Isotope Fractionation Effects Research Articles

Mechanisms of carbon isotopic fractionation in the process of natural gas generation: Geochemical evidence from thermal simulation experiment

Abstract Low maturity coal samples were taken from the Ordos Basin to conduct gold tube thermal simulation experiment in a closed system, and the characteristics of the products were analyzed to find out the fractionation mechanism of carbon isotopes and the causes of abnormal carbon isotopic compositions of natural gas. At the heating rates of 2 °C/h (slow) and 20 °C/h (rapid), the low maturity coal samples of the Ordos Basin had the maximum yields of alkane gas of 302.74 mL/g and 230.16 mL/g, the δ13C1 ranges of −34.8‰ to −23.6‰ and −35.5‰ to −24.0‰; δ13C2 ranges of −28.0‰ to −9.0‰ and −28.9‰ to −8.3‰; and δ13C3 ranges of −25.8‰ to −14.7‰ and −26.4‰ to −13.2‰, respectively. Alkane gas in the thermal simulation products of rapid temperature rise process showed obvious partial reversal of carbon isotope series at 550°C, and at other temperatures showed positive carbon isotope series. In the two heating processes, the δ13C1 turned lighter first and then heavier, and the non-monotonic variation of the δ13C1 values is because the early CH4 is from different parent materials resulted from heterogeneity of organic matter or the carbon isotope fractionation formed by activation energy difference of early enriched 12CH4 and late enriched 13CH4. The reversal of carbon isotope values of heavy hydrocarbon gas can occur not only in high to over mature shale gas (oil-type gas), but also in coal-derived gas. Through thermal simulation experiment of toluene, it is confirmed that the carbon isotope value of heavy hydrocarbon gas can be reversed and inversed at high to over mature stage. The isotope fractionation effect caused by demethylation and methyl linkage of aromatic hydrocarbons may be an important reason for carbon isotope inversion and reversal of alkane gas at the high to over mature stage.

Application of online two-dimensional high-performance liquid chromatography as purification procedure to determine the origin of 19-norandrosterone in urine by gas chromatography-combustion-isotope ratio mass spectrometry.

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.

Stable isotopic (δ18O and δ2H) and geospatial approach for evaluating extreme rainfall events

Natural disasters are more frequent in the tropical region in the recent past due to anthropogenic forcing and consequent climate change. In this context, understanding the genesis and pattern of extreme rainfall events are important to devise disaster management plans. Two natural disaster events, Cyclone Ockhi (05 December 2017) and a torrential rainfall event (944 mm in 24 h) occurred on 26 July 2005, that had caused large scale devastation, loss of lives and properties, in Mumbai, India were evaluated using stable isotopes (δ 18 O and δ 2 H) of precipitation and geospatial data. Both the events were analysed and compared to understand their occurrences, moisture source regions and deduce the atmospheric factors responsible for their formation. Distinctly depleted stable isotopic content in rains were observed during the Cyclone Ockhi, while, during the extreme rainfall event, it was same as that of the normal monsoon rains, which is attributed to the dynamic nature of the moisture. The depleted isotopic signature of the cyclone may be because of the isotopic fractionation effect caused by the organized convective condition and vapour recycling along the trajectory. The genesis, structure and pattern of both the extreme events were evaluated using Sea Surface Temperature (SST), Outgoing Longwave Radiation (OLR) and Relative Humidity (RH) data. Wind pattern and moisture transport pathways were derived using HYSPLIT backward trajectory model. Prevalence of high SST over the southwest Bay of Bengal and adjoining areas of Sri Lanka as well as in the Arabian Sea was responsible for the genesis and intensification of the Cyclone Ockhi. The extremely low OLR value indicated intense and deep convection over the region during the cyclone. The large scale altitudinal variation in RH created unstable condition in the troposphere, consequently, strengthened the cyclone. The intensive shallow convection, continuous supply of moisture from the Arabian Sea and the orographic uplifting of clouds at the Western Ghats were responsible for the occurrence of torrential rainfall on 26 July 2005 in Mumbai. The wind pattern during the cyclone was distinct; it was circular and ascending upward, while, it was more or less horizontal and was descending downwards during the torrential rainfall event. The atmospheric processes responsible for the occurrence of two natural disaster rain events were also conceptualized. Although, the isotope data is limited, the study is first of its kind and it provides better insights into the genesis, intensification of the extreme rainfall events that are essential for forecasting, planning disaster mitigation measures and in warning strategies of this region. • Genesis, structure and pattern of two natural disaster events (Cyclone Ockhi and a torrential rainfall) were evaluated. • The atmospheric processes responsible for the extreme events were conceptualized. • Prevalence of deep convection during Cyclone Ockhi and shallow convection during torrential rainfall event was deduced.

