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PubMed1996

Secondary H/T and D/T isotope effects in enzymatic enolization reactions. Coupled motion and tunneling in the triosephosphate isomerase reaction.

Secondary kH/kT kinetic isotope effects in H2O and kH/kT or kD/kT isotope effects in D2O have been measured for the triosephosphate isomerase-catalyzed conversion of dihydroxyacetone 3-phosphate (DHAP) to D-glyceraldehyde 3-phosphate. The proton transfer steps are made rate-limiting using [1(R)-2H]-labeled substrate in D2O to slow the chemical steps, relative to product release. After a small correction for the beta-equilibrium isotope effect for dehydration of DHAP, the H/T kinetic isotope effect kH/kT = 1.27 +/- 0.03 for [1(R)-2H,(S)-3H]-labeled substrate in D2O is subtantially larger than the equilibrium isotope effect for enolization of DHAP, KH/KT = 1.12. The H/T isotope effect is related to the D/T isotope effect with a Swain-Schaad exponent y = 4.4 +/- 1.3. These results are consistent with coupled motion of the C-1 primary and secondary hydrogens of DHAP and tunneling. Large secondary kinetic isotope effects are a general feature of enzymatic enolization reactions while nonenzymatic enolization reactions show secondary kinetic isotope effects that are substantially smaller than equilibrium effects [Alston, W. A., II, Haley, K., Kanski, R., Murray, C.J., & Pranata, J. (1996) J. Am. Chem Soc., 118, 6562-6569]. Possible origins for these differences in transition state structure are discussed.

Animals↗

PubMed1999

Nature of hydrogen transfer in soybean lipoxygenase 1: separation of primary and secondary isotope effects.

Previous measurements of the kinetics of oxidation of linoleic acid by soybean lipoxygenase 1 have indicated very large deuterium isotope effects, but have not been able to distinguish the primary isotope effect from the alpha-secondary effect. To address this question, singly deuterated linoleic acid was prepared, and enantiomerically resolved using the enzyme itself. Noncompetitive measurements of the primary deuterium isotope effect give a value of ca. 40 which is temperature-independent. The enthalpy of activation is low and isotope-independent, and there is a large isotope effect on the Arrhenius prefactor. A very large apparent secondary isotope effect (ca. 2.1) is measured with deuterium in the primary position, but a greatly reduced value (1.1) is observed with protium in the primary position. Mutagenesis of the active site leads to a significant reduction in k(cat) and perturbed isotope effects, in particular, a secondary effect of 5.6 when deuterium is in the primary position. The anomalous secondary isotope effects are shown to arise from imperfect stereoselectivity of hydrogen abstraction which, for the mutant, is attributed to a combination of inverse substrate binding and increased flexibility at the reactive carbon. After correction, a very large primary (76-84) and small secondary (1.1-1.2) kinetic isotope effects are calculated for both mutant and wild-type enzymes. The weight of the evidence is taken to favor hydrogen tunneling as the primary mechanism of hydrogen transfer.

Catalysis↗

PubMedDate not supplied

Deuterium isotope effect in the interaction of N-nitrosodimethylamine, ethanol, and related compounds with cytochrome P-450IIE1.

