Microbial carbon cycling in oligotrophic regional aquifers near the Tono Uranium Mine, Japan as inferred from δC and ΔC values of in situ phospholipid fatty acids and carbon sources

Microorganisms are ubiquitous in deep subsurface environments, but their role in the global carbon cycle is not well-understood. The natural abundance δ¹³C and Δ¹⁴C values of microbial membrane phospholipid fatty acids (PLFAs) were measured and used to assess the carbon sources of bacteria in sedimentary and granitic groundwaters sampled from three boreholes in the vicinity of the Tono Uranium Mine, Gifu, Japan. Sample storage experiments were performed and drill waters analyzed to characterize potential sources of microbial contamination. The most abundant PLFA structures in all waters sampled were 16:0, 16:1ω7c, cy17:0, and 18:1ω7c. A PLFA biomarker for type II methanotrophs, 18:1ω8c, comprised 3% and 18% of total PLFAs in anoxic sedimentary and granitic waters, respectively, sampled from the KNA-6 borehole. The presence of this biomarker was unexpected given that type II methanotrophs are considered obligate aerobes. However, a bacterium that grows aerobically with CH₄ as the sole energy source and which also produces 56% of its total PLFAs as 18:1ω8c was isolated from both waters, providing additional evidence for the presence of type II methanotrophs. The Δ¹⁴C values determined for type II methanotroph PLFAs in the sedimentary (−861‰) and granite (−867‰) waters were very similar to the Δ¹⁴C values of dissolved inorganic carbon (DIC) in each water (∼−850‰). This suggests that type II methanotrophs ultimately derive all their carbon from inorganic sources, whether directly from DIC and/or from CH₄ produced by the reduction of DIC. In contrast, δ¹³C values of type II PLFAs in the sedimentary (−93‰) and granite (−60‰) waters indicate that these organisms use different carbon assimilation schemes in each environment despite very similar δ¹³C_CH4 values (∼−95‰) for each water. The δ¹³C_PLFA values (−28‰ to −45‰) of non-methanotrophic bacteria in the KNA-6 LTL water do not clearly distinguish between heterotrophic and autotrophic metabolisms, but Δ¹⁴C_PLFA values indicate that >65% of total bacteria filtered from the KNA-6 LTL water are heterotrophs. Ancient Δ¹⁴C values (∼−1000‰) of some PLFAs suggest that many heterotrophs utilize ancient organic matter, perhaps from lignite seams within the sedimentary rocks. The more negative range of δ¹³C_PLFA values determined for the KNA-6 granitic water (−42‰ to −66‰) are likely the result of a microbial ecosystem dominated by chemolithoautotrophy, perhaps fuelled by abiogenic H₂. Results of sample storage experiments showed substantial shifts in microbial community composition and δ¹³C_PLFA values (as much as 5‰) during 2-4 days of dark, refrigerated, aseptic storage. However, water samples collected and immediately filtered back in the lab from freshly drilled MSB-2 borehole appeared to maintain the same relative relationships between δ¹³C_PLFA values for sedimentary and granitic host rocks as observed for samples directly filtered under artesian flow from the KNA-6 borehole of the Tono Uranium Mine.     

Stable isotopic evidence in support of active microbial methane cycling in low-temperature diffuse flow vents at 9°50′N East Pacific Rise

A unique dataset from paired low- and high-temperature vents at 9°50′N East Pacific Rise provides insight into the microbiological activity in low-temperature diffuse fluids. The stable carbon isotopic composition of CH₄ and CO₂ in 9°50′N hydrothermal fluids indicates microbial methane production, perhaps coupled with microbial methane consumption. Diffuse fluids are depleted in ¹³C by ∼10‰ in values of δ¹³C of CH₄, and by ∼0.55‰ in values of δ¹³C of CO₂, relative to the values of the high-temperature source fluid (δ¹³C of CH₄ =−20.1 ± 1.2‰, δ¹³C of CO₂ =−4.08 ± 0.15‰). Mixing of seawater or thermogenic sources cannot account for the depletions in ¹³C of both CH₄ and CO₂ at diffuse vents relative to adjacent high-temperature vents. The substrate utilization and ¹³C fractionation associated with the microbiological processes of methanogenesis and methane oxidation can explain observed steady-state CH₄ and CO₂ concentrations and carbon isotopic compositions. A mass-isotope numerical box model of these paired vent systems is consistent with the hypothesis that microbial methane cycling is active at diffuse vents at 9°50′N. The detectable ¹³C modification of fluid geochemistry by microbial metabolisms may provide a useful tool for detecting active methanogenesis.     

