Cd Isotopic Compositions Research Articles

Reply to the “Comment by on ‘the isotopic composition of Cadmium in the water column of the South China Sea’”

Our previous study has shown that the Cd isotopic composition in the surface water and the sinking particles in the South China Sea (SCS) are identical at ε114/110Cd∼+9. Therefore, net biological isotopic fractionation of Cd in the surface water was insignificant (Yang et al., 2012). Using a box-model calculation, Murphy et al. (2014) suggested that the particles should have relatively light εCd, +3 to +5, if the aerosol inputs were characterized with εCd from −4 to +4. Consequently, the observed +9 εCd is not representative to the overall SCS phytoplankton exports, and the heavy εCd observed in the surface water of the SCS must be the results of preferential uptake of light Cd by phytoplankton. Here we argue that their assumption about the aerosol εCd is most likely incorrect as in the case of SCS. In addition, it is questionable for the heavy εCd in the surface water reflecting solely by the preferential uptake of phytoplankton. Besides phytoplankton uptake, εCd in the surface water may also be influenced by microbial degradation and zooplankton grazing. Consequently, an integrated study focusing on the fractionation effects of the processes is necessary to fully understand the major controlling mechanisms on Cd isotopic fractionations in the oceans.

Cadmium isotope ratio measurements in environmental matrices by MC-ICP-MS

Various stages of an analytical method for high-precision cadmium (Cd) isotope ratio measurements by MC-ICP-MS (sample preparation, matrix separation, instrumental analysis and data evaluation) were critically evaluated and optimized for carbon-rich environmental samples. The method was used in a pilot study focusing on the assessment of factors affecting Cd isotope composition in birch leaves.

Comment on “The isotopic composition of cadmium in the water column of the South China Sea”

Yang et al. (2012) found that marine biogenic particles and corresponding surface seawater from the South China Sea were characterized by essentially identical Cd isotope compositions, and inferred that biological Cd isotope fractionation is insignificant at this location. Based on results obtained for a box model that represents Cd cycling in the surface layer, and using published data to constrain the Cd source and sink fluxes, we show that this conclusion is likely to be incorrect. The modeling results indicate that the heavy Cd isotope signature of ε114/110Cd≈+9 observed by Yang et al. (2012) for the surface South China Sea is most likely a consequence of substantial isotope fractionation (of about 4–6 ε) during uptake of dissolved seawater Cd by phytoplankton.

Biogeochemical cycling of cadmium isotopes in the Southern Ocean along the Zero Meridian

We present depth profiles of Cd isotopes and concentrations from the Southern Ocean at four stations in the Atlantic sector along the Greenwich Meridian (47°S to 68°S) located across the main Antarctic frontal zones and productivity belt. The vertical profiles of Cd concentration typically show low values in surface waters, elevated values at intermediate depths, reflecting remineralization of sinking particulate organic matter, and constant values in deep waters. The surface-to-deep isotopic gradient shows “heavy” Cd isotope signatures in the mixed surface layer, becoming more pronounced northward, with values up to ɛ112/110Cd of around +4.1 in the Subantarctic sector of the Southern Ocean. Deep Antarctic waters display a uniform and “light” ε112/110Cd of +1.18±0.38 and Cd concentrations of 0.761±0.101nmol/kg (n=23, 2SD). Intermediate waters are characterized by ε112/110Cd lying between those of surface and deep waters, with a constant value of about +0.8 in the High Nutrient Low Chlorophyll sector and a notably higher value of +2.3 in the Subantarctic sector.The Cd isotope fractionation in the Southern Ocean closely follows a simple closed-system Rayleigh model, in which biological uptake of Cd imparts the ε112/110Cd signature to the surface layer while that of deep waters is determined by the flux of regenerated isotopically-light Cd from sinking organic matter from the surface ocean and the degree of mixing of distinct water masses.The vertical gradient documented for Cd isotopes and nutrient ratios, along with the meridional gradient in surface waters, highlights the important role played by upwelling in the Southern Ocean in closing the meridional overturning circulation via the export of Antarctic intermediate and mode waters which have a distinctive chemical (low Cd:P) and Cd isotope (“heavy”) signature.The combined Cd–Zn isotope systematics provide evidence for a strong link between the magnitude of biological Cd stable isotope fractionation and Zn availability in the contrasted nutrient and ecological regimes of the Southern Ocean. Substitution of Cd for Zn in the enzyme carbonic anhydrase appears to be the driving mechanism for Cd isotope fractionation in the Antarctic Circumpolar Current, while an “excess-uptake” mechanism seems to predominate in the Weddell Gyre.Our study highlights some of the complexities of the biogeochemical cycling of Cd in the oceans. Nevertheless, systematic variations in Cd isotopic compositions with water mass distribution in the Southern Ocean suggest that Cd isotopes could, with some caveats, be useful tracers of changes in past nutrient utilization and deep water circulation.

