Distribution of stable carbon isotope in coalbed methane from the east margin of Ordos Basin

The commercial recovery of methane from coal is well established in the coalbed methane (CBM) blocks at the east margin of Ordos Basin, China. CBM forms with various carbon isotopic ratios (δ¹³C_PDB) due to the carbon isotopic fractionation in biogenical or thermogenical processes. Based on the geologic evolution of coalbed reservoir and studies on the characteristics of δ¹³C_PDB values distributed spatially (e.g., horizontal CBM well location area, vertical coal burial zone, coal rank, etc.) and temporally (e.g., geologic evolution history), we explored the formation mechanism of carbon isotopic of methane. The relatively low δ¹³C_PDB values are widely distributed along the research area, indicating a trend of “lighter-heavier-lighter” from north to south. From a combination analysis of the relationship between δ¹³C_PDB and the relative effects, the essential aspects in determining CBM carbon isotope being light in the study area are: the genesis of secondary biogas in the north; water soluble effects in the active hydrodynamic areas in the middle; desorption fractionation effect promoted by tectonic evolution in the south; and the sudden warming hydrocarbon fractionation accelerated by magmatic event in particular areas (e.g., Linxian).     

Kinetic disequilibrium of C, He, Ar and carbon isotopes during degassing of mid-ocean ridge basalts

Vesicle characteristics (vesicularity, largest vesicle size, number of vesicles/cm²), CO₂-δ¹³C and CO₂-⁴He-⁴⁰Ar-⁴⁰Ar/³⁶Ar in vesicles and CO₂-δ¹³C in the glass have been measured in 19 tholeiitic basalt glasses from the Easter Microplate East Ridge (East Pacific Rise) collected at 3 different sites (26°S East Ridge, Pito Seamount and Pito Deep at 23°S).Carbon supersaturation values (C_melt/C_solubility) vary from 1.3 to 4.3. Carbon supersaturation values are strongly correlated with the number of vesicles/cm². There is also a correlation between number of vesicles/cm² and vesicle size. At the Pito Seamount site, there is a negative correlation between carbon supersaturation values and observed carbon isotope fractionation between CO₂ in vesicles and carbon dissolved in the glass (Δ¹³C_observed). High ⁴He/⁴⁰Ar* ratios in vesicles (from 49 to 190) are observed in both the most and least carbon supersaturated samples, while samples with intermediate carbon supersaturation have the lowest ⁴He/⁴⁰Ar* ratios (16±1). These correlations show that most quenched melts record different disequilibrium to equilibrium states during closed-system degassing.The samples showing the highest carbon supersaturation (4.3) have the highest ⁴He/⁴⁰Ar* (from 94 to 190). This observation shows for the first time that the ⁴He/⁴⁰Ar* ratio can be kinetically fractionated during incomplete degassing of magmas from the magma chamber to the seafloor. This result implies that high ⁴He/⁴⁰Ar* ratios are not a systematic indicator of open-system degassing (Rayleigh distillation) and that caution should be taken when using this ratio for any degassing correction.A two-stage degassing model, with the first stage being a closed-system degassing occurring between the source and the magma chamber, and the second stage of degassing (with a mode varying from open-system degassing to different degrees of kinetic closed-system degassing) taking place between the magma chamber and eruption on the seafloor, is the most appropriate to describe the degassing of MORB. Reconstructing initial carbon content of the magma prior to degassing and extrapolating the results to the entire ridge system results in a carbon flux of 1.6_-0.3^+0.6×10¹⁴ g/year. This value implies vigorous exchange of carbon between the mantle and the surface throughout geological times.     

