Carbon isotope fractionation in synthetic magnesian calcite

Mg-calcite was precipitated at 25 °C in closed system, free-drift experiments, from solutions containing NaHCO₃, CaCl₂ and MgCl₂. The carbon stable isotope composition of bulk solid and solution were analyzed from subsamples collected during time course experiments of 24 h duration. Considering only the Mg-content and δ¹³C values for the bulk solid, the carbon isotope fractionation factor for the Mg-calcite-HCO_3(aq)⁻ system (as ) increased with average mol percentage of Mg (X_Mg) in the solid at a rate of (0.024 ± 0.011) per mol% MgCO₃. Extrapolation of this relationship to the pure calcite end member yields a value of 0.82 ± 0.09, which is similar to published values for the calcite-HCO_3(aq)⁻ system. Although did not vary for precipitation rates that ranged from 10^3.21 to 10^4.60 μmol m⁻² h⁻¹, it was not possible to hold Mg-content of the solid constant, so kinetic effect on 10³ ln α could not be evaluated from these experiments.     

Carbon isotope fractionation in wood during carbonization

A significant uncertainty exists as to whether δ¹³C values in charcoal meaningfully represent the stable isotopic content of the original material, with studies suggesting variable responses to both natural and laboratory heating. An extensive study was undertaken using fully homogenised samples of wood taken from Eucalyptus spp., Quercus robur and Pinus radiata. The results demonstrate that the duration of heating had no tangible effect on the final composition of the charred material, with the δ¹³C and carbon content of wood fixed after 30 min of heating. Furthermore, all three wood types become progressively depleted in ¹³C with increasing temperature. The results demonstrate that even at temperatures commonly reached in natural fires (<450 °C) isotopic fractionation of up to 1.3‰ can take place indicating that the absolute values obtained from charcoal extracted for paleoenvironmental reconstruction must be interpreted with caution.     

Carbon isotope fractionation between dissolved and gaseous carbon dioxide

Fractionation of the stable carbon isotopes between dissolved and gaseous carbon dioxide has been measured at temperature 25°C by two methods. In the first method the open system conditions and different methods of CO₂ sampling were arranged. In the second method—the closed system conditions and CO₂ gas extraction were used. The results obtained by these methods are very consistent. Gaseous CO₂ is enriched in heavy isotope by 1.03±0.02 permil in comparison to dissolved carbon dioxide.     

Magnesium stable isotope fractionation in marine biogenic calcite and aragonite

This survey of magnesium stable isotope compositions in marine biogenic aragonite and calcite includes samples from corals, sclerosponges, benthic porcelaneous and planktonic perforate foraminifera, coccolith oozes, red algae, and an echinoid and brachiopod test. The analyses were carried out using MC-ICP-MS with an external repeatability of ±0.22‰ (2SD for δ²⁶Mg; n = 37), obtained from a coral reference sample (JCp-1).Magnesium isotope fractionation in calcitic corals and sclerosponges agrees with published data for calcitic speleothems with an average Δ²⁶Mg_{calcite-seawater} = −2.6 ± 0.3‰ that appears to be weakly related to temperature. With one exception (Vaceletia spp.), aragonitic corals and sclerosponges also display uniform Mg isotope fractionations relative to seawater with Δ²⁶Mg_{biogenic aragonite-seawater} = −0.9 ± 0.2.Magnesium isotopes in high-Mg calcites from red algae, echinoids and perhaps some porcelaneous foraminifera as well as in all low-Mg calcites (perforate foraminifera, coccoliths and brachiopods) display significant biological influences. For planktonic foraminifera, the Mg isotope data is consistent with the fixation of Mg by organic material under equilibrium conditions, but appears to be inconsistent with Mg removal from vacuoles. Our preferred model, however, suggests that planktonic foraminifera synthesize biomolecules that increase the energetic barrier for Mg incorporation. In this model, the need to remove large quantities of Mg from vacuole solutions is avoided. For the high-Mg calcites from echinoids, the precipitation of amorphous calcium carbonate may be responsible for their weaker Mg isotope fractionation.Disregarding superimposed biological effects, it appears that cation light isotope enrichments in CaCO₃ principally result from a chemical kinetic isotope effect, related to the incorporation of cations at kink sites. In this model, the systematics of cation isotope fractionations in CaCO₃ relate to the activation energy required for cation incorporation, which probably reflects the dehydration of the cation and the crystal surface and bond formation at the incorporation site. This kinetic incorporation model predicts (i) no intrinsic dependence on growth rate, unless significant back reaction upon slow growth reduces the isotope fractionation towards that characteristic for equilibrium isotope partitioning (this may be observed for Ca isotopes in calcites), (ii) a small decrease of isotope fractionation with increasing temperature that may be amplified if higher temperatures promote back reaction and (iii) a sensitivity to changes in the activation barrier caused by additives such as anions or biomolecules or by the initial formation of amorphous CaCO₃.     

