Dissolved Carbon Species Research Articles

Isotope distribution of dissolved carbonate species in southeastern coastal aquifers of Sicily (Italy)

AbstractConcentrations of major ions and the δ13C composition of dissolved inorganic carbon in groundwater and submarine groundwater discharges in the area between Siracusa and Ragusa provinces, southeastern Sicily, representing coastal carbonate aquifers, are presented and discussed. Most of groundwater analysed belongs to calcium bicarbonate type, in agreement with the geological nature of carbonate host rocks. Carbonate groundwater acquires, besides the dissolution of carbonate minerals, dissolved carbon (and the relative isotopic composition) from the atmosphere and from soil biological activity. In fact, δ13C values and total dissolved inorganic carbon contents show that both these sources contribute to carbon dissolved species in the waters studied. Finally, mixing with seawater in the second main factor of groundwater mineralization Copyright © 2007 John Wiley & Sons, Ltd.

Long-Term Effects of Dissolved Carbonate Species on the Degradation of Trichloroethylene by Zerovalent Iron

The effect of different concentrations of total inorganic carbon (TIC) and flow rates on the reactivity of iron metal with trichloroethylene (TCE) was studied in column experiments to verify whether concentration or mass flux of TIC is the major key parameter for barrier performance. First-order rate coefficients (kobs) for TCE degradation vary initially between 0.15 and 0.32 h-' and are positively related to TIC influent concentration. Maximal kobs were reached after 164 and 591 PV, varied between 0.55 and 1.1 h(-1), and were positively correlated to the TIC mass flux, followed by a decrease resulting in values similar to the reference system at the end of the experiments. Enhancement of iron corrosion (0.7 to 3.5 mmol kgFe(-1) d(-1) and formation of gas bubbles during the initial experimental phase were observed and were also positively correlated to TIC mass flux. The higher gas bubble formation probably has a more significant effect on porosity than mineral precipitations in Fe0-systems. The results suggest that higher TIC mass fluxes cause a more pronounced acceleration in CHC degradation, but also a faster inhibition in the longer-term. This faster inhibition has serious implication for the design of funnel and gate systems.

Carbon incorporation during ethene oxidation on Pd(111) studied by in situ X-ray photoelectron spectroscopy at [formula omitted

The oxidation of ethene on the Pd(111) surface was studied in the temperature range of 330–923 K by in situ XPS and mass spectrometry during both heating and cooling in a reaction mixture of 5 × 10 −4 mbar C 2H 4 and 1.5 × 10 −3 mbar O 2. Carbon-containing surface species were found to be strongly predominant over oxygen species within the whole temperature range, despite the excess of oxygen in the gas phase. Diffusion of carbon into the palladium bulk started at 480 K, leading to the appearance of an electronically altered, dissolved carbon phase with a C1s binding energy of 284.45 eV at ∼ 500 K , which extended over several layers in the near-surface region and was stable up to ∼ 650 K . This spectroscopic trend was clearly related to a pronounced shift of catalytic selectivity toward CO. Above 660 K, the dissolved carbon species decomposed, and the reaction occurred on an adsorbate-depleted Pd metal surface, with CO as the main product. During the cooling ramp, the same near-surface carbon modification formed at a 70-degree-lower threshold temperature, inducing pronounced hysteresis of the catalytic selectivity.

Experimental studies of low-T oxygen isotope fractionation between carbonate minerals and water

