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Ammonium nitrate (AN) is one of the most commonly used explosives in criminal cases. The comparison and source-tracing of AN is important for investigation of attribution and fingerprinting of an explosive used at different events. The stable isotope signature of AN is an important index for comparison and tracing. However, the characteristics of the stable nitrogen isotopic ratios of AN (δ15 NNH4NO3 ) alone are not sufficient to achieve a fine comparison between different AN samples. To increase the comparison index and further improve the discriminability between stable nitrogen isotopic ratios of different ANs, a method of isolation and analysis of nitrate ions in AN was established using stable-isotope-ratio mass spectrometry (IRMS). The method was based on the principle that strong alkali react with AN to produce ammonia and nitrate. After the isolation, stable nitrogen isotopes of nitrate ions (δ15 NNO3 ) were obtained using IRMS, and then the stable nitrogen isotopes of ammonium ions from AN (δ15 NNH4 ) was calculated according to the principle of mass balance. The results show that the method is effective for the isolation of nitrate ions without notable isotope fractionation. The developed method was applied to analyze and discriminate AN samples from eight different cities in China. Three samples out of the initial eight AN samples with similar δ15 NNH4NO3 values were further distinguished by their δ15 NNH4 and δ15 NNO3 values. The isolation and stable-nitrogen isotopic analysis method developed for nitrate ions in AN is simple and effective, thereby increasing the discriminability of the stable isotope ratios in AN.

Despite increasing recognition of the critical role of coastal wetlands in mitigating climate change, sea-level rise, and salinity increase, soil organic carbon (SOC) sequestration mechanisms in estuarine wetlands remain poorly understood. Here, we present new results on the source, decomposition, and storage of SOC in estuarine wetlands with four vegetation types, including single Phragmites australis (P, habitat I), a mixture of P. australis and Suaeda salsa (P + S, habitat II), single S. salsa (S, habitat III), and tidal flat (TF, habitat IV) across a salinity gradient. Values of δ13 C increased with depth in aerobic soil layers (0-40 cm) but slightly decreased in anaerobic soil layers (40-100 cm). The δ15 N was significantly enriched in soil organic matter at all depths than in the living plant tissues, indicating a preferential decomposition of 14 N-enriched organic components. Thus, the kinetic isotope fractionation during microbial degradation and the preferential substrate utilization are the dominant mechanisms in regulating isotopic compositions in aerobic and anaerobic conditions, respectively. Stable isotopic (δ13 C and δ15 N), elemental (C and N), and lignin composition (inherited (Ad/Al)s and C/V) were not completely consistent in reflecting the differences in SOC decomposition or accumulation among four vegetation types, possibly due to differences in litter inputs, root distributions, substrate quality, water-table level, salinity, and microbial community composition/activity. Organic C contents and storage decreased from upstream to downstream, likely due to primarily changes in autochthonous sources (e.g., decreased onsite plant biomass input) and allochthonous materials (e.g., decreased fluvially transported upland river inputs, and increased tidally induced marine algae and phytoplankton). Our results revealed that multiple indicators are essential to unravel the degree of SOC decomposition and accumulation, and a combination of C:N ratios, δ13 C, δ15 N, and lignin biomarker provides a robust approach to decipher the decomposition and source of sedimentary organic matter along the river-estuary-ocean continuum.

Soil water stable isotopes are widely used across disciplines (e.g., hydrology, ecology, soil science, and biogeochemistry). However, the full potential of stables isotopes as a tool for characterizing the origin, flow path, transport processes and residence times of water in different eco‐, hydro‐, and geological compartments has not yet been exploited. This is mainly due to the large variety of different methods for pore water extraction. While recent work has shown that matric potential affects the equilibrium fractionation, little work has examined how different water retention characteristics might affect the sampled water isotopic composition. Here, we present a simple laboratory experiment with two well‐studied standard soils differing in their physico‐chemical properties (e.g., clayey loam and silty sand). Samples were sieved, oven‐dried and spiked with water of known isotopic composition to full saturation. For investigating the effect of water retention characteristics on the extracted water isotopic composition, we used pressure extractors to sample isotopically labelled soil water along the pF curve. After pressure extraction, we further extracted the soil samples via cryogenic vacuum extraction. The null hypothesis guiding our work was that water held at different tensions shows the same isotopic composition. Our results showed that the sampled soil water differed isotopically from the introduced isotopic label over time and sequentially along the pF curve. Our and previous studies suggest caution in interpreting isotope results of extracted soil water and a need to better characterize processes that govern isotope fractionation with respect to soil water retention characteristics. In the future, knowledge about soil water retention characteristics with respect to soil water isotopic composition could be applied to predict soil water fractionation effects under natural and non‐stationary conditions. In this regard, isotope retention characteristics as an analog to water retention characteristics have been proposed as a way forward since matric potential affects the equilibrium fractionation between the bound water and the water vapour.

19-Norandrosterone (19-NA) is the main metabolite of nandrolone and/or its precursors, which can be found naturally in human urine in trace amount. Gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) confirmation procedure can be used to identify a potential exogenous origin of 19-NA in urine sample. Sample purification for GC-C-IRMS analysis is crucial to the whole confirmation procedure because the concentration of 19-NA in the urine to be tested is very low. Online two-dimensional high-performance liquid chromatography (2D-HPLC) clean-up procedure with high separation capacity is used to isolate and enrich 19-NA as a sample pretreatment process. Linearity, lowest detectable concentration, uncertainty, and selectivity of the method are validated according to the World Anti-doping Agency's (WADA) requirement. Isotope fractionation effect was not observed during the 2D-HPLC purification process. The validated method provides a high efficient and convenient confirmation procedure to determine the origin of 19-NA.

RATIONALE Analyses of the isotope ratios of nitrogen (15 N/14 N) and oxygen (18 O/16 O) in nitrate (NO3- ) with the denitrifier method require relatively high sample volumes at low concentrations (≤ 1 μM) to afford sufficient analyte for mass spectrometry, resulting in isotopic offsets compared to more concentrated samples of the same isotopic composition. METHODS To uncover the origins of isotopic offsets, we analyzed the N and O isotope ratios of the NO3- reference materials spanning concentrations of 0.5-20 μM. We substantiated the incidence of volume-dependent isotopic offsets, then investigated whether they resulted from (a) incomplete sample recovery during N2 O sparging, (b) blanks - bacterial, atmospheric, or in reference material solutions - and (c) oxygen atom exchange with water during the bacterial conversion of NO3- to N2 O. RESULTS Larger sample volumes resulted in modest offsets in d15 N, but substantial offsets in d18 O. N2 O recovery from sparging was less complete at higher volumes, resulting in decreases in d15 N and d18 O due to associated isotope fractionation. Blanks increased detectably with volume, whereas oxygen atom exchange with water remained constant within batch analyses, being neither sensitive to sample volume nor salinity. The sizeable offsets in d18 O with volume are only partially explained by the factors considered in our analysis. CONCLUSION Our observations argue for bracketing of NO3- samples with reference materials that emulate sample volumes (concentrations) to achieve improved measurement accuracy and foster inter-comparability.