Characteristics of Cd isotopic compositions and their genetic significance in the lead-zinc deposits of SW China

National Basic Research Program of China [2009CB421005]; Chinese Academy of Sciences for Key Topics in Innovation Engineering [KZCX2-YW-Q04-01]

Cd isotopes as a potential source tracer of metal pollution in river sediments

Tracing the sources of heavy metals in water environment is key important for our understanding of their pollution behavior. In this present study, Cd concentrations and Cd isotopic compositions in sediments were determined to effectively identify possible Cd sources. Results showed that elevated concentrations and high enrichment factor for Cd were found in all sediments, suggesting anthropogenic Cd origin. Cd isotopic compositions in sediments yielded relative variations ranged from −0.35‰ to 0.07‰ in term of δ114/110Cd (the mean: −0.08‰). Large fractionated Cd was found in sediments collected from a smelter and an E-waste town. Cd isotopic compositions and Cd concentrations measured in sediments allowed the identification of three main origins (dust from metal refining (δ114/110Cd < 0), slag of metal refining (δ114/110Cd > 0), and those δ114/110Cd = 0, such as background and mining activity). According to the actual precision obtained, Cd isotopes could be a potential tool for tracing metal pollution sources in water environment.

Reply to Morel: Cadmium as a micronutrient and macrotoxin in the oceans

We thank Francois Morel for his interest in our study. Morel states that our conclusions are based on the approximate match between the Cd-isotope composition of cultured bacteria and the fractionation of Cd isotopes seen in seawater (1). This match is only a minor component of our argument, and we welcome the opportunity to reiterate our case.

Nonspecific uptake and homeostasis drive the oceanic cadmium cycle

The global marine distributions of Cd and phosphate are closely correlated, which has led to Cd being considered as a marine micronutrient, despite its toxicity to life. The explanation for this nutrient-like behavior is unknown because there is only one identified biochemical function for Cd, an unusual Cd/Zn carbonic anhydrase. Recent developments in Cd isotope mass spectrometry have revealed that Cd uptake by phytoplankton causes isotopic fractionation in the open ocean and in culture. Here we investigate the physiochemical pathways that fractionate Cd isotopes by performing subcellular Cd isotope analysis on genetically modified microorganisms. We find that expression of the Cd/Zn carbonic anhydrase makes no difference to the Cd isotope composition of whole cells. Instead, a large proportion of the Cd is partitioned into cell membranes with a similar direction and magnitude of Cd isotopic fractionation to that seen in surface seawater. This observation is well explained if Cd is mistakenly imported with other divalent metals and subsequently managed by binding within the cell to avoid toxicity. This process may apply to other divalent metals, whereby nonspecific uptake and subsequent homeostasis may contribute to elemental and isotopic distributions in seawater, even for elements commonly considered as micronutrients.