Natural gas seepage in the Gulf of Mexico

Hydrocarbon compositions and δ¹³C values for methane of fourteen natural seep gases and four underwater vents in the northwestern Gulf of Mexico are reported. The C₁/(C₂ + C₃) ratios of the seep gas samples ranged from 68 to greater than 1000, whereas δ_PDB¹³C values varied from ?39.9 to ?65.5‰. Compositions suggest that eleven of the natural gas seeps are produced by microbial degradation whereas the remaining three have a significant thermocatalytically produced component. Contradictions in the inferences drawn from molecular and isotopic compositions make strict interpretation of the origins of a few of the samples impossible.     

Carbon isotope composition of graphite and carbonate minerals from 3.8-AE metamorphosed sediments,Isukasia, Greenland

Graphite comprises about 2% of some of the 3.8-AE metamorphosed sedimentary rocks of Isukasia, west Greenland. δ¹³C_PDB of carbon in this graphite ranges from ?16.0 to ?9.3‰. For those samples that contain both graphite and siderite, Δ_{carbonate-graphite} is about 6; this fractionation is consistent with an inorganic equilibrium between siderite and graphite at roughly 400–500°C. It is likely that graphite found in these rocks formed by the reaction: 6FeCO₃ = 2Fe₃O₄ + 5CO₂ +C in which case it is of little help in determining whether or not organisms were active 3.8 AE ago. The presence of quartz-magnetite-cummingtonite-iron formation in the Isukasia metasedimentary sequence may, ultimately, be one of the most powerful environmental indicators remaining in these rocks.     

Leaky savannas: the significance of lateral carbon fluxes in the seasonal tropics

Globally, dissolved inorganic carbon (DIC) accounts for more than half the annual flux of carbon exported from terrestrial ecosystems via rivers. Here, we assess the relative influences of biogeochemical and hydrological processes on DIC fluxes exported from a tropical river catchment characterized by distinct land cover, climate and geology transition from the wet tropical mountains to the low‐lying savanna plains. Processes controlling changes in river DIC were investigated using dissolved organic carbon, particulate organic carbon and DIC concentrations and stable isotope ratios of DIC (δ¹³C_DIC) at two time scales: seasonal and diel. The recently developed Isotopic Continuous Dissolved Inorganic Carbon Analyser was used to measure diel DIC concentration and δ¹³C_DIC changes at a 15‐min temporal resolution. Results highlight the predominance of biologically mediated processes (photosynthesis and respiration) controlling diel changes in DIC. These resulted in DIC concentrations varying between 3.55 and 3.82 mg/l and δ¹³C_DIC values ranging from ?19.7 ± 0.31‰ to ?17.1 ± 0.08‰. In contrast, at the seasonal scale, we observed wet season DIC variations predominantly from mixing processes and dry season DIC variations due to both mixing processes and biological processes. The observed wet season increases in DIC concentrations (by 6.81 mg/l) and δ¹³C_DIC values of river water (by 5.4‰) largely result from proportional increases in subsurface inflows from the savanna plains (C₄ vegetation) region relative to inflows from the rainforest (C₃ vegetation) highlands. The high DIC river load during the wet season resulted in the transfer of 97% of the annual river carbon load. Therefore, in this gaining river, there are significant seasonal variations in both the hydrological and carbon cycles, and there is evidence of substantial coupling between the carbon cycles of the terrestrial and the fluvial environments. Recent identification of a substantial carbon sink in the savannas of northern Australia during wetter years in the recent past does not take into account the possibility of a substantial, rapid, lateral flux of carbon to rivers and back to the atmosphere. Copyright © 2015 John Wiley & Sons, Ltd.     