BACKGROUND: Quantitative determination with site‐specific natural isotope fractionation nuclear magnetic resonance (SNIF‐NMR) has been exploited widely in detecting adulteration and for the classification of natural products by their geographical origin. RESULTS: We compared isotopic parameters such as deuterium / hydrogen (D/H) site specific ratios and the R parameter for alcoholic beverages, obtained using (i) a 500 MHz spectrometer equipped with a dedicated probe for isotopic measurements, and (ii) a 700 MHz spectrometer equipped with a standard probe. The factors affecting the accuracy and precision of quantitative NMR with the second instrument have been explored. CONCLUSIONS: It has been demonstrated that, in laboratories with a spectrometer that is not equipped with a specific deuterium probe, the selection of the appropriate experimental parameters enables measurements with a similar precision and accuracy as in the case of the official method adopted by the International Organisation of Vine and Wine. © 2019 Society of Chemical Industry

RATIONALE γ-Hydroxybutyric acid (GHB) is a naturally endogenous neurotransmitter that is popular as a recreational drug due to its sedative, hypnotic, and euphoric effects. GHB derived from endogenous production or exogenous ingestion has been effectively discriminated by carbon isotopic compositions (δ13 C values) through gas chromatography/combustion-isotope ratio mass spectrometry (GC/C-IRMS). However, an unintended uncertainty of isotopic signatures caused by a wide range of GHB quantities remains unsolved when using only single-isotope corrections of the di-TMS derivative. METHODS The δ13 C values of the original GHB standard were first determined by elemental analyzer/isotope ratio mass spectrometry (EA/IRMS). The δ13 C values of silylated GHB in concentrations from 10 to 500 ppm were determined by GC/C-IRMS. With respect to the silylated reaction products, the correction of δ13 C values for the introduced carbons was calculated from a stoichiometric mass balance equation. RESULTS The results showed a significant quantity-dependent trend in δ13 C values of introduced carbon (δ13 Cdi-TMS values) with increased GHB standard concentrations (r2 = 0.70, p

Rationale: Boron (B) is an essential micronutrient in plants and its isotope variations are used to gain insights into plant metabolism, which is important for crop plant cultivation. B isotope variations were used to trace intra‐plant fractionation mechanisms in response to the B concentration in the irrigation water spanning the range from B depletion to toxic levels. Methods: A fully validated analytical procedure based on multi‐collector inductively coupled plasma mass spectrometry (MC‐ICP‐MS), sample decomposition and B Matrix separation was applied to study B isotope fractionation. The Validation was accomplished by establishing a complete uncertainty budget and by applying reference materials, yielding expanded measurement uncertainties of 0.8‰ for pure boric acid solutions and ≤1.5‰ for processed samples. With this validated procedure SI traceable B isotope amount ratios were determined in plant reference materials for the first time. Results: The B isotope compositions of Irrigation water and bell pepper samples suggest passive diffusion of the heavy 11B isotope into the roots during low to high B concentrations while uptake of the light 10B isotope was promoted during B depletion, probably by active processes. A systematic enrichment of the heavy 11B isotope in higher located plant parts was observed (average Δ11Bleaf‐roots = 20.3 ± 2.8‰ (1 SD)), possibly by a facilitated transport of the heavy 11B isotope to growing Meristems by B transporters. Conclusions: The B isotopes can be used to identify plant metabolism in Response to the B concentration in the irrigation water and during intra‐plant B transfer. The large B isotope fractionation within the plants demonstrates the importance of biological B cycling for the global B cycle.

Aim The aim was to decipher Europe‐wide spatio‐temporal patterns of forest growth dynamics and their associations with carbon isotope fractionation processes inferred from tree rings as modulated by climate warming. Location Europe and North Africa (30‒70° N, 10° W‒35° E). Time period 1901‒2003. Major taxa studied Temperate and Euro‐Siberian trees. Methods We characterize changes in the relationship between tree growth and carbon isotope fractionation over the 20th century using a European network consisting of 20 site chronologies. Using indexed tree‐ring widths (TRWi), we assess shifts in the temporal coherence of radial growth across sites (synchrony) for five forest ecosystems (Atlantic, boreal, cold continental, Mediterranean and temperate). We also examine whether TRWi shows variable coupling with leaf‐level gas exchange, inferred from indexed carbon isotope discrimination of tree‐ring cellulose (Δ13Ci). Results We find spatial autocorrelation for TRWi and Δ13Ci extending over a maximum of 1,000 km among forest stands. However, growth synchrony is not uniform across Europe, but increases along a latitudinal gradient concurrent with decreasing temperature and evapotranspiration. Latitudinal relationships between TRWi and Δ13Ci (changing from negative to positive southwards) point to drought impairing carbon uptake via stomatal regulation for water saving occurring at forests below 60° N in continental Europe. An increase in forest growth synchrony over the 20th century together with increasingly positive relationships between TRWi and Δ13Ci indicate intensifying impacts of drought on tree performance. These effects are noticeable in drought‐prone biomes (Mediterranean, temperate and cold continental). Main conclusions At the turn of this century, convergence in growth synchrony across European forest ecosystems is coupled with coordinated warming‐induced effects of drought on leaf physiology and tree growth spreading northwards. Such a tendency towards exacerbated moisture‐sensitive growth and physiology could override positive effects of enhanced leaf intercellular CO2 concentrations, possibly resulting in Europe‐wide declines of forest carbon gain in the coming decades. Spanish Government, Grant/Award Number: AGL2015‐68274 ‐C3 ‐3‐R; Sixth Framework Programme, Grant/AwardNumber: EVK2‐2001 ‐00237; Seventh Framework Programme, Grant/AwardNumber: COST ‐STSM ‐ECOST ‐STSM ‐FP1304‐140915‐066395 and ERANET‐Mundus program (Grant agreement 20112573)

The stable isotopes of hydrogen and oxygen (δ²H and δ¹⁸O) are useful conservative tracers for tracking the movement of water in soil. But although the tracking of water infiltrating through the soil profile and its movement as run‐off and groundwater recharge are well developed, water movement through the soil can also include evaporative fractionation. Soil water fractionation factors have, until now, been largely empirical. Unlike open water evaporation where temperature, humidity, and vapour pressure gradient define fractionation, soil water evaporation includes fractionation by soil matrix effects. These effects are still poorly characterized. Here, we present preliminary results from a simple laboratory experiment with four soil admixtures with grain sizes that range from sand to silt and clay. Our results show that soil tension seems to control the isotope fractionation of resident soil water. The relationship between soil tension and equilibrium fractionation appears to be independent of soil texture and appears well supported by thermodynamic theory. Although these results are preliminary, they suggest that future work should go after soil tension effects as a possible explanatory factor of soil water and water vapour fractionation.