Isotopic analysis of Cd in the mixing zone of Siberian rivers with the Arctic Ocean—New constraints on marine Cd cycling and the isotope composition of riverine Cd

The Cd concentrations and isotopic compositions were determined for 19 water samples which cover the mixing zones of four major rivers with the coastal seas of the Siberian Shelf. The waters span salinities from about 1–32, with Cd concentrations of about 0.02–0.46nmol/kg and small but resolvable Cd isotope fractionations, with ε114/110Cd values of between +1.4 and +5.7. The data for the majority of the samples are in accord with the systematics expected for quasi-binary mixing of Arctic seawater (ε114/110Cd≈5.5±0.5 and Cd concentration between 0.1 and 0.25nmol/kg) with river waters characterized by ε114/110Cd≈+2±1 and low pristine Cd contents of about 0.02–0.06nmol/kg. The river values are similar to the inferred composition of the continental crust, which implies that weathering produces no or only limited Cd isotope fractionation. The results for five samples provide clear evidence for non-conservative behaviour of Cd, as the element is released from suspended riverine particles during mixing with seawater. The isotopic data furthermore show that the desorbed Cd is characterized by ε114/110Cd≈+3, in accord with a natural origin. This implies that the (natural) net riverine Cd fluxes of the Siberian rivers to seawater are also likely to be characterized by ε114/110Cd≈+2±1, a value that is either identical to or intermediate between the composition of the continental crust and marine deep waters. Additional data for the boreal Kalix River in Sweden contrasts with the results obtained for the Siberian rivers, as the former exhibits a much lighter Cd isotope composition of ε114/110Cd=–3.8 coupled with a much higher Cd content of ∼0.24nmol/kg. These characteristics appear to be a consequence of the distinct hydro-geological setting of the Kalix drainage basin, which suggests that the riverine input of Cd isotopes to the oceans might display significant regional variability. In summary, our study underlines the important role that stable isotope analyses can play in biogeochemical investigations of trace metals. Here, the Cd isotope results provide important constraints, which are not available from concentration data alone, on the cycling of Cd in riverine and shelf environments.

The isotopic composition of Cadmium in the water column of the South China Sea

We determined the Cd isotopic composition of seawater and sinking particles collected in the deep basin of the northern South China Sea (SCS) to investigate the controlling mechanisms on the Cd isotopic composition in the water column. The isotopic composition in the water column decreased with depth, with ε114/110Cd values (ε114/110Cd=[(114Cd/110Cd)sample/(114Cd/110Cd)JMC Cd Münster−1]×104) ranging from +8.7 to +9.9 in the top 80m, from +4.6 to +5.5 between 100 and 150m, decreasing from +5.5 to +3.6 at depths from 150 to 1000m, and remaining at +3.4±0.5 from 1000 to 3500m. The isotopic composition and concentrations of Cd observed in the deep waters of the SCS are similar to the values that were previously reported in the North Pacific Ocean. In the thermocline, the variations in the Cd isotopic composition and concentrations were consistent with the relative volumetric percentages of the subsurface water, the intermediate water, and the deep water in the water column, indicating that water mixing is the dominant process determining the isotopic composition in the thermocline. Comparable to the isotopic composition value in the seawater of the mixed layer, the ε114/110Cd in the sinking particles collected at 30m was +9.3±0.9. Because our previous studies demonstrated that the particulate Cd was predominantly biogenic organic matter, the comparable isotopic composition between the surface seawater and the sinking particles indicates that net biological isotopic fractionation on Cd in the surface water was insignificant. The result indicates that phytoplankton do not necessarily take up relatively light Cd in the oceanic surface waters. It is necessary to directly and systematically investigate how marine phytoplankton fractionate Cd isotopes.