What can we learn from N2 O isotope data? - Analytics, processes and modelling.

The isotopic composition of nitrous oxide (N2 O) provides useful information for evaluating N2 O sources and budgets. Due to the co-occurrence of multiple N2 O transformation pathways, it is, however, challenging to use isotopic information to quantify the contribution of distinct processes across variable spatiotemporal scales. Here, we present an overview of recent progress in N2 O isotopic studies and provide suggestions for future research, mainly focusing on: analytical techniques; production and consumption processes; and interpretation and modelling approaches. Comparing isotope-ratio mass spectrometry (IRMS) with laser absorption spectroscopy (LAS), we conclude that IRMS is a precise technique for laboratory analysis of N2 O isotopes, while LAS is more suitable for in situ/inline studies and offers advantages for site-specific analyses. When reviewing the link between the N2 O isotopic composition and underlying mechanisms/processes, we find that, at the molecular scale, the specific enzymes and mechanisms involved determine isotopic fractionation effects. In contrast, at plot-to-global scales, mixing of N2 O derived from different processes and their isotopic variability must be considered. We also find that dual isotope plots are effective for semi-quantitative attribution of co-occurring N2 O production and reduction processes. More recently, process-based N2 O isotopic models have been developed for natural abundance and 15 N-tracing studies, and have been shown to be effective, particularly for data with adequate temporal resolution. Despite the significant progress made over the last decade, there is still great need and potential for future work, including development of analytical techniques, reference materials and inter-laboratory comparisons, further exploration of N2 O formation and destruction mechanisms, more observations across scales, and design and validation of interpretation and modelling approaches. Synthesizing all these efforts, we are confident that the N2 O isotope community will continue to advance our understanding of N2 O transformation processes in all spheres of the Earth, and in turn to gain improved constraints on regional and global budgets.

Improvement in the Analytical Capabilities of LA-ICP-MS for High Spatial Resolution U-Pb Dating of Zircon Using Mixed-Gas Plasma

In this work, a novel method for high spatial resolution U-Pb dating of zircon by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at 10–16 μm spot diameter has been proposed. This was achieved by introducing 2% (v/v) water-ethanol vapours into ICP in combination with the shielded torch system to increase the sensitivity and suppress the isotopic fractionation effect. Precise and accurate concordant U-Pb ages for Plešovice, GJ-1, and 91500 zircons were obtained using the proposed method, and the results agreed well with the isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and LA-ICP-MS within 2σ, except for the ages obtained using dry plasma at 10 μm spot diameter. Additionally, the effects of plasma condition (dry plasma or 2% (v/v) ethanol plasma) and spot size (10, 16, 24, or 32 μm) on precision (RSD), accuracy (RE), and uncertainty (2σ) of the 206Pb/238U ages have been studied. The results indicated that increasing the spot diameter and introducing 2% (v/v) water-ethanol vapours into ICP significantly improved the precision, accuracy, and uncertainty for small spot diameters (10 and 16 μm), while it exhibited little influence for intermediate spot diameters (24 and 32 μm). Furthermore, the effects of spot size and plasma condition on the precision, accuracy, and uncertainty strongly depended on the sensitivity of analytes.

Nitrate source apportionment in groundwater using Bayesian isotope mixing model based on nitrogen isotope fractionation

Accurate identification of nitrate (NO3−) sources is critical to address the issue of groundwater pollution. The nitrogen (N) isotopic enrichment factor (ɛp/s) is an important parameter to explain the N cycle and determine the proportional contribution of NO3− sources. Considering the isotopic fractionation effects in N transformation processes, this study quantitatively analyzed the NO3− sources in groundwater using stable isotopes (δ15N-NO3− and δ18O-NO3−) and the Bayesian isotope mixing model (SIAR). For the first time, the ɛp/s values (0.0‰, −8.7‰, −8.7‰, and 14.7‰) of atmospheric deposition (AD), soil nitrogen (SN), chemical fertilizers (CF), and manure and sewage (M&S) were calculated to determine the NO3− source apportionment in groundwater. It was proved that the isotopic fractionation effect could produce a more accurate NO3− source apportionment. We also found that the NO3− source contributions were closely related to the cropping system. In the vegetable cultivation area, CF (54.32%) and SN (37.75%) were the dominant NO3− source, while in the grain cultivation area, NO3− pollution was largely influenced by SN (33.67%), CF (33.27%), and M&S (30.16%). According to this study, the isotope fractionation is strongly recommended for NO3− source apportionment in groundwater system.