Rationale Nitrate pollution represents one of the most important issues for ground and surface water quality and source identification is essential for developing effective mitigation practices. Nitrate isotopic fingerprinting can be utilized to identify the sources of nitrate pollution in aquifers. However, it is crucial to assess the performances (precision and accuracy) of the analytical procedure applied to measure the δ15 N and δ18 O values of nitrates from field samples to correctly apply this tool. Methods Nitrates were extracted from a large number of KNO3 samples using the AgNO3 method, and the δ15 N and δ18 O values of these nitrate extracts were measured by isotope ratio mass spectrometry. The availability of this dataset, comprising 693 unprocessed quality control (QC) KNO3 samples and 618 processed samples, allowed us to rigorously quantify the performance of the procedures employed. A salt doping experiment was also performed from which the effects of contaminants on the performance of the method could be ascertained. Results The overall instrumental reproducibility for the analysis of unprocessed QC samples was 0.5‰ and 2‰ for δ15 N and δ18 O values, respectively, and a strict dependence on signal amplitude was observed. No isotope fractionation was reported for reference samples that were processed according to the "identical treatment" principle (ITP) but normalized by unprocessed reference materials. A significant increase in the standard deviation (SD) was, however, observed compared with that for unprocessed samples. The SD of the processed QC samples allowed us to quantify the reproducibility of the entire procedure as 0.6‰ and 1.0‰ for δ15 N and δ18 O values, respectively. This was comparable with the system reproducibility when normalization using processed reference materials was applied according to the ITP. Conclusions Normalization with processed standards is essential to achieve high-precision measurements of the δ15 N and δ18 O values of nitrates extracted from unknown samples. This procedure allowed good accuracy to be guaranteed, and precision levels comparable with the observed instrumental performance to be achieved. A salt doping experiment showed a significant influence of the SO4 2- content on the δ15 N values.

RATIONALE Nitrate is an oxyanion similar to CO3- and thus should undergo stable N and O isotope fractionation during dissolution or precipitation. This process should dominate abiotic soil nitrate processes in hyperarid regions of Earth and possibly Mars. METHODS The N and O isotope fractionations during the precipitation of nitrate salt from saturated solutions at ~20°C were determined by two methods: rapid precipitation by antisolvent crystallization and slow uninhibited precipitation in a desiccator. In the antisolvent crystallization procedure, increasing volumes of acetone were added to samples of saturated sodium and strontium nitrate solutions to instantaneously precipitate nitrate salt. In the slow procedure (requiring weeks), slow evaporative water loss drove the process. RESULTS There was little difference between the two procedures. Using a Rayleigh model, the calculated N fractionation (15 eproduct-residual ) ranged from 1.69‰ to 2.77‰, whereas for O, the 18 eproduct-residual values were between 1.27‰ and 4.61‰. The N isotope fractionation between NO3- and the metal solid is similar to that between C in dissolved CO3-2 and carbonates. We found that O versus N isotope plots of soil nitrate in a cold/dry Antarctic chronosequence had slopes similar to those from the experiments, revealing abiotic transport. In the Atacama Desert, where the soil nitrates are a mix of biological and tropospheric nitrate, there is an inverse relationship between soil N and O isotopes. These two relationships were compared with the isotope composition of nitrate from Martian meteorite EETA79001. CONCLUSIONS While the N and O isotope composition of the Martian nitrate is remarkably similar to that of the present Atacama Desert, the interpretation of the slope of the O versus N isotopes remains ambiguous due to the limited number of samples. Additional NO3 samples from Martian meteorites are needed to address the question of abiotic versus biotic alteration of NO3- on Mars.

Rationale Decabrominated diphenyl ether (BDE-209) is a notorious persistent organic pollutant widely found in the environment. Developing a compound-specific isotope analysis (CSIA) method is much needed in order to trace its transport and degradation processes and to evaluate the effectiveness of the remediation of BDE-209 in the environment. However, the conventional CSIA method, i.e. gas chromatography (GC) combustion isotope ratio mass spectrometry, is not appropriate for BDE-209 because of its high thermal instability and incomplete combustion. Methods We developed a high-performance liquid chromatography (HPLC) method for the separation and purification of BDE-209 that prevents its thermal reactivity as occurred in prior GC-based methods. The δ13 C value of the purified BDE-209 was determined using offline elemental analyzer isotope ratio mass spectrometry (EA/IRMS). This two-step method was applied to determine the δ13 C values of BDE-209 in two commercial samples and to characterize carbon isotope fractionation associated with the debromination of BDE-209 via nanoscale zero-valent iron. Results The mean values of daily δ13 C analyses of six replicates of a BDE-209 standard varied from -27.66‰ to -27.92‰, with a standard deviation ranging from 0.07‰ to 0.16‰, indicating a good reproducibility of EA/IRMS. The EA/IRMS analysis of the purified BDE-209 standard indicated no obvious isotope fractionation during the sample purification. The impurity content in commercial BDE-209 samples may contribute additional variation of the δ13 C values of BDE-209. The δ13 C values of BDE-209 gradually changed from -27.47 ± 0.37‰ to -24.59 ± 0.19‰ when 74% of the BDE-209 standard was degraded within 36 h. The estimated carbon isotope enrichment factor was -1.72 ± 0.18‰. Conclusions The two-step method based on HPLC and EA/IRMS avoids the thermal instability of BDE-209 in the traditional CSIA method. It offers a novel approach for elucidating the degradation mechanisms of BDE-209 in the environment and for source identification in contaminated sites.