Characteristics of δ13CDIC in lakes on the Tibetan Plateau and its implications for the carbon cycle

Dissolved inorganic carbon isotope (δ¹³C_DIC) is an important tool to reveal the carbon cycle in lake systems. However, there are only few studies focusing on the spatial variation of δ¹³C_DIC of closed lakes. Here we analyze the characteristics of δ¹³C_DIC of 24 sampled lakes (mainly closed lakes) across the Qiangtang Plateau (QTP) and identify the driving factors for its spatial variation. The δ¹³C_DIC value of these observed lakes varies in the range of ? 15·0 to 3·2‰, with an average value of ? 1·2‰. The δ¹³C_DIC value of closed lakes is close to the atmospheric isotopic equilibrium value, much higher than that in rivers and freshwater lakes reported before. The high δ¹³C_DIC value of closed lakes is mainly attributed to the significant contribution of carbonate weathering in the catchment and the evasion of dissolved CO₂ induced by the strong evaporation of lake water. The δ¹³C_DIC value of closed lakes has a logarithmic correlation with water chemistry (TDS, DIC and pCO₂), also suggesting that the evapo‐concentration of lake water can influence the δ¹³C_DIC value. The δ¹³C_DIC value shows two opposite logarithmic correlations with lake size depending on the δ¹³C_DIC range. This study suggests that the δ¹³C in carbonates in lacustrine sediments can be taken as an indicator of lake volume variation in closed lakes on QTP. Copyright © 2011 John Wiley & Sons, Ltd.     

Sulfur and carbon isotopic systematics of Guadalupian‐Lopingian (Permian) mid‐Panthalassa: δ34S and δ13C profiles in accreted paleo‐atoll carbonates in Japan

Immediately before the extinction of the end‐Guadalupian (Middle Permian; ca 260 Ma), a significant change to the global carbon cycle occurred in the superocean Panthalassa, as indicated by a prominent positive δ¹³C excursion called the Kamura event. However, the causes of this event and its connection to the major extinction of marine invertebrates remain unclear. To understand the mutual relationships between these changes, we analyzed the sulfur isotope ratio of the carbonate‐associated sulfate (CAS) and HCl‐insoluble residue, as well as the carbon isotope ratio of bulk organic matter, for the Middle‐Upper Permian carbonates of an accreted mid‐oceanic paleo‐atoll complex from Japan, where the Kamura event was first documented. We detected the following unique aspects of the stable carbon and sulfur isotope records. First, the extremely high δ¹³C values of carbonate (δ¹³C_carb) over +5 ‰ during the Capitanian (late Guadalupian) were associated with large isotopic differences between carbonate and organic matter (Δ¹³C = δ¹³C_carb ? δ¹³C_org). We infer that the Capitanian Kamura event reflected an unusually large amount of dissolved organic matter in the expanded oxygen minimum zone at mid‐depth. Second, the δ³⁴S values of CAS (δ³⁴S_CAS) were inversely correlated with the δ¹³C_carb values during the Capitanian to early Wuchiapingian (early Late Permian) interval. The Capitanian trend may have appeared under increased oceanic sulfate conditions, which were accelerated by intense volcanic outgassing. Bacterial sulfate reduction with increased sulfate concentrations in seawater may have stimulated the production of pyrite that may have incorporated iron in pre‐existing iron hydroxide/oxide. This stimulated phosphorus release, which enhanced organic matter production and resulted in high δ¹³C_carb. Low δ³⁴S_CAS values under high sulfate concentrations were maintained and the continuous supply of sulfate cannot by explained only by the volcanic eruption of the Emeishan Trap, which has been proposed as a cause of the extinction. The Wuchiapingian δ³⁴S_CAS–δ¹³C_carb correlation, likely related to low sulfate concentration, may have been caused by the removal of oceanic sulfate through the massive evaporite deposition.     

