Isotope Fractionation Effects Research Articles

Insight of microbial degradation of n-hexadecane and n-heneicosane in soil during natural attenuation and bioaugmentation by Compound-specific Stable Isotope Analysis (CSIA)

Microbial degradation of n-hexadecane (n-C16) and n-heneicosane (n-C21) in soil during natural attenuation (NA) and bioaugmentation (BA) were investigated by compound-specific stable isotope analysis (CSIA), microbial community and key enzyme genes analysis. In both NA and BA treatments, more than 95% of n-C16 and n-C21 were degraded within 3 and 6 days, respectively, with a slight faster degradation in BA than NA. According to CSIA, n-C16 showed a reverse hydrogen isotope fractionation effect, while n-C21 had a normal hydrogen isotope fractionation effect, and the degree of hydrogen isotope fractionation in BA were greater than NA. Isotope enrichment factors (εH) of n-C16 and n-C21were 3.50 ± 0.60‰ and − 2.11 ± 0.57‰ in NA, and 5.18 ± 0.46‰ and − 2.73 ± 0.39‰ in BA, respectively. The dominant genera in BA and NA were significantly different for n-C16 and n-C21 degradation and few strains added during BA remained active, such as Rhodococcus in both n-C16 and n-C21, and Brevundimonas only in n-C21, which showing high application potential in alkanes contamination remediation. Both alkB and almA genes played more key roles than Cytochromes P450 in n-C16 and n-C21 biodegradation, but alkB was much more promoted by n-C16 while almA was more motivated by n-C21. The different types of alkane-degrading bacteria with different enzyme reaction kinetics might be the reason of opposite hydrogen isotope fractionation of n-C16 and n-C21 during their biodegradation. Above all, CSIA can be a proper technology to quantitatively evaluate the biodegradation of different alkane during NA and BA bioremediation process in contamination sites.

Quantitative analysis of hydrogen isotope gas mixtures by low resolution quadrupole mass spectrometer

Quadrupole mass spectrometry (QMS) has an important application in the field of rapid on-line analysis of hydrogen isotopes. However, due to the interference of isotope fractionation effect and mass overlap peaks, accurate quantitative analysis of hydrogen isotopes by quadrupole mass spectrometry has always been a challenging problem. For the calibration of a low-resolution quadrupole mass spectrometry, standard samples of hydrogen isotopes gas mixtures with different HD contents were prepared by platinum catalyst that accelerated hydrogen isotope exchange reaction. The results show that the fraction of ionized fragments of various hydrogen isotopes in the mixture is stable and less affected by the coexistence of other isotopes. Taking the main fragment peak as the calibration basis, considering the isotope fractionation effect and mass overlap peak interference, the calibration curve between hydrogen isotope abundance and mass spectral signal was obtained by systematic correction method, and the linear correlation coefficient is 0.9991. The average relative errors between the correction values and the standard values of isotope abundance measured are 1.87% (hydrogen content) and 3.43% (deuterium content), respectively. The method developed in this work can be used for rapid on-line quantitative analysis of hydrogen isotope gas mixtures.

Freshwater Uptake of Mangrove Growing in an Extremely Arid Area

This study evaluated the freshwater use of two mangrove species (R. mucronate: Rhizophora mucronata and A. marina: Avicennia marina) growing in a coastal area of Djibouti City and on Moucha Island in Djibouti. We accomplished this by comparing hydrogen and oxygen stable isotope ratios (δ2H and δ18O) in the body water with those of creek water, soil water, groundwater, and rainwater samples. Line-conditioned excess (lc-excess) was calculated for the δ2H and δ18O values of the groundwater and rainwater samples. Although two regression lines were drawn using the δ2H and δ18O values of the plant body water as well as the environmental water, no significant differences were found, suggesting that the effects of isotope fractionation due to evaporation in all the samples were almost similar. Comparison of lc-excess and δ18O suggested that the body water from the two mangrove species growing on Moucha Is. were strongly influenced by seawater, but contained some freshwater sources other than the seawater, the soil water, and the creek water. It was also found that Avicennia marina growing in the coastal area was strongly influenced by groundwater. The origin of the freshwater absorbed by the mangroves on the island was not clear, but it could be groundwater recharged in a faraway place.

