of interspecies variation in the hydrogen isotopic composition of leaf wax n-alkanes in CAM-intermediate plants.     

Stable isotopic signatures of the modern land snail Eremina desertorum from a low-latitude (hot) dry desert—A study from the Petrified Forest,New Cairo,Egypt

This study was conducted on recent desert samples—including (1) soils, (2) plants, (3) the shell, and (4) organic matter from modern specimens of the land snail Eremina desertorum—which were collected at several altitudes (316–360 m above sea level) from a site in the New Cairo Petrified Forest. The soils and shell_{E. desertorum} were analyzed for carbonate composition and isotopic composition (δ¹⁸O, δ¹³C). The plants and organic matter_{E. desertorum} were analyzed for organic carbon content and δ¹³C. The soil carbonate, consisting of calcite plus minor dolomite, has δ¹⁸O values from −3.19 to −1.78‰ and δ¹³C values −1.79 to −0.27‰; covariance between the two values accords with arid climatic conditions. The local plants include C3 and C4 types, with the latter being dominant. Each type has distinctive bulk organic carbon δ¹³C values: −26.51 to −25.36‰ for C3-type, and −13.74 to −12.43‰ for C4-type plants.The carbonate of the shell_{E. desertorum} is composed of aragonite plus minor calcite, with relatively homogenous isotopic compositions (δ¹⁸O_mean = −0.28 ± 0.22‰; δ¹³C_mean = −4.46 ± 0.58‰). Most of the δ¹⁸O values (based on a model for oxygen isotope fractionation in an aragonite-water system) are consistent with evaporated water signatures. The organic matter_{E. desertorum} varies only slightly in bulk organic carbon δ¹³C values (−21.78 ± 1.20‰) and these values suggest that the snail consumed more of C3-type than C4-type plants. The overall offset in δ¹³C values (−17.32‰) observed between shell_{E. desertorum} carbonate and organic matter_{E. desertorum} exceeds the value expected for vegetation input, and implies that 30% of carbon in the shell_{E. desertorum} carbonate comes from the consumption of limestone material.     

Water column methane oxidation adjacent to an area of active hydrate dissociation,Eel river Basin

The role of methane clathrate hydrates in the global methane budget is poorly understood because little is known about how much methane from decomposing hydrates actually reaches the atmosphere. In an attempt to quantify the role of water column methanotrophy (microbial methane oxidation) as a control on methane release, we measured water column methane profiles (concentration and δ¹³C) and oxidation rates at eight stations in an area of active methane venting in the Eel River Basin, off the coast of northern California. The oxidation rate measurements were made with tracer additions of ³H-CH₄.Small numbers of instantaneous rate measurements are difficult to interpret in a dynamic, advecting coastal environment, but combined with the concentration and stable isotope measurements, they do offer insights into the importance of methanotrophy as a control on methane release. Fractional oxidation rates ranged from 0.2 to 0.4% of ambient methane per day in the deep water (depths >370 m), where methane concentration was high (20–300 nM), to near-undetectable rates in the upper portion of the water column (depths <370 m), where methane concentration was low (3–10 nM). Methane turnover time averaged 1.5 yr in the deep water but was on the order of decades in the upper portion of the water column. The depth-integrated water column methane oxidation rates for the deep water averaged 5.2 mmol CH₄ m⁻² yr⁻¹, whereas the upper portion of the water column averaged only 0.14 mmol CH₄ m⁻² yr⁻¹; the depth-integrated oxidation rate for deep water in the 25-km² area encompassing the venting field averaged 2 × 10⁶ g CH₄ yr⁻¹. Stable isotope values (δ¹³C-CH₄) for individual samples ranged from −34 to −52‰ (vs. PDB, Peedee belemnite standard) in the region. These values are isotopically enriched relative to hydrates in the region (δ¹³C-CH₄ about −57 to −69‰), further supporting our observations of extensive methane oxidation in this environment.     

Isotope (C and O) composition of auriferous quartz carbonate veins,central lode system,Gadag Gold Field,Dharwar Craton,India: Implications to source of ore fluids