Dissolved inorganic carbon evolution in neutral discharge from mine tailings piles

We measured the concentrations of dissolved inorganic carbon (DIC) and major ions and the stable carbon isotope ratios of DIC (δ¹³C_DIC) in two creeks discharging from carbonate‐rich sulphide‐containing mine tailings piles. Our aim was to assess downstream carbon evolution of the tailings discharge as it interacted with the atmosphere. The discharge had pH of 6.5–8.1 and was saturated with respect to carbonates. Over the reach of one creek, the DIC concentrations decreased by 1.1 mmol C/l and δ¹³C_DIC increased by ~4.0‰ 200 m from the seep source. The decrease in the DIC concentrations was concomitant with decreases in the partial pressure of CO_2(aq) because of the loss of excess CO_2(aq) from the discharge. The corresponding enrichment in the δ¹³C_DIC is because of kinetic isotope fractionation accompanying the loss of CO_2(g). Over the reach of the other creek, there was no significant decrease in the DIC concentrations or notable changes in the δ¹³C_DIC. The insignificant change in the DIC concentrations and the δ¹³C_DIC is because the first water sample was collected 160 m away from the discharge seep, not accessible during this research. In this case, most of the excess CO_2(aq) was lost before our first sampling station. Our results indicate that neutral discharges from tailings piles quickly lose excess CO_2(aq) to the atmosphere and the DIC becomes enrich in ¹³C. We suggest that a significant amount of carbon cycling in neutral discharges from tailings piles occur close to the locations where the discharge seeps to the surface. Copyright © 2015 John Wiley & Sons, Ltd.     

Investigating landfill‐impacted groundwater seepage into headwater streams using stable carbon isotopes

The impact of landfill contaminated groundwater along a reach of a small stream adjacent to a municipal landfill was investigated using stable carbon isotopes as a tracer. Groundwater below the stream channel, groundwater seeping into the stream, groundwater from the stream banks and stream water were sampled and analysed for dissolved inorganic carbon (DIC) and the isotope ratio of DIC (δ¹³C_DIC). Representative samples of groundwater seeping into the stream were collected using a device (a ‘seepage well’) specifically designed for collecting samples of groundwater seeping into shallow streams with soft sediments. The DIC and δ¹³C_DIC of water samples ranged from 52 to 205 mg C/L and ?16·9 to +5·7‰ relative to VPDB standard, respectively. Groundwater from the stream bank adjacent to the landfill and some samples of groundwater below the stream channel and seepage into the stream showed evidence of δ¹³C enriched DIC (δ¹³C_DIC = ?2·3 to +5·7‰), which we attribute to landfill impact. Stream water and groundwater from the stream bank opposite the landfill did not show evidence of landfill carbon (δ¹³C_DIC = ?10·0 to ?16·9‰). A simple mixing model using DIC and δ¹³C_DIC showed that groundwater below the stream and groundwater seeping into the stream could be described as a mixture of groundwater with a landfill carbon signature and uncontaminated groundwater. This study suggests that the hyporheic zone at the stream–groundwater interface probably was impacted by landfill contaminated groundwater and may have significant ecological implications for this ecotone. Copyright © 2004 John Wiley & Sons, Ltd.     

Tectonic control of bioalteration in modern and ancient oceanic crust as evidenced by carbon isotopes

Abstract We review the carbon‐isotope data for finely disseminated carbonates from bioaltered, glassy pillow rims of basaltic lava flows from in situ slow‐ and intermediate‐spreading oceanic crust of the central Atlantic Ocean (CAO) and the Costa Rica Rift (CRR). The δ¹³C values of the bioaltered glassy samples from the CAO show a large range, between ?17 and +3‰ (Vienna Peedee belemnite standard), whereas those from the CRR define a much narrower range, between ?17‰ and ?7‰. This variation can be interpreted as the product of different microbial metabolisms during microbial alteration of the glass. In the present study, the generally low δ¹³C values (less than ?7‰) are attributed to carbonate precipitated from microbially produced CO₂ during oxidation of organic matter. Positive δ¹³C values >0‰ likely result from lithotrophic utilization of CO₂ by methanogenic Archaea that produce CH₄ from H₂ and CO₂. High production of H₂ at the slow‐spreading CAO crust may be a consequence of fault‐bounded, high‐level serpentinized peridotites near or on the sea floor, in contrast to the CRR crust, which exhibits a layer‐cake pseudostratigraphy with much less faulting and supposedly less H₂ production. A comparison of the δ¹³C data from glassy pillow margins in two ophiolites interpreted to have formed at different spreading rates supports this interpretation. The Jurassic Mirdita ophiolite complex in Albania shows a structural architecture similar to that of the slow‐spreading CAO crust, with a similar range in δ¹³C values of biogenic carbonates. The Late Ordvician Solund–Stavfjord ophiolite complex in western Norway exhibits structural and geochemical evidence for evolution at an intermediate‐spreading mid‐ocean ridge and displays δ¹³C signatures in biogenic carbonates similar to those of the CRR. Based on the results of this comparative study, it is tentatively concluded that the spreading rate‐dependent tectonic evolution of oceanic lithosphere has a significant control on the evolution of microbial life and hence on the δ¹³C biosignatures preserved in disseminated biogenic carbonates in glassy, bioaltered lavas.     