Approche de la santé au travail des artistes de cirque, en France

Environmental monitoring and remediation require techniques to identify the source and fate of metals emissions. The measurement of heavy metal isotopic signatures, made possible by the advent of the MC-ICP-MS, is a powerful new geochemical tool, which may be used to trace the source of these metals in the environment. In a multi-tracer study, Cd, Zn and Pb isotopic compositions (MC-ICP-MS) and elemental concentrations (HR-ICP-MS) are used to distinguish between natural and anthropogenic sources of these metals in bivalves collected from western Canada (British Columbia), Hawaii, and the USA East Coast.Variability in the δ114/110Cd values of bivalves (−1.20‰ to −0.09‰) is attributed to differences in the relative contributions of Cd from natural and anthropogenic sources between sites. Cadmium isotopic compositions (δ114/110Cd = −0.69‰ to −0.09‰) identify high Cd levels in B.C. oysters as primarily natural (i.e., upwelling of Cd rich intermediate waters in the North Pacific), with some variability attributed to anthropogenic sources (e.g., mining and smelting). Variability in the δ66/64Zn values exhibited by the B.C. bivalves is relatively small (0.28–0.36‰). Despite the low Pb levels found in B.C. oysters, Pb isotopes are used to identify emissions from industrial processes and the consumption of unleaded gasoline and diesel fuel as significant metal sources. Although the Cd concentrations of the USA East Coast bivalves are primarily lower than those of B.C. oysters, their relatively light Cd isotopic compositions (δ114/110Cd = −1.20‰ to −0.54‰) indicate the significance of anthropogenic Cd sources and are attributed to the high prevalence of industry on this coast. The δ114/110Cd values of USA East Coast bivalves include the lightest ever reported, with the exception of values reported for extraterrestrial materials. In addition, the Pb isotopic compositions of bivalves from the USA East Coast indicate Pb emissions from the combustion of coal are an important source of Pb, consistent with the high consumption of coal for power production on this coast.This study demonstrates the effective use of Cd and Zn isotopes to trace anthropogenic sources in the environment and the benefit of combining these tools with Pb “fingerprinting” techniques.

Isotopic fractionation of cadmium into calcite

Cadmium mimics the distribution of the macronutrient phosphate in the oceans, and has uses as a palaeoproxy of past ocean circulation and nutrient utilization. Isotopic analyses of dissolved Cd in modern seawater show potential as a new tool for disentangling phytoplankton utilization of Cd from abiotic processes, such as ocean mixing. Extending this information into the past requires the Cd isotope signal to be captured and faithfully preserved in a suitable sedimentary archive. However, the role that environmental factors, such as temperature, may play in controlling Cd isotope fractionation into such archives has not been assessed. To this end, we have performed controlled inorganic CaCO 3 precipitation experiments in artificial seawater solutions. We grew calcite under different precipitation rates, temperatures, salinities, and ambient [Mg 2 + ], before measuring Cd isotopic compositions by double spike MC-ICPMS. We find that the isotopic fractionation factor for Cd into calcite ( α C a C O 3 – C d aq ) in seawater is always less than one (i.e. light isotopes of Cd are preferred in calcite). The fractionation factor has a value of 0.99955 ± 0.00012 and shows no response to temperature, [Mg 2 + ], or precipitation rate across the range studied. The constancy of this fractionation in seawater suggests that marine calcites may provide a record of the local seawater composition, without the need to correct for effects due to environmental variables. We also performed CaCO 3 growth in freshwater and, in contrast to calcite precipitated from artificial seawater solutions, no isotopic offset was recorded between the growth solution and calcite ( α C a C O 3 – C d aq = 1.0000 ± 0.0001 ). Cadmium isotope fractionation during calcite growth can be explained by a kinetic isotope effect during the largely unidirectional incorporation of Cd at the mineral surface. Further, the rate of Cd uptake and isotopic fractionation can be modulated by increased ion blocking of crystal surface sites at high salinity. The fractionation of Cd isotopes observed during precipitation of calcite has the same direction and similar magnitude to that implicated for Cd removal from the surface ocean by seawater measurements. However, flux calculations show that CaCO 3 precipitation is unlikely to play a significant role in setting the Cd isotope composition in seawater, compared to Cd utilization in phytoplankton soft tissue. Marine carbonates therefore record seawater Cd isotope chemistry – with potential as a palaeoceanographic proxy – rather than drive oceanic Cd isotope compositions.