Carbon and hydrogen isotope fractionation by moderately thermophilic methanogens

A series of laboratory studies were conducted to increase understanding of stable carbon (¹³C/¹²C) and hydrogen (D/H) isotope fractionation arising from methanogenesis by moderately thermophilic acetate- and hydrogen-consuming methanogens. Studies of the aceticlastic reaction were conducted with two closely related strains of Methanosaeta thermophila. Results demonstrate a carbon isotope fractionation of only 7‰ (α = 1.007) between the methyl position of acetate and the resulting methane. Methane formed by this process is enriched in ¹³C when compared with other natural sources of methane; the magnitude of this isotope effect raises the possibility that methane produced at elevated temperature by the aceticlastic reaction could be mistaken for thermogenic methane based on carbon isotopic content. Studies of H₂/CO₂ methanogenesis were conducted with Methanothermobacter marburgensis. The fractionation of carbon isotopes between CO₂ and CH₄ was found to range from 22 to 58‰ (1.023 ≤ α ≤ 1.064). Greater fractionation was associated with low levels of molecular hydrogen and steady-state metabolism. The fractionation of hydrogen isotopes between source H₂O and CH₄ was found to range from 127 to 275‰ (1.16 ≤ α ≤ 1.43). Fractionation was dependent on growth phase with greater fractionation associated with later growth stages. The maximum observed fractionation factor was 1.43, independent of the δD-H₂ supplied to the culture. Fractionation was positively correlated with temperature and/or metabolic rate. Results demonstrate significant variability in both hydrogen and carbon isotope fractionation during methanogenesis from H₂/CO₂. The relatively small fractionation associated with deuterium during H₂/CO₂ methanogenesis provides an explanation for the relatively enriched deuterium content of biogenic natural gas originating from a variety of thermal environments. Results from these experiments are used to develop a hypothesis that differential reversibility in the enzymatic steps of the H₂/CO₂ pathway gives rise to variability in the observed carbon isotope fractionation. Results are further used to constrain the overall efficiency of electron consumption by way of the hydrogenase system in M. marburgensis, which is calculated to be less than 55%.     

Stable isotope geochemistry of ground and surface waters associated with undisturbed massive sulfide deposits; constraints on origin of waters and water-rock reactions

Stable isotopes (H, O, C) were determined for ground and surface waters collected from two relatively undisturbed massive sulfide deposits (Halfmile Lake and Restigouche) in the Bathurst Mining Camp (BMC), New Brunswick, Canada. Additional waters from active and inactive mines in the BMC were also collected. Oxygen and hydrogen isotopes of surface and shallow groundwaters from both the Halfmile Lake and Restigouche deposits are remarkably uniform (− 13 to − 14‰ and − 85 to − 95‰ for δ¹⁸O_VSMOW and δ²H_VSMOW, respectively). These values are lighter than predicted for northern New Brunswick and, combined with elevated deuterium excess values, suggest that recharge waters are dominated by winter precipitation, recharged during spring melting. Deeper groundwaters from the Restigouche deposit, and from active and inactive mines have heavier δ¹⁸O_VSMOW ratios (up to − 10.8‰) than shallow groundwaters suggesting recharge under warmer climate or mixing with Shield-type brines. Some of the co-variation in Cl concentrations and δ¹⁸O_VSMOW ratios can be explained by mixing between saline and shallow recharge water end-members. Carbon isotopic compositions of dissolved inorganic carbon (DIC) are variable, ranging from − 15 to − 5‰ δ¹³C_VPDB for most ground and surface waters. Much of the variation in the carbon isotopes is consistent with closed system groundwater evolution involving soil zone CO₂ and fracture zone carbonate minerals (calcite, dolomite and siderite; average = − 6.5‰ δ¹³C_VPDB). The DIC of saline Restigouche deposit groundwater is isotopically heavy (∼+ 12‰ δ¹³C_VPDB), indicating carbon isotopic fractionation from methanogenesis via CO₂ reduction, consistent with the lack of dissolved sulfate in these waters and the observation of CH₄-degassing during sampling.     

Carbon-Isotope Characteristics of CO2 and CH4 in Geothermal Springs from the Central Andes

Carbon stable-isotope compositions of coexisting carbon dioxide and methane from geothermal springs across the Central Andes of northern Chile and Bolivia are reported. A total of 60 samples were analyzed for δ¹³C_CO2 and, of these, 10 were selected for δ¹³C_CH4 analyses. The Central Andes are characterized by an active volcanic arc and an unusually thick (up to 75 km) continental crust behind the arc, beneath the high plateau region of the Altiplano. Furthermore, helium-isotope evidence suggests active mantle degassing in a 350-km-wide zone beneath the thick continental crust in the Central Andes (Hoke et al., 1994).

The present results show a wide range of δ¹³C_CO2 (-14.9 to -0.6‰) and a surprisingly heavy δ¹³C_CH4 (?20.9 to ?12.3‰). The difference between δ¹³C_CO2 and δ¹³C_CH4 (δ¹³C_CO2-CH4 ) for individual samples varies between 1.5‰ and 13.5‰. The δ¹³C_CO2 results show wide and overlapping ranges in the samples collected from the Precordillera, the Volcanic Arc (or Western Cordillera), the Altiplano, and the Eastern Cordillera. The widest ranges occur in the Eastern Cordillera (?15.0 to ?4.8‰) and the Altiplano (?20 to ?6‰). The δ¹³C_CO2 results for geothermal samples from the Volcanic Arc range between ?8.0‰ (Surire) and ?0.6‰ (Abra de Nappa), whereas δ¹³C_CO2 measured in gases collected from geothermal springs in the Precordillera range from ?10 to ?5‰.

The relationships between 3He/4He, δ¹³C_CO2 , and δ¹³C_CH4 are used to distinguish between crustal and mantle origins. The wide (21‰) range in the is interpreted to reflect contributions from different CO₂ sources that include organic and inorganic crustal and mantle carbon. Assuming isotopic equilibrium between coexisting methane and carbon dioxide, Δ¹³C_CO2-CH4 suggests very high equilibrium temperatures, in excess of 530°C, for some geothermal systems that also are characterized by a high (up to 63%) mantle-derived helium component.

