Mass-independent Isotope Research Articles

Mass-dependent and mass-independent sulfur isotope fractionation (δ34S and δ33S) from Brazilian Archean and Proterozoic sulfide deposits by laser ablation multi-collector ICP-MS

Mass-independent sulfur isotope fractionation (MIF) has been observed in rocks of the geological record older than about 2.45Ga, a characteristic which is thought to be related to processes in the Neoarchean atmosphere. Samples recording a MIF effect therefore have to contain sulfur of the exogenic sulfur cycle, while endogenic sulfur should not show this effect. The sulfides analysed from six Brazilian deposits represent either exogenic or endogenic sulfur sources, with supposed ages ranging from about 1.9 to 2.7Ga. Sulfur isotopes were analysed by in‐situ laser ablation MC-ICP-MS. A range of experiments were conducted using international and in-house isotope standards, which were run under various conditions and set-ups. These include the reference materials IAEA-S1, IAEA-S3, NBS123 (sphalerite), NBS127 (barite), and in-house standards BSB-py (pyrite) and BSB-cpy (chalcopyrite). During six days of analysis, an internal precision of sulfide analyses of 0.10–0.15‰ (1s) for δ34S and 0.40–0.60‰ (1s) for δ33S, and an accuracy of ~0.30‰ for δ34S and δ33S was achieved. The standard measurements define a δ33S/δ34S relationship of δ33S=0.513*δ34S+0.149, with R²=0.9997, which is close to the theoretical relationship for mass-dependent fractionation. The by far best analytical errors were obtained for natural pyrite, reaching a within-run precision of about 0.05–0.15‰ for δ34S (1s), and 0.10–0.15‰ (1s) for δ33S determinations. Including all precision and accuracy data, we arrive at the following 1s error limits to which sulfur isotope analyses for MIF studies obtained with this method are reliable: 0.32, 0.34 and 0.46‰ for pyrite (δ34S, δ33S and Δ33S determinations, respectively), 0.34, 0.42 and 0.54‰ for pyrrhotite, and 0.34, 0.50 and 0.58‰ for chalcopyrite.Pyrites and pyrrhotites from 2200Ma old sediments and volcanics associated with the Pilar greenstone belt, Goiás, have δ34S from −15.0 to +2.0‰, and δ33S from −8.0 to +1.0‰, with Δ33S being slightly but not significantly positive. Pyrites from supposed ca. 2060Ma old clastic sediments of the Águas Claras Formation (Carajás, Pará), have δ34S=9.5–12.5‰ and δ33S=5.0–6.5‰ with Δ33S close to zero. Chalcopyrites from the Salobo and Sequerinha sulfide deposits, also Carajás, are genetically related to 2570Ma old A-type alkaline granites with endogenic sources. They have δ34S=1.6–2.9‰ and δ33S=1.0–1.5‰, and δ34S=0.2–0.9‰ and δ33S=0.65‰, respectively, with Δ33S ranging from −0.15 to 0.56. The only Δ33S values of this study which clearly suggest sulfur isotope MIF come from the mineralized banded iron-formations of the Quadrilátero Ferrífero, Minas Gerais. Pyrites from the Mina de Cuiabá deposit (2700Ma) have δ34S=3.8–5.1‰ and δ33S=2.9–4.8‰ with Δ33S=0.9–2.2‰, while pyrites and chalcopyrites from São Bento (2720Ma) have δ34S=1.6–3.2‰ and δ33S=1.5–2.8‰ with Δ33S=0.2–1.5‰. The sulfides represent an epigenetic mineralization, derived from supracrustal rocks making part of an exogenic sulfur cycle. The study shows, that in-situ laser ablation ICP-MS is sufficiently sensitive for detecting MIF effects in Neoarchean rocks. Whether or not such samples show MIF-related Δ33S values depends on the age and provenance of the sulfur.