Tracing cadmium, zinc and lead sources in bivalves from the coasts of western Canada and the USA using isotopes

Environmental monitoring and remediation require techniques to identify the source and fate of metals emissions. The measurement of heavy metal isotopic signatures, made possible by the advent of the MC-ICP-MS, is a powerful new geochemical tool, which may be used to trace the source of these metals in the environment. In a multi-tracer study, Cd, Zn and Pb isotopic compositions (MC-ICP-MS) and elemental concentrations (HR-ICP-MS) are used to distinguish between natural and anthropogenic sources of these metals in bivalves collected from western Canada (British Columbia), Hawaii, and the USA East Coast. Variability in the δ 114/110Cd values of bivalves (−1.20‰ to −0.09‰) is attributed to differences in the relative contributions of Cd from natural and anthropogenic sources between sites. Cadmium isotopic compositions (δ 114/110Cd = −0.69‰ to −0.09‰) identify high Cd levels in B.C. oysters as primarily natural (i.e., upwelling of Cd rich intermediate waters in the North Pacific), with some variability attributed to anthropogenic sources (e.g., mining and smelting). Variability in the δ 66/64Zn values exhibited by the B.C. bivalves is relatively small (0.28–0.36‰). Despite the low Pb levels found in B.C. oysters, Pb isotopes are used to identify emissions from industrial processes and the consumption of unleaded gasoline and diesel fuel as significant metal sources. Although the Cd concentrations of the USA East Coast bivalves are primarily lower than those of B.C. oysters, their relatively light Cd isotopic compositions (δ 114/110Cd = −1.20‰ to −0.54‰) indicate the significance of anthropogenic Cd sources and are attributed to the high prevalence of industry on this coast. The δ 114/110Cd values of USA East Coast bivalves include the lightest ever reported, with the exception of values reported for extraterrestrial materials. In addition, the Pb isotopic compositions of bivalves from the USA East Coast indicate Pb emissions from the combustion of coal are an important source of Pb, consistent with the high consumption of coal for power production on this coast. This study demonstrates the effective use of Cd and Zn isotopes to trace anthropogenic sources in the environment and the benefit of combining these tools with Pb “fingerprinting” techniques.

Ferromanganese crusts as archives of deep water Cd isotope compositions

The geochemistry of Cd in seawater has attracted significant attention owing to the nutrient‐like properties of this element. Recent culturing studies have demonstrated that Cd is a biologically important trace metal that plays a role in the sequestration of inorganic carbon. This conclusion is supported by recent isotope data for Cd dissolved in seawater and incorporated in cultured phytoplankton. These results show that plankton features isotopically light Cd while Cd‐depleted surface waters typically exhibit complimentary heavy Cd isotope compositions. Seawater samples from below 900 m depth display a uniform and intermediate isotope composition of ε114/110Cd = +3.3 ± 0.5. This study investigates whether ferromanganese (Fe‐Mn) crusts are robust archives of deep water Cd isotope compositions. To this end, Cd isotope data were obtained for the recent growth surfaces of 15 Fe‐Mn crusts from the Atlantic, Pacific, Indian, and Southern oceans and two USGS Fe‐Mn reference nodules using double spike multiple collector inductively coupled plasma mass spectrometry. The Fe‐Mn crusts yield a mean ε114/110Cd of +3.2 ± 0.4 (2 SE, n = 14). Data for all but one of the samples are identical, within the analytical uncertainty of ±1.1ε114/110Cd (2 SD), to the mean deep water Cd isotope value. This indicates that Fe‐Mn crusts record seawater Cd isotope compositions without significant isotope fractionation. A single sample from the Southern Ocean exhibits a light Cd isotope composition of ε114/110Cd = 0.2 ± 1.1. The origin of this signature is unclear, but it may reflect variations in deep water Cd isotope compositions related to differences in surface water Cd utilization or long‐term changes in seawater ε114/110Cd. The results suggest that time series analyses of Fe‐Mn crusts may be utilized to study changes in marine Cd utilization.