Quantitative assessment of selective degradation behavior of etoxazole in different classes of organisms by compound-specific isotope analysis

In this paper, the stereoselective degradation and quantitative identification of chiral pesticide etoxazole in organisms with different classes of organisms (soil, chlorella algal fluid and mice) were carried out by compound-specific isotope analysis (CSIA). The degradation behavior and stable isotope fractionation effect of etoxazole in soil, chlorella and mice were investigated. The R-etoxazole degraded faster than S-etoxazole in different classes of organisms. The metabolites M1, M2 and M3 were detected in all three substrates. Biodegradation is the main factor for the change of stable isotope ratio of chiral pesticide etoxazole. Furthermore, the relationship between fractionation value of carbon isotope and residual concentration of etoxazole is established by Rayleigh equation, and the biodegradation rate of etoxazole could be calculated by using CSIA without measuring the concentration of etoxazole. Therefore, the use of CSIA can accurately assess the degradation behavior of pesticide pollution in the environment and provide a certain scientific evidence and technical support in the process of environmental remediation.

Ultra small mass AMS 14C sample preparation and analyses at NIES-TERRA AMS facility

We have made substantial improvements to the NEC 5 MV tandem Pelletron AMS system, such as installing a new type of MC-SNICS ionizer, reconstructing beam lines with the development of the graphite preparation method using an online elemental analyzer for small mass samples down to 1 μgC. To obtain an adequately stable and strong beam current intensity for reliable measurement of small mass samples, we examined graphite position depths (0.35–2.0 mm) related to the optimal geometric conditions to enhance the efficiency of the iron catalyst. Our AMS system generated the most stable and intense beam current at a graphite depth of 1.5–2.0 mm and 2 mg of the iron catalyst, with ionization efficiency of 18–22 %. Under these conditions, the current produced with the 1–10 μgC samples showed a more stable beam current (10–20 μA) and a fivefold increase in beam output duration (greater than 30 min compared to 5 min at 1 μgC). Effects of small carbon graphite-induced isotopic fractionation and higher background was corrected by measuring samples relative to standards of similar mass and a blank. The pMC results were obtained for ultra-small samples ranging from 1 to 50 μgC, calculated relative to adjacent HOxII (NIST SRM 4990C) and IAEA-C1 standards of similar amounts for corrected 14C contents, were approximately in agreement with consensus values overall. By using the method developed in this study, we confirmed the 14C content of ultra-small carbon samples of (1-10 μgC) of IAEA-C8 standards can be measured with precision and accuracy of a few percent.

Origin of highly variable and unusually low δ7Li in mineral separates from ultramafic intrusive rocks in a convergent tectonic setting in the Tibetan plateau