Calcium isotope measurement by combined HR-MC-ICPMS and TIMS

We report a novel approach for the chemical purification of Ca from silicate rocks by ion-exchange chromatography, and a highly-precise method for the isotopic analysis of Ca—including the smallest isotope 46Ca (0.003%)—by high-resolution multiple collector inductively coupled plasma source mass spectrometry (HR-MC-ICPMS), in combination with thermal ionization mass spectrometry (TIMS). Using this approach, we measured the Ca isotope composition of a number of terrestrial rock standards and seawater. Based on these data, we show that the non-mass-dependent abundances of μ43Ca, μ46Ca, and μ48Ca (normalized to 42Ca/44Ca) can be measured with an external reproducibility of 1.8, 45 and 12.5 ppm, respectively, when measured by HR-MC-ICPMS and μ40Ca and μ43Ca to 80 and 23 ppm, respectively, when measured by TIMS (μ notation is the per 106 deviation from the reference material). Comparison with earlier studies demonstrate that it is possible to measure the mass-dependent Ca isotope composition of terrestrial materials using HR-MC-ICPMS with an external reproducibility comparable to that typically obtained with double spike TIMS techniques. The resolution of the mass-independent 43Ca, 46Ca and 48Ca data obtained by HR-MC-ICPMS represents more than a 45-, 120-, and 18-fold improvement, respectively, relative to earlier measurements obtained by TIMS. This improvement allows for a better understanding of the mass fractionation laws responsible for the mass-dependent fractionation of Ca present in natural samples and synthetic standards. For example, the presence of an apparent excess of ∼60 ppm in the μ48Ca composition of the SRM 915a suggests that equilibrium fractionation processes have generated the mass-dependent fractionation of this material. In contrast, the absence of residual anomalies in the mass-independent composition of seawater implies that biogenic and inorganic processes of carbonate formation fractionate Ca kinetically from seawater. Finally, we note that SRM 915b has a mass-dependent and mass-independent Ca isotope composition that is within the resolution of our method identical to that of bulk silicate Earth (BSE). This observation, together with the potential heterogeneity in the 40Ca composition of the SRM 915a inferred from our measurements, suggests that the SRM 915b is a better reference material to study the Ca isotope composition of terrestrial and non-terrestrial materials.

Hg Speciation and Stable Isotope Signatures in Human Hair As a Tracer for Dietary and Occupational Exposure to Mercury

Exposure of humans and wildlife to various inorganic and organometallic forms of mercury (Hg) may induce adverse health effects. While human populations in developed countries are mainly exposed to marine fish monomethylmercury (MMHg), this is not necessarily the case for developing countries and diverse indigenous people. Identification of Hg exposure sources from biomonitor media such as urine or hair would be useful in combating exposure. Here we report on the Hg stable isotope signatures and Hg speciation in human hair across different gold miner, indigenous and urban populations in Bolivia and France. We found evidence for both mass-dependent isotope fractionation (MDF) and mass-independent isotope fractionation (MIF) in all hair samples. Three limiting cases of dominant exposure to inorganic Hg (IHg), freshwater fish MMHg, and marine fish MMHg sources are used to define approximate Hg isotope source signatures. Knowing the source signatures, we then estimated Hg exposure sources for the Bolivian gold miner populations. Modeled IHg levels in hair correspond well to measured IHg concentrations (R = 0.9), demonstrating that IHg exposure sources to gold miners can be monitored in hair samples following either its chemical speciation or isotopic composition. Different MMHg and inorganic exposure levels among gold miners appear to correspond to living and working conditions, including proximity to small towns, and artisanal vs large scale mining activity.

Evidence of magnetic isotope effects during thermochemical sulfate reduction

Thermochemical sulfate reduction experiments with simple amino acid and dilute concentrations of sulfate reveal significant degrees of mass-independent sulfur isotope fractionation. Enrichments of up to 13‰ for (33)S are attributed to a magnetic isotope effect (MIE) associated with the formation of thiol-disulfide, ion-radical pairs. Observed (36)S depletions in products are explained here by classical (mass-dependent) isotope effects and mixing processes. The experimental data contrasts strongly with multiple sulfur isotope trends in Archean samples, which exhibit significant (36)S anomalies. These results support an origin other than thermochemical sulfate reduction for the mass-independent signals observed for early Earth samples.