The functional role(s) of plant calcium oxalate (CaOx) crystals are still poorly understood. Recently, it was shown that crystals function as dynamic carbon pools whose decomposition could provide CO2 to photosynthesis when stomata are closed (e.g. under drought conditions) and CO2 starvation conditions may be created within the mesophyll. This biochemical process, named as 'alarm photosynthesis', can become crucial for plant survival under adverse conditions. Here, we study crystal decomposition under controlled CO2 starvation conditions (either in the shoot or in the root) to obtain a better insight into the process of crystal formation and function. Hydroponically grown pigweed plants were kept in CO2 -free air and/or CO2 -free nutrient medium for 9 days. Crystal volume was monitored daily, and carbon stable isotope composition (δ13 C) and Fourier transformation Raman spectra were obtained at the end of the experiment. A considerable reduction in the leaf crystal volume was observed in shoot-CO2 -starved plants at the end of the experiment. The smallest crystals were isolated from the plants in which carbon was excluded from both the shoot and the root and contained potassium nitrate. Crystal δ13 C of CO2 -starved plants was altered in a predicted way. Specifically, it depended on the average calculated isotope fractionation of all carbon fixation processes considered to be contributing in each experimental treatment. The results of the present study confirmed the correlation between CO2 starvation conditions and the CaOx crystal decomposition. Inorganic carbon fixed in the root may represent a major carbon source for CaOx formation.

Earth's atmospheric composition has changed significantly over geologic time. Many redox active atmospheric constituents have left evidence of their presence, while inert constituents such as dinitrogen gas (N2 ) are more elusive. In this study, we examine two potential biological indicators of atmospheric N2 : the morphological and isotopic signatures of heterocystous cyanobacteria. Biological nitrogen fixation constitutes the primary source of fixed nitrogen to the global biosphere and is catalyzed by the oxygen-sensitive enzyme nitrogenase. To protect this enzyme, some filamentous cyanobacteria restrict nitrogen fixation to microoxic cells (heterocysts) while carrying out oxygenic photosynthesis in vegetative cells. Heterocysts terminally differentiate in a pattern that is maintained as the filaments grow, and nitrogen fixation imparts a measurable isotope effect, creating two biosignatures that have previously been interrogated under modern N2 partial pressure (pN2 ) conditions. Here, we examine the effect of variable pN2 on these biosignatures for two species of the filamentous cyanobacterium Anabaena. We provide the first in vivo estimate of the intrinsic isotope fractionation factor of Mo-nitrogenase (efix = -2.71 ± 0.09‰) and show that, with decreasing pN2 , the net nitrogen isotope fractionation decreases for both species, while the heterocyst spacing decreases for Anabaena cylindrica and remains unchanged for Anabaena variabilis. These results are consistent with the nitrogen fixation mechanisms available in the two species. Application of these quantifiable effects to the geologic record may lead to new paleobarometric measurements for pN2 , ultimately contributing to a better understanding of Earth's atmospheric evolution.

Remobilization of zinc (Zn) from shoot to grain contributes significantly to Zn grain concentrations and thereby to food quality. On the other hand, strong accumulation of cadmium (Cd) in grain is detrimental for food quality. Zinc concentrations and isotope ratios were measured in wheat shoots (Triticum aestivum) at different growth stages to elucidate Zn pathways and processes in the shoot during grain filling. Zinc mass significantly decreased while heavy Zn isotopes accumulated in straw during grain filling (Δ66 Znfull maturity-flowering = 0.21-0.31‰). Three quarters of the Zn mass in the shoot moved to the grains, which were enriched in light Zn isotopes relative to the straw (Δ66 Zngrain-straw -0.21 to -0.31‰). Light Zn isotopes accumulated in phloem sinks while heavy isotopes were retained in phloem sources likely because of apoplastic retention and compartmentalization. Unlike for Zn, an accumulation of heavy Cd isotopes in grains has previously been shown. The opposing isotope fractionation of Zn and Cd might be caused by distinct affinities of Zn and Cd to oxygen, nitrogen, and sulfur ligands. Thus, combined Zn and Cd isotope analysis provides a novel tool to study biochemical processes that separate these elements in plants.

The ability of micro-organisms to oxidize manganese (Mn) from Mn(II) to Mn(III/IV) oxides transcends boundaries of biological clade or domain. Many bacteria and fungi oxidize Mn(II) to Mn(III/IV) oxides directly through enzymatic activity or indirectly through the production of reactive oxygen species. Here, we determine the oxygen isotope fractionation factors associated with Mn(II) oxidation via various biotic (bacteria and fungi) and abiotic Mn(II) reaction pathways. As oxygen in Mn(III/IV) oxides may be derived from precursor water and molecular oxygen, we use a twofold approach to determine the isotope fractionation with respect to each oxygen source. Using both 18 O-labeled water and closed-system Rayleigh distillation approaches, we constrain the kinetic isotope fractionation factors associated with O atom incorporation during Mn(II) oxidation to -17.3‰ to -25.9‰ for O2 and -1.9‰ to +1.8‰ for water. Results demonstrate that stable oxygen isotopes of Mn(III/IV) oxides have potential to distinguish between two main classes of biotic Mn(II) oxidation: direct enzymatic oxidation in which O2 is the oxidant and indirect enzymatic oxidation in which superoxide is the oxidant. The fraction of Mn(III/IV) oxide-associated oxygen derived from water varies significantly (38%-62%) among these bio-oxides with only weak relationship to Mn oxidation state, suggesting Mn(III) disproportionation may account for differences in the fraction of mineral-bound oxygen from water and O2 . Additionally, direct incorporation of molecular O2 suggests that Mn(III/IV) oxides contain a yet untapped proxy of δ18OO2 of environmental O2 , a parameter reflecting the integrated influence of global respiration, photorespiration, and several other biogeochemical reactions of global significance.

Summary Our understanding of how temporal variations of atmospheric water vapour and its isotopic composition (δ18OV) influence water and assimilates in plants remains limited, restricting our ability to use δ18O as a tracer of ecophysiological processes. We exposed oak (Quercus robur) saplings under wet and dry soil moisture conditions to 18O-depleted water vapour (c. − 200‰) at high relative humidity (c. 93%) for 5 h, simulating a fog event. We then traced the step change in δ18OV into water and assimilates (e.g. sucrose, hexoses, quercitol and starch) in the leaf lamina, main veins and twigs over 24 h. The immediate δ18OV effect was highest for δ18O of leaf lamina water, but 40% lower on δ18O of main vein water. To a smaller extent, we also observed changes in δ18O of twig xylem water. Depending on the individual assimilation rate of each plant, the 18O-label was partitioned among different assimilates, with highest changes in δ18O of starch/sucrose and lowest in δ18O of quercitol. Additionally, 18O-label partitioning and allocation towards leaf starch and twig phloem sugars was influenced by the plant water status. Our results have important implications for water isotope heterogeneity in plants and for our understanding of how the δ18O signal is incorporated into biomarkers.