Lithium isotopes are increasingly used as a tracer for the sources of mafic-ultramafic intrusive rocks, but the validity of such approach has never been evaluated directly. We have addressed this issue by analyzing mineral separates from an arc-type mafic-ultramafic complex (namely Xiarihamu) in the Tibetan plateau for Li isotopes. The Xiarihamu complex consists of a gently-dipping, ultramafic chonolith in the center and older gabbro in the margins. The chonolith was interpreted to be a feeder for olivine-charged magma by previous studies. Our new data show minor to strong isotopic disequilibrium between coexisting minerals in the chonolith, with lower δ7Li in clinopyroxene than in coexisting orthopyroxene by up to 7‰. The total range of δ7Li in the pyroxene and olivine separates together is from 5.0‰ to −16.2‰, which is significantly different from that of known arc lavas in the world (3.6 ± 1.2 ‰). Samples from a subvertical drill core of ∼190 m in length penetrating the chonolith show a concentric decrease of δ7Li in orthopyroxene from −16.2‰ in the middle to +1.5‰ and + 3.8‰ in both ends. The δ7Li values of mineral separates show no correlation with whole-rock SrNd isotopes and incompatible trace element ratios, as well as olivine forsterite contents and orthopyroxene Mg/(Mg + Fe) molar ratios, indicating that the observed great variability of δ7Li in the minerals was not caused by variable subduction inputs or magma differentiation alone. A positive correlation exists between orthopyroxene δ7Li and whole-rock LOI, illustrating that serpentinization-talc alteration caused δ7Li in the residual mineral to increase, contributing to the great variability of δ7Li on the mineral scales. Subducting sediment input during magma generation and crustal contamination together cannot adequately account for the observed extremely low δ7Li (−2 to −16.2‰) in the minerals from many of the rock samples as well as the unusually low average δ7Li (−3.5‰) of all of the samples, because the δ7Li of the crust and subducting sediments are much higher. Kinetic fractionation of Li isotopes in response to a temperature gradient across the chonolith as well as to subsequent diffusion of Li from interstitial melt to the mineral phases is the best explanation. Since the kinetic fractionation is independent of tectonic settings, such effect is inevitably present in all mafic-ultramafic intrusions on Earth, illustrating that the effect of kinetic Li isotope fractionation must be considered before using Li isotopic data to constrain mantle sources and magmatic evolution, especially if mineral rather than whole-rock data are employed.

Physiological and metabolic responses of chemolithoautotrophic reducers to high hydrostatic pressure.

We investigated the impact of pressure on thermophilic, chemolithoautotrophic reducing bacteria of the phyla Campylobacterota and Aquificota isolated from deep-sea hydrothermal vents. Batch incubations at 5 and 20 MPa resulted in decreased consumption, lower cell concentrations, and overall slower growth in Caminibacter mediatlanticus (Campylobacterota) and Thermovibrio ammonificans (Aquificota), relative to batch incubations near standard pressure (0.2MPa) conditions. Nitrogen isotope fractionation effects from chemolithoautotrophic reduction by both microorganisms were, on the contrary, maintained under all pressure conditions. Comparable chemolithoautotrophic reducing activities between previously reported natural hydrothermal vent fluid microbial communities dominated by Campylobacterota at 25 MPa and Campylobacterota laboratory isolates at 0.2MPa, suggest robust similarities in cell-specific reduction rates and doubling times between microbial populations and communities growing maximally under similar temperature conditions. Physiological and metabolic comparisons of our results with other studies of pressure effects on anaerobic chemolithoautotrophic processes (i.e., microbial S0 -oxidation coupled to Fe(III) reduction and hydrogenotrophic methanogenesis) suggest that anaerobic chemolithoautotrophs relying on oxidation-reduction (redox) reactions that yield higher Gibbs energies experience larger shifts in cell-specific respiration rates and doubling times at increased pressures. Overall, our results advance understanding of the role of pressure, its relationship with temperature and redox conditions, and their effects on seafloor chemolithoautotrophic reduction and other anaerobic chemolithoautotrophic processes.

Organic geochemical characteristics of coal biogasification

To investigate the gas production mechanism and geochemical characteristics of biogenic coalbed methane, two pieces of lignite were selected as the degradation substrate of coal biogasification, and the coal seam native bacteria were used as the bacteria source to carry out the anaerobic gas production simulation experiment for 90 days. The results show that the gas generation mode is mostly acetic acid fermentation under anaerobic closed circumstances, and the gas production efficiency is low. The major components of biogas are CH4 and CO2, and the gas composition is dry. The entire gas production process can be divided into three stages: rapid growth, fluctuating growth, and decline. After entering the steady stage of gas production, the contents of CH4 and CO2 exhibit a clear synchronous change law. In this process, two significant inherited isotope fractionation effects caused the changes in the carbon isotopic composition of CH4 and CO2, which eventually led to the fractionation of light carbon isotopes of methyl into CH4 and heavy carbon isotopes of carboxyl into CO2, The lighter the δ13CCH4, The heavier the δ13CCO2, and the biochemical methane tends to enrich light carbon isotopes. In addition, as the biodegradation time increased, the Pr/nC17 and Ph/nC18 ratios of the two coal samples generally increased, while the OEP value and ΣnC21-/ΣnC22+ ratios generally decreased, indicating that the native bacteria of coal seam are better at degrading odd carbon of short-chain n-alkanes in soluble organic matter. At the same time, these microorganisms can also make use of aromatic compounds in coal, so it can be seen that the way of coal seam origin bacteria using coal is diversified.