Deuteration of N-nitrosodimethylamine (NDMA) decreases its carcinogenicity and produces an isotope effect on its metabolism. Our previous results showed that deuteration causes a 5-fold increase in the apparent Km, but not the Vmax, for the demethylation and denitrosation of NDMA in microsomes. In the present work, we studied the nature of this deuterium isotope effect with several compounds using acetone-induced microsomes as a source of cytochrome P-450IIE1. In the microsomal N-nitrosodiethylamine deethylase reaction, NDMA and [2H6]NDMA were competitive inhibitors and displayed apparent Ki values of 59 and 441 mM, respectively, showing an isotope effect of 0.13. Similarly, in the p-nitrophenol hydroxylase reaction, a deuterium isotope effect of 0.21 on the Ki was observed. With acetone as an inhibitor for p-nitrophenol hydroxylase, the isotope effect on the Ki was 0.11. Similar deuterium isotope effects were also observed with acetone and dimethylformamide as competitive inhibitors for NDMA demethylase. When the oxidation of ethanol, [1,1-2H2]ethanol, [2,2,2-2H3]ethanol, and [2H6]ethanol was compared, an isotope effect of about 5 was found in the Vmax/Km due to the deuteration of the methylene group (carbon 1) but not due to the methyl group. However, the Vmax was not affected. A corresponding deuterium isotope effect was observed in the Ki when these compounds were used as competitive inhibitors for the NDMA demethylase reaction. The results demonstrate that deuteration of NDMA, ethanol, and related compounds results in an increase in the Km or Ki with little change in the Vmax of P-450IIE1-catalyzed reactions. The molecular basis of this isotope effect is discussed.

Acetone↗

PubMed2001

pH and kinetic isotope effects in d-amino acid oxidase catalysis.

The effects of pH, solvent isotope, and primary isotope replacement on substrate dehydrogenation by Rhodotorula gracilis d-amino acid oxidase were investigated. The rate constant for enzyme-FAD reduction by d-alanine increases approximately fourfold with pH, reflecting apparent pKa values of approximately 6 and approximately 8, and reaches plateaus at high and low pH. Such profiles are observed in all presteady-state and steady-state kinetic experiments, using both d-alanine and d-asparagine as substrates, and are inconsistent with the operation of a base essential to catalysis. A solvent deuterium isotope effect of 3.1 +/- 1.1 is observed on the reaction with d-alanine at pH 6; it decreases to 1.2 +/- 0.2 at pH 10. The primary substrate isotope effect on the reduction rate with [2-D]d-alanine is 9.1 +/- 1.5 at low and 2.3 +/- 0.3 at high pH. At pH 6.0, the solvent isotope effect is 2.9 +/- 0.8 with [2-D]d-alanine, and the primary isotope effect is 8.4 +/- 2.4 in D2O. Thus, primary and solvent kinetic isotope effects (KIEs) are independent of the presence of the other isotope, i.e. the 'double' kinetic isotope effect is the product of the individual KIEs, consistent with a transition state in which rupture of the two bonds of the substrate to hydrogen is concerted. These results support a hydride transfer mechanism for the dehydrogenation reaction in d-amino acid oxidase and argue against the occurrence of any intermediates in the process. A pKa,app of approximately 8 is interpreted to arise from the microscopic ionization of the substrate amino acid alpha-amino group, but also includes contributions from kinetic parameters.

Alanine↗

PubMed2004

Some symmetry-induced isotope effects in the kinetics of recombination reactions.

Symmetry-induced isotope effects in recombination and collision-induced dissociation reactions are discussed. Progress on understanding the anomalous isotope effects in ozone is reviewed. Then, calculations are performed for the simpler reaction xNe+yNe+H<-->xNeyNe+H, where x and y label either identical or different isotopes. The atomic masses in the model are chosen so that symmetry is the only difference between the systems. Starting from a single potential energy surface, the properties of the bound, quasibound, and continuum states of the neon dimer are calculated. Then, the vibration rotation infinite order sudden approximation is used to calculate cross sections for all possible inelastic and dissociative processes. A rate constant matrix that exactly satisfies detailed balance is constructed. It allows recombination to occur both via direct three-body collisions and via tunneling into the quasibound states of the energy transfer mechanism. The eigenvalue rate coefficients are determined. Significant isotope effects are clearly found, and their behavior depends on the pressure, temperature, and mechanism of the reaction. Both spin statistics and symmetry breaking produce isotope effects. Under most conditions the breaking of symmetry enhances the rates, but a wide spectrum of effects is observed; they range from isotope effects with a normal mass dependence to huge, mass-independent isotope effects to cancellation and even to reversal of the isotope effects. This is the first calculation of symmetry-induced isotope effects in recombination rates from first principles. The relevance of the present effects to ozone recombination is discussed.