δ¹³C_CH4 results suggest that methane has not formed by bacteriogenic processes or by thermal decomposition of organic matter, but rather abiogenically through the high-temperature reaction between H₂ and CO₂. The δ¹³C_CH4 results for the samples from the Volcanic Arc and from two CO₂-rich geothermal springs in the Altiplano (Coipasa-2 and Belen de Andamarca) are similar to those reported from hydrothermal fluids emitted from the East Pacific Rise (Welhan, 1988) and White Island, New Zealand (Hulston and McCabe, 1962), suggesting a mantle-derived carbon component in the methane.     

Experimental constraints on Fe isotope fractionation during magnetite and Fe carbonate formation coupled to dissimilatory hydrous ferric oxide reduction

Iron isotope fractionation between aqueous Fe(II) and biogenic magnetite and Fe carbonates produced during reduction of hydrous ferric oxide (HFO) by Shewanella putrefaciens, Shewanella algae, and Geobacter sulfurreducens in laboratory experiments is a function of Fe(III) reduction rates and pathways by which biogenic minerals are formed. High Fe(III) reduction rates produced ⁵⁶Fe/⁵⁴Fe ratios for Fe(II)_aq that are 2-3‰ lower than the HFO substrate, reflecting a kinetic isotope fractionation that was associated with rapid sorption of Fe(II) to HFO. In long-term experiments at low Fe(III) reduction rates, the Fe(II)_aq-magnetite fractionation is −1.3‰, and this is interpreted to be the equilibrium fractionation factor at 22°C in the biologic reduction systems studied here. In experiments where Fe carbonate was the major ferrous product of HFO reduction, the estimated equilibrium Fe(II)_aq-Fe carbonate fractionations were ca. 0.0‰ for siderite (FeCO₃) and ca. +0.9‰ for Ca-substituted siderite (Ca_0.15Fe_0.85CO₃) at 22°C. Formation of precursor phases such as amorphous nonmagnetic, noncarbonate Fe(II) solids are important in the pathways to formation of biogenic magnetite or siderite, particularly at high Fe(III) reduction rates, and these solids may have ⁵⁶Fe/⁵⁴Fe ratios that are up to 1‰ lower than Fe(II)_aq. Under low Fe(III) reduction rates, where equilibrium is likely to be attained, it appears that both sorbed Fe(II) and amorphous Fe(II)(s) components have isotopic compositions that are similar to those of Fe(II)_aq.The relative order of δ⁵⁶Fe values for these biogenic minerals and aqueous Fe(II) is: magnetite > siderite ≈ Fe(II)_aq > Ca-bearing Fe carbonate, and this is similar to that observed for minerals from natural samples such as Banded Iron Formations (BIFs). Where magnetite from BIFs has δ⁵⁶Fe >0‰, the calculated δ⁵⁶Fe value for aqueous Fe(II) suggests a source from midocean ridge (MOR) hydrothermal fluids. In contrast, magnetite from BIFs that has δ⁵⁶Fe ≤0‰ apparently requires formation from aqueous Fe(II) that had very low δ⁵⁶Fe values. Based on this experimental study, formation of low-δ⁵⁶Fe Fe(II)_aq in nonsulfidic systems seems most likely to have been produced by dissimilatory reduction of ferric oxides by Fe(III)-reducing bacteria.     

Kinetic isotopic fractionation during carbonate dissolution in laboratory experiments: Implications for detection of microbial CO2 signatures using δC-DIC

Laboratory experiments on reagent-grade calcium carbonate and carbonate rich glacial sediments demonstrate previously unreported kinetic fractionation of carbon isotopes during the initial hydrolysis and early stages of carbonate dissolution driven by atmospheric CO₂. There is preferential dissolution of Ca¹²CO₃ during hydrolysis, resulting in δ¹³C-DIC values that are significantly lighter isotopically than the bulk carbonate. The fractionation factor for this kinetic isotopic effect is defined as ε_carb. ε_carb is greater on average for glacial sediments (−17.4‰) than for calcium carbonate (−7.8‰) for the < 63 μm size fraction, a sediment concentration of 5 g L⁻¹ and closed system conditions at 5°C. This difference is most likely due to the preferential dissolution of highly reactive ultra-fine particles with damaged surfaces that are common in subglacial sediments. The kinetic isotopic fractionation has a greater impact on δ¹³C-DIC at higher CaCO₃:water ratios and is significant during at least the first 6 h of carbonate dissolution driven by atmospheric CO₂ at sediment concentrations of 5 g L⁻¹. Atmospheric CO₂ dissolving into solution following carbonate hydrolysis does not exhibit any significant equilibrium isotopic fractionation for at least ∼ 6 h after the start of the experiment at 5°C. This is considerably longer than previously reported in the literature. Thus, kinetic fractionation processes will likely dominate the δ¹³C-DIC signal in natural environments where rock:water contact times are short <6-24 h (e.g., glacial systems, headwaters in fluvial catchments) and there is an excess of carbonate in the sediments. It will be difficult apply conventional isotope mass balance techniques in these types of environment to identify microbial CO₂ signatures in DIC from δ¹³C-DIC data.     