13C/12C ratios of rocks and inclusions in popping rocks of the Mid-Atlantic Ridge and their bearing on the problem of isotopic composition of deep-seated carbon

CO₂-rich inclusions recovered from “popping” and related tholeiitic rocks from the Mid-Atlantic Ridge have δ¹³C values of ?7.6 ± 0.5%. relative to PDB. δ¹³C values of total carbon in the same rocks range from ?12 to ?13.7‰. These values are discussed in the light of the known δ¹³C variations in rocks of deep-seated origin. The ?7.6‰ value is interpreted as a reasonable estimate of the primary value of δ¹³C of deep-seated carbon in the ridge area.     

Deep pooling of low degree melts and volatile fluxes at the 85°E segment of the Gakkel Ridge: Evidence from olivine-hosted melt inclusions and glasses

We present new analyses of volatile, major, and trace elements for a suite of glasses and melt inclusions from the 85°E segment of the ultra-slow spreading Gakkel Ridge. Samples from this segment include limu o pele and glass shards, proposed to result from CO₂-driven explosive activity. The major element and volatile compositions of the melt inclusions are more variable and consistently more primitive than the glass data. CO₂ contents in the melt inclusions extend to higher values (167–1596 ppm) than in the co-existing glasses (187–227 ppm), indicating that the melt inclusions were trapped at greater depths. These melt inclusions record the highest CO₂ melt concentrations observed for a ridge environment. Based on a vapor saturation model, we estimate that the melt inclusions were trapped between seafloor depths (～ 4 km) and ～ 9 km below the seafloor. However, the glasses are all in equilibrium with their eruption depths, which is inconsistent with the rapid magma ascent rates expected for explosive activity. Melting conditions inferred from thermobarometry suggest relatively deep (25–40 km) and cold (1240°–1325 °C) melting conditions, consistent with a thermal structure calculated for the Gakkel Ridge. The water contents and trace element compositions of the melt inclusions and glasses are remarkably homogeneous; this is an unexpected result for ultra-slow spreading ridges, where magma mixing is generally thought to be less efficient based on the assumption that steady-state crustal magma chambers are absent in these environments. All melts can be described by a single liquid line of descent originating from a pooled melt composition that is consistent with the aggregate melt calculated from a geodynamic model for the Gakkel Ridge. These data suggest a model in which deep, low degree melts are efficiently pooled in the upper mantle (9–20 km depth), after which crystallization commences and continues during ascent and eruption. Based on our melting model and the assumption that CO₂ is perfectly incompatible, we show that the highest CO₂ concentrations of the melt inclusions (～ 1600 ppm) are consistent with the calculated CO₂ concentrations of primary undegassed melts. The highest measured CO₂/Nb ratio (443) of Gakkel Ridge melt inclusions predicts a mantle CO₂ content of 134 ppm and would result in a global ridge flux of 2.0 × 10¹² mol CO₂/yr.     