Redox controls on H and N speciation and intermolecular isotopic fractionations in aqueous fluids at high pressure and high temperature: Insights from in-situ experiments

Aqueous magmatic fluids are essential to the transport of hydrogen (H), carbon (C), and nitrogen (N) from the mantle to the surface, during which changes in pressure, temperature, and redox conditions affect the chemical speciation and intermolecular isotopic fractionations of H, C, and N. Here, we performed a series of hydrothermal diamond-anvil cell experiments to evaluate the role of pressure, temperature, and redox conditions on the speciation and intermolecular fractionations of H and N during the decompression and cooling of aqueous fluids from 780 MPa to 800°C to 150 MPa and 200°C. We used Raman spectroscopy to investigate the distribution and exchange reactions of H and N isotopologues between water, methane, ammonia, and di-nitrogen molecules under changing physicochemical conditions. Our experiments show that upon decompression, a C- and N-bearing fluid will preferentially degas D-rich methane and 15N-rich N2, depleting the residual aqueous fluid in those isotopes. If this fluid precipitates N-rich (i.e., NH4+-bearing) minerals, the observed N isotopic fractionation is opposite to that during N2 degassing, enriching the aqueous fluid in 15N. Because these fractionations result from changes in H, C, and N speciation in the aqueous fluid, their magnitudes depend on redox conditions as well as pressure and temperature. Our new in-situ experimental results are consistent with the large H and N isotopic fractionations observed between water, methane, and ammonia species in aqueous fluids at high pressures and temperatures, although the magnitude of the fractionations in our experiments cannot be quantified. Nonetheless, our results suggest that statistical thermodynamic models likely underestimate isotopic fractionation effects for isotopic molecules under these conditions, and should account for solubility and isotopic effects of the solvent associated with the solvation of water, methane, and ammonia isotopologues in aqueous fluids.This work has significant implications for interpreting isotopic measurements of natural samples from hydrothermal systems because it offers insights into isotopic fractionations in multicomponent and multiphase systems under hydrothermal temperatures and pressures.

Phase-specific stable isotope fractionation effects during combined gas-liquid phase exchange and biodegradation

Stable isotope fractionation of toluene under dynamic phase exchange was studied aiming at ascertaining the effects of gas-liquid partitioning and biodegradation of toluene stable isotope composition in liquid-air phase exchange reactors (Laper). The liquid phase consisted of a mixture of aqueous minimal media, a known amount of a mixture of deuterated (toluene-d) and non-deuterated toluene (toluene-h), and bacteria of toluene degrading strain Pseudomonas putida KT2442. During biodegradation experiments, the liquid and air-phase concentrations of both toluene isotopologues were monitored to determine the observable stable isotope fractionation in each phase. The results show a strong fractionation in both phases with apparent enrichment factors beyond −800‰. An offset was observed between enrichment factors in the liquid and the gas phase with gas-phase values showing a stronger fractionation in the gas than in the liquid phase. Numerical simulation and parameter fitting routine was used to challenge hypotheses to explain the unexpected experimental data. The numerical results showed that either a very strong, yet unlikely, fractionation of the phase exchange process or a – so far unreported – direct consumption of gas phase compounds by aqueous phase microorganisms could explain the observed fractionation effects. The observed effect can be of relevance for the analysis of volatile contaminant biodegradation using stable isotope analysis in unsaturated subsurface compartments or other environmental compartment containing a gas and a liquid phase.