Oxygen isotope fractionation factors between calcium carbonates and water have been applied to ancient marine geochemistry principally for the purpose of geothermometry. The problem was encountered, however, with respect to the direction and magnitude of O isotope fractionation between calcite and aragonite at thermodynamic equilibrium. This study presents a series of synthesis experiments at 0–70 C to resolve these intriguing issues. The experiments were designed with oneand two-step approaches, respectively, corresponding to the two complex anions of dissolved carbonate prior to aragonite precipitation. Witherite and metastable aragonite were directly synthesized by the direct precipitation and the overgrowth techniques, respectively. The twostep experiments delivered equilibrium fractionations between the precipitated carbonate and water, whereas one-step experiments did not. Significant difference was observed in experiment-derived calcite–aragonite fractionations at above 25 C; the two-step experiments at different periods from 20 to 60 days demonstrate that the dissolved [CO3] 2 and H2O have achieved O isotope equilibrium prior to the aragonite precipitation. Thus this difference cannot be ascribed to rapid (disequilibrium) precipitation of aragonite from either [CO3] 2 or [HCO3] dominant solutions. The experimental results show that the equilibrium fractionations derived from the two-step experiments are independent of the effects of high solution pH, the low O/O ratio of [CO3] 2 and rapid precipitation. Our experiments suggest negligible effect of Mg concentrations on the O isotope fractionations within the analytical error, indicating that no salt effect of Mg cation occurred under the experimental conditions. The O isotope composition of precipitated aragonite does not depend on the concentration of dissolved carbonate species in aqueous solutions provided that the O isotope equilibrium is really achieved thermodynamically between dissolved carbonate and water. Although the effects of the chemical composition of solutions may occur in synthesis experiments on the carbonate–water fractionations, enough attention must be paid to the kinetic mechanism of O isotope exchange and equilibrium in carbonate-water systems. It is of critical importance in judging the achievement of O isotope equilibrium between the dissolved carbonate complex anions and water prior to the precipitation of either aragonite or calcite. According to the currently available observations, it appears that the direction of O isotope fractionation between calcite and aragonite is positive rather than negative at thermodynamic equilibrium.

Mineralogical and geochemical consequences of the long-term presence of CO 2 in natural reservoirs: An example from the Springerville–St. Johns Field, Arizona, and New Mexico, U.S.A.

The Springerville–St. Johns CO 2 field in eastern Arizona and western New Mexico is one of more than a dozen gas fields developed within the Colorado Plateau and Southern Rocky Mountain region. Extensive travertine (CaCO 3) deposits record a long history of CO 2 migration and leakage to the atmosphere. The oldest travertine deposits may have formed during the initial filling of the CO 2 reservoir when groundwaters exsolved CO 2 upon reaching the surface. The youngest travertine deposits are associated with springs on the floor of the Little Colorado River valley, but travertine deposition appears to be insignificant today. Older deposits occur up to 325 m above the valley floor. Geologic relationships suggest travertine deposition began in the late Pleistocene after volcanic activity ended at ∼0.3 Ma. Most of the CaCO 3 could have been derived from dissolution of the underlying limestones and dolomites. Interactions between the reservoir fluids and rocks were observed in core samples from one of the intervals that produced dry gas. These reactions resulted in the dissolution of carbonate cements and detrital feldspars and the formation of dawsonite and kaolinite. Geochemical simulations suggest that the dawsonite could have been deposited when the CO 2 fugacity reached 20 bars and that the kaolinite formed as the CO 2 fugacity decreased. Corrosion of drill pipe by acidic waters and a pronounced HCO 3 anomaly above the CO 2 reservoir provide evidence of a continuing flux of CO 2 from depth. CO 2 storage occurs primarily as dissolved carbonate species and as gas accumulations. Only a small percentage of the CO 2 was sequestered in secondary minerals.

Isotopic, chemical and dissolved gas constraints on spring water from Popocatepetl volcano (Mexico): evidence of gas–water interaction between magmatic component and shallow fluids

Geochemical research was carried out on cold and hot springs at Popocatepetl (Popo) volcano (Mexico) in 1999 to identify a possible relationship with magmatic activity. The chemical and isotopic composition of the fluids is compatible with strong gas–water interaction between deep and shallow fluids. In fact, the isotopic composition of He and dissolved carbon species is consistent with a magmatic origin. The presence of a geothermal system having a temperature of 80–100° C was estimated on the basis of liquid geothermometers. A large amount of dissolved CO 2 in the springs was also detected and associated with high CO 2 degassing.

