Anomalous carbon isotope fractionation between atmospheric CO2 and dissolved inorganic carbon induced by intense photosynthesis

A stable isotope mass-balance of dissolved inorganic carbon during a blue-green algae bloom in a softwater lake demonstrates that at low partial pressure of carbon dioxide there must be a large net negative carbon isotope fractionation between atmospheric CO₂ and the CO₂ absorbed by lake water at pH = 9.5. The net fractionation of CO₂(g) with respect to HCO⁻₃ was about −13%. compared with about +8%. for water at equilibrium with atmospheric CO₂ at pH ≈ 7. Chemical enhancement of CO₂ invasion at high pH by the reaction CO₂ + OH⁻→ HCO⁻₃ at large apparent film thicknesses may result in carbon isotope fractionation approaching that for a hydroxide solution. This phenomenon, coupled with a decrease in the photosynthetic fractionation, forced the surface water of a softwater lake to achieve increasingly negative δ¹³C values during an algal bloom, which is in the opposite sense to the trend that results from photosynthesis under less extreme conditions. This and other similar systems must operate under non-equilibrium (kinetic) conditions, causing a large kinetic fractionation during CO₂ invasion at pH > 8 and relatively large film thicknesses (i.e., low wind stress).     

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

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 δ¹⁸O of the foraminiferal calcite decreases with increasing CO₃²⁻ concentration or pH. 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 = [H₂CO₃] + [HCO₃⁻] + [CO₃²⁻] decreases with increasing pH. 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 CaCO₃ also decreases with increasing pH. The slope of shell δ¹⁸O vs. [CO₃²⁻] 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 pH. 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 pH of the solution from which it is formed.     

Does bicarbonate affect the nitrate utilization and photosynthesis of <Emphasis Type="Italic">Orychophragmus violaceus</Emphasis>?

The effect of bicarbonate (HCO₃^?) on the growth and development of plants varies by species. To better understand inorganic carbon and nitrogen assimilation changes of karst-adaptable plants under different HCO₃^? treatments, we conducted experiments on seedlings and in vitro plantlets of Orychophragmus violaceus (Ov). We found that the vital photosynthesis potential (as measured by net photosynthetic rate, actual photochemical efficiency of photosystem-II, photochemical quenching coefficient, and the instantaneous carbon isotope ratio of 3-phosphoglycerate) was consistent under different HCO₃^? treatments of Ov. Bicarbonate’s lack of effect on carbon assimilation of Ov may be related to carbonic anhydrase in Ov converting HCO₃^? to H₂O and CO₂. In this way, Ov could prevent HCO₃^? ion toxicity and high pH from harming its growth and development under HCO₃^? stress. This study also found that high HCO₃^? concentrations could promote nitrogen assimilation and utilization of Ov through changes in related indexes (foliar nitrogen isotope fractionation ratio, stable nitrogen isotope assimilation ratio, foliar stable nitrogen isotope fractionation, nitrate nitrogen utilization efficiency, and nitrate utilization share) under different HCO₃^? treatments. Bicarbonate has different effects on photosynthesis and on inorganic nitrogen assimilation of Ov, which may be connected to photosynthesis providing electrons for nitrate/nitrite reduction through the photosynthetic chain.     

Carbonate and carbon isotopic evolution of groundwater contaminated by produced water brine with hydrocarbons

The major ionic and dissolved inorganic carbon (DIC) concentrations and the stable carbon isotope composition of DIC (δ¹³C_DIC) were measured in a freshwater aquifer contaminated by produced water brine with petroleum hydrocarbons. Our aim was to determine the effects of produced water brine contamination on the carbonate evolution of groundwater. The groundwater was characterized by three distinct anion facies: HCO₃⁻-rich, SO₄²⁻-rich and Cl⁻-rich. The HCO₃⁻-rich groundwater is undergoing closed system carbonate evolution from soil CO_2(g) and weathering of aquifer carbonates. The SO₄²⁻-rich groundwater evolves from gypsum induced dedolomitization and pyrite oxidation. The Cl⁻-rich groundwater is contaminated by produced water brine and undergoes common ion induced carbonate precipitation. The δ¹³C_DIC of the HCO₃⁻-rich groundwater was controlled by nearly equal contribution of carbon from soil CO_2(g) and the aquifer carbonates, such that the δ¹³C of carbon added to the groundwater was −11.6‰. In the SO₄²⁻-rich groundwater, gypsum induced dedolomitization increased the ¹³C such that the δ¹³C of carbon added to the groundwater was −9.4‰. In the produced water brine contaminated Cl⁻-rich groundwater, common ion induced precipitation of calcite depleted the ¹³C such that the δ¹³C of carbon added to the groundwater was −12.7‰. The results of this study demonstrate that produced water brine contamination of fresh groundwater in carbonate aquifers alters the carbonate and carbon isotopic evolution.     