Journal Article↗

PubMed1991

Effects of Mild Water Stress and Diurnal Changes in Temperature and Humidity on the Stable Oxygen and Hydrogen Isotopic Composition of Leaf Water in Cornus stolonifera L.

In this paper we make comparisons between the observed stable isotopic composition of leaf water and the predictions of the Craig-Gordon model of isotopic enrichment when plants (Cornus stolonifera L.) were exposed to natural, diurnal changes in temperature and humidity in a glasshouse. In addition, we determined the effects of mild water stress on the isotopic composition of leaf water. The model predicted different patterns of diurnal change for the oxygen and hydrogen isotopic composition of leaf water. The observed leaf water isotopic composition followed qualitatively similar patterns of diurnal change to those predicted by the model. At midday, however, the model always predicted a higher degree of heavy isotope enrichment than was actually observed in leaves. There was no effect of mild water stress on the hydrogen isotopic composition of leaf water. For the oxygen isotopic composition of leaf water, there was either no significant difference between control and water-stressed plants or the stressed plants had lower delta(18)O values, despite the enriched stem water isotopic composition observed for the stressed plants.

Journal Article↗

PubMed2005

Isotopic fractionation of carbon and nitrogen during the illicit processing of cocaine and heroin in South America.

The forensic application of stable isotope analysis to cocaine and heroin for geolocation of exhibits must take into account the possible enrichment and/or depletion of 13C and 15N during the illicit manufacturing process. Continuous-flow elemental analysis-isotope ratio mass spectrometry was utilized to measure changes in the stable isotope ratios of carbon and nitrogen for both cocaine (N = 92) and heroin/morphine (N = 81) exhibits derived from illicit manufacturing processes utilized by South American clandestine chemists. In controlled settings in South America, there was no siginficiant carbon isotope fractionation during the conversion of cocaine base to cocaine HCI using current illict methodologies. In contrast, nitrogen isotope fractionation for this conversion was 1 per thousand. There was a kinetic carbon isotope ratio fractionation during the acetylation of Colombian morphine to heroin and as a result heroin exhibits will almost always have more negative delta13C values than the original morphine. There was an isotopic fractionation against 15N during the acetylation of morphine base to heroin base, but this effect was not expressed since all of the heroin base was precipitated during the manufacturing process. However, the clandestine process of converting a single batch of heroin base usually involved two consecutive crops of heroin HCl and the latter crop was isotopically depleted as expected from a Rayleigh distillation process. When heroin was deacetylated to morphine, the morphine produced resulted in delta13C values that were indistinguishable from the original morphine. The kinetic carbon isotope fractionation factor for the South American process of morphine acetylation was -1.8 per thousand, allowing calculation of the delta13C values of the acetic anhydride from deacetylated heroin delta13C values.

Journal Article↗

PubMed1987

A kinetic isotope effect study and transition state analysis of the S-adenosylmethionine synthetase reaction.