Geochemical characterization of secondary microbial gas occurrence in the Songliao Basin, NE China

A suite of natural gases from the northern Songliao Basin in NE China were characterized for their molecular and carbon isotopic composition. Gases from shallow reservoirs display clear geochemical evidence of alteration by biodegradation, with very high dryness (C₁/C₂₊ > 100), high C₂/C₃ and i-C₄/n-C₄ ratios, high nitrogen content and variable carbon dioxide content. Isotopic values show wide range variations (δ¹³C_CH4 from −79.5‰ to −45.0‰, δ¹³C_C2H6 from −53.7‰ to −32.2‰, δ¹³C_C3H8 from −36.5‰ to −20.1‰, δ¹³C_nC4H10 from −32.7‰ to −24.5‰, and δ¹³C_CO2 from −21.6‰ to +10.5‰). A variety of genetic types can be recognized on the basis of chemical and isotopic composition together with their geological occurrence. Secondary microbial gas generation was masked by primary microbial gas and the mixing of newly generated methane with thermogenic methane already in place in the reservoir can cause very complicated isotopic signatures. System openness also was considered for shallow biodegraded gas accumulations. Gases from the Daqing Anticline are relatively wet with ¹³C enriched methane and ¹³C depleted CO₂, representing typically thermogenic origin. Gases within the Longhupao-Da’an Terrace have variable dryness, ¹³C enriched methane and variable δ¹³C of CO₂, suggesting dominant thermogenic origin and minor secondary microbial methane augment. The Puqian-Ao’nan Uplift contains relatively dry gas with ¹³C depleted methane and ¹³C enriched CO₂, typical for secondary microbial gas with a minor part of thermogenic methane. Gas accumulations in the Western Slope are very dry with low carbon dioxide concentrations. Some gases contain ¹³C depleted methane, ethane and propane, indicating low maturity/primary microbial origin. Recognition of varying genetic gas types in the Songliao Basin helps explain the observed dominance of gas in the shallow reservoir and could serve as an analogue for other similar shallow gas systems.     

Stable carbon isotope composition and concentrations of CO2 and CH4 in the deep catotelm of a peat bog

Vertical profiles of concentration and C-isotopic composition of dissolved methane and carbon dioxide were observed over 26 months in the catotelm of a deep (6.5 m) peat bog in Switzerland. The dissolved concentrations of these gases increase with depth while CO₂ predominates over CH₄ (CO₂ ca. 5 times CH₄). This pattern can be reproduced by a reaction-advection-ebullition model, where CO₂ and CH₄ are formed in a ratio of 1:1. The less soluble methane is preferentially lost via outgassing (bubbles). The isotopic fractionation between CO₂ and CH₄ also increases with depth, with α_C values ranging from 1.045 to 1.075. The isotopic composition of the gases traces the passage of respiration-derived CO₂ (from the near surface) through a shallow zone with methanogenesis of low isotopic fractionation (splitting of fermentation-derived acetate). This solution then moves through the catotelm, where methanogenesis occurs by CO₂ reduction (large isotopic fractionation). In the upper part of the catotelm the C-13-depleted respiration-derived CO₂ pool buffers the isotopic composition of CO₂; the δ¹³C of CO₂ increases only slowly. At the same time strongly depleted CH₄ is formed as CO₂ reduction consumes the depleted CO₂. In the lower part of the catotelm, the respiration-derived CO₂ and shallow CH₄ become less important and CO₂ reduction is the dominant source of CO₂ and CH₄. Now, the δ¹³C values of both gases increase until equilibrium is reached with respect to the isotopic composition of the substrate. Thus, the δ¹³C values of methane reach a minimum at intermediate depth, and the deep methane has δ¹³C values comparable to shallow methane. A simple mixing model for the isotopic evolution is suggested. Only minor changes of the observed patterns of methanogenesis (in terms of concentration and isotopic composition) occur over the seasons. The most pronounced of these is a slightly higher rate of acetate splitting in spring.     

Controls on dissolved inorganic carbon and δC in cave waters from DeSoto Caverns: Implications for speleothem δC assessments

Unraveling the factors controlling the carbon chemistry and transport of carbon within extant karst systems has important implications concerning the assessment of time-series δ¹³C records of speleothems. Here we report the results of a 3-year study of total dissolved inorganic carbon [DIC] and δ¹³C_DIC from cave waters at DeSoto Caverns (Southeastern USA) that offer valuable insight on carbon transport and the accompanied isotope fractionations from end-member sources to speleothems.[DIC] and δ¹³C_DIC values of cave waters range from 0.2 to 6.0 mM and 2.7 to −12.9 (‰ VPDB), respectively. [DIC] and δ¹³C_DIC of “seasonal drips” show seasonal, albeit noisy, variability and are inversely related (δ¹³C_DIC = −2.49[DIC] + 0.64, r² = 0.84). A shallow pool fed by multiple drips shows a bimodal δ¹³C_DIC distribution with an isotopically heavier mode during winter (−4‰ to −5‰ VPDB) relative to summer months (−9‰ to −10‰ VPDB). A multi-year trend of decreasing water availability during the study period is not reflected in a response of cave water carbon chemistry suggesting that rainfall amount may not be a significant controlling factor of the carbon chemistry. Coupled cave air winter ventilation/summer stagnation and varying CO₂ fluxes through the soil horizon and epikarst exert the strongest influence on seasonal [DIC] and δ¹³C_DIC variability. Measured values of high [DIC] and low δ¹³C_DIC from cave waters collected during the summer/early fall closely approximate isotopic equilibrium conditions. Conversely, low [DIC] and high δ¹³C_DIC values during winter/early months indicate kinetically enhanced isotopic fractionations within the cave waters. The kinetically enhanced isotopic fractionation of partitioned between degassed CO₂ and precipitated CaCO₃(1000lnα_{[(CO2-HCO3)+(CaCO3(AR)-HCO3)]/2}) is greater by about a factor of two (−6.7 ± 0.3‰) relative to the same isotopic fractionation under equilibrium conditions (−3.1‰).On the basis of ¹⁴C mass balance and paired ¹⁴C-U/Th measurements we estimate that on average about ∼23% of C delivered annually by the drips to the aragonite stalagmites is derived from ¹⁴C-dead dolomite cap while the remainder of ∼77% is derived from ¹⁴C-live biomass. δ¹³C measurements of aragonite (n = 12) sampled from the tips of active speleothems during the summer months are consistent with theoretical aragonite δ¹³C values calculated using the shallow pool summer/early fall data thus confirming the δ¹³C seasonality in both drips and coeval aragonite. δ¹³C values of an active stalagmite section spanning the last 200 years show a normal distribution with a mean of −7.1 ± 1.2‰ (n = 81) and a mode of −7‰ to −8‰ that are statistically indistinguishable from the annual mean and mode of all dripwaters. Thus secular time-series δ¹³C records of stalagmites at DeSoto Caverns with resolving power >10⁻¹ year will likely carry the imprints of drip annual means that record climate-driven δ¹³C seasonal biases.     