Isotope shifts in the Late Permian of the Delaware Basin,Texas, precisely timed by varved sediments

Closely spaced samples (285 in number) of varved sediments from the Upper Permian in Delaware Basin, Texas, have been analyzed for δ¹³C_carb, δ¹³C_org, δ¹⁸O_carb, C_org, C_carb, and calcite/dolomite. δ¹³C records a dramatic rise from ?2.8 to +5.7‰ in only 4400 years, detected in three sections across the basin, extrapolating smoothly through a 600-year interruption by a local (west side of the basin) fresh-water inflow evidenced by low δ¹⁸O. This continuity and low C_org within the basin, both indicate that the excess net deposition of C_org, necessary to generate the rise in δ¹³C, took place in the ocean external to the Delaware Basin. Correlation with similar records from the Zechstein Basin suggest that the event was world-wide, although this poses obvious difficulties for the carbon cycle. The rate of rise of δ¹³C, and its sustained high level, must imply conversions of oxidized carbon to reduced carbon that are very large depending on which reservoirs were involved.     

Biogeochemical processes involving dissolved CO2 and CH4 at Albano, Averno, and Monticchio meromictic volcanic lakes (Central–Southern Italy)

This paper focuses on the chemical and isotopic features of dissolved gases (CH₄ and CO₂) from four meromictic lakes hosted in volcanic systems of Central–Southern Italy: Lake Albano (Alban Hills), Lake Averno (Phlegrean Fields), and Monticchio Grande and Piccolo lakes (Mt. Vulture). Deep waters in these lakes are characterized by the presence of a significant reservoir of extra-atmospheric dissolved gases mainly consisting of CH₄ and CO₂. The δ¹³C-CH₄ and δD-CH₄ values of dissolved gas samples from the maximum depths of the investigated lakes (from ?66.8 to ?55.6?‰ V-PDB and from ?279 to ?195?‰ V-SMOW, respectively) suggest that CH₄ is mainly produced by microbial activity. The δ¹³C-CO₂ values of Lake Grande, Lake Piccolo, and Lake Albano (ranging from ?5.8 to ?0.4?‰ V-PDB) indicate a significant CO₂ contribution from sublacustrine vents originating from (1) mantle degassing and (2) thermometamorphic reactions involving limestone, i.e., the same CO₂ source feeding the regional thermal and cold CO₂-rich fluid emissions. In contrast, the relatively low δ¹³C-CO₂ values (from ?13.4 to ?8.2?‰ V-PDB) of Lake Averno indicate a prevalent organic CO₂. Chemical and isotopic compositions of dissolved CO₂ and CH₄ at different depths are mainly depending on (1) CO₂ inputs from external sources (hydrothermal and/or anthropogenic); (2) CO₂–CH₄ isotopic exchange; and (3) methanogenic and methanotrophic activity. In the epilimnion, vertical water mixing, free oxygen availability, and photosynthesis cause the dramatic decrease of both CO₂ and CH₄ concentrations. In the hypolimnion, where the δ¹³C-CO₂ values progressively increase with depth and the δ¹³C-CH₄ values show an opposite trend, biogenic CO₂ production from CH₄ using different electron donor species, such as sulfate, tend to counteract the methanogenesis process whose efficiency achieves its climax at the water–bottom sediment interface. Theoretical values, calculated on the basis of δ¹³C-CO₂ values, and measured δ¹³C_TDIC values are not consistent, indicating that CO₂ and the main carbon-bearing ion species (HCO₃ ^?) are not in isotopic equilibrium, likely due to the fast kinetics of biochemical processes involving both CO₂ and CH₄. This study demonstrates that the vertical patterns of the CO₂/CH₄ ratio and of δ¹³C-CO₂ and δ¹³C-CH₄ are to be regarded as promising tools to detect perturbations, related to different causes, such as changes in the CO₂ input from sublacustrine springs, that may affect aerobic and anaerobic layers of meromictic volcanic lakes.     