Carbon (δ¹³C_PDB) and oxygen (δ¹⁸O_SMOW) isotopic compositions of auriferous quartz-carbonate veins (QCVs) of gold deposits from Sangli, Kabuliyatkatti, Nagavi, Nabapur and Mysore mining areas developed on the Central Lode system of the Gadag Gold Field (GGF) in the Neoarchaean Gadag schist belt of the Dharwar Craton, southern India have been examined for the first time to understand the origin of the mineralising fluids. In majority of the samples (46 out of 49), δ¹³C_pdb of carbonates of the QCVs fall in the range from − 2.2‰ to − 9.7‰ and the δ¹⁸O values range from 12.0‰ to 30.5‰ SMOW. The calculated fluid δ¹³C_∑ C compositions for these deposits range from − 2.1‰ to − 9.6‰ and δ¹⁸O_H2O from 6.8‰ to 25.9‰, respectively. Carbonate δ¹³C and fluid δ¹³C_∑ C compositions of the carbonates of the QCVs of the GGF are not only distinct from the carbon isotope range of marine carbonates or meta-sedimentary carbonates of the Chitradurga schist belt, but are consistent with C-isotope values of magmatic (− 5 ± 3‰, Burrows et al., 1986) and/or mantle (− 6 ± 2‰, Ohmoto, 1986) carbonates. As dissolution/decarbonation reactions during metamorphism of pre-existing carbonate/carbonated rocks produce CO₂ with δ¹³C values similar to or more enriched than parent rock, the carbonate or fluid δ¹³C ratios of the QCVs (which fall in the compositional range of mantle/magmatic derived CO₂ or carbonates) obtained in this work cannot be the result of metamorphism. The present study corroborates our previous reports from Ajjanahalli and G.R. Halli gold deposits (Sarangi et al., 2012) occurring in the vicinity of the southern extension of the same crustal scale shear zone on which all the GGF deposits are located.The age of gold mineralisation in this area has been reported to be 2522 ± 6 Ma by Sarma et al., 2011. Chardon et al. (2011) have proposed large-scale remobilization of the older gneissic basement, as well as, emplacement of juvenile granites between 2559 Ma and 2507 Ma, close to the crustal scale shear zone along the eastern margin of the Chitradurga schist belt. Based on these observations and our isotope studies, it is proposed that gold mineralising fluids were derived from mantle/juvenile magmatic melts and were channelled through crustal scale shear zones to give rise to the gold deposits in the GGF.     

Experimental investigation on the carbon isotope fractionation of methane during gas migration by diffusion through sedimentary rocks at elevated temperature and pressure

Molecular transport (diffusion) of methane in water-saturated sedimentary rocks results in carbon isotope fractionation. In order to quantify the diffusive isotope fractionation effect and its dependence on total organic carbon (TOC) content, experimental measurements have been performed on three natural shale samples with TOC values ranging from 0.3 to 5.74%. The experiments were conducted at 90°C and fluid pressures of 9 MPa (90 bar). Based on the instantaneous and cumulative composition of the diffused methane, effective diffusion coefficients of the ¹²CH₄ and ¹³CH₄ species, respectively, have been calculated.Compared with the carbon isotopic composition of the source methane (δ¹³C₁ = −39.1‰), a significant depletion of the heavier carbon isotope (¹³C) in the diffused methane was observed for all three shales. The degree of depletion is highest during the initial non-steady state of the diffusion process. It then gradually decreases and reaches a constant difference (Δ δ = δ¹³C_diff −δ¹³C_source) when approaching the steady-state. The degree of the isotopic fractionation of methane due to molecular diffusion increases with the TOC content of the shales. The carbon isotope fractionation of methane during molecular migration results practically exclusively from differences in molecular mobility (effective diffusion coefficients) of the ¹²CH₄ and ¹³CH₄ entities. No measurable solubility fractionation was observed.The experimental isotope-specific diffusion data were used in two hypothetical scenarios to illustrate the extent of isotopic fractionation to be expected as a result of molecular transport in geological systems with shales of different TOC contents. The first scenario considers the progression of a diffusion front from a constant source (gas reservoir) into a homogeneous “semi-infinite” shale caprock over a period of 10 Ma.In the second example, gas diffusion across a 100 m caprock sequence is analyzed in terms of absolute quantities and isotope fractionation effects. The examples demonstrate that methane losses by molecular diffusion are small in comparison with the contents of commercial size gas accumulations. The degree of isotopic fractionation is related inversely to the quantity of diffused gas so that strong fractionation effects are only observed for relatively small portions of gas.The experimental data can be readily used in numerical basin analysis to examine the effects of diffusion-related isotopic fractionation on the composition of natural gas reservoirs.     

Nanhuan manganese deposits within restricted basins of the southeastern Yangtze Platform,China: Constraints from geological and geochemical evidence