The isotopic composition of respired carbon dioxide in scleractinian corals: Implications for cycling of organic carbon in corals

The origin of δ¹³C variations within the skeletons of zooxanthellate scleractinian corals is still a matter of considerable debate. In particular, the role respired CO₂ plays in controlling the eventual δ¹³C of the skeleton remains unclear. In this study, the temporal variability of the δ¹³C of respired CO₂ produced by Montastraea faveolata has been measured at approximately monthly intervals over a 1-year period. In these experiments, three corals maintained on a platform at 8 m depth near Molasses Reef in the Florida Keys were incubated in closed chambers for 24-h periods and samples of the incubation water analyzed for the δ¹³C of the dissolved inorganic carbon (ΣCO₂) at ∼3-h intervals. Throughout the incubation, the concentration of O₂ was measured continuously within the chamber. Our results show that during daylight, the δ¹³C of the ΣCO₂ in the incubation water becomes enriched in ¹³C as a result of fractionation during the fixation of C by photosynthesis, whereas at night the δ¹³C of the ΣCO₂ becomes more negative. The δ¹³C of the respiratory CO₂ ranges from −9‰ in the late spring to values as low as −17‰ in the autumn. The lighter values are significantly more negative than those reported by previous workers for coral tissue and zooxanthellae. An explanation for this discrepancy may be that the corals respire a significant proportion of isotopically negative substances, such as lipids, which are known to have values up to 10‰ lighter compared to the bulk δ¹³C of the tissue. The clear seasonal cycle in the δ¹³C of the respiratory CO₂ suggests that there is also seasonal variability in either the δ¹³C of the coral tissue or the type and/or amount of organic material being respired. A similar temporal pattern and magnitude of change was observed in the δ¹³C of the coral tissue samples collected from a nearby reef at monthly intervals between 1995 and 1997. These patterns are similar in timing to the δ¹³C measured in the coral skeletons. We have also calculated an annual mean value for the fractionation factor between dissolved CO₂⁻ in the external environment and photosynthate fixed by the zooxanthellae of 1.0121 (±0.003). This value is inversely correlated with the ratio of photosynthesis to respiration (P/R) of the entire organism and shows the highest values during the summer months.     

Graphite-bearing CO2-fluid inclusions in granulites: Insights on graphite precipitation and carbon isotope evolution

Graphite in deep crustal enderbitic (orthopyroxene + garnet + plagioclase + quartz) granulites (740°C, 8.9 kb) of Nilgiri hills, southern India were investigated for their spectroscopic and isotopic characteristics. Four types of graphite crystals were identified. The first type (Gr_I), which is interstitial to other mineral grains, can be grouped into two subtypes, Gr_IA and Gr_IB. Gr_IA is either irregular in shape or deformed, and rough textured with average δ¹³C values of −12.7 ± 0.4‰ (n = 3). A later generation of interstitial graphite (Gr_IB) shows polygonal crystal shapes and highly reflecting smooth surface features. These graphite grains are more common and have δ¹³C values of −11.9 ± 0.3‰ (n = 14). Both subtypes show well-defined Raman shifts suggesting a highly crystalline nature. Cores of interstitial graphite grains have, on average, lower δ¹³C values by ∼0.5‰ compared to that of the rim. The second type of graphite (Gr_II) occurs as solid inclusions in silicate minerals, commonly forming regular hexagonal crystals with a slightly disordered structure. The third type of graphite (Gr_III) is associated with solid inclusions (up to 100 μm) that have decrepitation halos of numerous small (<15 μm) satellite fluid inclusions of pure CO₂ with varying density (1.105 to 0.75 g/cm³). The fourth type of graphite (Gr_IV) is found as daughter crystals within primary type CO₂-fluid inclusions in garnet and quartz. These fluid inclusions have a range of densities (1.05 to 0.90 g/cm³), but in general are significantly less dense than graphite-free primary, pure CO₂ fluid inclusions (1.12 g/cm³). Raman spectral characteristics of graphite inside fluid inclusions suggest graphite crystallization at low temperature (∼ 500°C). The precipitation of graphite probably occurred during the isobaric cooling of CO₂-rich peak metamorphic fluid as a result of oxyexsolution of oxide phases. The oxyexsolution process is evidenced by the magnetite-ilmenite granular exsolution textures and the systematic presence of numerous micron-sized rutile and other oxide inclusions in association with fluid inclusions within garnet, plagioclase, and quartz.The carbon isotope compositions of coexisting CO₂ (in fluid inclusions) and graphite show a fractionation (α₂CO−gr) of ∼6‰ in garnet, consistent with the existing theoretical estimates of α₂CO−gr at 800°C. A subsequent generation of CO₂ inclusions trapped in matrix quartz and quartz segregation have higher δ¹³C values, −4‰ and −2.9‰ respectively. Graphite in quartz segregations also has higher δ¹³C values (−9.8‰) than those in enderbite (−12.7‰). Micro-graphite crystals included in garnet, quartz (enderbite), and quartz (segregation) have average δ¹³C values of −11.1, −10.4, and −8.7‰ respectively, indicating progressive enrichment in ¹³C with a decrease in temperature of recrystallization of respective minerals. This progressive enrichment is also observed in carbon isotope compositions of fluid inclusion CO₂, suggesting isotopic equilibrium during graphite precipitation from CO₂ fluids. Thus, the carbon isotope record preserved in these rocks by the interstitial graphite, CO₂ fluid in enderbite, graphite microcrystals, graphite in quartz segregation, and CO₂ fluid in quartz segregation, suggests a temperature-controlled isotopic evolution. This evolution is in accordance with a closed system Rayleigh-type graphite precipitation process which progressively enriched residual CO₂ in ¹³C.     