Isotope fractionation of palladium in chemical exchange reaction

Abstract Palladium isotopes were fractionated by the solvent extraction technique with a crown ether. After purification by ion-exchange chemistry, the isotopic ratios of 105Pd/106Pd, 108Pd/106Pd, and 110Pd/106Pd were determined by multiple-collector inductively coupled plasma mass spectrometry. Isotope fractionations between the two phases were found to be larger than 0.1‰. The isotope fractionation of the odd atomic mass isotope (105Pd) showed a deviation from that estimated from the even atomic mass isotopes (106Pd, 108Pd, and 110Pd). The mass-independent isotope fractionation found was attributable to the nuclear field shift effect. The quantum chemical calculations for the different Pd species supported the validity of the isotope fractionation factors obtained.

Iron and sulphur isotopes from the Carajás mining province (Pará, Brazil): Implications for the oxidation of the ocean and the atmosphere across the Archaean–Proterozoic transition

Mineralogical analyses and major and trace element geochemistry combined with iron and sulfur isotopes were applied to banded iron formations (BIFs) and pyrite-bearing sandstones from the Carajás mineral province, Brazil. The BIFs belong to the 2.7 Ga old Grão Pará Group and show a rather simple mineralogy (hematite and quartz or cryptocrystalline silica). Their REE + Y patterns suggest deposition under marine sub-oxic conditions. Fe isotopes show highly positive values (δ 57Fe IRMM-14 = 2.06 ± 0.34‰ in average) which can be explained by the partial oxidation of Fe 2+ aq in the surface ocean, subsequent to the upwelling of deep anoxic seawater. The resulting iron oxy-hydroxides likely formed in the shallow photic zone as a seasonal process and acquired highly positive iron isotope signature values consistent with a Rayleigh distillation model. The pyrite-bearing sandstones belong to the Aguas Claras Formation of (Palaeoproterozoic) younger age and were cored at the base of the giant Azul manganese deposit. Diagenetic pyrite is characterized by positive δ 57Fe IRMM-14 = 1.02 ± 0.2‰ and δ 34S VCTD (11.97 ± 0.12‰) average values, that are different from values of modern marine biogenic pyrite. Results likely indicate that dissolved sulfate was the limiting factor controlling biogenic pyrite formation at that time. Near-zero Δ 33S values (0.013 ± 0.003‰) indicate minimal mass-independent sulfur isotope fractionation (MIF-S), consistent with the 2.1 Ga age previously obtained from the Aguas Claras pyrites. This result suggests deposition at a time when the oxygen partial pressure in the atmosphere was already high enough (10 − 5 to 10 − 2 PAL- or Present Atmospheric Level-) to hinder MIF-S by photochemical reactions. Incidentally, this also confirms that the giant Mn deposit of Carajás is in the same age range (2.1 ± 0.2 Ga) as other Mn ores in Gabon and South Africa, pointing to the importance of Mn precipitation in Paleoproterozoic times.

Constraining atmospheric oxygen and seawater sulfate concentrations during Paleoproterozoic glaciation: In situ sulfur three-isotope microanalysis of pyrite from the Turee Creek Group, Western Australia

Previous efforts to constrain the timing of Paleoproterozoic atmospheric oxygenation have documented the disappearance of large, mass-independent sulfur isotope fractionation and an increase in mass-dependent sulfur isotope fractionation associated with multiple glaciations. At least one of these glacial events is preserved in diamictites of the ∼2.4 Ga Meteorite Bore Member of the Kungarra Formation, Turee Creek Group, Western Australia. Outcrop exposures of this unit show the transition from the Boolgeeda Iron Formation of the upper Hamersley Group into clastic, glaciomarine sedimentary rocks of the Turee Creek Group. Here we report in situ multiple sulfur isotope and elemental abundance measurements of sedimentary pyrite at high spatial resolution, as well as the occurrence of detrital pyrite in the Meteorite Bore Member. The 15.3‰ range of Δ 33S in one sample containing detrital pyrite (−3.6‰ to 11.7‰) is larger than previously reported worldwide, and there is evidence for mass-independent sulfur isotope fractionation in authigenic pyrite throughout the section (Δ 33S from −0.8‰ to 1.0‰). The 90‰ range in δ 34S observed (−45.5‰ to 46.4‰) strongly suggests microbial sulfate reduction under non-sulfate limiting conditions, indicating significant oxidative weathering of sulfides on the continents. Multiple generations of pyrite are preserved, typically represented by primary cores with low δ 34S (<−20‰) overgrown by euhedral rims with higher δ 34S (4–7‰) and enrichments in As, Ni, and Co. The preservation of extremely sharp sulfur isotope gradients (30‰/<4 μm) implies limited sulfur diffusion and provides time and temperature constraints on the metamorphic history of the Meteorite Bore Member. Together, these results suggest that the Meteorite Bore Member was deposited during the final stages of the “Great Oxidation Event,” when pO 2 first became sufficiently high to permit pervasive oxidative weathering of continental sulfides, yet remained low enough to permit the production and preservation of mass-independent sulfur isotope fractionation.