The thallium isotope composition of carbonaceous chondrites — New evidence for live 205Pb in the early solar system

The extinct radionuclide 205Pb, which decays to 205Tl with a half-life of 15 Ma, is of considerable cosmochemical interest, as it is the only short-lived isotope that is produced exclusively by s-process nucleosynthesis. Evidence for the existence of 205Pb in the early solar system has only recently been obtained from analyses of IAB iron meteorites, but significant uncertainties remain about the initial 205Pb abundance and Tl isotope composition of the solar system. In an attempt to better constrain these values, a comprehensive 205Pb– 205Tl isochron study was carried out on ten carbonaceous chondrites of groups CI, CM, CV, CO and CR. The Pb and Cd isotope compositions of the meteorites were also determined, to correct for terrestrial Pb contamination and eliminate samples that exhibit fractionated Tl isotope compositions from thermal processing. The analyses revealed only limited variation in ε 205Tl, with values of between − 4.0 and + 1.2, but nonetheless the Tl isotope compositions correlate with Pb/Tl ratios. This correlation is unlikely to be due to stable isotope fractionation from terrestrial weathering or early solar system processes, and is most readily explained by in situ decay of 205Pb to 205Tl. Previous 53Mn– 53Cr and 107Pd– 107Ag studies of bulk carbonaceous chondrites provide evidence that the Pb–Tl isochron records volatile fractionation in the solar nebula at close to 4567 Ma. The isochron thus yields the initial 205Pb abundance and Tl isotope composition of the solar system, with values of 205Pb/ 204Pb SS,0 = (1.0 ± 0.4) × 10 − 3 and ε 205Tl SS,0 = − 7.6 ± 2.1, respectively. These results confirm the previous Pb–Tl data for IAB iron meteorites, which provided the first clear evidence for the existence of live 205Pb in the early solar system. The initial 205Pb SS,0 abundance inferred from carbonaceous chondrites demonstrates that the 205Pb– 205Tl decay system is well suited for chronological studies of early solar system processes that produce fractionations in Pb/Tl ratios, including core crystallization and the mobilization of volatiles during thermal processing. The 205Pb SS,0 abundance is close to the upper limit of nucleosynthetic production estimates for AGB stars and thus in accord with contributions of such stars to the early solar system budget of freshly synthesized radioisotopes.

Mass-dependent cadmium isotopic variations in nature with emphasis on the marine environment

We report a survey of natural mass-dependent cadmium isotope fractionation measured by thermal ionization mass spectrometry using a double-spike technique (DS-TIMS). Over sixty samples of natural terrestrial Cd from diverse environments, including MORB, OIB, continental loess, hydrogenic and hydrothermal ferromanganese deposits, and sphalerites (both oceanic and from major continental ore deposits) were analysed. Our results are expressed in terms of ε 112/110Cd, which are deviations in 112Cd/ 110Cd from our in-house JMC Cd standard in parts per 10 4. The total ε 112/110Cd variation is relatively small, with a range of only 5 ε-units, and is one-to-two orders of magnitude smaller than that previously found in meteorites. The MORB, OIB and loess ε 112/110Cd values are similar and provide a good estimate for the bulk silicate Earth (BSE) value which is − 0.95 ± 0.12 relative to our Cd standard (ε 112/110Cd = + 0.16 relative to Münster JMC Cd). Taken together, these data suggest little Cd isotope fractionation takes place during crust–mantle segregation. Cd isotopic compositions of continental sphalerite (ZnS) deposits worldwide and high-temperature oceanic hydrothermal sulphides show remarkably similar ε 112/110Cd values, consistent with our estimate for the BSE. In contrast, mid-temperature oceanic sulphides from a single extinct hydrothermal chimney display over 4 ε-units variation — along with the most negative values. These variations are most probably caused by precipitation/redissolution of sulphide phases en route within the hydrothermal system. The ε 112/110Cd variability found in worldwide marine Fe–Mn deposits reflects the seawater Cd isotope signal upon precipitation from ambient seawater. A decrease in ε 112/110Cd is observed in passing from shallow-water Fe–Mn deposits to those from deeper waters (> 2000 m depth). This shift is explained by biological fractionation related to the uptake of dissolved seawater Cd by phytoplankton in the upper water column. The relatively uniform ε 112/110Cd values close to zero at great depths are consistent with regeneration and remineralization of Cd at depth. Our data suggest that Cd isotopes – much like the Cd/Ca ratio in foraminifera – could potentially serve as a proxy for past changes in biological productivity. The temporal Cd isotope record in a Fe–Mn crust archive at 2000 m depth from the NE Atlantic suggests no gross long-term changes in Cd cycling took place over the past 8 Ma.