The biosynthesis of S-adenosylmethionine occurs in a unique enzymatic reaction in which the synthesis of the sulfonium center results from displacement of the entire polyphosphate chain from MgATP. The mechanism of S-adenosylmethionine synthetase (ATP:L-methionine s-adenosyltransferase) from Escherichia coli has been characterized by kinetic isotope effect and substrate trapping measurements. Replacement of 12C by 14C at the 5' carbon of ATP yields a primary Vmax/Km isotope effect (12C/14C) of 1.128 +/- 0.003 in the absence of added monovalent cation activator (K+). At saturating K+ concentrations (10 mM) the primary isotope effect diminishes slightly to 1.108 +/- 0.003, indicating that the step in the mechanism involving bond breaking at the 5' carbon of MgATP has a small commitment to catalysis at conditions near Vmax. No alpha-secondary 3H isotope effect from [5'-3H]ATP was detected, (1H/3H) = 1.000 +/- 0.002, even in the absence of KCl. There was no significant primary sulfur isotope effect from [35S]methionine at KCl concentrations from 0 to 10 mM. Substitution of the methyl group of methionine with tritium yielded a beta-secondary isotope effect (CH3/C3H3) = 1.009 +/- 0.008 independent of KCl concentration. The reaction of selenomethionine and [5'-14C]ATP gave a primary isotope effect of 1.097 +/- 0.006, independent of KCl concentration. Substrate trapping experiments demonstrated that the step in the mechanism involving bond making to sulfur of methionine does not have a significant commitment to catalysis at 0.25 mM KCl, therefore intrinsic isotope effects were observed. Substrate trapping experiments indicated that the step involving bond breaking at carbon 5' of MgATP has a 10% commitment to catalysis at 0.25 mM KCl. The isotope effects are interpreted in terms of an Sn2-like transition state structure in which bonding of the C5' is symmetric with respect to the departing tripolyphosphate group and the incoming sulfur of methionine. With selenomethionine as substrate an earlier transition state is implicated.

Carbon Radioisotopes↗

PubMed2006

Correlation between multielement stable isotope ratio and geographical origin in Peretta cows' milk cheese.

The aim of this study was to characterize the isotopic composition and protect "Peretta" cows' milk cheese, a typical product of Sardinia, against other cheeses of the same appearance sold under the same name, but made of raw materials from northern Europe. The study was concerned with 3 types of cheese: those produced in local dairies from milk from free-grazing or pasture-grazing cows in Sardinia (local dairy product), cheeses made on an industrial scale from milk produced by intensive farming in Sardinia (factory cheese), and cheeses made with raw materials imported from other countries (imported product). To distinguish the Sardinian cheeses from the imported product, the stable isotope ratios 13C/12C, 15N/14N, D/H, 34S/32S, and (18)O/(16)O were used. Determination of the isotopic data delta13C, delta15N, delta2H, and delta34S was performed in the casein fraction, whereas delta(18)O and delta13C were determined in the glycerol fraction. Measurements were performed by isotope ratio mass spectrometry. A comparison between mean values of the isotope ratios by statistical analysis (ANOVA and Tukey's test) showed that the greatest difference between the 3 types of cheese (local dairy, factory, and imported products) was in the 13C/12C, 34S/32S, and (18)O/(16)O isotope ratios. In the other parameters, either no differences (delta15N) or minimal differences (delta2H) were found. Evaluation of the data by multivariate statistical analysis (principal component analysis and hierarchical cluster analysis) revealed that the isotope characteristics of the factory products were similar to those of the cheeses produced from imported raw materials, whereas a difference was found between the local dairy-produced cheeses and the products in the other 2 categories.

Analysis of Variance↗

PubMed1997

Safety of stable isotope use.

UNLABELLED: The increased employment of stable isotope tracers for diagnostic and research purposes frequently raises questions on potential risks associated with their use, which is of particular importance in the paediatric age group. Biological effects and the potential of adverse events has been evaluated in a large number of animal and, in part, also human studies. Possible differences in physical, chemical and biochemical behaviour resulting in kinetic and thermodynamic isotope effects between stable isotopes of the same element are related to the relative differences in atomic weight. Deuterium (2H), which differs markedly in mass from the predominant hydrogen isotope 1H, may induce serious side-effects at high concentrations in body fluids. The threshold dose for the occurrence of side-effects lies well above the usual tracer dosages for clinical use. In contrast to deuterium, heavier stable isotopes such as 13C, 15N or 18O that differ relatively little in mass from the predominant isotopes such as 12C, does not show any adverse biological effects even at highest enrichments. CONCLUSION: The doses of stable isotope tracer substances that are used for clinical diagnostic and research purposes appear safe and without any adverse effects. Stable isotope tracers should only be used in children if the trace is safe at the doses applied, and tracer is chemically pure and stable. In the case of intravenous application, the tracer preparation must also be sterile and pyrogen free.