The kinetics study of the S + S2 → S3 reaction by the chaperone mechanism

The recombination of S atoms has been found to be stepwise from the smallest unit, the elemental S atom, to the most abundant molecule S(8). The reaction between S + S(2) → S(3) has not been reported either experimentally or by theory, but may be a key intermediate step in the formation of sulfur aerosols in low-O(2) atmospheres. In this work, the kinetics of this reaction is reported with Ar gas used as the chaperone molecule in the production of S(3) via two complex intermediates: SAr + S(2) and S(2)Ar + S. Quasi-classical and classical trajectory methods are used. The rate constant of the S + S(2) + Ar → S(3) + Ar reaction is determined to be 2.66 × 10(-33) cm(6) mol(-1) s(-1) at 298.15 K. The temperature dependence of the reaction is found to be 2.67 × 10(-33) exp[143.56(1∕T-1∕298.15)]. The second-order rate constant of S + S(2) → S(3) is 6.47 × 10(-14) cm(3) molecule(-1) s(-1) at 298.15 K and the Arrhenius-type rate constant is calculated to be 6.25 × 10(-14) exp[450.15(1∕T-1∕298.15)] cm(3) molecule(-1) s(-1). This work provides a rate coefficient for a key intermediate species in studies of sulfur formation in the modern Venus atmosphere and the primitive Earth atmosphere, for which assumed model rate coefficients have spanned nearly 4 orders of magnitude. Although a symmetry-induced mass-independent isotope effect is not expected for a chaperone mechanism, the present work is an important step toward evaluating whether mass-independence is expected for thiozone formation as is observed for ozone formation.

Mass-independent isotope fractionation of heavy elements measured by MC-ICPMS: a unique probe in environmental sciences

This article overviews recent developments in the use of multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) in studies of mass-independent isotope chemistry of heavy elements. Origins of mass-independent isotope effects and their relevance to isotope ratio measurements by MC-ICPMS are briefly described. The extent to which these effects can affect instrumental mass bias in MC-ICPMS is critically discussed on the basis of the experimental observations. Furthermore, key findings reported in studies of mass-independent isotope fractionation (MIF) of mercury in the field of environmental sciences are reviewed. MIF of heavy elements is not only of interest from a fundamental point of view, but also provides scientists with a new and effective means of studying the biogeochemistry of these elements.

Mercury isotope fractionation during abiotic transmethylation reactions

Experiments modeling abiotic methylation of mercury were performed by using methylcobalamin, methyltin, acetic acid and dimethylsulfoxide as the methyl donor compounds. Mercury isotope ratios were measured by using multicollector ICP-MS for both methylmercury and parent inorganic Hg(II). Abiotic methylation of mercury in the dark was accompanied by mass-dependent Hg isotope fractionation with isotope fractionation factors, α product/substrate, ranging from 0.9985 to 0.9995 in terms of δ 202/198Hg values and with undetectable Hg isotope anomalies. The radical substitution reactions, releasing a methyl radical in solution in the dark, facilitated formation of methylmercury and increased the magnitude of the concomitant mass-dependent Hg isotope effect but were not capable of producing any measurable mass-independent anomaly in the isotopic composition of mercury. In contrast to methylation in the dark, photochemical methylation of mercury was accompanied by both mass-dependent and mass-independent Hg isotope fractionation. The latter resulted in selective enrichment of 199Hg and 201Hg isotopes in methylmercury and was attributed to the magnetic isotope effect. These data highlight the fact that the light-assisted methylation of mercury produces a unique mass-independent Hg isotope signature that can be used in tracing the origin of this highly toxic compound.