Theoretical analysis of the kinetic isotope effect on carboxylation in RubisCO

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) fixes atmospheric carbon dioxide into bioavailable sugar molecules. It is also well known that a kinetic isotope effect (KIE; CO2 carbon atoms) accompanies the carboxylation process. To describe the reaction and the KIE α, two different types of molecular dynamics (MD) simulations (ab initio MD and classical MD) have been performed with an Own N-layered Integrated molecular Orbitals and molecular Mechanics (ONIOM)-hybrid model. A channel structure for CO2 transport has been observed during the MD simulation in RubisCO, and assuming the reaction path from the inlet to the product through the coordinate complex with Mg2+ , simulations have been performed on several molecular configuration models fixing several distances between CO2 and ribulose-1,5-bisphosphate along the channel. Free energy analysis and diffusion coefficient analysis have been evaluated for different phases of the process. It is confirmed that the isotopic fractionation effect for CO2 containing either 13 C or 12 C would appear through the transiting path in the channel structure identified in RubisCO. The estimated isotope fractionation constant was quite close to the experimental value.

Improved Mass Accuracy and Isotope Confirmation through Alignment of Ultrahigh-Resolution Mass Spectra of Complex Natural Mixtures

Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) is one of the state-of-the-art methods to analyze complex natural organic mixtures. The precision of detected masses is crucial for molecular formula attribution. Random errors can be reduced by averaging multiple measurements of the same mass, but because of limited availability of ultrahigh-resolution mass spectrometers, most studies cannot afford analyzing each sample multiple times. Here we show that random errors can be eliminated also by averaging mass spectral data from independent environmental samples. By averaging the spectra of 30 samples analyzed on our 15 T instrument we reach a mass precision comparable to a single spectrum of a 21 T instrument. We also show that it is possible to accurately and reproducibly determine isotope ratios with FT-ICR-MS. Intensity ratios of isotopologues were improved to a degree that measured deviations were within the range of natural isotope fractionation effects. In analogy to δ13C in environmental studies, we propose Δ13C as an analytical measure for isotope ratio deviances instead of widely employed C deviances. In conclusion, here we present a simple tool, extensible to Orbitrap-based mass spectrometers, for postdetection data processing that significantly improves mass accuracy and the precision of intensity ratios of isotopologues at no extra cost.

A critical evaluation of copper isotopes in Precambrian Iron Formations as a paleoceanographic proxy

Trace metals in Iron Formations (IF) have been widely used as proxies for oceanic redox processes and oxygen evolution leading to the Great Oxidation Event (GOE). Copper has hitherto received comparatively little attention, with a single study reporting variation in δ65Cu between pre- and post-GOE black shales. This is attributed to postulated isotope fractionation effects in the pre-GOE ocean during widespread iron oxide deposition in IF. Here we focus on the application of Cu isotopes in two classic IF-containing sequences of the Neoarchean-Paleoproterozoic, namely the Hamersley (Australia) and Transvaal (South Africa) Supergroups. We specifically targeted the oxide-rich Joffre and Kuruman IF, the carbonate-rich Griquatown IF, and the Mn-rich Hotazel IF, which collectively record over 100 Ma of sustained IF deposition in the pre-GOE ocean. The aim was to assess the utility of Cu isotopes in IF as a paleoceanographic proxy in view of existing oxygen evolution models. Iron formation Cu concentrations are low compared to average crust and modern oceanic Fe-Mn oxide deposits, with average values between 1 and 5 ppm for all four IF data sets. Copper concentrations show no systematic variability with mineralogy, no statistical correlation with bulk Fe and Mn contents, good correlations with Ti (rCu-Ti = 0.73) for the Joffre data set and with Cr, Ni, and V for the South African data sets, and shale-like Cu/Ti ratios. Isotopic results show statistically invariant average δ65Cu values very close to 0‰ for all but the Joffre IF which has a marginally negative average δ65Cu value (−0.24 ± 0.22‰). Combined with other trace transition metal systematics, the Cu isotope data point to a Cu source that was controlled largely by inputs of fine volcanic-derived particles (ash), thus placing limitations on its utility as a paleoceanographic redox proxy during IF genesis.