Desulfurization of black powder for isotopic profiling using isotope ratio mass spectrometry

The stable isotope analysis of black powder is of utmost significance with regard to its further comparison and traceability in explosion-related cases. To date, only a limited number of studies have reported the stable isotope analysis of black powder using isotope ratio mass spectrometry (IRMS). However, because the sulfur content in black powder is high, sulfur deposition is extremely likely during the analysis process, thereby damaging the instrument and affecting the reproducibility of the results. Therefore, desulfurization of black powder samples prior to IRMS analysis is extremely desirable. Herein, based on the difference in the solubility of black powder components, chloroform was selected to desulfurize black powder. The results demonstrated that chloroform extraction could effectively eliminate sulfur from the black powder. The isotope fractionation effect caused by desulfurization was also studied. The changes in δ13C, δ15N, and δ18O after desulfurization were close to the standard deviation values and did not affect the analysis of black powder. Desulfurization and stable isotope analysis were performed on 12 black powder samples obtained from cases that happened in eight different areas in China, and the carbon, nitrogen, and oxygen isotope ratios in all the samples were effectively distinguished. Therefore, the established method can successfully distinguish the stable isotope ratios in black powder samples using IRMS after desulfurization while avoiding damage to the instrument and enhancing the stability of the results.

Dual isotopes of nitrite in the Amundsen Sea in summer

Nitrite (NO2−) is a key intermediate in the nitrogen (N) cycle, and its transformation is accomplished by microbial communities. However, due to few studies on the nitrite cycle, a clear assessment of the contribution to the marine biogeochemical cycle is missing. Here, we present data on nitrogen and oxygen isotopic composition of NO2− in the Amundsen Sea in summer, and explore the biogeochemical processes that influence the NO2− cycle. Extremely low δ15NNO2 and abnormally high δ18ONO2 were found in the upper waters of the Amundsen Sea, with δ15NNO2 as low as −58.4 ‰ and δ18ONO2 as high as 44.4 ‰. Enzymatic isotopic exchange reactions between nitrate and nitrite have been proposed to be responsible for these isotopic anomalies. The mirror-symmetrical variation between δ15NNO2 and δ18ONO2 suggests that the isotopic fractionation effects of nitrogen and oxygen are opposite in isotope exchange reactions. Dual isotopes of nitrite indicate that ammonia oxidation is the main source of nitrite, thus nitrification plays an important role in the formation of primary nitrite maximum in the upper Amundsen Sea. The nitrogen and oxygen isotopic compositions of nitrite provide support for clarifying multiple processes of marine nitrogen cycle.

Distinct oxygen isotope fractionations driven by different electron donors during microbial nitrate reduction in lake sediments.

Microbial nitrate reduction can be driven by organic carbon oxidation, as well as by inorganic electron donors, such as reduced forms of sulfur and iron. An apparent inverse oxygen isotope fractionation effect was observed during nitrate reduction in sediment incubations from five sampling sites of a freshwater lake, Hongze Lake, China. Incubations with organic and inorganic electron donor additions were performed. Especially, the inverse oxygen isotope effect was intensified after glucose addition, whereas the incubations with sulfide and Fe2+ showed normal fractionation factors. Nitrate reductase encoding genes, napA and narG, were analysed with metagenomics. Higher napA/narG ratios were associated with higher oxygen fractionation factors. The most abundant clade (59%) of NapA in the incubation with glucose was affiliated with Rhodocyclales. In contrast, it only accounted for 8%-9% of NapA in the incubations with sulfide and Fe2+ . Differences in nitrate reductases might explain different oxygen isotope effects. Our findings also suggested that large variance of O-nitrate isotope fractionations might have to be considered in the interpretation of natural isotope records.

Resolving impact volatilization and condensation from target rock mixing and hydrothermal overprinting within the Chicxulub impact structure