Tracing and Quantifying Anthropogenic Mercury Sources in Soils of Northern France Using Isotopic Signatures

The mercury (Hg) isotopic composition was investigated in topsoils from two case studies in north of France. The Hg isotope composition was first determined in agricultural topsoils contaminated by a close by Pb-Zn smelter. The Hg isotopic composition was also measured in topsoils from an urban area in northeastern France (Metz). In both cases, no significant mass independent isotope fractionation could be found in the soils. However, the soil isotopic composition (δ(202)Hg) was enriched in the heavier isotopes as the Hg concentration increased in the soils. A linear relationship between the δ(202)Hg in soils and 1/[Hg] indicated a mixing between a contamination source and the Hg derived from the geogenic background soils. Such findings demonstrate that the contamination signature was preserved in the soils and that the deposition of anthropogenic Hg was predominant compared to reactions leading to isotope fractionation such as biotic and abiotic reduction of Hg(II) and resulting in Hg mobility or evasion from the soils. It was therefore possible, for the first time in the case of Hg, to evaluate the contribution of the contamination source relative to the background Hg source in urban topsoils using relative isotope abundances.

Modern mass spectrometry for studying mass-independent fractionation of heavy stable isotopes in environmental and biological sciences

This is the first review to focus specifically on the application of modern mass spectrometry techniques for studying mass-independent or anomalous isotope fractionation for ‘heavy’ elements. This review covers a period of ten years, starting from 2000. In the first part of the manuscript, we address theoretical aspects of mass-independent isotope fractionation, such as nuclear volume and nuclear spin effects. Secondly, most published articles on this topic to date are reviewed. Mass-independent isotope fractionation was determined by ICP-MS, MC-ICP-MS or TIMS for nineteen elements so far: Hg, Sn, Cd, Zn, Te, Sr, Pb, Cr, U, Ti, Ni, Mo, Ru, Ba, Nd, Sm, Gd, Yb and Hf, as reported in about fifty publications. In addition, mass-independent fractionation was observed for some other elements, silicon, germanium and sulfur, by other techniques.

Magnetic isotope effect and theory of atomic orbital hybridization to predict a mechanism of chemical exchange reactions

A novel approach is suggested to investigate the mechanisms of chemical complexation reactions based on the results of Fujii with co-workers; they have experimentally observed that several metals and metalloids demonstrate mass-independent isotope fractionation during the reactions with the DC18C6 crown ether using solvent-solvent extraction. In this manuscript, the isotope fractionation caused by the magnetic isotope effect is used to understand the mechanisms of chemical exchange reactions. Due to the rule that reactions are allowed for certain electron spin states, and forbidden for others, magnetic isotopes show chemical anomalies during these reactions. Mass-independent fractionation is suggested to take place due to the hyperfine interaction of the nuclear spin with the electron spin of the intermediate product. Moreover, the sign of the mass-independent fractionation is found to be dependent on the element and its species, which is also explained by the magnetic isotope effect. For example, highly negative mass-independent isotope fractionation of magnetic isotopes was observed for reactions of DC18C6 with SnCl(2) species and with several Ru(III) chloro-species, and highly positive for reactions of this ether with TeCl(6)(2-), and with several Cd(II) and Pd(II) species. The atomic radius of an element is also a critical parameter for the reaction with crown ether, particularly the element ions with [Kr]4d(n)5s(m) electron shell fits the best with the DC18C6 crown ring. It is demonstrated that the magnetic isotope effect in combination with the theory of orbital hybridization can help to understand the mechanism of complexation reactions. The suggested approach is also applied to explain previously published mass-independent fractionation of Hg isotopes in other types of chemical exchange reactions.