Key Factors of Precipitation Stable Isotope Fractionation in Central-Eastern Africa and Central Mediterranean

The processes of isotope fractionation in the hydrological cycle naturally occur during vapor formation, vapor condensation, and moisture transportation. These processes are therefore dependent on local and regional surface and atmospheric physical features such as temperature, pressure, wind speed, and land morphology, and hence on the climate. Because of the strong influence of climate on the isotope fractionation, latitudinal and altitudinal effects on the δ18O and δ2H values of precipitation at a global scale are observed. In this study, we present and compare the processes governing precipitation isotope fractionation from two contrasting climatic regions: Virunga in Central-Eastern Africa and the Central Mediterranean (Stromboli and Sicily, Italy). While Virunga is a forested rainy tropical region located between Central and Eastern Africa, the Mediterranean region is characterized by a rainy mild winter and a dry hot summer. The reported δ18O and δ2H dataset are from precipitation collected on rain gauges sampled either on a monthly or an approximately bimonthly basis and published in previous papers. Both regions show clearly defined temporal and altitudinal variations of δ18O and δ2H, depending on precipitation amounts. The Central Mediterranean shows a clear contribution of local vapor forming at the sea–air interface, and Virunga shows a contribution from both local and regional vapor. The vapor of Virunga is from two competing sources: the first is the continental recycled moisture from soil/plant evaporation that dominates during the rainy season, and the second is from the East African Great Lakes evaporation that dominates during the dry season.

Strong and selective isotope effect in the vacuum ultraviolet photodissociation branching ratios of carbon monoxide

Rare isotope (13C, 17O and 18O) substitutions can substantially change absorption line positions, oscillator strengths and photodissociation rates of carbon monoxide (CO) in the vacuum ultraviolet (VUV) region, which has been well accounted for in recent photochemical models for understanding the large isotopic fractionation effects that are apparent in carbon and oxygen in the solar system and molecular clouds. Here, we demonstrate a strong isotope effect associated with the VUV photodissociation of CO by measuring the branching ratios of 12C16O and 13C16O in the Rydberg 4p(2), 5p(0) and 5s(0) complex region. The measurements show that the quantum yields of electronically excited C atoms in the photodissociation of 13C16O are dramatically different from those of 12C16O, revealing strong isotope effect. This isotope effect strongly depends on specific quantum states of CO being excited, which implies that such effect must be considered in the photochemical models on a state by state basis.

Effect of Mass-Independent Isotope Fractionation of Sulfur (Δ33S and Δ36S) during SO2 Photolysis in Experiments with a Broadband Light Source

Abstract–This paper presents the results of experimental studies of the behavior of mass-independent isotope effects of sulfur Δ33S and Δ36S during photochemical processes initiated by a broadband ultraviolet radiation. Experiments were performed in a flow photochemical reactor using a high-pressure mercury lamp, which is a source of radiation of a wide range with maximum radiation intensity in the wavelength range of 270−330 nm and weaker intensity in the range of 190−250 nm. The temperature and SO2 pressure dependences of the sulfur isotope ratios in the elemental sulfur products are revealed. Based on a comparative analysis of our isotope data with data from previous experimental studies with xenon and hydrogen lamps, it was shown that the correlation between the values of δ34S, Δ33S and Δ36S in elemental sulfur depends on the relative spectral distribution of the radiation intensity. Based on a comparison of our isotope data with data from previous experimental studies with xenon and hydrogen lamps, it was shown that the correlation between the values of δ34S, Δ33S and Δ36S in elemental sulfur depends on the relative spectral distribution of the radiation intensity. Our experiments suggest that photochemical processes in the range of 250−330 nm could play a significant role in the production of an isotope sulfur anomaly in the Archean atmosphere. The conditions in which 250− 330 nm radiation prevails over the 190−220 nm radiation are consistent with the assumption that the level of solar radiation reaching the Earth’s surface in Archean was several orders of magnitude higher in the wavelength range 200−300 nm compared with the current level of radiation in this range.

Carbon, nitrogen and sulfur isotopic features and the associated geochemical processes in a coastal aquifer system of the Pearl River Delta, China