Carbon isotope fractionation of sapropelic organic matter during early diagenesis

Study of an algal, sapropelic sediment from Mangrove Lake, Bermuda shows that the mass balance of carbon and stable carbon isotopes in the major organic constituents is accounted for by a relatively straightforward model of selective preservation during diagenesis. The loss of ¹³C-enriched carbohydrates is the principal factor controlling the intermolecular mass balance of ¹³C in the sapropel. Results indicate that labile components are decomposed leaving as a residual concentrate in the sediment an insoluble humic substance that may be an original biochemical component of algae and associated bacteria. An overall decrease of up to about 4‰ in the δ ¹³C values of the organic matter is observed as a result of early diagenesis.     

Copper isotope fractionation by desert shrubs

Copper has two naturally occurring stable isotopes of masses 63 and 65 which can undergo mass dependent fractionation during various biotic and abiotic chemical reactions. These interactions and their resulting Cu isotope fractionations can be used to determine the mechanisms involved in the cycling of Cu in natural systems. In this study, Cu isotope changes were investigated at the organismal level in the metal-accumulating desert plant, Prosopis pubescens. Initial results suggest that the lighter Cu isotope was preferentially incorporated into the leaves of the plant, which may suggest that Cu was actively transported via intracellular proteins. The roots and stems show a smaller degree of Cu isotope fractionation and the direction and magnitude of the fractionations was dependent upon the levels of Cu exposure. Based on this and previous work with bacteria and yeast, a trend is emerging that suggests the lighter Cu isotope is preferentially incorporated into biological components, while the heavier Cu isotope tends to become enriched in aqueous solutions. In bacteria, plants and animals, intracellular Cu concentrations are strictly regulated via dozens of enzymes that can bind, transport, and store Cu. Many of these enzymes reduce Cu(II) to Cu(I). These initial results seem to fit into a broader picture of Cu isotope cycling in natural systems where oxidation/reduction reactions are fundamental in controlling the distributions of Cu isotopes.     

Carbon and oxygen isotope fractionation in scleractinian corals: a review

The present theories on the fractionation of stable isotopes in scleractinian corals are critically discussed in the light of data available on primary productivity, respiration and stable isotope chemistry. These data support a model of fractionation in which the carbon and oxygen isotopes are decoupled. Calcification occurs from a reservoir of carbon dioxide derived from both organic and inorganic sources. Photosynthesis preferentially fixes¹³C and thereby leaves behind¹³C. Increases in the rate of photosynthesis therefore also enrich the carbon isotope ratio of the skeleton. From theoretical considerations, photosynthesis has little effect on the oxygen isotope ratio of the skeleton, a fact confirmed by available data. The process of respiration adds depleted carbon and oxygen to the calcification reservoirs. The varying correlations between carbon and oxygen isotopes seen in hermatypic corals are caused by changes in the relationship between photosynthesis and respiration at different geographical localities. The isotopic compositions in the skeletons of non-zooxanthellate corals, which show a consistent positive correlation, can also be explained by the above scenario.     

Stable isotope fractionation between mollusc shells and marine waters from Martinique Island

Forty-nine aragonitic and calcitic shells from 14 species of marine tropical molluscs (Bivalvia, Gastropoda, Polyplacophora) and ambient waters from Martinique have been analyzed for their carbon and oxygen isotope compositions. Mineralogy of shells was systematically determined by Raman spectroscopy that reveals composite shell structures and early processes of diagenetic alteration. In mangrove, brackish waters result from the mixing between 89±1% of seawater and 11±1% of freshwater, a hydrological budget quantified by both oxygen isotope and salinity mass balance calculations. Mollusc shells from the mangrove environment (S=31‰; δ¹⁸O=0.5‰) are characterized by mean δ¹³C values (−1.2‰) lower than those (+2.6‰) living in the open sea (S=35‰; δ¹⁸O=1‰). These low carbon isotope compositions result from the oxidation of organic matter into bicarbonate ions used in the building of mollusc shells. The oxygen isotope compositions of the studied mollusc species are mainly controlled by the temperature and composition of seawater whereas the role of the so-called “vital effects” is negligible. Contrasting with carbon isotopes, variability in the δ¹⁸O values among and within species of mollusc shells is very low (1σ=0.15) for a given littoral environment. Using ambient temperatures of seawater (28-30 °C), oxygen isotope fractionations between all studied living species and environmental waters match those extrapolated from the fractionation equation established for molluscs by Grossman and Ku [Chem. Geol., Isot. Geosci. Sect. 59 (1986) 59] in the range 3-20 °C. By analyzing calcite and aragonite layers from the same shell or by comparing shells from different species living in the same environment, there is no evidence that oxygen isotope fractionation between aragonite and water differs from that between calcite and water. On the basis of these results, we conclude that the oxygen isotope compositions of shells from most fossil mollusc species are suitable to estimate past seawater temperatures at any paleolatitude.     