The Nanhuan manganese deposits in the southeastern Yangtze Platform occur in the black shale series in the lower part of the Datangpo Formation. In order to constrain the genesis of the deposits, a detailed study was undertaken that involved field observations, major and trace element analyses, organic carbon analyses, and isotope analyses (C, O, S). The major findings are as follows. (1) The ore-bearing rock series, morphology of the ore bodies, and characteristics of ores in several deposits are similar. The ore minerals are rhodochrosite and manganocalcite. The gangue minerals are mainly quartz, feldspar, dolomite, and illite. Minor apatite and bastnaesite occur in the manganese ores. (2) The ores are enriched in Ca and Mg, whereas they are depleted in Si, Al, K, and Ti compared to wall rocks. The ores normalized to average Post-Archean Australian shale (PAAS) are enriched in Co, Mo, and Sr. The chondrite-normalized rare earth element (REE) patterns for ores and wall rocks are between those of typical hydrogenous and hydrothermal type manganese deposits. Additionally, the ores have positive Ce anomalies with an average Ce/Ce* of 1.23 and positive Eu anomalies with an average Eu/Eu* of 1.18 (normalized to PAAS). (3) The average content of organic carbon is 2.21% in the samples, and the average organic carbon isotopic value (δ¹³C_V-PDB) is − 33.44‰. The average inorganic carbon isotopic value (δ¹³C_V-PDB) of carbonates in Gucheng is − 3.07‰, while the values are similar in the other deposits with an average of − 8.36‰. The oxygen isotopic compositions (δ¹⁸O_V-PDB) are similar in different deposits with an average of − 7.72‰. (4) The sulfur isotopic values (δ³⁴S_V-CDT) of pyrite are very high and range from + 37.9‰ to + 62.6‰ (average of 52.7‰), which suggests that the pyrite was formed in restricted basins where sulfate replenishment was limited. The sulfate concentrations in the restricted basins were extremely low and enriched in δ³⁴S, which resulted in the very high δ³⁴S values for the pyrite that formed in the manganese deposits. Therefore, a terrigenous weathering origin for manganese can be excluded; otherwise, the sulfate would have been introduced into the basins together with terrigenous manganese, which would have decreased the δ³⁴S values of pyrites. The manganese, which originated from hydrothermal processes, was enriched in the restricted and anoxic basins, and then, it was oxidized to manganese oxyhydroxide in the overlying oxic waters whereby the products precipitated into the sediments. The manganese oxyhydroxide in the sediment was then reduced to Mn^2 + and released to the pore waters during the process of diagenesis. Some organic carbon was oxidized to CO₃^2 −, which made the depletion of ¹³C in manganese carbonates. Therefore, we suggest that the Nanhuan manganese deposits are hydrothermal–sedimentary/diagenetic type deposits.     

Predominance of archaea-derived hydrocarbons in an Early Triassic microbialite

We investigated the distribution of lipids in Lower Triassic sedimentary rocks (252–247 myr) from South China, including a shallow water microbialite in the uppermost section of the outcrop. Archaeal derived hydrocarbons were the major constituents of the microbialite from the latest Early Triassic. Among these, we detected (i) abundant C₄₀ acyclic and monocyclic biphytanes (possibly derived from glycerol dialkyl glycerol tetraether lipids) and their degradation products, C_30–39 pseudohomologues and (ii) a C₂₅ head-to-tail linked (regular) isoprenoid hydrocarbon [possibly derived from dialkyl glycerol diether lipids (DGDs)] and its degradation products, C_21–24 pseudohomologues and abundant pristane and phytane. Through combination of compound-specific stable carbon isotope analysis of isoprenoid hydrocarbons, which had average δ¹³C values of −35‰ to −30‰, and their molecular distribution, it was not possible to unambiguously define the archaeal source for the biphytanes in the microbialite. The δ¹³C values for pristane and phytane were similar to those for head-to-tail linked C_21–25 isoprenoids; potential source organisms for these compounds were halophilic archaea. Except for methane seep microbialites, no other ancient or recent phototrophic microbialites have been reported to contain predominantly archaeal isoprenoid hydrocarbons. Our findings suggest the presence of a new type of microbialite.     

Stable chlorine and carbon isotope measurements of selected chlorinated organic solvents

Stable Cl and C isotope ratio results for 3 selected chlorinated solvents, perchloroethylene (PCE), trichloroethylene (TCE) and 1,1,1-trichloroethane (TCA) provided by 4 different manufacturers are presented. The isotope ratio for all compounds range between −3.5 and +6.0‰ forδ³⁷Cl and from −37.2 to −23.3%. forδ¹³C. The greatest³⁷Cl difference between manufacturers is observed in the TCE samples which showδ³⁷CI values of −2.5%o for PPG, +2.43‰ for ICI and +4.4‰ for DOW. TCAs show a smaller range (−2.4 to +2.0‰), while the TCEs have slightly different³⁷Cl contents. The¹³C data show the most distinctδ¹³C values for PCEs (−23.3 for DOW, −24.1 for Vulcan, −33.8 for PPG and −37.2‰ for ICI) while both TCEs and TCAs show a smallerδ¹³C range, but still distinct differences. These preliminary data suggest that each manufacturer and solvent type may have distinctiveδ⁶³⁷Cl andδ¹³C values. These results show that by using a combination of³⁷Cl and¹³C, there is a potential to indicate a specific source of chlorinated solvents, as well as an ability to delineate contamination episodes caused by these compounds in groundwaters.