Hydrogen isotope fractionation between kaolinite and water revisited

The equilibrium hydrogen isotope fractionation factor (α) between kaolinite and water in the temperature range 330 to 0°C is 1000 In α_kaol-water = −2.2 × 10⁶T⁻² − 7.7. This monotonic expression is based on a combination of experimental data with >75% of exchange and empirical calibrations. The previously proposed and widely accepted complex fractionation expression is considered to reflect the role of surface and intersite fractionation effects in the low percent of exchange experiments(Liu and Epstein, 1984), and incorrect δD water values for the empirical values (Lambert and Epstein, 1980). There is no measurable fractionation between dickite and kaolinite. The temperature dependence of the kaolinite-water hydrogen isotope fractionation factor can probably be used as a model for other phyllosilicate-water systems below 350°C.     

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

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

An empirical method for the determination of single ion hydrogen isotope salt effects in aqueous electrolyte solutions

An empirical method is presented that allows the determination of the individual contributions of anions and cations to the effect of dissolved salts on hydrogen isotope fractionation in aqueous systems (isotope salt effect). The method is solely based on experimental data and does not involve the choice of arbitrary reference values or theoretical assumptions. Plotting experimental liquid–vapor D/H fractionation factors for aqueous solutions of sodium salts vs. O–D stretching frequencies of water molecules in the hydration shells of the anions shows an excellent linear correlation. The distance between this line and the pure water liquid–vapor fractionation data point in the same plot gives the cation contribution to the isotope salt effects. The anion contribution can then simply be derived as the difference between the total salt effect and the cation salt effect. The validity of the concept is demonstrated using precise literature data for the O–D stretching frequencies in the hydration shells of individual ions at 20°C [Bergström, P.A., 1991. Single ion hydration properties in aqueous solution: a quantitative infrared spectroscopic study. PhD Thesis. Uppsala University] and for the liquid–vapor hydrogen isotope fractionation between aqueous solutions and water vapor at the same temperature [Stewart, M.K., Friedman, I., 1975. Deuterium fractionation between aqueous salt solutions and water vapor. Journal of Geophysical Research 80, 3812–3818]. Within the limits of experimental uncertainties, the data set shows internal consistency. Cation salt effects, 1000 ln Γ at 20°C, are (in per mil per mole per liter, using the convention of Horita et al. [Horita, J., Cole, D.R., Wesolowski, D.J., 1993a. The activity–composition relationship of oxygen and hydrogen isotopes in aqueous salt solutions: II. Vapor–liquid water equilibration of mixed salt solutions from 50–100°C. Geochimica et Cosmochimica Acta 57, 4703–4711]): Na⁺+0.7; K⁺+0.7; Mg²⁺+6.5; Ca²⁺+1.8; Al³⁺+12. The salt effect of H⁺ cannot be determined unequivocally. The combined effect of the fractionation of H⁺ itself plus its salt effect is +4.9. Anion effects are +1.4 for Cl⁻, +2.7 for Br⁻, +3.5 for I⁻ and −1.4 for SO₄²⁻. Further single anion salt effects are being predicted as −1.8 for F⁻, +4.9 for NO₃⁻, +6.9 for ClO₄⁻ and +5.4 for the triflate ion (CF₃SO₃⁻).     

Kinetic fractionation of carbon-13 during calcium carbonate precipitation

Kinetic isotope fractionation of ¹³C during precipitation of CaCO₃ under open system conditions has been investigated. The isotope enrichment factor ?_HCO3^?-CaCO₃ varies between ?0.35 ± 0.23 and ?3.37 ± 0.36%. at 25°C depending on the rate of precipitation and mineralogy, the enrichment of ¹³C in the solid carbonate phase decreasing with increasing precipitation rate. An estimate of equilibrium ?_HCO3-Calcite of between ?1.83 ± 0.32 and ?2.26 ± 0.31%. is calculated from slow precipitation runs. A surface diffusion crystal growth model is used to describe the combination of kinetic isotope effects on thermodynamic isotope fractionation during rapid diffusion controlled crystal growth. Under slow precipitation conditions ?_{Calcite-Aragonite} was estimated as ?1.4%.; however, during rapid precipitation this fractionation appears to diminish and aragonite becomes less enriched in ¹³C.     