後期始生代の堆積岩中に見られるイオウ質量非依存同位体分別(MIF-S)の特徴

後期始生代の堆積岩中に見られるイオウ質量非依存同位体分別(MIF-S)の特徴

Technique for High-Precision Analysis of Triple Oxygen Isotope Ratios in Carbon Dioxide

Since the discovery of mass-independent isotope effects in stratospheric and tropospheric gases, the analysis of triple oxygen isotope abundance in carbon dioxide gained in importance. However, precise triple oxygen isotope determination in carbon dioxide is a challenging task due to mass-interference of (17)O and (13)C variations. Here, we present a novel analytical technique that allows us to determine slight deviations of CO(2) from the terrestrial fractionation line [TFL]. Our approach is based on isotopic equilibration between CO(2) gas and CeO(2) powder at 685 degrees C and subsequent mass spectrometric analysis of ceria powder by infrared-laser fluorination. We found that beta(CO2-CeO2), the exponent in the relation alpha(17/16) = (alpha(18/16))(beta), amounts to 0.5240 +/- 0.0011 at 685 degrees C. The oxygen isotope anomaly of CO(2) (Delta(17)O) can be determined for a single analysis of CeO(2) with a precision of +/-0.05 per thousand (1sigma). Our CO(2)-CeO(2) equilibration procedure is performed with an excess of CO(2) so that one analysis of Delta(17)O on CO(2) requires at least 3.5 mmol of CO(2) gas. Our new technique allows accurate and precise determination of Delta(17)O in CO(2) and opens up a new field for investigating triple oxygen isotope abundance in various types of natural CO(2).

Isotope Fractionation of Mercury during Its Photochemical Reduction by Low-Molecular-Weight Organic Compounds

Photochemical reduction of Hg(II) by various low-molecular-weight organic compounds (LMWOC) was investigated to evaluate the effect of specific functional groups that are typically encountered in natural dissolved organic matters (DOM) on the photoreactivity and isotope fractionation of Hg. LMWOC with reduced sulfur functional groups (e.g., cysteine, glutathione) resulted in slower photochemical reduction of Hg(II) than those without reduced sulfur groups (e.g., serine, oxalic acid). Reduction rate constants were specifically determined for two contrasting LMWOC: dl-serine (0.640 h(-1)) and l-cysteine (0.047 h(-1)). Different mass independent isotope effects of Hg were induced by the two types of LMWOC. S-containing ligands specifically enriched magnetic isotopes ((199)Hg and (201)Hg) in the product (Hg(0)) while sulfurless ligands enriched (199)Hg and (201)Hg in the reactant (Hg(II)), suggesting that opposite magnetic isotope effects were produced by different types of ligands. The nuclear field shift effect was also observed in the photochemical reduction by serine. These isotope effects are related to specific functional groups and reduction mechanisms, and may be used to distinguish between primary and secondary photochemical reduction mechanisms of Hg(II) and to explain isotope fractionation during the photochemical reduction of Hg(II) by natural DOM, which provides mixed bonding conditions.

Experimental study of mass-independence of Hg isotope fractionation during photodecomposition of dissolved methylmercury

Experiments modelling photolytic decomposition of methylmercury chloride in aqueous solutions of different chemical composition have been performed. Ion-exchange chromatographic separation using Chelex® 100 resin was used in order to separate methylmercury from inorganic mercury prior to the isotope ratio measurements by solution nebulization multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The performance of the chromatographic separation has been evaluated in terms of recovery of both methylmercury and inorganic Hg using synthetic solutions. Both mass-dependent and mass-independent fractionation of Hg isotopes concomitant with the decomposition process have been observed. Mass-independent Hg isotope fractionation (MIF) resulted in selective enrichment of 199Hg and 201Hg relative to the other isotopes in the methylmercury molecules and has been attributed to the magnetic isotope effect. The highest extent of MIF of Hg isotopes, expressed as Δ199Hg and Δ201Hg values, has been observed in acidified solution with low concentration of total dissolved solids (TDS). Progressive decrease in Δ199Hg and Δ201Hg values in acidified solution with higher concentration of TDS, alkaline solutions of both low and high concentration of TDS, and in a solution of ascorbic acid has been attributed to suppression of the radical pair reaction mechanism, responsible for the occurrence of the magnetic isotope effect, by substances acting as radical scavengers, such as OH− or ascorbic acid. The data obtained in this study demonstrate the significance of spin chemistry effects in the isotope fractionation of mercury.