Zinc (Zn) isotope fractionation by the coccolithophore Emiliania huxleyi (a Tasman Sea isolate) was investigated by culturing it across a range of free Zn2+ ion concentrations (1.2 pmol L−1 to 3.2 nmol L−1), which span the natural range observed in the global oceans. Across the range of free Zn2+ ion concentrations tested, the amount of bio-available Zn did not have any appreciable influence on the specific growth rate or cell morphology of E. huxleyi. However, a suite of secondary physiological indicators reflected changes in Zn bioavailability. An increase in the photosynthetic efficiency of photosystem II (Fv/Fm) was observed with increasing free Zn2+ ion concentration. The time constants for electron transport from QA to QB and from QB to the plastoquinone (PQ) pool decreased as the free Zn2+ ion concentration increased, reflecting enhanced photosynthetic electron transport. The effect of Zn bioavailability on photosynthetic physiology was speculated to be due to increased activity of carbonic anhydrase with increasing Zn bioavailability. Zinc uptake by E. huxleyi resulted in Zn isotope fractionation and the Δ66Zncells-media ranged between −0.6‰ and −0.2‰. The major finding from this work is that the lighter Zn isotope is taken up preferentially by this coccolithophore, making the dissolved Zn composition of the surrounding seawater isotopically heavier.

Rationale The conventional high-temperature conversion (HTC) approach towards hydrogen compound-specific isotope analysis (CSIA) of halogen-bearing (F, Cl, Br, I) organics suffers from incomplete H2 yields and associated hydrogen isotope fractionation due to generation of HF, HCl, HBr, and HI byproducts. Moreover, the traditional off-line combustion of highly halogenated compounds results in incomplete recovery of water as an intermediary compound for hydrogen isotope ratio mass spectrometry (IRMS), and hence also leads to isotope fractionation. This study presents an optimized chromium-based high-temperature conversion (Cr/HTC) approach for hydrogen CSIA of various fluorinated, chlorinated, brominated and iodinated organic compounds. The Cr/HTC approach is fast, economical, and not affected by low H2 yields and associated isotope fractionation. Methods The performance of the modified gas chromatography/chromium-based high-temperature conversion (GC-Cr/HTC) system was monitored and optimized using an ion trap mass spectrometer. Quantitative conversion of organic hydrogen into H2 analyte gas was achieved for all halogen-bearing compounds. The corresponding accuracy of CSIA was validated using (i) manual dual-inlet (DI)-IRMS after off-line conversion into H2 , and (ii) elemental analyzer (EA)-Cr/HTC-IRMS (on-line conversion). Results The overall hydrogen isotope analysis of F-, Cl-, Br- and I-bearing organics via GC-Cr/HTC-IRMS achieved a precision σ ≤ 3 mUr and an accuracy within ±5 mUr along the VSMOW-SLAP scale compared with the measured isotope compositions resulting from both validation methods, off-line and on-line. The same analytical performance as for single-compound GC-Cr/HTC-IRMS was achieved compound-specifically for mixtures of halogenated organics following GC separation to baseline resolution. Conclusions GC-Cr/HTC technology can be implemented in existing analytical equipment using commercially available materials to provide a versatile tool for hydrogen CSIA of halogenated and non-halogenated organics. Copyright © 2017 John Wiley & Sons, Ltd.

Almost no δ(18) O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ(18) O relationship between leaf water and cellulose. We measured δ(18) O values of bulk leaf water (δ(18) OLW ) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ(18) O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally (18) O-enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (ebio ) of more than 27‰ relative to δ(18) OLW , which might be explained by isotopic leaf water and sucrose synthesis gradients. δ(18) OLW and δ(18) O values of carbohydrates and cellulose in grasses were strongly related, indicating that the leaf water signal in carbohydrates was transferred to cellulose (ebio = 25.1‰). Interestingly, damping factor pex px , which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ(18) OLW but not if modelled directly from δ(18) O of individual carbohydrates. We conclude that δ(18) OLW is not always a good substitute for δ(18) O of synthesis water due to isotopic leaf water gradients. Thus, compound-specific δ(18) O analyses of individual carbohydrates are helpful to better constrain (post-)photosynthetic isotope fractionation processes in plants.

We tested for isotope exchange between bound (immobile) and mobile soil water, and whether there is isotope fractionation during plant water uptake. These are critical assumptions to the formulation of the 'two water worlds' hypothesis based on isotope profiles of soil water. In two different soil types, soil-bound water in two sets of 19-l pots, each with a 2-yr-old avocado plant (Persea americana), were identically labeled with tap water. After which, one set received isotopically enriched water whereas the other set received tap water as the mobile phase water. After a dry down period, we analyzed plant stem water as a proxy for soil-bound water as well as total soil water by cryogenic distillation. Seventy-five to 95% of the bound water isotopically exchanged with the mobile water phase. In addition, plants discriminated against 18 O and 2 H during water uptake, and this discrimination is a function of the soil water loss and soil type. The present experiment shows that the assumptions for the 'two water worlds' hypothesis are not supported. We propose a novel explanation for the discrepancy between isotope ratios of the soil water profile and other water compartments in the hydrological cycle.

Source nitrogen (N) identification of leachate or groundwater nitrate is complicated by N source mixing and N and oxygen (O) isotope fractionation caused by microbial N transformations. This experiment examined the δ¹⁵NNO₃ and δ¹⁸ONO₃ values in leachate collected over 1 yr at 55 cm below raspberry (Rubus idaeus L.) plots receiving either synthetic fertilizer (FT) or poultry manure (MT). The large ranges of δ¹⁵NNO₃ (FT: −2.4 to +8.7‰, MT: +1.6 to +9.6‰) and δ¹⁸ONO₃ (FT: −9.9 to −0.3‰, MT: −10.9 to +1.7‰) values in leachate collected under crop rows prohibited the reliable identification of the applied N sources on individual sampling dates. However, the mass-weighted average δ¹⁵NNO₃ (FT: +3.2‰, MT: +7.3‰) values in leachate were significantly different and can be explained by accounting for the estimated contributions of nitrate and δ¹⁵NNO₃ values of the various N sources, including applied fertilizer (−0.7‰) or manure (+7.9‰), nitrate-rich irrigation water (+9.0‰), and nitrate from soil N mineralization and nitrification (FT: +3.7‰, MT: +4.6‰; the seasonal timing of which is unknown). This study illustrates the importance of characterizing all major N sources and considering the seasonal variation of these sources and of N cycling processes, as they contribute to the δ¹⁵NNO₃ values of leachate.