Age Factors↗

PubMed2004

Applications of C and N stable isotopes to ecological and environmental studies in seagrass ecosystems.

Stable isotopes of carbon and nitrogen are increasingly used in marine ecosystems, for ecological and environmental studies. Here, we examine some applications of stable isotopes as ecological integrators or tracers in seagrass ecosystem studies. We focus on both the use of natural isotope abundance as food web integrators or environmental tracers and on the use of stable isotopes as experimental tools. As ecosystem integrators, stable isotopes have helped to elucidate the general structure of trophic webs in temperate, Mediterranean and tropical seagrass ecosystems. As environmental tracers, stable isotopes have proven their utility in sewage impact measuring and mapping. However, to make such environmental studies more comprehensible, future works on understanding of basic reasons for variations of N and C stable isotopes in seagrasses should be encouraged. At least, as experimental tracers, stable isotopes allow the study of many aspects of N and C cycles at the scale of a plant or at the scale of the seagrass ecosystem.

Alismatales↗

PubMed1985

Use of intermediate partitioning to calculate intrinsic isotope effects for the reaction catalyzed by malic enzyme.

For those enzymes that proceed via a stepwise reaction mechanism with a discrete chemical intermediate and where deuterium and 13C isotope effects are on separate steps, a new method has been developed to solve for the intrinsic deuterium and 13C kinetic isotope effects that relies on directly observing the partitioning of the intermediate between the forward and reverse directions. This observed partitioning ratio, along with the values of the primary deuterium, tritium, and 13C kinetic isotope effects on V/K for the substrate with the label being followed, allows an exact solution for the intrinsic deuterium and 13C isotope effects, the forward commitment for the deuterium-sensitive step, and the partition ratio for the intermediate in the reaction. This method allows portions of the reaction coordinate diagram to be defined precisely and the relative energy levels of certain activation barriers to be assigned exactly. With chicken liver triphosphopyridine nucleotide (TPN) malic enzyme activated by Mg2+, the partitioning of oxalacetate to pyruvate vs. malate in the presence of TPNH, 0.47, plus previously determined isotope effects gives an intrinsic deuterium isotope effect of 5.7 on hydride transfer and a 13C isotope effect of 1.044 on decarboxylation. Reverse hydride transfer is 10 times faster than decarboxylation, and the forward commitment for hydride transfer is 3.3. The 13C isotope effect is not significantly different with reduced acetylpyridine adenine dinucleotide phosphate replacing TPNH (although the pyruvate/malate partitioning ratio for oxalactate is now 9.9), but replacement of Mg2+ by Mn2+ raises the value to 1.065 (partition ratio 0.99).

Animals↗

PubMed1985

Use of nitrogen-15 and deuterium isotope effects to determine the chemical mechanism of phenylalanine ammonia-lyase.

Phenylalanine ammonia-lyase has been shown to catalyze the elimination of ammonia from the slow alternate substrate 3-(1,4-cyclohexadienyl)alanine by an E1 cb mechanism with a carbanion intermediate. This conclusion resulted from comparison of 15N isotope effects with deuterated (0.9921) and unlabeled substrates (1.0047), and a deuterium isotope effect of 2.0 from dideuteration at C-3, with the equations for concerted, carbanion, and carbonium ion mechanisms. The 15N equilibrium isotope effect on the addition of the substrate to the dehydroalanine prosthetic group on the enzyme is 0.979, while the kinetic 15N isotope effect on the reverse of this step is 1.03-1.04 and the intrinsic deuterium isotope effect on proton removal is in the range 4-6. Isotope effects with phenylalanine itself are small (15N ones of 1.0021 and 1.0010 when unlabeled or 3-dideuterated and a deuterium isotope effect of 1.15) but are consistent with the same mechanism with drastically increased commitments, including a sizable external one (i.e., phenylalanine is sticky). pH profiles show that the amino group of the substrate must be unprotonated to react but that a group on the enzyme with a pK of 9 must be protonated, possibly to catalyze addition of the substrate to dehydroalanine. Incorrectly protonated enzyme-substrate complexes do not form. Equilibrium 15N isotope effects are 1.016 for the deprotonation of phenylalanine or its cyclohexadienyl analogue, 1.0192 for deprotonation of NH4+, 1.0163 for the conversion of the monoanion of phenylalanine to NH3, and 1.0138 for the conversion of the monoanion of aspartate to NH4+.(ABSTRACT TRUNCATED AT 250 WORDS)