PH control on oxygen isotopic composition of symbiotic corals

Boron, carbon and oxygen isotopic compositions were determined at the micrometre scale by high-resolution ion microprobe in a sample of modern coral (massive hermatypic coral, Porites lutea). The ion probe data show for B and O much larger isotopic variations at the micrometre scale than those measured at the millimetre scale by conventional techniques: δ18OPDB values range from −10.6±0.9‰ to −0.2±0.5‰ and δ11B values range from +18.6±1.5‰ to +30.6±1.6‰. By contrast, δ13C values show the same range of variations, from −4.6±0.65‰ to −2.2±0.67‰ at the micrometre and millimetre scales. The range of δ11B values indicates that significant pH variations, from ≈7.1 to ≈9.0, are present at the sites of calcification. The largest δ18O variations correspond to the highest δ11B values, i.e. to the highest pHs. This measurement of pH allows modelling the oxygen isotopic fractionation occurring during aragonite precipitation. Taking into account the rate of O isotopic equilibrium between dissolved carbonate species (H2CO3, HCO3− and CO32−) and water via the two reactions of hydration and hydroxylation, the full range of δ18O values measured at the micrometre scale can be modelled for residence times of dissolved carbonates in the calcifying fluid ranging between ≈1 h and at maximum ≈12 h. The pH controls the δ18O of the growing carbonate through the relative fractions of dissolved carbonate species and through the kinetics of their isotopic equilibration with water via hydration and hydroxylation. The so-called ‘vital effect’ systematically observed for δ18O in corals can thus be understood as representing an average of rapid pH variations due to coral biology during coral growth. Selectively measuring δ18O values in the zones of coral skeletons that have low δ11B values (i.e. formed at low pH) should significantly improve the quality of palaeoclimatic reconstructions based on δ18O values.

Identification of the deep-seated component in paleo fluidscirculated through a potential nuclear waste disposal site: Yucca Mountain, Nevada, USA

Using a combination of mineralogical, fluid inclusion and stable isotope techniques, chemical data have been obtained thathelp characterize the hydrogeochemistry of paleogroundwaters that circulated through the vadose zone of Yucca Mountain, Nevada, USA—a prospective site for geological disposal of high-level nuclear waste. The results depict unidirectional evolution of groundwaters from reducing to oxidizing states concomitantly with an overall cooling from ∼ 85 °C to < 35–50 °C. Early calcite has strongly positive δ 13C values (up to +9 %o PDB), which are interpreted as an indication of the partitioning between reduced and oxidized dissolved carbon species in anoxic (reducing) environment. Reducing character of fluids is further supported by the chemistry of gases trapped in inclusions (dominant CH 4, CO 2, very little O 2). The maximum homogenization temperatures of fluid inclusions and reconstructed paleoheat flows form a non-uniform field with prominent maximum near the block-bounding Paintbrush fault. The data support the model of invasion(s) of the deep-seated thermal fluids, into the vadose zone, along the permeable fault zone. Chemical properties of early portions of the fluid were acquired during its long residence in the deep-seated part of the Earth's crust. At advanced stages, the fluid evolved in response to the overall cooling and mixing with oxidized waters from shallow aquifers.

Effects of carbonate species on the kinetics of dechlorination of 1,1,1-trichloroethane by zero-valent iron.

The effect of precipitates on the reactivity of iron metal (Fe0) with 1,1,1-trichloroethane (TCA) was studied in batch systems designed to model groundwaters that contain dissolved carbonate species (i.e., C(IV)). At representative concentrations for high-C(IV) groundwaters (approximately 10(-2) M), the pH in batch reactors containing Fe0 was effectively buffered until most of the aqueous C(IV) precipitated. The precipitate was mainly FeCO3 (siderite) but may also have included some carbonate green rust. Exposure of the Fe0 to dissolved C(IV) accelerated reduction of TCA, and the products formed under these conditions consisted mainly of ethane and ethene, with minor amounts of several butenes. The kinetics of TCA reduction were first-order when C(IV)-enhanced corrosion predominated but showed mixed-order kinetics (zero- and first-order) in experiments performed with passivated Fe0 (i.e., before the onset of pitting corrosion and after repassivation by precipitation of FeCO3). All these data were described by fitting a Michaelis-Menten-type kinetic model and approximating the first-order rate constant as the ratio of the maximum reaction rate (Vm) and the concentration of TCA at half of the maximum rate (K(1/2)). The decrease in Vm/K(1/2) with increasing C(IV) exposure time was fit to a heuristic model assuming proportionality between changes in TCA reduction rate and changes in surface coverage with FeCO3.