The Odds and Evens of Mercury Isotopes: Applications of Mass-Dependent and Mass-Independent Isotope Fractionation

Mercury (Hg) is a redox-active trace metal that is viewed internationally as a priority pollutant. Research into Hg stable isotope biogeochemistry is rapidly providing new insight into the behavior of Hg. With the recent discovery that Hg can exhibit both mass-dependent (MDF) and mass-independent fractionation (MIF) (range of >6‰ for both), Hg isotopes are providing a valuable new tool for tracing this important toxin through the environment. MDF alone, which occurs during redox transformations, biological cycling, and volatilization of Hg, can be exploited to increase understanding of the processes that control Hg distribution and bioaccumulation. The addition of MIF signatures greatly increases the usefulness of Hg isotopes because MIF provides a unique fingerprint of specific chemical pathways, such as photochemical reduction.

Anomalous Mercury Isotopic Compositions of Fish and Human Hair in the Bolivian Amazon

We report mercury (Hg) mass-dependent isotope fractionation (MDF) and mass-independent isotope fractionation (MIF) in hair samples of the Bolivian Esse Ejjas native people and in several tropical fish species that constitute their daily diet. MDF with delta(202)Hg ranging from -0.40 to -0.92 per thousand for fish and +1.04 to +1.42 per thousand for hair was observed. Hair samples of native people with a fish-dominated diet are enriched by +2.0 +/- 0.2 per thousand in delta(202)Hg relative to the fish consumed. Both odd Hg isotopes, (199)Hg and (201)Hg, display MIF in fish (from -0.14 to +0.38 per thousand for Delta(201)Hg and from -0.09 to +0.55 per thousand for Delta(199)Hg) and in hair (from +0.12 to +0.66 per thousand for Delta(201)Hg and from +0.14 to +0.81 per thousand for Delta(199)Hg). No significant difference in MIF anomalies is observed between Hg in fish and in human hair, suggesting that the anomalies act as conservative source tracers between upper trophic levels of the tropical food chain. Fish Hg MIF anomalies are 10-fold lower than those published for fish species from midlatitude lakes. Grouping all Amazonian fish species per location shows that Delta(199)Hg:Delta(201)Hg regression slopes for the clear water Itenez River basin (0.95 +/- 0.08) are significantly lower than those for the white water Beni River basin (1.28 +/- 0.12). Assuming that the observed MIF originates from aquatic photoreactions, we calculated limited photodemethylation of monomethylmercury (MMHg) in the Beni River floodplains and insignificant photodemethylation in the Itenez River floodplains. This is possibly related to lower residence times of MMHg in the Itenez compared to the Beni River floodplains. Finally, a significantly negative Delta(201)Hg of -0.14 per thousand in Beni River fish suggests that the inorganic Hg precursor to the MMHg that bioaccumulates up the food chain defines an ecosystem specific non-zero Delta(201)Hg baseline. Calculation of photodemethylation intensities from Hg or MMHg MIF, therefore, requires a baseline correction.

Mass-Dependent and Mass-Independent Isotope Effects of Zinc in a Redox Reaction

We report the isotope fractionation of zinc (Zn) associated with a redox reaction between Zn(0) and Zn(II). Zn isotopes were found fractionated in pyrometallurgical biphase extraction between liquid zinc and molten chloride. The isotopic composition of Zn in the molten chloride phase was analyzed by multiple collector inductively coupled plasma mass spectrometry and reported as (m)Zn/64Zn (m = 66, 67, and 68) ratios. The observed isotope fractionation consists of the mass-dependent and mass-independent isotope effects. The contributions of the nuclear mass and the nuclear volume to the overall isotope effect were evaluated by employing first-principles quantum calculations and using reported isotope shifts in atomic spectra. The magnitude of the mass-dependent isotope effect was explained by the sum of the isotope effect via intramolecular vibrations and the correction to the Born-Oppenheimer electronic energy. The mass-independent isotope effect was correlated with the Gibbs free energy change in the redox reaction.