Ammonia-Lyases↗

PubMed1988

Nitrogen isotope effects on glutamate decarboxylase from Escherichia coli.

The nitrogen isotope effect on the decarboxylation of glutamic acid by glutamate decarboxylase from Escherichia coli has been measured by comparison of the isotopic composition of the amino nitrogen of the product gamma-aminobutyric acid isolated after 10-20% reaction with that of the starting glutamic acid. At pH 4.7, 37 degrees C, the isotope effect is k14/k15 = 0.9855 +/- 0.0006 when compared to unprotonated glutamic acid. Interpretation of this result requires knowledge of the equilibrium nitrogen isotope effect for Schiff base formation. This equilibrium isotope effect is k14/k15 = 0.9824 for the formation of the unprotonated Schiff base between unprotonated valine and salicylaldehyde. Analysis of the nitrogen isotope effect on decarboxylation of glutamic acid and of the previously measured carbon isotope effect on this same reaction [O'Leary, M.H., Yamada, H., & Yapp, C.J. (1981) Biochemistry 20, 1476] shows that decarboxylation and Schiff base formation are jointly rate limiting. The enzyme-bound Schiff base between glutamate and pyridoxal 5'-phosphate partitions approximately 2:1 between decarboxylation and return to the starting state. The nitrogen isotope effect also reveals that the Schiff base nitrogen is protonated in this intermediate.

Escherichia coli↗

PubMed1980

Use of isotope effects to deduce the chemical mechanism of fumarase.

The pH variation of primary 18O and primary and secondary deuterium isotope effects has been determined by the use of the equilibrium perturbation method for the reaction catalyzed by fumarase. The primary 18O effect is 1.073 from the malate side at pH 5 (with an equilibrium 18O effect of 1.033) but decreases rapidly to near unity above pH 7. The primary deuterium isotope effect is near unity at pH 5 and 9 but is strongly inverse at neutral pH (0.915 from the malate said at pH 7, compared to the equilibrium isotope effect of 0.98). Secondary isotope effects with dideuterated substrates from (Brant malate side were 1.31 at pH 5-6 but decreased to a value of 1.08 at pH 9.6 (the equilibrium isotope effect is 1.45). These data are interpreted to mean that the 3R proton of malate is transferred to a group (probably carboxyl) on the enzyme with a fractionation factor relative to water of at least 1.2, to give a carbanion intermediate with an acicarboxylate structure which is tetrahedral at C-2 and trigonal at C-3. Carbon-oxygen bond cleavage accompanied by proton transfer from a group (probably imidazole) on the enzyme then gives water and fumarate. By quantities analysis of the isotope effects, partition ratios for forward and reverse reaction of the E-malate, EH-carbanion, and EH-H2O-fumarate intermediates are calculated as a function of pH. The commitments to catalysis of malate, fumarate, water, and the proton on the enzyme are small at pH 5, and carbon-oxygen bond breaking is totally rate limiting. At neutral and high pH, however, the commitment factors (except that for water) are large, so that no 18O isotope effect is seen, and the other isotope effects are equilibrium ones, with the exact value seen depending on the ratio of forward and reverse commitments.

Deuterium↗

PubMed2006

Sulfur and oxygen isotope fractionation during benzene, toluene, ethyl benzene, and xylene degradation by sulfate-reducing bacteria.