Surface complexation modeling of carbonate effects on the adsorption of Cr(VI), Pb(II), and U(VI) on goethite.

Dissolved carbonate species are known to affect the sorption behavior of trace species. The macroscopic description of these interactions with a thermodynamic approach has been limited by the lack of data on the binary interaction between carbonate and relevant mineral surfaces. This work follows from two detailed studies of carbonate adsorption on goethite (4, 13). It shows that independent triple-layer surface complexation modeling (TLM) of carbonate adsorption allows successful descriptions of carbonate-trace element ternary sorption on this oxide, using relatively simple and optimal stoichiometries. Carbonate adsorption was considerably enhanced in the presence of Pb(II), despite an invariant total Pb(II) sorption to equilibration with up to 1% C02(g). Both the Pb(II)-carbonate system behavior and the anion-like pH adsorption behavior of U(VI) in the presence of C02 were successfully modeled using binary and ternary metal-bound surface complexes. The significant reduction of Cr(VI) adsorption edges to lower pH values in the presence of C02 was accurately simulated and explained via site competition and surface electrostatic repulsion effects on the predicted inner- and outer-sphere Cr(VI) surface complexes formed. The results of this research are highly relevant to modeling of metal transport field data and of potential soil remediation schemes using carbonate.

Robust and fast FORTRAN and MATLAB ® libraries to calculate pH distributions in marine systems

Three “easy-to-use solvers” are presented for the approximation of pH and the concentrations of protolytic species in marine aqueous systems. The first one is based on the conservation of alkalinity, the second one describes the progress (advancement) of a chemical reaction towards its state of equilibrium, and the third one is based on the conservation of the charges in the system (charge balance). They are developed for steady state or non-steady state modelling, but they are also useful for pH calculations from measured data sets. To demonstrate their numerical stability over a wide range of pH, the solvers are tested with random input values. Internal consistency is checked by mass and charge conservation and by comparing the results of each solver. In closed systems, equilibrium calculations lead to identical pH values and species concentrations, as expected. The three approaches are also tested in a non-steady-state transport-reaction model that simulates early diagenesis in oxic deep-sea sediments as well as in reactive sediments from the deep Arabian Sea. In the model, dissolved carbonate species are either transported as individual species (CO 2, HCO 3 −, CO 3 2−) or as summed variables (TCO 2, alkalinity). In the first case, the species are transported according to their individual molecular diffusion coefficients and concentration gradients. In the second case, protolytic species are transported along the gradient of summed concentrations using weighted average diffusion coefficients. The advancement approach is coupled with transport of the individual species, whereas the alkalinity conservation and the charge balance approaches are applied to transport of summed variables. The model results show that the approaches lead to non-negligible differences in the species distribution and the pH. With intensive calcium carbonate dissolution, the thermodynamically-controlled dissolution rates dominate the system; in this case, the differences in the species distribution between the three solvers and the two transport schemes can be neglected, but strong differences in the calculated fluxes between sediment and bottom water can be observed.

An evaluation of the inorganic and organic geochemistry of the San Vicente mississippi valley-type zinc-lead district, central Peru; implications for ore fluid composition, mixing processes, and sulfate reduction