Geochemical monitoring of the 2002–2003 eruption at Stromboli volcano (Italy): precursory changes in the carbon and helium isotopic composition of fumarole gases and thermal waters

Significant changes in the helium and carbon isotopic composition of shallow thermal waters vs. gas and a crater fumarolic gas have been recorded at Stromboli prior and during the 2002–2003 eruption. The³He/⁴He ratios corrected for air contamination (Rc/Ra), and δ¹³C of fumarolic gases gradually increased from May to November 2002 before the eruption onset. These variations imply early degassing of a gas-rich magma at depth that likely fed both the intense Strombolian activity and small lava overflows recorded during that period. The lava effusion of late December 2002 was shortly preceded by a marked Rc/Ra decrease both in water and fumarolic gases. Comparison of He/CO₂ and CH₄/CO₂ ratios in dissolved gas and with values rules out the Rc/Ra decrease due to an increasing input of radiogenic⁴He. The Rc/Ra decrease is attributed to the He isotope fractionation during rapid magma ascent and degassing. A new uprising of ³He-rich magma probably occurred in January to February 2003, when Rc/Ra ratios displayed the highest values in dissolved gases ever measured before (4.56 Rc/Ra). The increase in He/CO₂ and CH₄/CO₂ ratios and decrease in δ¹³C of dissolved CO₂ was recorded after the 5 April 2003 explosive paroxysm, likely caused by enhanced gas-water interaction inducing CO₂ dissolution. No anomalous Rc/Ra values were recorded in the same period, when usual Strombolian activity gradually resumed.Editorial responsibility: H Shinohara     

Stable carbon isotope signatures of cellulose in macrophytes collected from three eutrophic lakes in China

Aquatic plants are essential for maintaining the diversity and stability of a lake ecosystem. Stable carbon isotopes (δ¹³C) of macrophytes have been widely used as a powerful tool to study ecological processes and paleoenvironmental evolution in lakes. Varying results are obtained when using the δ¹³C of macrophytes to study the changes in the lake environment at different spatio-temporal scales. Thus, sample preparation and subsequent laboratory analyses are crucial for studying environmental changes using the isotopic signal retained in the macrophytes, and are essential for the interpretation of isotope-environment relationships. This study analyzed the δ¹³C of different tissue components of macrophytes in three lakes of the lower Yangtze River basin, and a correlation analysis was performed on aquatic environments influencing the δ¹³C values in the different tissue components of macrophytes. The test results showed the difference between the δ¹³C values of the whole sample and cellulose. Relative analyses indicated that the major factors contributing to the δ¹³C variability in macrophytes were pH and the concentration of dissolved inorganic carbon (DIC). The δ¹³C of α-cellulose (δ¹³C_AC) is more sensitive to environmental variables than that of the whole sample (δ¹³C_W) and holocellulose (δ¹³C_HC). The results of this study imply that extraction of α-cellulose is a prerequisite for research on the changes in lake environment using δ¹³C of macrophytes. This study aims to provide theoretical and data basis for further research on the environmental and ecological change using stable carbon isotopes of aquatic plants.     