Rationale The ability to recover the isotopic signature of water added to soil samples that have previously been oven-dried decreases with the increasing presence of silt and clay. The effects on the isotopic signature of water associated with physicochemical soil properties are not yet fully understood, for either hydration or dehydration of soil samples. Methods The soil sample chemistry and the crystallinity of minerals were measured by X-ray fluorescence and X-ray diffraction. The organic carbon and the cation-exchange capacity were also determined. Water of known isotopic signature was used to spike an oven-dried substrate and subsequently extracted by cryogenic vacuum extraction at a temperature of 105°C. In addition, the soils were oven-dried at 205°C and water extractions were also performed at 205°C. The isotopic signatures of the water samples were determined by cavity-ring-down spectrometry. Results The isotope effects caused by the cryogenic vacuum extraction method applied to soils with elevated clay content were reduced. First, by increasing the extraction temperature to 205°C, we improved the precision of the cryogenic vacuum extraction method and the recovery of the known isotopic signature of the spike water. Secondly, the post-correction of data based on the physicochemical soil properties and a common extraction temperature of 105°C improved the measurement trueness. Conclusions The isotopic signature of soil water is influenced by mineral-water interaction. During the hydration of clay, different minerals deplete free water in heavy isotopes. The extracted soil water (dehydration water) gathered from clay-rich soils is generally more depleted in the heavy isotopes than the spike water, making results obtained for different soil types difficult to compare. Isotope effects observed at the mineral-water interface highlight potential explanations for eco-hydrological separation of water pools. Copyright © 2016 John Wiley & Sons, Ltd.

We report mass-independent and mass-dependent Ca isotopic compositions for thirteen geological reference materials, including carbonates (NIST SRM 915a and 915b), Atlantic seawater as well as ten rock reference materials ranging from peridotite to sandstone, using traditional e and δ values relative to NIST SRM 915a, respectively. Isotope ratio determinations were conducted by independent unspiked and 43Ca-48Ca double-spiked measurements using a customised Triton Plus TIMS. The mean of twelve measurement results gave e40/44Ca values within ± 1.1, except for GSP-2 that had e40/44Ca = 4.04 ± 0.15 (2SE). Significant radiogenic 40Ca enrichment was evident in some high K/Ca samples. At an uncertainty level of ± 0.6, all reference materials had the same e43/44Ca and e48/44Ca values. We suggest the use of δ44/42Ca to report mass-dependent Ca isotopic compositions. The precision under intermediate measurement conditions for δ44/42Ca over eight months in our laboratory was ± 0.03‰ (with n ≥ 8 repeat measurements). Measured igneous reference materials gave δ44/42Ca values ranging from 0.27‰ to 0.54‰. Significant Ca isotope fractionation may occur during magmatic and metasomatism processes. Studied reference materials with higher (Dyn/Ybn) tend to have lower δ44/42Ca, implying a potential role of garnet in producing magmas with low δ44/42Ca. Sandstone GBW07106 had a δ44/42Ca value of 0.22‰, lower than all igneous rocks studied so far.

Tufa samples from 16 consecutive barrages along a 13 km section of a groundwater-fed Krka River (Slovenia) were analysed for their petrographical, mineralogical, elemental and stable carbon (δ13C) and oxygen (δ18O) isotope composition, to establish their relation to current climatic and hydrological conditions. Waters constantly oversaturated with calcite and the steep morphology of the Krka riverbed stimulate rapid CO2 degassing and subsequent tufa precipitation. The carbon isotope fractionation (Δ13C) between DIC and tufa in the Krka River evolves toward isotopic equilibrium being controlled by continuous CO2 degassing and tufa precipitation rate downstream. The Δ13C increased from 1.9 to 2.5‰ (Vienna PeeDee Belemnite); however, since tufa precipitation rates remain similar downstream, the major controlling factor of carbon isotope exchange is most probably related to the continuous 12CO2 degassing downstream leaving the carbon pool enriched in 13C. In the case of oxygen, the isotope fractionation was found to be from 1.0 to 2.3‰ smaller than reported in the literature. The observed discrepancies are due to different precipitation rates of calcite deposits as Krka tufas on cascades grow relatively faster compared to slowly precipitated calcite deposits in cave or stream pools. Due to non-equilibrium oxygen isotope exchange between Krka tufa and water, the δ18O proxy showed from 1.2 to 8.2°C higher calculated water temperatures compared to measured water temperatures, demonstrating that δ18O proxy-based temperature equations are not reliable for water temperature calculations of fast growing tufa on cascades. Because Mg is bound to the terrigenous dolomite fraction in the Krka tufa samples, the Mg/Ca was also found to be an unreliable temperature proxy yielding over up to 20°C higher calculated water temperatures. This article is protected by copyright. All rights reserved.

The most widely used method for isotope analysis at natural abundance is isotope ratio monitoring by Mass Spectrometry (irm-MS) which provides bulk isotopic composition in 2 H, 13 C, 15 N, 18 O or 34 S. However, in the 1980s, the direct access to Site-specific Natural Isotope Fractionation by Nuclear Magnetic Resonance (SNIF-NMRTM ) was immediately recognized as a powerful technique to authenticate the origin of natural or synthetic products. The initial - and still most popular - application consisted in detecting the chaptalization of wines by irm-2 H NMR. The approach has been extended to a wide range of methodologies over the last decade, paving the way to a wide range of applications, not only in the field of authentication but also to study metabolism. In particular, the emerging irm-13 C NMR approach delivers direct access to position-specific 13 C isotope content at natural abundance. After highlighting the application scope of irm-NMR (2 H and 13 C), this article describes the major improvements which made possible to reach the required accuracy of 1‰ (0.1%) in irm-13 C NMR. The last part of the manuscript summarizes the different steps to perform isotope analysis as a function of the sample properties (concentration, peak overlap) and the kind of targeted isotopic information (authentication, affiliation). Copyright © 2016 John Wiley & Sons, Ltd.

The method of reconstructing paleoatmospheric CO2 levels using carbon isotope signatures of organic matter buried in sediments has been questioned due to the dubious foundation that carbon isotope fractionation during phytoplankton photosynthesis (ep) is controlled primarily by aquatic CO2 concentration ([CO2(aq)]). Consequently, what carbon isotope data from bulk sedimentary organic matter reflects is a puzzle. In this study, we determined the carbon isotope compositions of dissolved inorganic carbon and particulate organic carbon in a lake located in a carbonate area. Partial correlation analysis was employed to distinguish between direct and indirect factors in controlling ep. The results show that ep is more closely, and more steadily related with pH than with [CO2(aq)], which is in accordance with recent advances in our understanding of the physiology of carbon utilization by phytoplankton for CO2 and HCO3−. Therefore, we propose that carbon isotope fractionation in phytoplankton is more suitable as a proxy of pH than of [CO2(aq)]. One advantage of this amendment is that information on HCO3−, the main species of carbon uptake by phytoplankton, is likewise included. In the future, culture experiments aiming at revealing the relationship between pH and cellular carbon isotope signatures is necessary to construct a new isotope fractionation formula to couple the different effects of CO2 and HCO3−, which is of critical importance to improve the understanding of carbon isotope fractionation, and to more precisely model pH and CO2.