We examined the oxygen and sulfur isotope fractionation of sulfate during anaerobic degradation of toluene by sulfate-reducing bacteria in culture experiments with Desulfobacula toluolica as a type strain and with an enrichment culture Zz5-7 obtained from a benzene, toluene, ethylbenzene, and xylene (BTEX)-contaminated aquifer. Sulfur isotope fractionation can show considerable variation upon sulfate reduction and may react extremely sensitively to changes in environmental conditions. In contrast, oxygen isotope fractionation seems to be less sensitive to environmental changes. Our results clearly indicate that oxygen isotope fractionation is dominated by isotope exchange with ambient water. To verify our experimental results and to test the applicability of oxygen and sulfur isotope investigations under realistic field conditions, we evaluated isotope data from two BTEX-contaminated aquifers presented in the recent literature. On a field scale, bacterial sulfate reduction may be superimposed by processes such as dispersion, adsorption, reoxidation, or mixing. The dual isotope approach enables the identification of such sulfur transformation processes. This identification is vital for a general qualitative evaluation of the natural attenuation potential of the contaminated aquifer.

Benzene↗

PubMed2000

Stable isotopes examined across a migratory divide in Scandinavian willow warblers (Phylloscopus trochilus trochilus and Phylloscopus trochilus acredula) reflect their African winter quarters.

The C and N isotopes of feathers from two subspecies of willow warblers Phylloscopus trochilus trochilus and Phylloscopus trochilus acredula) are isotopically distinct. Our analysis of 138 adult males from 14 sites distributed across Sweden shows that the mean delta15N and delta13C values of subspecies acredula (from latitudes above 63 degrees N) were significantly higher than the mean delta15N and delta13C values of subspecies trochilus (from latitudes below 61 degrees N). The analysed willow warbler feathers had been moulted in the winter quarters and the observed isotopic signatures should thus reflect the isotopic pattern of food assimilated in Africa. The isotopic data observed in Sweden match the cline in morphology, both showing abrupt changes around 62 degrees N. This result agrees with data from ringing recoveries indicating that the two subspecies occupy geographically and isotopically distinct wintering grounds in Africa. Our isotopic data suggest that analysis of stable isotopes of C and N is a promising method to track wintering quarters of European birds that migrate to Africa.

Africa↗

PubMed1985

Millimeter-scale variations of stable isotope abundances in carbonates from banded iron-formations in the Hamersley Group of Western Australia.

Several diamond drill cores from formations within the Hamersley Group of Western Australia have been studied for evidence of short-range variations in the isotopic compositions of the carbonates. For a set of 32 adjacent microbands analyzed in a specimen from the Marra Mamba Iron Formation, carbon isotope compositions of individual microbands ranged from -2.8 to -19.8 per mil compared to PDB and oxygen isotope compositions ranged from 10.2 to 20.8 per mil compared to SMOW. A pattern of alternating abundances was present, with the average isotopic contrasts between adjacent microbands being 3.0 per mil for carbon and 3.1 per mil for oxygen. Similar results were obtained for a suite of 34 microbands (in four groups) from the Bruno's Band unit of the Mount Sylvia Formation. Difficulties were experienced in preparing samples of single microbands from the Dales Gorge Member of the Brockman Iron Formation, but overall isotopic compositions were in good agreement with values reported by previous authors. Chemical analyses showed that isotopically light carbon and oxygen were correlated with increased concentrations of iron. The preservation of these millimeter-scale variations in isotopic abundances is interpreted as inconsistent with a metamorphic origin for the isotopically light carbon in the BIF carbonates. A biological origin is favored for the correlated variations in 13C and Fe, and it is suggested that the 13C-depleted carbonates may derive either from fermentative metabolism or from anaerobic respiration. A model is presented in which these processes occur near the sediment-water interface and are coupled with an initial oxidative precipitation of the iron.

Australia↗