The kinetics of calcite precipitation from a high salinity water

Calcium carbonate is one of the most common and important scale-forming minerals in oilfield produced water, but the kinetics of CaCO₃ precipitation has been ignored in most scale prediction models because of the lack of reliable precipitation rate model. There are none in the open literature for oilfield conditions (temperature > 100°C, pressure > 200 bar and salinity > 0.5 mol kg− 1). In this work the kinetics of calcite (CaCO₃) precipitation from high salinity waters (up to 2 mol kg⁻¹) have been studied by a pH-free-drift method in a closed water system. This method. is much easier to operate than the often used steady-state method. The experimental results indicate that the calcite precipitation rate is not only affected by the solution CaCO₃ saturation level, but also by the solution pH, ionic strength and the concentration ratios of Ca to HCO₃₋ ions (C_Ca²⁺/C_HCO3⁻). When the concentration ratios of Ca to HCO₃⁻ ions are close to their chemical stoichiometric ratio of 0.5, the calcite growth from a supersaturated solution is believed to be surface reaction controlled. However, at higher C_Ca²⁺C_HCO3⁻ ratios, the transportation of the lattice ions to calcite crystal surface has to be considered.     

Fractionation of stable carbon isotopes during anaerobic production and degradation of propionate in defined microbial cultures

In many anoxic environments propionate is, after acetate, the second most important fermentation product, being degraded further to finally result in CH₄ production. In principle, isotope discrimination can be used to assess the path of organic matter degradation to acetate, CO₂ and CH₄. However, nothing is known about the isotope fractionation in primary and secondary fermentation steps involving propionate, although it is an important precursor of acetate. We therefore studied the degradation of propionate with a syntrophic coculture of Syntrophobacter fumaroxidans and Methanobacterium formicicum. The isotope enrichment factor for propionate degradation to acetate, CO₂ and CH₄ was almost negligible (ε_prop 0.9‰). The fermentative production of propionate was studied in cultures with Opitutus terrae growing on pectin, xylan and starch. These polysaccharides were fermented to acetate, succinate, propionate, H₂ and CO₂. While the δ¹³C value of the initially produced propionate was similar to that of the organic substrates (ca. −28 to −25‰), the δ¹³C value of the other fermentation products was higher. The δ¹³C values of all products generally decreased during the course of fermentation. Finally, a small depletion in ¹³C (ca. 6‰) with respect to the organic substrate was observed for propionate, while the other fermentation products where slightly enriched. Overall, stable carbon isotope discrimination was small during both fermentative production and consumption of propionate in the anaerobic microbial cultures, so that propionate turnover probably only marginally affects isotope fractionation during anaerobic degradation of organic matter.     

Fractionation of hydrogen,oxygen and carbon isotopes in n-alkanes and cellulose of three Sphagnum species

Compound-specific isotope measurements of organic compounds are increasingly important in palaeoclimate reconstruction. Searching for more accurate peat-based palaeoenvironmental proxies, compound-specific fractionation of stable C, H and O isotopes of organic compounds synthesized by Sphagnum were determined in a greenhouse study. Three Sphagnum species were grown under controlled climate conditions. Stable isotope ratios of cellulose, bulk organic matter (OM) and C₂₁–C₂₅ n-alkanes were measured to explore whether fractionation in Sphagnum is species-specific, as a result of either environmental conditions or genetic variation. The oxygen isotopic composition (δ¹⁸O) of cellulose was equal for all species and all treatments. The hydrogen isotopic composition (δD) of the n-alkanes displayed an unexpected variation among the species, with values between −154‰ for Sphagnum rubellum and −184‰ for Sphagnum fallax for the C₂₃ n-alkane, irrespective of groundwater level. The stable carbon isotopic composition (δ¹³C) of the latter also showed a species-specific pattern. The pattern was similar for the carbon isotope fractionation of bulk OM, although the C₂₃ n-alkane was >10‰ more depleted than the bulk OM. The variation in H fractionation may originate in the lipid biosynthesis, whereas C fractionation is also related to humidity conditions. Our findings clearly emphasize the importance of species identification in palaeoclimate studies based on stable isotopes from peat cores.     