Leaf water contains naturally occurring stable isotopes of oxygen and hydrogen in abundances that vary spatially and temporally. When sufficiently understood, these can be harnessed for a wide range of applications. Here, we review the current state of knowledge of stable isotope enrichment of leaf water, and its relevance for isotopic signals incorporated into plant organic matter and atmospheric gases. Models describing evaporative enrichment of leaf water have become increasingly complex over time, reflecting enhanced spatial and temporal resolution. We recommend that practitioners choose a model with a level of complexity suited to their application, and provide guidance. At the same time, there exists some lingering uncertainty about the biophysical processes relevant to patterns of isotopic enrichment in leaf water. An important goal for future research is to link observed variations in isotopic composition to specific anatomical and physiological features of leaves that reflect differences in hydraulic design. New measurement techniques are developing rapidly, enabling determinations of both transpired and leaf water δ(18) O and δ(2) H to be made more easily and at higher temporal resolution than previously possible. We expect these technological advances to spur new developments in our understanding of patterns of stable isotope fractionation in leaf water.

Microbial nitrite reduction plays an important role in the nitrogen cycle, producing the first gaseous product in the denitrification pathway. The role of nitrite reduction in the environment can be assessed using stable isotope measurements of nitrite. Here, we present estimates for nitrogen (N) and oxygen (O) isotope fractionation during nitrite reduction catalyzed by copper-containing nitrite reductase (Cu-NIR) and cytochrome cd1-containing nitrite reductase (Fe-NIR). A Rayleigh fractionation model was used to calculate the N and O isotope effects, 15e and 18e respectively, from time-course measurements of nitrite concentration and isotopic composition in batch culture experiments. For three strains of denitrifier carrying the Cu-NIR, 15e = 22 ± 2‰ and 18e = 2 ± 2‰ (95% confidence interval). For three strains of denitrifier carrying the Fe-NIR, 15e = 8 ± 2 and 18e = 6 ± 2‰ (95% confidence interval). These isotope effects for nitrite reduction are significantly different from each other. Furthermore, 15e and 18e do not show a 1 : 1 relationship, as has been assumed. The difference between the isotope effects for these two families of enzymes is likely due to a mechanical difference in how the enzymes bind nitrite. The Cu-NIR binds to both O atoms and the Fe-NIR only binds to the N, allowing either NO bond to be cleaved and imparting a larger isotope effect for O than for the Cu-NIR. Utilizing these new N isotope effects for nitrite reduction in oxygen minimum zone N cycle models results in higher rates of nitrite oxidation than previously modeled.

Rationale Isotopic signatures of N2O can help distinguish between two sources (fertiliser N or endogenous soil N) of N2O emissions. The contribution of each source to N2O emissions after N‐application is difficult to determine. Here, isotopologue signatures of emitted N2O are used in an improved isotopic model based on Rayleigh‐type equations. Methods The effects of a partial (33% of surface area, treatment 1c) or total (100% of surface area, treatment 3c) dispersal of N and C on gaseous emissions from denitrification were measured in a laboratory incubation system (DENIS) allowing simultaneous measurements of NO, N2O, N2 and CO2 over a 12‐day incubation period. To determine the source of N2O emissions those results were combined with both the isotope ratio mass spectrometry analysis of the isotopocules of emitted N2O and those from the 15N‐tracing technique. Results The spatial dispersal of N and C significantly affected the quantity, but not the timing, of gas fluxes. Cumulative emissions are larger for treatment 3c than treatment 1c. The 15N‐enrichment analysis shows that initially ~70% of the emitted N2O derived from the applied amendment followed by a constant decrease. The decrease in contribution of the fertiliser N‐pool after an initial increase is sooner and larger for treatment 1c. The Rayleigh‐type model applied to N2O isotopocules data (δ15Nbulk‐N2O values) shows poor agreement with the measurements for the original one‐pool model for treatment 1c; the two‐pool models gives better results when using a third‐order polynomial equation. In contrast, in treatment 3c little difference is observed between the two modelling approaches. Conclusions The importance of N2O emissions from different N‐pools in soil for the interpretation of N2O isotopocules data was demonstrated using a Rayleigh‐type model. Earlier statements concerning exponential increase in native soil nitrate pool activity highlighted in previous studies should be replaced with a polynomial increase with dependency on both N‐pool sizes., Biotechnology and Biological Sciences Research Council (BBSRC), Grant/Award Number: BB/K001051/1.D.

This work was funded by a The Leverhulme Trust Research Project Grant to I. Crawford (F/07 112/AA). C. Cousins is funded by a Royal Society of Edinburgh Research Fellowship. Stable isotope analyses at the Carnegie Institution of Washington, USA, were funded by a grant from the W. M. Keck Foundation (2007-6-29) and through the NASA Astrobiology grant (NNH082ZDA0002C) to M. Fogel. We investigated bacterial and archaeal communities along an ice‐fed surficial hot spring at Kverkfjöll volcano—a partially ice‐covered basaltic volcano at Vatnajökull glacier, Iceland, using biomolecular (16S rRNA, apsA, mcrA, amoA, nifH genes) and stable isotope techniques. The hot spring environment is characterized by high temperatures and low dissolved oxygen concentrations at the source (68°C and

This review retraces the role of stable isotopes and mass spectrometry in the life sciences. The timeline is divided into four segments covering the years 1920-1950, 1950-1980, 1980-2000, and 2000 until today. For each period methodic progress and typical applications are discussed. Application of stable isotopes is driven by improvements of mass spectrometry, chromatography, and related fields in sensitivity, mass accuracy, structural specificity, complex sample handling ability, data output, and data evaluation. We currently experience the vision of omics-type analyses, that is, the comprehensive identification and quantification of a complete compound class within one or a few analytical runs. This development is driven by stable isotopes without competition by radioisotopes. In metabolic studies as classic field of isotopic tracer experiments, stable isotopes and radioisotopes were competing solutions, with stable isotopes as the long-term junior partner. Since the 1990s the number of metabolic studies with radioisotopes decreases, whereas stable isotope studies retain their slow but stable upward tendency. Unique fields of stable isotopes are metabolic tests in newborns, metabolic experiments in healthy controls, newborn screening for inborn errors, quantification of drugs and drug metabolites in doping control, natural isotope fractionation in geology, ecology, food authentication, or doping control, and more recently the field of quantitative omics-type analyses. There, cells or whole organisms are systematically labeled with stable isotopes to study proteomic differences or specific responses to stimuli or genetic manipulation. The duo of stable isotopes and mass spectrometry will probably continue to grow in the life sciences, since it delivers reference-quality quantitative data with molecular specificity, often combined with informative isotope effects. © 2016 Wiley Periodicals, Inc. Mass Spec Rev 36:58-85, 2017.