Mississippi Valley-type zinc-lead deposits and ore occurrences in the San Vicente belt are hosted in dolostones of the eastern Upper Triassic to Lower Jurassic Pucara basin, central Peru. Combined inorganic and organic geochemical data from 22 sites, including the main San Vicente deposit, minor ore occurrences, and barren localities, provide better understanding of fluid pathways and composition, ore precipitation mechanisms, Eh-pH changes during mineralization, and relationships between organic matter and ore formation. Ore-stage dark replacement dolomite and white sparry dolomite are Fe and rare earth element (REE) depleted, and Mn enriched, compared to the host dolomite. In the main deposit, they display significant negative Ce and probably Eu anomalies. Mixing of an incoming hot, slightly oxidizing, acidic brine (H 2CO 3 being the dominant dissolved carbon species), probably poor in REE and Fe, with local intraformational, alkaline, reducing waters explains the overall carbon and oxygen isotope variation and the distributions of REE and other trace elements in the different hydrothermal carbonate generations. The incoming ore fluid flowed through major aquifers, probably basal basin detrital units, with limited interaction with the carbonate host rocks. The hydrothermal carbonates show a strong regional chemical homogeneity, indicating access of the ore fluids by interconnected channelways near the ore occurrences. Negative Ce anomalies in the main deposit, that are absent at the district scale, indicate local ore-fluid chemical differences. Oxidation of both migrated and indigenous hydrocarbons by the incoming fluid provided the local reducing conditions necessary for sulfate reduction to H 2S, pyrobitumen precipitation, and reduction of Eu 3+ to Eu 2+. Fe-Mn covariations, combined with the REE contents of the hydrothermal carbonates, are consistent with the mineralizing system shifting from reducing/rock-dominated to oxidizing/fluid-dominated conditions following ore deposition. Sulfate and sulfide sulfur isotopes support sulfide origin from evaporite-derived sulfate by thermochemical organic reduction; further evidence includes the presence of 13C-depleted calcite cements (~-12‰ δ 13C) as sulfate pseudomorphs, elemental sulfur, altered organic matter in the host dolomite, and isotopically heavier, late, solid bitumen. Significant alteration of the indigenous and extrinsic hydrocarbons, with absent bacterial membrane biomarkers (hopanes) is observed. The light δ 34S of sulfides from small mines and occurrences compared to the main deposit reflect a local contribution of isotopically light sulfur, evidence of local differences in the ore-fluid chemistry.

Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane

The diagenetic cycling of carbon within recent unconsolidated sediments and soils generally can be followed more effectively by discerning changes in the dissolved constituents of the interstitial fluids, rather than by monitoring changes in the bulk or solid organic components. The major dissolved carbon species in diagenetic settings are represented by the two carbon redox end-members CH4 and CO2. Bacterial uptake by methanogens of either CO2 or “preformed” reduced carbon substrates such as acetate, methanol or methylated amines can be tracked with the aid of carbon (13C/12C) and hydrogen (D/H≡2H/1H) isotopes. The bacterial reduction of CO2 to CH4 is associated with a kinetic isotope effect (KIE) for carbon which discriminates against 13C. This leads to carbon isotope separation between CO2 and CH4 (εC) exceeding 95 and gives rise to δ13CCH4 values as negative as −110‰ vs. PDB. The carbon KIE associated with fermentation of methylated substrates is lower (εC is ca. 40 to 60, with δ13CCH4 values of −50‰ to −60‰). Hydrogen isotope effects during methanogenesis of methylated substrates can lead to deuterium depletions as large as δDCH4=−531‰ vs. SMOW, whereas, bacterial D/H discrimination for the CO2-reduction pathway is significantly less (δDCH4 ca. −170‰ to −250‰). These field observations have been confirmed by culture experiments with labeled isotopes, although hydrogen isotope exchange and other factors may influence the hydrogen distributions. Bacterial consumption of CH4, both aerobic and anaerobic, is also associated with KIEs for C and H isotopes that enrich the residual CH4 in the heavier isotopes. Carbon fractionation factors related to CH4 oxidation are generally less than εC=10, although values >20 are known. The KIE for hydrogen (εH) during aerobic and anaerobic CH4 oxidation is between 95 and 285. The differences in C and H isotope ratios of CH4, in combination with the isotope ratios of the coexisting H2O and CO2 pairs, differentiate the various bacterial CH4 generation and consumption pathways, and elucidate the cycling of labile sedimentary carbon.

The Stable Isotopic Geochemistry of the Sulfur and Carbon Cycles in a Modern Karst Environment