This work presents isotopic data for the non-traditional isotope systems Fe, Cu, and Zn on a set of Chicxulub impactites and target lithologies with the aim of better documenting the dynamic processes taking place during hypervelocity impact events, as well as those affecting impact structures during the post-impact phase. The focus lies on material from the recent IODP-ICDP Expedition 364 Hole M0077A drill core obtained from the offshore Chicxulub peak ring. Two ejecta blanket samples from the UNAM 5 and 7 cores were used to compare the crater lithologies with those outside of the impact structure. The datasets of bulk Fe, Cu, and Zn isotope ratios are coupled with petrographic observations and bulk major and trace element compositions to disentangle equilibrium isotope fractionation effects from kinetic processes. The observed Fe and Cu isotopic signatures, with δ56/54Fe ranging from −0.95‰ to 0.58‰ and δ65/63Cu from −0.73‰ to 0.14‰, mostly reflect felsic, mafic, and carbonate target lithology mixing and secondary sulfide mineral formation, the latter associated to the extensive and long-lived (>105 years) hydrothermal system within Chicxulub structure. On the other hand, the stable Zn isotope ratios provide evidence for volatility-governed isotopic fractionation. The heavier Zn isotopic compositions observed for the uppermost part of the impactite sequence and a metamorphic clast (δ66/64Zn of up to 0.80‰ and 0.87‰, respectively) relative to most basement lithologies and impact melt rock units indicate partial vaporization of Zn, comparable to what has been observed for Cretaceous-Paleogene boundary layer sediments around the world, as well as for tektites from various strewn fields. In contrast to previous work, our data indicate that an isotopically light Zn reservoir (δ66/64Zn down to −0.49‰), of which the existence has previously been suggested based on mass balance considerations, may reside within the upper impact melt rock (UIM) unit. This observation is restricted to a few UIM samples only and cannot be extended to other target or impact melt rock units. Light isotopic signatures of moderately volatile elements in tektites and microtektites have previously been linked to (back-)condensation under distinct kinetic regimes. Although some of the signatures observed may have been partially overprinted during post-impact processes, our bulk data confirm impact volatilization and condensation of Zn, which may be even more pronounced at the microscale, with variable degrees of mixing between isotopically distinct reservoirs, not only at proximal to distal ejecta sites, but also within the lithologies associated with the Chicxulub impact crater.

Nitrous Oxide from Abiotic Processes of Hydroxylamine and Nitrite in Estuarine and Coastal Ecosystems: A Review

Abiotic processes of nitrogen (N) are suggested to contribute to nitrous oxide (N2O) production; however, the important role of these processes in N2O emissions is invariably ignored. This review synthesized the main abiotic processes of hydroxylamine and nitrite and associated biogeochemical controls in estuarine and coastal ecosystems. Abiotic processes of hydroxylamine and nitrite are availably detected in estuarine and coastal environments. The abiotic processes of hydroxylamine contribute more to N2O production than the abiotic processes of nitrite in estuarine and coastal environments, suggesting that hydroxylamine plays an important role in N2O production. The isotopic fractionation effects of N can occur during the abiotic processes of hydroxylamine and nitrite and are enriched with the increasing rates of N reactions. In addition, abiotic processes of hydroxylamine and nitrite are highly dependent on pH, oxygen, Fe2+, Fe3+, and Mn4+ and are also triggered by the increasing substrate contents. These results suggest that abiotic processes of hydroxylamine and nitrite have been greatly concerned for the estuarine and coastal environments, whereas the dynamics of these processes are still sparse for projecting N fates and dynamics in response to environmental factors changes. This review highlights the importance of abiotic processes of N and associated environmental implications and presents the future trend of N cycling in estuarine and coastal environments.

Carbon and oxygen isotope fractionation in the Late Devonian heterocoral Oligophylloides: Implications for the skeletogenesis and evolution of the Heterocorallia