CHe in volatile fluxes from the solid Earth: implications for carbon geodynamics

In order to picture C geodynamics past and present, theC³He ratios of the relevant reservoirs are considered. Evaluation of publishedC³He ratio in conjunction with new results for MORB glasses worldwide, suggests that this ratio is unfractionated during magma outgassing, a best estimate being 2 × 10⁹.C³He ratios from other volcanic emissions (hot spots and arcs) do not appear significantly different when the subducted component is omitted.This result permits scaling of the CO₂ degassing flux to that of³He and yields a value of 2 × 10¹² mol/yr which corresponds to a model degassing duration of 3.9 Gyr when recycling to the mantle is disregarded.A bulk Earth chondritic ratio of about 2 × 10⁹ is calculated, very close to the MORB value. On the other hand the reconstructed exospheric (“Rubey inventory”) value of4 ± 1 × 10⁷ is very different from both basaltic and chondritic values.Among the possible interpretations of these results the following two are retained: (1) CO₂ was not released in the early age of the Earth because of the reducing conditions prevailing at that time in the mantle. Formation of the core changed this picture and permitted subsequent degassing of CO₂. (2) Carbonates need a continental crust of significant size to become stabilized in the exosphere. Therefore accumulation in the exosphere was delayed until crustal formation.Alternatively, a similar degassing behaviour for both He and CO₂ requires a massive recycling of carbonates throughout time. This possibility is in contradiction with the present-day maximum recycling rate and the severe imbalance with the observed outgassing flux on one hand and with the small fraction of carbon now present in the exosphere on the other.We conclude that carbon has never been severely degassed. The mantle acts as a buffer for C and most carbon is still retained there, possibly as graphite (or diamond?) or dissolved in minerals.     

13C/12C- und 18O16O-Signaturen von Calcit-Abscheidungen in Drainagesystemen

¹³C/¹²C- and ¹⁸O/16O-signatures of Calcite Precipitations in Drainage Systems Measurements of drainage waters show two distinct processes of calcite precipitation: 1. reprecipitation of calcium carbonate previously dissolved in groundwaters and 2. absorption of atmospheric CO₂ by alkaline solutions. Both processes may be distinguished by the stable isotopes of oxygen and carbon. Calcite precipitated from carbonate groundwater yields δ¹³C ≈ ?13%₀ (PDB) and δ¹⁸O ≈ 24%₀ (SMOW), whereas calcite produced by CO₂-absorption shows δ¹³C ≈ ?25%₀ (PDB) and δ¹⁸O ≈ 10%₀ (SMOW).     

Cenomanian (Upper Cretaceous) carbon isotope stratigraphy of terrestrial organic matter for the Yezo Group, Hokkaido, Japan

Abstract Carbon isotope fluctuations of sedimentary organic matter along the two geological traverses in the Yezo Group, Hokkaido, northern Japan, elucidate a detailed chemostratigraphy for the Cenomanian Stage on the northwestern Pacific margin. Visual characterization of the kerogen from mudstone samples shows that the major constituents of sedimentary organic matter originated as terrestrial higher plants. The atomic hydrogen/carbon ratios of the kerogen suggest that the original δ¹³C values of terrestrial organic matter (TOM) have not been affected significantly by thermal diagenesis. The patterns in two δ¹³C_TOM curves are similar and independent of changes in lithology and total organic carbon contents, which suggests that TOM was mixed sufficiently before the deposition in the Yezo forearc basin for the δ¹³C composition having been homogenized. In addition, this implies that the Hokkaido δ¹³C_TOM profiles represent the averaged temporal δ¹³C variations of terrestrial higher‐plant vegetation in the hinterlands of northeast Asia during Cenomanian time. Three shorter‐term (ca. 0.1 my duration) positive‐and‐negative δ¹³C_TOM fluctuations of ∼1‰ are present in the Lower to Middle Cenomanian interval in the Yezo Group. On the basis of the age‐diagnostic taxa (ammonoids, inoceramids and planktic foraminifers), these discrete δ¹³C_TOM events are interpreted to be correlated with those in the δ¹³C curves of pelagic carbonates from European basins. The correlation of δ¹³C events between the European and Yezo Group sections suggests that the shorter‐term δ¹³C fluctuations in Cenomanian ocean‐atmosphere carbon reservoirs are useful for global chemostratigraphic correlation of marine strata. In particular, the correlation of δ¹³C fluctuations of the so‐called ‘Mid‐Cenomanian event’ (MCE) implies: (i) the δ¹³C variations of global carbon reservoir during the MCE are precisely recorded in the δ¹³C_TOM records; and (ii) the MCE δ¹³C_TOM event is an efficient chronostratigraphic index for the Lower/Middle Cenomanian boundary of the Mid‐Cretaceous sequences.