The stable isotopic geochemistry of the sulfur and carbon cycles in a modern karst environment at the southwest border of the Harz Mountains (Germany) has been studied during a 7-year period. Beside major, minor and trace elements, river and sulfate-carbonate ground water were analyzed for the 13C/12C ratios of the dissolved carbonate species, the 18O/16O ratio of water, and the 34S/32S and 18O/16O ratios of dissolved sulfate. Additionally, the carbon, sulfur and oxygen isotopic composition of the major karst aquifers (carbonate and gypsum anhydrite rocks) of the Permian Zechstein formation was measured. Nowadays, the stable isotope biogeochemistry of the river and karst water in the studied area results from the complex interactions between dissolution of biogenic CO2 in the water-unsaturated zone, (minor) subterrestrial microbial decomposition reactions of organic matter (recent DOC or fossil organic matter), interactions with carbonate and sulfate aquifer minerals, and input of acids from atmospheric pollution (e.g., sulfuric acid). Subsurface precipitation of secondary calcite due to the incongruent dissolution of gypsum and/or dolomite is deduced from the hydrogeochemical composition of selected ground waters and the isotopical composition of calcite found in gypsum aquifer material. Spring and river waters were additionally influenced by the liberation of carbon dioxide into the Earth's atmosphere, a process which is accompanied by the preferrential desorbtion of 12CO2. The sulfur isotopic composition of dissolved sulfate from ground and spring waters indicates a mixture of sulfate derived from surface waters (mainly originating from atmospheric pollution) and geogenic sulfate from the subsurface dissolution of gypsum/anhydrite of the Zechstein formation. The oxygen isotopic composition of dissolved sulfate is generally far away from the exchange equilibrium with water, but consistent with the two-component mixing model derived from the sulfur isotope ratios.

An explanation of the effect of seawater carbonate concentration on foraminiferal oxygen isotopes

Abstract Stable oxygen isotope ratios of foraminiferal calcite are widely used in paleoceanography to provide a chronology of temperature changes during ocean history. It was recently demonstrated that the stable oxygen isotope ratios in planktonic foraminifera are affected by changes of the seawater chemistry carbonate system: the δ 18 O of the foraminiferal calcite decreases with increasing CO 3 2− concentration or p H. This paper provides a simple explanation for seawater chemistry dependent stable oxygen isotope variations in the planktonic foraminifera Orbulina universa which is derived from oxygen isotope partitioning during inorganic precipitation. The oxygen isotope fractionation between water and the dissolved carbonate species S = [H 2 CO 3 ] + [HCO 3 − ] + [CO 3 2− ] decreases with increasing p H. Provided that calcium carbonate is formed from a mixture of the carbonate species in proportion to their relative contribution to S , the oxygen isotopic composition of CaCO 3 also decreases with increasing p H. The slope of shell δ 18 O vs. [CO 3 2− ] of Orbulina universa observed in culture experiments is −0.0022‰ (μmol kg −1 ) −1 (Spero et al., 1997) , whereas the slope derived from inorganic precipitation is −0.0024‰ (μmol kg −1 ) − . The theory also provides an explanation of the nonequilibrium fractionation effects in synthetic carbonates described by Kim and O’Neil (1997) which can be understood in terms of equilibrium fractionation at different p H. The results presented here emphasize that the oxygen isotope fractionation between calcium carbonate and water does not only depend on the temperature but also on the p H of the solution from which it is formed.

Chemical Evolution of Groundwater Near a Sinkhole Lake, Northern Florida: 2. Chemical Patterns, Mass Transfer Modeling, and Rates of Mass Transfer Reactions

Chemical patterns along evolutionary groundwater flow paths in silicate and carbonate aquifers were interpreted using solute tracers, carbon and sulfur isotopes, and mass balance reaction modeling for a complex hydrologic system involving groundwater inflow to and outflow from a sinkhole lake in northern Florida. Rates of dominant reactions along defined flow paths were estimated from modeled mass transfer and ages obtained from CFC‐modeled recharge dates. Groundwater upgradient from Lake Barco remains oxic as it moves downward, reacting with silicate minerals in a system open to carbon dioxide (CO2), producing only small increases in dissolved species. Beneath and downgradient of Lake Barco the oxic groundwater mixes with lake water leakage in a highly reducing, silicate‐carbonate mineral environment. A mixing model, developed for anoxic groundwater downgradient from the lake, accounted for the observed chemical and isotopic composition by combining different proportions of lake water leakage and infiltrating meteoric water. The evolution of major ion chemistry and the 13C isotopic composition of dissolved carbon species in groundwater downgradient from the lake can be explained by the aerobic oxidation of organic matter in the lake, anaerobic microbial oxidation of organic carbon, and incongruent dissolution of smectite minerals to kaolinite. The dominant process for the generation of methane was by the CO2 reduction pathway based on the isotopic composition of hydrogen (δ2H(CH4) = −186 to −234‰) and carbon (δ13C(CH4) = −65.7 to −72.3‰). Rates of microbial metabolism of organic matter, estimated from the mass transfer reaction models, ranged from 0.0047 to 0.039 mmol L−1 yr−1 for groundwater downgradient from the lake.