Carbon and hydrogen isotopic compositions of products of open-system catalytic hydrogenation of CO2: Implications for abiogenic hydrocarbons in Earth’s crust

This paper reports the isotope effects in an open-system Fischer-Tropsch type (FTT) synthesis, with implications for the origin of natural abiogenic hydrocarbons. The starting form of carbon was CO₂, with carbon and hydrogen isotopic compositions measured for products of catalytic hydrogenation of CO₂ on iron and cobalt catalysts (FTCO₂-Fe and FTCO₂-Co) at 350 and 245 °C, respectively, and 10 MPa. The carbon isotopic composition of the resulting saturated hydrocarbons (alkanes) as a function of carbon number shows a positive trend for both FTCO₂-Fe and FTCO₂-Co, with a fractionation of 2-4‰ and 3-6‰ between CH₄ and C₂H₆ over the Fe and Co catalysts, respectively. The unsaturated hydrocarbons (alkenes) do not show any trend. A strong kinetic isotope fractionation (>40‰) occurred between CO₂ and CH₄ in both experiments. The hydrogen isotope fractionation between alkanes appeared to be similar to that found in natural (thermogenic and biogenic) gases, with enrichment in deuterium of longer hydrocarbon chains; the dominant H/D fractionation occurred between CH₄ and C₂H₆. Alkenes in the products of the FTCO₂-Fe reaction are enriched in deuterium (∼50‰) and do not show any trend versus carbon number. We suggest that other than FTT reactions or a simple mixing are responsible for the occurrence of the inverse isotopic trends in both δ¹³C and δD found in light hydrocarbons in some terrestrial environments and meteorites.     

Genesis of fumarolic emissions as inferred by isotope mass balances: CO2 and water at Vulcano Island, Italy

We have developed a quantitative model of CO₂ and H₂O isotopic mixing between magmatic and hydrothermal gases for the fumarolic emissions of the La Fossa crater (Vulcano Island, Italy). On the basis of isotope balance equations, the model takes into account the isotope equilibrium between H₂O and CO₂ and extends the recent model of chemical and energy two-end-member mixing by Nuccio et al. (1999). As a result, the H₂O and CO₂ content and the δD, δ¹⁸O, and δ¹³C isotope compositions for both magmatic and hydrothermal end-members have been assessed. Low contributions of meteoric steam, added at a shallow depth, have been also recognized and quantified in the fumaroles throughout the period from 1988 to 1998. Nonequilibrium oxygen isotope exchange also seems to be occurring between ascending gases and wall rocks along some fumarolic conduits.The δ¹³C_CO2 of the magmatic gases varies around −3 to 1‰ vs. Peedee belemnite (PDB), following a perfect synchronism with the variations of the CO₂ concentration in the magmatic gases. This suggests a process of isotope fractionation because of vapor exsolution caused by magma depressurization. The hydrogen isotopes in the magmatic gases (−1 to −‰ vs. standard mean ocean water [SMOW]), as well as the above δ¹³C_CO2 value, are coherent with a convergent tectonic setting of magma generation, where the local mantle is widely contaminated by fluids released from the subducted slab. Magma contamination in the crust probably amplifies this effect.The computed isotope composition of carbon and hydrogen in the hydrothermal vapors has been used to calculate the δD and δ¹³C of the entire hydrothermal system, including mixed H₂O-CO₂ vapor, liquid water, and dissolved carbon. We have computed values of about 10‰ vs. SMOW for water and −2 to −6.5‰ vs. PDB for CO₂. On these grounds, we think that Mediterranean marine water (δD_H2O ≈ 10‰) feeds the hydrothermal system. It infiltrates at depth throughout the local rocks, reaching oxygen isotope equilibrium at high temperatures. Interaction processes between magmatic gases and the evolving seawater also seem to occur, causing the dissolution of isotopically fractionated aqueous CO₂ and providing the source for hydrothermal carbon. These results have important implications concerning fluid circulation beneath Vulcano and address the more convenient routine of geochemical surveillance.     