Matrix effects on the multi-collector inductively coupled plasma mass spectrometric analysis of high-precision cadmium and zinc isotope ratios

Resin-derived contaminants added to samples during column chemistry are shown to cause matrix effects that lead to inaccuracy in multi-collector inductively coupled plasma mass spectrometry measurement of small natural variations in Cd and Zn isotopic compositions. These matrix effects were evaluated by comparing pure Cd and Zn standards and standards doped with bulk column blank from the anion exchange chromatography procedure. Doped standards exhibit signal enhancements (Cd, Ag, Zn and Cu), instrumental mass bias changes and inaccurate isotopic compositions relative to undoped standards, all of which are attributed to the combined presence of resin-derived organics and inorganics. The matrix effect associated with the inorganic component of the column blanks was evaluated separately by doping standards with metals at the trace levels detected in the column blanks. Mass bias effects introduced by the inorganic column blank matrix are smaller than for the bulk column blank matrix but can still lead to significant changes in ion signal intensity, instrumental mass bias and isotopic ratios. Chemical treatment with refluxed HNO 3 or HClO 4/HNO 3 removes resin-derived organic components resulting in matrix effects similar in magnitude to those associated with the inorganic component of the column blank. Mass bias correction using combined external normalization-SSB does not correct for these matrix effects because the instrumental mass biases experienced by Cd and Zn are decoupled from those of Ag and Cu, respectively. Our results demonstrate that ion exchange chromatography and associated resin-derived contaminants can be a source of error in MC-ICP-MS measurement of heavy stable element isotopic compositions.

Precise determination of cadmium and lead isotopic compositions in river sediments

A method for the accurate determination of Cd and Pb isotope compositions in sediment samples is presented. Separation of Cd and Pb was designed by using an anionic exchange chromatographic procedure. Measurements of Cd isotopic compositions were carried out by multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS), by using standard-sample bracketing technology for mass bias correction and Pb isotopic ratios were determined by thermal ionization mass spectrometry (TIMS). The factors that affect the accurate and precise Cd isotope compositions analysis, such as instrumental mass fractionation and isobaric interferences, were carefully evaluated and corrected. The Cd isotopic results were reported relative to an internal Cd solution and expressed as the δ 114/110Cd. Five Cd reference solutions and one Pb standard were repeatedly measured in order to assess the accuracy of the measurements. Uncertainties obtained were estimated to be lesser than 0.11‰ (2s) for the δ 114/110Cd value. Analytical uncertainties in 2s for Pb isotopic ratios were better than 0.5‰. The method has been successfully applied to the investigation of Cd and Pb isotope compositions in sediment samples collected from North River in south China.