We measured the concentrations of dissolved inorganic carbon (DIC) and major ions and the stable carbon isotope ratios of DIC (δ13CDIC) in two creeks discharging from carbonate-rich sulphide-containing mine tailings piles. Our aim was to assess downstream carbon evolution of the tailings discharge as it interacted with the atmosphere. The discharge had pH of 6.5–8.1 and was saturated with respect to carbonates. Over the reach of one creek, the DIC concentrations decreased by 1.1 mmol C/l and δ13CDIC increased by ~4.0‰ 200 m from the seep source. The decrease in the DIC concentrations was concomitant with decreases in the partial pressure of CO2(aq) because of the loss of excess CO2(aq) from the discharge. The corresponding enrichment in the δ13CDIC is because of kinetic isotope fractionation accompanying the loss of CO2(g). Over the reach of the other creek, there was no significant decrease in the DIC concentrations or notable changes in the δ13CDIC. The insignificant change in the DIC concentrations and the δ13CDIC is because the first water sample was collected 160 m away from the discharge seep, not accessible during this research. In this case, most of the excess CO2(aq) was lost before our first sampling station. Our results indicate that neutral discharges from tailings piles quickly lose excess CO2(aq) to the atmosphere and the DIC becomes enrich in 13C. We suggest that a significant amount of carbon cycling in neutral discharges from tailings piles occur close to the locations where the discharge seeps to the surface. Copyright © 2015 John Wiley & Sons, Ltd.

RationaleThe aim of this study was to determine the impact of isotope fractionation associated with N2O reduction during soil denitrification on N2O site preference (SP) values and hence quantify the potential bias on SP-based N2O source partitioning. MethodsThe N2O SP values (n=431) were derived from six soil incubation studies in N-2-free atmosphere, and determined by isotope ratio mass spectrometry (IRMS). The N-2 and N2O concentrations were measured directly by gas chromatography. Net isotope effects (NIE) during N2O reduction to N-2 were compensated for using three different approaches: a closed-system model, an open-system model and a dynamic apparent NIE function. The resulting SP values were used for N2O source partitioning based on a two end-member isotopic mass balance. ResultsThe average SP0 value, i.e. the average SP values of N2O prior to N2O reduction, was recalculated with the closed-system model, resulting in -2.6 (+/- 9.5), while the open-system model and the dynamic apparent NIE model gave average SP0 values of 2.9 parts per thousand (+/- 6.3) and 1.7 parts per thousand (+/- 6.3), respectively. The average source contribution of N2O from nitrification/fungal denitrification was 18.7% (+/- 21.0) according to the closed-system model, while the open-system model and the dynamic apparent NIE function resulted in values of 31.0% (+/- 14.0) and 28.3% (+/- 14.0), respectively. ConclusionsUsing a closed-system model with a fixed SP isotope effect may significantly overestimate the N2O reduction effect on SP values, especially when N2O reduction rates are high. This is probably due to soil inhomogeneity and can be compensated for by the application of a dynamic apparent NIE function, which takes the variable reduction rates in soil micropores into account. Copyright (c) 2016 John Wiley & Sons, Ltd.

In this study the homogeneity of the zinc isotopic composition in the NIST SRM 683 reference material was examined by measuring the Zn isotopic signature in microdrilled sample powders from two metal nuggets. Zinc was purified using AG MP-1M resin and then measured by MC-ICP-MS. Instrumental mass bias was corrected using the “sample-standard bracketing” method and empirical external normalisation with Cu doping. After evaluating the potential effects of varying acid mass fractions and different matrices, high-precision Zn isotope data were obtained with an intermediate measurement precision better than ± 0.05‰ (d 66 Zn, 2s) over a period of 5 months. The d 66 ZnJMC-Lyon mean values of eighty-four and fourteen drilled powders from two nuggets were 0.11 ± 0.02‰ and 0.12 ± 0.02‰, respectively, indicating that NIST SRM 683 is a good isotopic reference material with homogeneous Zn isotopes. The Zn isotopic compositions of seventeen rock reference materials were also determined, and their d 66 Zn values were in agreement with most previously published data within 2s. The d 66 Zn values of most of the rock reference materials analysed were in the range 0.22–0.36‰, except for GSP-2 (1.07 ± 0.06‰, n = 12), NOD-A-1 (0.96 ± 0.03‰, n = 6) and NOD-P-1 (0.78 ± 0.03‰, n = 6). These comprehensive data should serve as reference values for quality assurance and interlaboratory calibration exercises.

The Arctic is particularly sensitive to climate change, but the independent effects of increasing atmospheric CO2 concentration (pCO2 ) and temperature on high-latitude forests are poorly understood. Here, we present a new, annually resolved record of stable carbon isotope (δ(13) C) data determined from Larix cajanderi tree cores collected from far northeastern Siberia in order to investigate the physiological response of these trees to regional warming. The tree-ring record, which extends from 1912 through 1961 (50 years), targets early twentieth-century warming (ETCW), a natural warming event in the 1920s to 1940s that was limited to Northern hemisphere high latitudes. Our data show that net carbon isotope fractionation (Δ(13) C), decreased by 1.7‰ across the ETCW, which is consistent with increased water stress in response to climate warming and dryer soils. To investigate whether this signal is present across the northern boreal forest, we compiled published carbon isotope data from 14 high-latitude sites within Europe, Asia, and North America. The resulting dataset covered the entire twentieth century and spanned both natural ETCW and anthropogenic Late Twentieth-Century Warming (~0.7 °C per decade). After correcting for a ~1‰ increase in Δ(13) C in response to twentieth century pCO2 rise, a significant negative relationship (r = -0.53, P < 0.0001) between the average, annual Δ(13) C values and regional annual temperature anomalies is observed, suggesting a strong control of temperature on the Δ(13) C value of trees growing at high latitudes. We calculate a 17% increase in intrinsic water-use efficiency within these forests across the twentieth century, of which approximately half is attributed to a decrease in stomatal conductance in order to conserve water in response to drying conditions, with the other half being attributed to increasing pCO2 . We conclude that annual tree-ring records from northern high-latitude forests record the effects of climate warming and pCO2 rise across the twentieth century.