The order Heterocorallia is an enigmatic, anatomically distinct group of calcifying Palaeozoic corals with unclear affinities to other, either extinct or modern cnidarian lineages. Here, we present results of microstructure and stable isotope studies on exceptionally well-preserved skeletons of two species of the heterocoral Oligophylloides from the Famennian (Upper Devonian) of the Anti-Atlas, Morocco and the Holy Cross Mountains, Poland. The investigations provide new information on the skeletogenesis and isotope fractionation effects in Palaeozoic heterocorals. In order to estimate the possible role of diagenetic alteration on the observed carbon and oxygen isotope ratios, we have conducted several preservation tests, including transmitted-light, scanning-electron, fluorescence and cathodoluminescence microscopy, Raman spectroscopy, and trace element analyses. The least- and most-altered skeleton portions identified based on this approach are typified by clearly different isotopic signatures. The least-altered parts of the specimens reveal carbon and oxygen isotope ratios close to those of contemporaneous marine-equilibrated calcites. The moderately-pronounced, positive correlations between the δ13C and δ18O values observed for these coralla may reflect relatively minor vital fractionation effects, obscured to some degree by diagenetic overprint. The results suggest that the isotope fractionation mechanisms of the heterocorals differed markedly from the significant kinetic fractionation characteristic of the modern scleractinian corals. Rather, the moderate, if any, extent of non-equilibrium fractionation during skeletogenesis in the Heterocorallia resembled the typical, environmental and metabolic CO2-dominated fractionation effects shown by most other marine calcifiers. These include the extinct rugose corals, modern hydrocorals and, in particular, modern octocorals, with which the heterocorals show notable architectural and structural similarities.

Chemical and Isotopic Evaluation of a Microsampling Method using Laser Ablation and Membrane Filter

Microscale isotopic measurements in geological materials are becoming of significant importance in geochemistry. This study evaluated the potential of a new microsampling method called laser ablation coupled with filter (LAF) for precise isotopic measurements of small‐sized geological materials, compared with conventional micromilling techniques. We optimised the system to maximise the sample collection rate and minimise the extent of the contamination. Isotope ratios (87Sr/86Sr, 142Nd/144Nd, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) of the glass reference material NIST SRM 610 collected using LAF were measured by thermal ionisation mass spectrometry, and no isotopic fractionation or blank effect was observed. This indicates that LAF can be applied for high‐precision isotope ratio measurement of these elements. Considering the total procedural blanks in LAF sampling, the minimum amounts required for high‐precision isotope measurement were 50, 2 and 20 ng for Sr, Nd and Pb, respectively. The LAF method has advantages over the conventional method. It also causes less damage in the test sample’s depth direction and can be applied to samples with complex shapes. It is expected therefore that the LAF sampling method will be applied to geology and other research fields that require microsampling.

Oxidation pathways and emission sources of atmospheric particulate nitrate in Seoul: based on <i>δ</i><sup>15</sup>N and Δ<sup>17</sup>O measurements

Abstract. PM2.5 haze pollution driven by secondary inorganic NO3- has been a great concern in East Asia. It is, therefore, imperative to identify its sources and oxidation processes, for which nitrogen and oxygen stable isotopes are powerful tracers. Here, we determined the δ15N (NO3-) and Δ17O (NO3-) of PM2.5 in Seoul during the summer of 2018 and the winter of 2018–2019 and estimated quantitatively the relative contribution of oxidation pathways for particulate NO3- and investigated major NOx emission sources. In the range of PM2.5 mass concentration from 7.5 µg m−3 (summer) to 139.0 µg m−3 (winter), the mean δ15N was −0.7 ‰ ± 3.3 ‰ and 3.8 ‰ ± 3.7 ‰, and the mean Δ17O was 23.2 ‰ ± 2.2 ‰ and 27.7 ‰ ± 2.2 ‰ in the summer and winter, respectively. While OH oxidation was the dominant pathway for NO3- during the summer (87 %), nighttime formation via N2O5 and NO3 was relatively more important (38 %) during the winter, when aerosol liquid water content (ALWC) and nitrogen oxidation ratio (NOR) were higher. Interestingly, the highest Δ17O was coupled with the lowest δ15N and highest NOR during the record-breaking winter PM2.5 episodes, revealing the critical role of photochemical oxidation process in severe winter haze development. For NOx sources, atmospheric δ15N (NOx) estimated from measured δ15N (NO3-) considering isotope fractionation effects indicates vehicle emissions as the most important emission source of NOx in Seoul. The contribution from biogenic soil and coal combustion was slightly increased in summer and winter, respectively. Our results built on a multiple-isotope approach provide the first explicit evidence for NO3- formation processes and major NOx emission sources in the Seoul megacity and suggest an effective mitigation measure to improve PM2.5 pollution.