Cadmium stable isotope cosmochemistry

Cadmium stable isotope compositions are reported for a comprehensive suite of carbonaceous, ordinary, enstatite, and Rumuruti chondrites as well as achondrites and lunar samples (soils, breccias, pristine anorthosite). The Cd isotope analyses were performed by multiple collector ICP-MS with an external reproducibility of ±0.43‰ (2 sd) for δ114/110Cd. None of the samples shows evidence of nucleosynthetic anomalies and cosmogenic isotope effects from neutron-capture by 113Cd were only observed for two lunar samples.The Cd stable isotope compositions of type 1, 2, and some type 3 carbonaceous chondrites, EH4 enstatite chondrites, eucrites, and the Earth are essentially identical at δ114/110Cd≈0.0±0.4. This suggests that the portion of the solar nebula from which the inner solar system bodies accreted was homogeneous with respect to its Cd isotope composition. It also indicates that the primary volatile element depletion of the inner solar system did not involve partial kinetic Rayleigh evaporation or condensation. Furthermore no resolvable Cd isotope effects were generated during the accretion and initial differentiation of the planetary bodies.In contrast, the analyses reveal large Cd isotope effects for ordinary and some enstatite chondrites, which display δ114/110Cd values between about −8 and +16. Smaller fractionations are observed for the Rumuruti and some type 3 to 5 carbonaceous chondrites. These Cd isotope variations are thought to reflect secondary depletion or redistribution of Cd, due to open system thermal metamorphism on the meteorite parent bodies.One CAI and chondrule separates from the Allende meteorite have unexpectedly high Cd concentrations and fractionated light Cd isotope compositions with δ114/110Cd≈−1 to −4. These characteristics may have been established by the interaction of originally Cd-poor materials with a volatile-rich gas prior to the final accretion of the Allende parent body. The general Cd enrichment of the lunar soil and regolith mainly reflects early volcanic activity. However, decreasing Cd abundances in lunar soils correlate well with an enrichment of the heavy Cd isotopes. This relationship is best explained by suppressed Rayleigh fractionation in response to space weathering.

Cadmium isotope fractionation in seawater — A signature of biological activity

Investigations of cadmium isotope variations in the oceans may provide new insights into the factors that control the marine distribution and cycling of this element. Here we present the results of Cd isotope and concentration analyses for 22 seawater samples from the Atlantic, Southern, Pacific, and Arctic Oceans. The results reveal, for the first time, large and well resolved Cd isotope fractionations in the marine environment. The majority of the seawater samples display an inverse relationship between dissolved Cd contents and isotope compositions, which range from ε 114/110Cd ≈ + 3 ± 0.5 for Cd-rich waters (0.8–1.0 nmol/kg) to ε 114/110Cd ≈ 38 ± 6 for surface water with a Cd concentration of only 0.003 nmol/kg (all ε 114/110Cd data are reported relative to the JMC Cd Münster standard). This suggests that the Cd isotope variations reflect kinetic isotope effects that are generated during closed system uptake of dissolved seawater Cd by phytoplankton. A few samples do not follow this trend, as they exhibit extremely low Cd contents (< 0.008 nmol/kg) and nearly un-fractionated Cd isotope compositions. Such complexities, which are not revealed by concentration data alone, require that the Cd distribution at the respective sites was affected by additional processes, such as water mass mixing, atmospheric inputs of Cd and/or adsorption. Uniform isotope compositions of ε 114/110Cd = + 3.3 ± 0.5 (1 S.D.) were determined for seawater from ≥ 900 m depth, despite of Cd concentrations that display the expected increase along the global deep-water pathway from the Atlantic (∼ 0.3 nmol/kg) to the Pacific Ocean (∼ 0.9 nmol/kg). This indicates that the biomass, which is remineralized in the deeper ocean, is also characterized by a very constant Cd isotope composition. This observation is in accord with the interpretation that the Cd distribution in surface waters is primarily governed by Rayleigh fractionation during near-quantitative uptake of dissolved seawater Cd.