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

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

Experimental studies of oxygen and hydrogen isotope fractionations between precipitated brucite and water at low temperatures

Oxygen and hydrogen isotope fractionation factors between brucite and water were experimentally determined by chemical synthesis techniques at low temperatures of 15° to 120°C. MgCl₂, Mg3N₂, and MgO were used as reactants, respectively, to produce brucite in aqueous solutions. All of the synthesis products were identified by x-ray diffraction (XRD) for crystal structure and by scanning electron microscope (SEM) for morphology. It is observed that oxygen isotope fractionations between brucite and water are temperature dependent regardless of variations in aging time, the chemical composition, and pH value of solutions. Brucites derived from three different starting materials yielded consistent fractionations with water at the same temperatures. These suggest that oxygen isotope equilibrium has been achieved between the synthesized brucite and water, resulting in the fractionation equation of 10³lnα=1.56×10⁶/T²−14.1. When the present results for the brucite–water system are compared with those for systems of gibbsite–water and goethite–water, it suggests the following sequence of ¹⁸O-enrichment in the M−OH bonds of hydroxides: Al³⁺ − OH > Fe³⁺ − OH > Mg²⁺ − OH.Hydrogen isotope fractionations between brucite and water obtained by the different synthesis methods have also achieved equilibrium, resulting in the fractionation equation of 10³lnα=−4.88×10⁶/T²−22.5. Because of the pressure effect on hydrogen isotope fractionations between minerals and water, the present calibrations at atmospheric pressure are systematically lower than fractionations extrapolated from hydrothermal exchange experiments at high temperatures of 510° to 100°C and high pressures of 1060 to 1000 bar. Comparison of the present results with existing calibrations involving other low-temperature minerals suggests the following sequence of D-enrichment in hydroxyl-bearing minerals: Al³⁺ − OH > Mg²⁺ − OH > Fe³⁺ − OH.     

Effects of carbon anhydrase on utilization of bicarbonate in microalgae: a case study in Lake Hongfeng

A bidirectional labeling method was established to distinguish the proportions of HCO₃^? and CO₂ utilization pathways of microalgae in Lake Hongfeng. The method was based on microalgae cultured in a medium by adding equal concentrations of NaH¹³CO₃ with different δ¹³C values simultaneously. The inorganic carbon sources were quantified according to the stable carbon isotope composition in the treated microalgae. The effects of extracellular carbonic anhydrase (CAex) on the HCO₃^? and CO₂ utilization pathways were distinguished using acetazolamide, a potent membrane-impermeable carbonic anhydrase inhibitor. The results show utilization of the added HCO₃^? was only 8% of the total carbon sources in karst lake. The proportion of the HCO₃^? utilization pathway was 52% of total inorganic carbon assimilation. Therefore, in the natural water of the karst area, the microalgae used less bicarbonate that preexisted in the aqueous medium than CO₂ derived from the atmosphere. CAex increased the utilization of inorganic carbon from the atmosphere. The microalgae with CAex had greater carbon sequestration capacity in this karst area.     

Experimental studies of oxygen isotope fractionation in the carbonic acid system at 15°, 25°, and 40°C

In light of recent studies that show oxygen isotope fractionation in carbonate minerals to be a function of HCO₃⁻ and CO₃²⁻ concentrations, the oxygen isotope fractionation and exchange between water and components of the carbonic acid system (HCO₃⁻, CO₃²⁻, and CO_2(aq)) were investigated at 15°, 25°, and 40°C. To investigate oxygen isotope exchange between HCO₃⁻, CO₃²⁻, and H₂O, NaHCO₃ solutions were prepared and the pH was adjusted over a range of 2 to 12 by the addition of small amounts of HCl or NaOH. After thermal, chemical, and isotopic equilibrium was attained, BaCl₂ was added to the NaHCO₃ solutions. This resulted in immediate BaCO₃ precipitation; thus, recording the isotopic composition of the dissolved inorganic carbon (DIC). Data from experiments at 15°, 25°, and 40°C (1 atm) show that the oxygen isotope fractionation between HCO₃⁻ and H₂O as a function of temperature is governed by the equation:

     