Argon isotope fractionation induced by stepwise heating

Noble gas isotopes are widely used to elucidate the history of the rocks in which they have been trapped, either from distinct reservoirs or by accumulation following radioactive decay. To extract noble gases from their host rocks, stepwise heating is the most commonly used technique to deconvolve isotopically different components, e.g., atmospheric, in situ radiogenic, or excess radiogenic from mantle or crustal reservoirs. The accurate determination of the isotopic composition of these different components is of crucial importance, e.g., for ages obtained by ⁴⁰Ar-³⁹Ar stepheating plateaus. However, diffusion theory-based model calculations predict that the stepwise thermal extraction process from mineral phases induces isotope fractionation and, hence, adulterates the original composition. Such effects are largely unconsidered, as they are small and a compelling experimental observation is lacking. We report the first unequivocal evidence for significant mass fractionation of argon isotopes during thermal extraction, observed on shungite, a carbon-rich Precambrian sedimentary rock. The degree of fractionation, as monitored by ³⁸Ar/³⁶Ar and ⁴⁰Ar/³⁶Ar ratios, very well agrees with theoretical predictions assuming an inverse square root dependence of diffusion coefficient and atomic mass, resulting in easier extraction of lighter isotopes. Hence, subatmospheric ⁴⁰Ar/³⁶Ar ratios obtained for argon extracted at low temperatures may not represent paleoatmospheric argon. Shungite argon resembles modern atmospheric composition, but constraints on the timing of trapping appear difficult to obtain, as shungites are multicomponent systems.In ⁴⁰Ar-³⁹Ar stepwise heating, the isotope fractionation effect could cause systematic underestimations of plateau ages, between 0.15 and 0.4% depending on age, or considerably higher if samples contain appreciable atmospheric Ar. The magnitude of this effect is similar to the presently achieved uncertainties of this increasingly precise dating technique. Our results also indicate the importance of thermally activated diffusion as a possible fractionation mechanism, e.g., for hydrothermal gas exhalations, or for carbonaceous carrier phases such as “Q” in meteorites that have been suggested as carriers of highly fractionated noble gas residues from the early solar nebula.     

Carbon isotope fractionation during low temperature carbonization of foxtail and common millets

Stable carbon isotopes of organic matter and fossilized plant remains can be used to effectively reconstruct local palaeoclimate changes, especially from plants using a single photosynthetic mode. The charred grains of foxtail and common millet are chemically stable in the environment and have been preserved widely and continuously throughout the Holocene in North China. The charred remains of these species are ideal materials for reconstructing the palaeoclimate based on δ¹³C of foxtail and common millets heated to temperatures up to around 250 °C. This study reports δ¹³C values of modern millets carbonized at different temperatures. The results indicate that there are no significant changes in δ¹³C of intact and charred samples of foxtail millet (?0.46‰) and common millet (?0.49‰) for temperatures below 300 °C. The δ¹³C of charred foxtail millet formed at 250 °C were 0.2‰ higher in δ¹³C than the source samples. In contrast, the δ¹³C of charred common millet formed at 250 °C were 0.2‰ lighter in δ¹³C than the source samples. The δ¹³C values of grains were determined in part by the carbon content (i.e., starches, lignins and lipids) and the variable thermal tolerances of these compounds to heating. However, the observed ¹³C carbonization associated with fractionation of only 0.2‰ in grains is much less than the natural variation typically found in wood. We therefore suggest that δ¹³C measured in carbonized grains can serve as an effective indicator for paleoenvironmental and archaeological reconstructions.     