Evaluation of coral skeletal δ88Sr as a paleoclimate proxy in the northern South China Sea

The climate and environmental significance of the stable strontium isotope composition (δ88Sr) in coral skeletons deserves further investigation. To improve our understanding of stable strontium isotope fractionation during the biomineralization process, correlations between monthly coral skeletal δ88Sr values and climatic and environmental variables (along with their corresponding geochemical proxies) between 2008 and 2009 were studied using a massive zooxanthellae Porites coral head from Sanya Bay on the south coast of Hainan Island in the northern South China Sea. Sea surface temperature and its corresponding geochemical proxies (Sr/Ca and δ18O), pH and its proxy δ11B, and precipitation (representing river runoff) show clear annual cycles, but their correlations with coral δ88Sr were not significant during 2008 and 2009. However, both total solar irradiance (TSI) and coral δ88Sr values followed a gradually increasing trend over the study period and showed a statistically significant correlation. These results indicate that the fractionation associated with coral skeleton δ88Sr is not temperature-dependent, but TSI may play a major role in controlling coral δ88Sr values at our study site. The underlying mechanism of TSI-induced 88Sr/86Sr fractionation in coral skeletons may be similar to that associated with coral δ13C. That is, symbiotic zooxanthellae preferentially uptakes 86Sr during photosynthesis leaving the internal strontium reservoirs enriched in 88Sr; consequently, increased uptake of 88Sr by the coral skeleton occurs when photosynthetic rates increase. However, the correlation between coral δ88Sr and δ13C values may be masked by the carbon input from terrestrial river runoff and the kinetic isotope fractionation effect during the calcification process.

Ca isotopic compositions of zoned granitoid intrusion: Implications for the emplacement and evolution of magma bodies

To investigate the mechanism of magma emplacement of the intruded zoned pluton, we report high-precision Ca isotopic data of 23 granitoids (including granodiorite and quartz diorite), 5 mafic enclaves, and 7 hornblende and plagioclase mineral pairs from the Early Cretaceous Fangshan pluton, Beijing, China. Δ44/40CaHbl-Pl of rocks span from 0.02 ± 0.15‰ to 0.23 ± 0.12‰, which likely reflects equilibrium inter-mineral isotope fractionation. The granodiorites exhibit ∼0.30‰ variation in Ca isotopic compositions, with δ44/40Ca ranging from 0.47 ± 0.08‰ to 0.77 ± 0.09‰. The diorites and the enclaves yield δ44/40Ca of 0.51 ± 0.03‰ to 0.64 ± 0.05‰, and 0.54 ± 0.06‰ to 0.66 ± 0.11‰, respectively, which are lower than that of the upper continental crust (0.72 ± 0.10‰). Ca isotopic variation cannot be ascribed to K decay due to their low εCa values (< ±1.0). Theoretical study showed that Hbl fractional crystallization could counteract Ca isotope fractionation effect of their co-existing Pl crystallization. Source heterogeneity cannot account for Ca isotopic variation because δ44/40Ca does not correlate to (87Sr/86Sr)i. Instead, the Ca isotopic variation is likely controlled by partial melting and residual mineralogy. Specifically, δ44/40Ca of the granodiorites are negatively correlated with (Dy/Yb)N, reflecting various magma pools derived from different crustal depths with clinopyroxene and garnet residua. Modeling calculations suggest that the low Ca isotopic signature can be caused by ∼10% partial melting of sources with various proportions of garnet and clinopyroxene (from 5% garnet + 95% clinopyroxene to 40% garnet + 60% clinopyroxene). The diorites intruded earlier than the granodiorites, which can be slightly contaminated by surrounding basement rock. Ca isotopic compositions of enclaves are similar to those of their host rocks. Considering the spatial structure of the Fangshan pluton, it should be assembled by at least four magma pulses derived from various crustal depths through ballooning. Our results show that Ca isotopes can be used as a potential tool to trace magma source depth and decipher the process of magma emplacement of intrusive pluton, which is important to understand the continental crustal growth.