Variations of the oxygen isotope fractionation between NaCO −3 and water due to the presence of NaCl at 100–300°C

The oxygen isotope fractionation between NaCO − 3 and water in the presence of NaCl concentrations up to 4 M at temperatures between 100° and 300°C has been investigated using a new, high-temperature titration technique. Dissolved carbonate is equilibrated at temperature with the experimental solution, and then the carbonate is quantitatively precipitated as BaCO 3 by the injection of BaCl 2 solution at temperature. The isotopic composition of the BaCO 3 reflects the composition of the free water, and hence reflects changes in the oxygen isotope composition of the free water due to the presence of NaCl. Under the experimental conditions used in this study, NaCO − 3 is the dominant dissolved carbonate species, and the oxygen isotope fractionation between NaCO − 3 and water between 100° and 300°C is given by: 1000 ln α = 2.7 (10 6T −2)−5.7 where T is temperature in K. Values of Δ 18O NaCl(solution-water) are generally small and negative (between −1 and 0‰). The small values of Δ 18O NaCl(solution-water) obtained in this study are consistent with data from the studies of Kendall et al. (1983), Zhang et al. (1989), and Horita et al. (1993a), rather than the studies of Truesdell (1974) and Kazahaya (1986), which indicate values of Δ 18O NaCl(solution-water) up to +3‰ for 4 M NaCl solutions at temperatures as high as 250°C. The results have implications for the use of oxygen isotope geothermometers in the presence of brines, and hence to the quantification of temperatures and water compositions in a variety of research areas, such as studies of ore deposits and sedimentary diagenesis, and to the quantification of water/rock ratios in geothermal and metamorphic fluid flow systems.

13C/12C partitioning during synthesis of Na2Ca(CO3)2·2H2O

During the synthesis of Na2Ca(CO3)2·2H2O (pirssonite) from aqueous solutions at 60 and 90 °C, 13C is enriched in the solid with respect to the dissolved carbonate species.

High-accuracy measurements of total dissolved inorganic carbon in the ocean: comparison of alternate detection methods

High-accuracy measurements of total dissolved inorganic carbon in the ocean are currently performed using an automated coulometric system based on that described by Johnson et al. (1987). These measurements require highly-trained technicians and the manipulation of expensive and hazardous chemicals. We tested an alternate detection method based on non-dispersive infra-red analysis. All of the dissolved carbonate species from a seawater sample were extracted as CO 2 gas by acidification and nitrogen stripping. The CO 2 gas was then quantitatively detected with either a coulometric system or an infra-red analyzer. The reproducibility of the two detection methods is similar. Although the infra-red system requires further testing, it holds promise as an alternative to coulometry. The detection of the CO 2 gas by infra-red analysis presents several advantages over coulometric detection: it simplifies and reduces the cost of the measurements, shortens the analysis time, reduces the sample size requirement by at least a factor of five, and allows us to consider complete automation of the system for underway surface seawater measurements.

Oxygen isotope exchange between carbonic acid, bicarbonate, carbonate, and water: A re-examination of the data of McCrea (1950) and an expression for the overall partitioning of oxygen isotopes between the carbonate species and water

An examination of McCREA's work (1950) shows that the 18O 16O ratio of rapidly precipitated calcium carbonate depends upon the pH value of the solution and discloses that the oxygen isotope equilibrium fractionation between the sum of the dissolved carbonate species and water is linearly related to the equilibrium between the carbonate species. This relation permits the calculation of fractionation factors for the individual carbonate species and the expression of an overall fractionation factor in terms of individual factors and the oxygen proportions of the carbonate species. The fractionation factors a H 2 CO 3- H 2 O = 1.0396, a HCO − 3- H 2 O = 1.0346, and a CO 2− 3- H 2 O = 1.0183 (25°C) compare well to recent data.