Sulphur isotope fractionation by Desulfovibrio and Desulfotomaculum species

Isotopic analysis of H₂S evolved during the growth of Desulfovibrio species and Desulfotomaculum species on a defined sulphate medium at their specific optimal growth temperature indicate no noticeable intrageneric or intergeneric differences in regard to isotopic fractionation. Changes in the composition of the growth medium were reflected in minor changes in the isotopic composition of the H₂S evolved and in the rate of sulphate reduction. Intergeneric differences were noted in resting cell experiments and in the organisms' ability to reduce sulphite.     

Low stable carbon isotope fractionation by coccolithophore RubisCO

The ¹³C/¹²C ratio of carbon compounds is used to identify sources and sinks in the global carbon cycle. However, the relatively enriched ¹³C content observed for marine organic carbon remains enigmatic. The majority of oceanic carbon is fixed by algae and cyanobacteria via the Calvin-Benson-Bassham cycle, yet isotopic discrimination by the CO₂ fixation enzyme, RubisCO (ribulose 1,5-bisphosphate carboxylase/oxygenase), has only been measured for a single marine cyanobacterium. Different forms of RubisCO occur in different phytoplankton species (overall amino acid identity varying by as much as ∼75%) and thus may vary in the degree to which they fractionate carbon. Here we measured isotope discrimination by RubisCO from the coccolithophore Emiliania huxleyi, a cosmopolitan species used as a marine algal model.E. huxleyi RubisCO discriminated substantially less (ε = 11.1‰) against ¹³CO₂ than other RubisCO enzymes (18-29‰), despite having Michaelis-Menten kinetic parameters (K_CO2 = 72 μM; V_max = 0.66 μmol min⁻¹ mg⁻¹ protein) similar to those measured for RubisCO enzymes from different organisms. If widespread, decreased isotope discrimination of ¹³C by phytoplankton RubisCO may be a major factor influencing the enriched ¹³C content of marine organic carbon. This finding emphasizes the necessity of (a) determining ε values for RubisCOs of other marine phytoplankton and (b) re-evaluation of δ¹³C values from physiological, environmental, and geological studies.     

Carbon isotopic fractionation by the marine diatom Phaeodactylum tricornutum under nutrient- and light-limited growth conditions

A theoretical model was developed to explain the characteristics of carbon isotopic fractionation (ε_P) by the marine diatom Phaeodactylum tricornutum under nutrient- and light-limited growth conditions. The model takes into consideration active transport and diffusion of inorganic carbon through the cell membrane and chloroplast membrane and the energetic tradeoff between production of Rubisco and operation of a carbon-concentrating mechanism to achieve a given growth rate. The model is able to explain 88% of the variance in experimental ε_p data reported in this study and in previous work and is able to account for the observed pattern of Rubisco activity in nitrate-limited chemostats. Two important implications of the model include the fact that ε_p is not a unique function of the ratio of growth rate to external CO₂ concentration (as opposed to the predictions of several previous models) and that changes in light-limited and nutrient-limited growth rates have opposite effects on the fraction of CO₂ taken up by the chloroplast that is lost to diffusion and hence on certain patterns of carbon isotopic fractionation.     

Carbon and hydrogen isotope fractionation by moderately thermophilic methanogens

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

Carbon and oxygen isotope ratios in diagenetic carbonates from marine sediments

The isotopic composition of some sixty samples of penecontemporaneous diagenetic carbonates from marine horizons shows that they are strikingly different from normal marine limestones. The variation of C¹³ ratio is 5·6 per cent, the lightest carbonate measuring -54%. A mechanism for the formation of such light carbon dioxide is postulated, involving the enzymic decarboxylation of organic compounds at low temperatures. A study of the oxygen isotopic composition of coexisting calcite and rhodochrosite indicates that, whereas the latter has retained its isotopic composition from the time of precipitation, the calcite has undergone equilibration during diagenesis. Isotopic measurements on such diagenetic carbonates would confuse palaeoenvironmental studies but may throw light on the processes of diagenesis.     