Temporal change of C-isotope signatures and methanogenic pathways in rice field soil incubated anoxically at different temperatures

Production of CH₄ and CO₂ was quantified in anoxically incubated soil samples taken from an Italian rice field. The rates increased with temperature between 10 and 37°C. The δ¹³C of the accumulated CO₂, CH₄ and acetate changed with time in a systematic way. The data were used in mass balance equations to constrain isotopic fractionation factors and pathways of CH₄ production. The calculations were further constrained by the determination of ¹⁴CH₄ production from ¹⁴CO₂ at steady state. At 50°C, CH₄ was exclusively produced from CO₂, indicating a fractionation factor of α_CO2/CH4 = 1.073. Between 10 and 37°C, the results showed a temporal change in the methanogenic pathway. A relatively high (40-60%) CO₂-derived fraction of CH₄ production in the beginning was followed by a phase in which contribution of CO₂-derived CH₄ decreased to low (<15%) values, and ultimately by the steady state phase in which values increased to <40% (the theoretically expected value). The rate of change from one phase to the next increased with temperature. Incubation temperature had a strong effect on the overall fractionation of ¹³C during the formation and consumption of acetate, with stronger fractionation at low than at high temperature. The results further showed that, especially at low temperatures, fractionation occurred during acetate turnover and acetoclastic methanogenesis, despite the fact that steady-state conditions caused (apparent) substrate-limitation.     

Soils at the hyperarid margin: The isotopic composition of soil carbonate from the Atacama Desert, Northern Chile

We evaluate the impact of exceptionally sparse plant cover (0-20%) and rainfall (2-114 mm/yr) on the stable carbon and oxygen composition of soil carbonate along elevation transects in what is among the driest places on the planet, the Atacama Desert in northern Chile. δ¹³C and δ¹⁸O values of carbonates from the Atacama are the highest of any desert in the world. δ¹³C (VPDB) values from soil carbonate range from −8.2‰ at the wettest sites to +7.9‰ at the driest. We measured plant composition and modeled respiration rates required to form these carbonate isotopic values using a modified version of the soil diffusion model of [Cerling (1984) Earth Planet. Sci. Lett.71, 229-240], in which we assumed an exponential form of the soil CO₂ production function, and relatively shallow (20-30 cm) average production depths. Overall, we find that respiration rates are the main predictor of the δ¹³C value of soil carbonate in the Atacama, whereas the fraction C₃ to C₄ biomass at individual sites has a subordinate influence. The high average δ¹³C value (+4.1‰) of carbonate from the driest study sites indicates it formed—perhaps abiotically—in the presence of pure atmospheric CO₂.δ¹⁸O (VPDB) values from soil carbonate range from −5.9‰ at the wettest sites to +7.3‰ at the driest and show much less regular variation with elevation change than δ¹³C values. δ¹⁸O values for soil carbonate predicted from local temperature and δ¹⁸O values of rainfall values suggest that extreme (>80% in some cases) soil dewatering by evaporation occurs at most sites prior to carbonate formation. The effects of evaporation compromise the use of δ¹⁸O values from ancient soil carbonate to reconstruct paleoelevation in such arid settings.     

Enrichment of nitrogen and 13C of methane in natural gases from the Khuff Formation,Saudi Arabia,caused by thermochemical sulfate reduction

Permian Khuff reservoirs along the east coast of Saudi Arabia and in the Arabian Gulf produce dry sour gas with highly variable nitrogen concentrations. Rough correlations between N₂/CH₄, CO₂/CH₄ and H₂S/CH₄ suggest that non-hydrocarbon gas abundances are controlled by thermochemical sulfate reduction (TSR). In Khuff gases judged to be unaltered by TSR, methane δ¹³C generally falls between −40‰ and −35‰ VPDB and carbon dioxide δ¹³C between −3‰ and 0‰ VPDB. As H₂S/CH₄ increases, methane δ¹³C increases to as much as −3‰ and carbon dioxide δ¹³C decreases to as little as −28‰. These changes are interpreted to reflect the oxidation of methane to carbon dioxide.Khuff reservoir temperatures, which locally exceed 150 °C, appear high enough to drive the reduction of sulfate by methane. Anhydrite is abundant in the Khuff and fine grained nodules are commonly rimmed with secondary calcite cement. Some cores contain abundant pyrite, sphalerite and galena. Assuming that nitrogen is inert, loss of methane by TSR should increase N₂/CH₄ of the residual gas and leave δ¹⁵N unaltered. δ¹⁵N of Paleozoic gases in Saudi Arabia varies from −7‰ to 1‰ vs. air and supports the TSR hypothesis. N₂/CH₄ in gases from stacked Khuff reservoirs varies by a factor of 19 yet the variation in δ¹⁵N (0.3–0.5‰) is trivial.Because the relative abundance of hydrogen sulfide is not a fully reliable extent of reaction parameter, we have attempted to assess the extent of TSR using plots of methane δ¹³C versus log(N₂/CH₄). Observed variations in these parameters can be fitted using simple Rayleigh models with kinetic carbon isotope fractionation factors between 0.98 and 0.99. We calculate that TSR may have destroyed more than 90% of the original methane charge in the most extreme instance. The possibility that methane may be completely destroyed by TSR has important implications for deep gas exploration and the origin of gases rich in nitrogen as well as hydrogen sulfide.     