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

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

Fractionation of stable carbon isotopes by marine phytoplankton

Stable carbon isotope fractionation by seventeen species of marine phytoplankton, representing the classes of Bacillariophyceae, Chlorophyceae, Prasinophyceae, Chrysophyceae, Haptophyceae and Dinophyceae have been determined in laboratory culture experiments using bicarbonate enriched artificial sea water. The values (Δ_HCO3^? = δ¹³C of algae vs HCO₃^?) range from ?22.1 to ?35.5%. Nitzschia closterium shows the smallest fractionation of ? 22.1% and Isochrysis galbana, the greatest of ?35.5%,. Since these algae were cultured under identical laboratory conditions, the wide range of Δ_HCO3^? values is seemingly due to the presence of different metabolic pathways within these organisms.A temperature dependent fractionation of 0.36% per °C with decreasing temperatures was measured for Skeletonema costatum whereas, smaller temperature dependencies of ?0.13, +0.15 and ?0.07%. per °C were observed for Dunaliella sp., Monochrysis lutheri and Glenodinium foliaceum, respectively.The consistency of values of Skeletonema costatum, Dunaliella sp. and Monochrysis lutheri grown at salinities of 22, 26, 32 and 36% indicates that natural salinity variations have negligible effects on the isotopic composition of marine phytoplankton.     

Natürliche Isotopengehalte von Nitrat als Indikatoren für dessen Herkunft

The δ¹⁵N-value has often tentatively been used for the assignment of nitrate to its origin. However, the very complex correlations between the different nitrogen pools, mostly accompanied by isotope discriminations, oppose a very limited application of this method. On the other hand, the oxygen isotope abundance should be more indicative, because industrially produced NO⁻₃ must nearly exclusively contain oxygen from O₂ (δ¹⁸O = +23.5%.), while NO⁻₃ originating from a nitrification process must have water (δ¹⁸O ≅ -10%.) as the main oxygen source.For the proof of this reflection a method for the precise oxygen isotope analysis of NO⁻₃ was developed. Its application to the δ-value determination of commercial fertilizers and NO⁻₃ formed by nitrification absolutely confirmed the above predictions. Similarly, the isotope abundance of NO⁻₃-samples from ground and drinking water of known origin corresponded to the expected values. On the basis of these results and taking into account the known isotope abundance shifts due to isotope effects of nitrification and denitrification, a diagram between δ¹⁵N- and δ¹⁸O-values for NO⁻₃ was developed, which permitted the assignment of NO⁻₃ in unknown water samples to its probable source and origin.     

Rare earth elements,neodymium and strontium isotopic systematics in mineral waters: evidence from the Massif Central,France

Rare Earth Elements (REEs), and Sr and Nd isotope distributions, have been studied in mineralized waters from the Massif Central (France). The CO₂-rich springs are characterized by a neutral pH (6–7) associated with total dissolved solids (TDS) from 1 to 7 g l⁻¹. The waters result from the mixing of very mineralized water pools, thought to have equilibrated at a temperature of around 200°C with superficial waters. These two mineral water pools evidenced by Sr isotopes and dissolved REEs could reflect 2 different stages of water–rock interaction and an equilibrium with different mineral assemblages.The concentrations of individual dissolved REEs and total dissolved REEs (ΣREE), in the mineral waters examined, vary over several orders of magnitude but are not dependent on the main parameters of the waters (TDS, T°C, pH, Total Organic C). The dissolved REE concentrations presented as upper continental crust normalized patterns show HREE enrichment in most of the samples. The time evolution of REE patterns does not show significant fluctuations except in 1 borehole, located in the Limagne d’Allier area, which was sampled on 16 occasions over an 18 month period. Ten samples are HREE-enriched, whereas 6 samples show flat patterns.The aqueous speciation of REEs shows that CO²⁻₃ complexes dominate (>80%) over the free metal, F⁻, SO²⁻₄ and HCO⁻₃ complexes. The detailed speciation demonstrates that the fractionation of REEs (i.e. the HREE enrichment) in CO₂-rich and pH neutral fluids is due essentially to the predominance of the CO²⁻₃ complexes.The Sr isotopic composition of the mineral waters in the Massif Central shows different mixing processes; in the Cézallier area at least 3 end-member water types exist. The most dilute end-member is likely to originate as poorly mineralized waters with minimal groundwater circulation. Two other mineralized end-members are identified, although the link between the geographical location of spring outflow and the mixing proportion between the 2 end-members is not systematic. The range in ϵ_Nd(0) for mineralized waters in the Massif Central correlates well with that of the known parent rocks except for 4 springs. One way to explain the ϵ_Nd(0) in these instances is a contribution from drainage of volcanic rocks. The isotopic systematics help to constrain the hydrogeological models for this area.