Carbon isotope fractionation between calcite,graphite and CO2: an experimental study

The partitioning of stable carbon isotopes between calcite, graphite and CO₂ was experimentally determined at temperatures from 500 to 1200 °C and 1 to 15 kbar pressure. Attainment of carbon isotope equilibrium in CO₂-calcite runs was proven by achieving the same fractionation from isotopically opposite directions. The resultant CO₂-calcite fractionation curve for carbon differs from Bottinga's calculation by 1.2 and confirms recent experiments of Chacko et al. and Mattey et al. In CO₂-graphite experiments equilibrium fractions were extrapolated by applying the partial-exchange technique of Northrop and Clayton and by optimizing the contribution of surface reaction in graphite. CO₂-graphite fractionations at temperatures up to 800 °C are in fair agreement with Bottinga's calculation, but yield a surprisingly high fractionation of 5 at upper mantle temperatures. The combination of CO₂-calcite (carbon) and CO₂-graphite fractionation results in a new experimentally determined calcite-grapite fractionation curve, expressed by the equation:

石英一水一盐体系氧同位素分馏作用

温度为180—550℃,盐度(wt.％)分别为0、5、25和40条件下,在高压釜内完成了由硅胶合成石英的氧同位素分馏作用实验研究,目的是了解:①盐同位素效应;②△t值对同位素分馏的影响;③温度与同位素分馏系数的关系。研究资料表明:低温条件下矿物和纯水之间同位素平衡作用不可能发生;影响含氧矿物(初)之间氧同位素平衡速率的因素包括盐度、△t值大小和温度等;我们的研究还表明,盐度对同位素分馏作用同系数无影响,即不存在所谓的“同位素盐效应”。在180—550℃温度范围内,不同盐度条件下获得的石英-水氧同位素分馏实验方程为:10001nα_(石英-水)=3.306×10~5T~(-2)—2.71。     

Interaction between zinc and freshwater and marine diatom species: Surface complexation and Zn isotope fractionation

This work is devoted to characterization of zinc interaction in aqueous solution with two marine planktonic (Thalassiosira weissflogii = TW, Skeletonema costatum = SC) and two freshwater periphytic species (Achnanthidium minutissimum = AMIN, Navicula minima = NMIN) by combining adsorption and electrophoretic measurements with surface complexation modeling and by assessing Zn isotopes fractionation during both long term uptake and short term adsorption on diatom cells and their frustules. Reversible adsorption experiments were performed at 25 and 5 °C as a function of exposure time (5 min to 140 h), pH (2 to 10), zinc concentration in solution (10 nM to 1 mM), ionic strength (I = 0.001 to 1.0 M) and the presence of light. While the shape of pH-dependent adsorption edge is almost the same for all four species, the constant-pH adsorption isotherm and maximal Zn binding capacities differ by an order of magnitude. The extent of adsorption increases with temperature from 5 to 25 °C and does not depend on light intensity. Zinc adsorption decreases with increase of ionic strength suggesting competition with sodium for surface sites. Cell number-normalized concentrations of sorbed zinc on whole cells and their silica frustules demonstrated only weak contribution of the latter (10-20%) to overall zinc binding by diatom cell wall. Measurements of electrophoretic mobilities (μ) revealed negative diatoms surface potential in the full range of zinc concentrations investigated (0.15-760 μmol/L), however, the absolute value of μ decreases at [Zn] > 15 μmol/L suggesting a change in surface speciation. These observations allowed us to construct a surface complexation model for Zn binding by diatom surfaces that postulates the constant capacitance of the electric double layer and considers Zn complexation with carboxylate and silanol groups. Thermodynamic and structural parameters of this model are based on previous acid-base titration and spectroscopic results and allow quantitative reproduction of all adsorption experiments. Although Zn adsorption constants on carboxylate groups are almost the same, Zn surface adsorption capacities are very different among diatom species which is related to the systematic differences in their cell wall composition and thickness. Measurements of Zn isotopic composition (⁶⁶Zn/(⁶⁴Zn)) performed using a multicollector ICP MS demonstrated that irreversible incorporation of Zn in cultured diatom cells produces enrichment in heavy isotope compared to growth media (Δ⁶⁶Zn(solid-solution) = 0.27 ± 0.05, 0.08 ± 0.05, 0.21 ± 0.05, and 0.19 ± 0.05‰ for TW, SC, NMIN, and AMIN species, respectively). Accordingly, an enrichment of cells in heavy isotopes (Δ⁶⁶Zn(solid-solution) = 0.43 ± 0.1 and 0.27 ± 0.1‰ for NMIN and AMIN, respectively) is observed following short-term Zn sorption on freshwater cells in nutrient media at pH ∼ 7.8. Finally, diatoms frustules are enriched in heavy isotopes compared to solution during Zn adsorption on silica shells at pH ∼ 5.5 (Δ⁶⁶Zn(solid-solution) = 0.35 ± 0.10‰). Measured isotopes fractionation can be related to the structure and stability of Zn complexes formed and they provide a firm basis for using Zn isotopes for biogeochemical tracing.