Incremental vacuum dehydration-decarbonation experiments on a natural gibbsite (α-Al(OH3)): CO2 abundance and δC values

Incremental vacuum dehydration-decarbonation experiments were performed at 190°C on chemically “cleaned” aliquots of a gibbsite-dominated, Eocene-age bauxite sample with evolution of CO₂ and H₂O. “Plateau” F (CO₂/H₂O ratios) and δ¹³C values of the CO₂ derived from gibbsite were attained over the dehydration interval, X_v(H₂) = 0.16 to 0.67 (i.e., 16 to 67% breakdown of gibbsite). The plateau value of F for gibbsite was 0.0043 ± 0.0003, while the corresponding δ¹³C value of evolved CO₂ was −16.0‰±0.4‰. Additional experiments on chemically cleaned aliquots included (1) treatment with a solution of 0.3M Na-Citrate + 0.1M Na-Dithionite and (2) an exchange experiment with 0.1 bar of ¹³C-depleted CO₂ (−46‰) at 105°C for 64.5 h. Neither of these additional treatments resulted in a measurable perturbation of plateau values of F or δ¹³C for CO₂ evolved from gibbsite during dehydroxylation. These results support published work on Holocene samples which suggested that CO₂ occluded in gibbsite may preserve information on δ¹³C values of CO₂ in ancient terrestrial systems. The plateau values of F observed in the Eocene gibbsite indicate that it may be possible to experimentally calibrate a relationship between the concentration of CO₂ occluded in gibbsite and CO₂ in the environment at the time of crystallization. Such a calibration would significantly enhance the value of gibbsite as a source of information on ancient oxidized carbon systems.     

On the possibility of a kinetic fractionation of nitrogen stable isotopes during natural diamond growth

In a diamond from New South Wales (Australia), cubic and octahedral growth sectors, as identified by cathodoluminescence (CL), show slight differences in N-contents of 29 and 42 ppm respectively but no significant differences in either δ¹³C, δ¹⁵N and nitrogen aggregation state with values at +1.96‰, +19.4‰, and 25% Type IaAB aggregation, respectively.Two gem cubes from the Orapa kimberlite (Botswana) were studied by CL revealing a nonfaceted cubic growth. Accordingly, nine other gem cubes were combusted and yielded δ¹³C-values from -5.33‰ to -6.63‰, δ¹⁵N from -1.0‰ to -5.5‰, and nitrogen contents from 914 to 1168 ppm, with nitrogen aggregation state being only Type IaA (zero % B). The gem cubes show striking similarities to fibrous/coated diamonds, not only in both δ¹³C ranges (less than 3‰ from -5 to -8‰), but also in the high levels of nitrogen (≈ 1000 ppm), suggesting that the two diamond types are related. Additionally, no δ¹⁵N variation was detected between the cube and octahedral growth sectors of the Australian diamond, in the cube sectors of the nine gem cubes from Botswana, nor in fibrous/coated diamonds previously studied. These analyses contrast with an earlier study on a synthetic diamond, which reported a strong kinetic fractionation of N-isotopes of about 40‰ between cube and octahedral growth. The present evidence, therefore, suggests that kinetic fractionation of N-isotopes does not operate during natural diamond formation.     

Grain-scale iron isotopic distribution of pyrite from Precambrian shallow marine carbonate revealed by a femtosecond laser ablation multicollector ICP-MS technique: Possible proxy for the redox state of ancient seawater

The redox state of Precambrian shallow seas has been linked with material cycle and evolution of the photosynthesis-based ecosystem. Iron is a redox-sensitive element and exists as a soluble Fe(II) species or insoluble Fe(III) species on Earth’s surface. Previous studies have shown that the iron isotopic ratio of marine sedimentary minerals is useful for understanding the ocean redox state, although the redox state of the Archean shallow sea is poorly known. This is partly because the conventional bulk isotope analytical technique has often been used, wherein the iron isotopic record may be dampened by the presence of isotopically different iron-bearing minerals within the same sample. Here we report a microscale iron isotopic ratio of individual pyrite grains in shallow marine stromatolitic carbonates over geological time using a newly developed, near-infrared femtosecond laser ablation multicollector ICP-MS technique (NIR-fs-LA-MC-ICP-MS).We have determined that the grain-scale iron isotopic distribution of pyrite from coeval samples shows a bimodal (2.7 and 2.3 Ga) or unimodal pattern (2.9, 2.6, and 0.7 Ga). In particular, pyrite from the 2.7 Ga Fortescue Group shows a unique bimodal distribution with highly positive (+1.0‰ defined as Type 1) and negative δ⁵⁶Fe values (−1.8‰ defined as Type 2). Type 1 and 2 pyrites occasionally occur within different siliceous layers in the same rock specimen. Layer-scale iron isotopic heterogeneity indicates that the iron isotopic ratios of the two types of pyrite are not homogenized by diagenesis after deposition. Some cubic pyrites have a core with a positive δ⁵⁶Fe value (1‰) and a rim with a crustal δ⁵⁶Fe value (0‰). The observed isotopic zoning suggests that the positive δ⁵⁶Fe value is a primary signature at the time of stromatolite formation, while secondary pyrite precipitated during diagenesis.The positive δ⁵⁶Fe value of Type 1 and the large iron isotopic difference between Type 1 and 2 (2.8‰.) suggest partial Fe(II) oxidation in the 2.7-Ga shallow sea, i.e., pyritization of ⁵⁶Fe-enriched ferric oxyhydroxide (Type 1) and ⁵⁶Fe depleted Fe²⁺aq in seawater (Type 2). Type 2 pyrite was probably not produced by microbial iron redox cycling during diagenesis because this scenario requires a higher abundance of pyrite with δ⁵⁶Fe of 0‰ than of −1.8‰. Consequently, the degree of Fe(II) oxidation in the 2.7-Ga shallow sea can be estimated by a Fe²⁺aq steady-state model. The model calculation shows that half the Fe²⁺aq influx was oxidized in the seawater. This implies that O₂ produced by photosynthesis would have been completely consumed by oxidation of the Fe²⁺aq influx. Grain-scale iron isotopic distribution of pyrite could be a useful index for reconstructing the redox state of the Archean shallow sea.