The coastal zone is not only a terrestrial-marine buffer system but also the main site of the organic matter decomposition process, which complicates the identification of the sources of bioactive elements (C, N and S) and the factors that control C-N-S cycling. Here, a multi-isotope approach is developed to distinguish C, N and S sources and the associated geochemical processes in a coastal aquifer system of the Pearl River Delta (PRD), China. The PRD is characterized as a typical multilayer groundwater system with a reducing environment in the confined aquifers and an oxidizing environment in the unconfined aquifer. For the groundwater samples from the unconfined aquifer, the δ15N values showed that the ammonium and nitrate originated from manure and sewage, the δ13C-DIC indicated that dissolved inorganic carbon (DIC) was mainly derived from soil CO2 and weathering of carbonate rocks, and the δ34S and δ18O signals of sulfate suggested that SO42− originated from the dissolution of continental evaporate. The oxidizing environment and the positive relationship between δ15N-NO3− and δ18O-NO3− suggested that nitrification and denitrification were the key factors influencing the transformation of nitrogen in the unconfined aquifer. No obvious correlation between C, N and S content and isotopic composition suggested C-N-S was loosely coupled in this aquifer.For the groundwater samples in the confined aquifers, the δD-H2O and δ18O-H2O signals suggested that partially confined groundwater samples were affected by seawater intrusion. In the non-seawater intrusion region (NSI zone), the C, N and S content combined with C, N and S isotopic composition indicated that ammonium was derived from the mineralization of organic matter, sulfate mainly originated from rock weathering, and DIC mainly originated from carbonate mineral dissolution and the CO2 produced by organic matter mineralization. Extremely high CH4 concentrations suggested the existence of methanogenesis, while the molar ratio of CH4/(CH4 + (1 + α) × CO2) indicated that both acetate fermentation and CO2 reduction were ubiquitous. CO2 reduction had a stronger isotopic fractionation effect on the δ13C-DIC values, thus, the relatively high δ13C-DIC values were observed. The electron acceptor was the key factor for organic matter mineralization, and CO2 acted as the main electron acceptor in the NSI Zone. Accordingly, acetate fermentation and CO2 reduction became the key factors driving ammonium release in the NSI zone. In the seawater intrusion region (SI zone), ammonium was derived from the mineralization of organic matter, sulfate was derived from the mixing of fresh water and seawater, and DIC was derived from carbonate mineral dissolution, seawater and the CO2 produced by organic matter mineralization. The δ34S-SO42− showed a positive correlation with δ18O-SO42− and a negative correlation with SO42−, suggesting the existence of bacterial sulfate reduction (BSR). High CH4 concentrations also suggested the existence of methanogenesis. The CH4/(CH4 + (1 + α) × CO2) ratios indicated that CO2 reduction existed in more than half of the confined groundwater samples in the SI zone. C-N-S was highly coupled in this region because both BSR and methanogenesis can increase the DIC content, while BSR enriches lighter carbon isotopes and methanogenesis enriches heavier carbon isotopes; thus, a wide range of δ13C-DIC values was found in SI zone. Sulfate or both sulfate and CO2 were the electron acceptors. Accordingly, acetate fermentation, sulfur reduction and CO2 reduction worked together to promote ammonium release for the groundwater samples in the SI Zone. However, the exhaustion of sulfate will make CO2 reduction the key factor for organic matter mineralization in the future.

Equilibrium thallium isotope fractionation and its constraint on Earth’s late veneer

Equilibrium isotope fractionation of thallium (Tl) includes the traditional mass-dependent isotope fractionation effect and the nuclear volume effect (NVE). The NVE dominates the overall isotope fractionation, especially at high temperatures. Heavy Tl isotopes tend to be enriched in oxidized Tl3+-bearing species. Our NVE fractionation results of oxidizing Tl+ to Tl3+ can explain the positive enrichments observed in ferromanganese sediments. Experimental results indicate that there could be 0.2–0.3 e-unit fractionation between sulfides and silicates at 1650 °C. It is consistent with our calculation results, which are in the range of 0.17–0.38 e-unit. Importantly, Tl’s concentration in the bulk silicate Earth (BSE) can be used to constrain the amount of materials delivered to Earth during the late veneer accretion stage. Because the Tl concentration in BSE is very low and its Tl isotope composition is similar with that of chondrites, suggesting either no Tl isotope fractionation occurred during numerous evaporation events, or the Tl in current BSE was totally delivered by late veneer. If it is the latter, the Tl-content-based estimation could challenge the magnitude of late veneer which had been constrained by the amount of highly siderophile elements in BSE. Our results show that the late-accreted mass is at least five-times larger than the previously suggested magnitude, i.e., 0.5 wt% of current Earth’s mass. The slightly lighter 205Tl composition of BSE relative to chondrites is probable a sign of occurrence of Tl-bearing sulfides, which probably were removed from the mantle in the last accretion stage of the Earth.

Stable carbon isotopic characteristics of plant-litter-soil continuum along a successional gradient of broadleaved Korean pine forests in Changbai Mountain, China.

Stable carbon isotope composition can accurately indicate ecosystem carbon cycling and provide key information for the study of the influence of forest succession on the carbon cycling and carbon sequestration potential. We measured the δ13C values and carbon and nitrogen contents of leaf, trunk, root, litter, and soil along a forest successional gradient in Changbai Mountain, which included a middle-aged poplar-birch secondary forest, a mature poplar-birch secondary forest, and an old-growth broad-leaved Korean pine forest. The results showed that leaf δ13C reduced with their position from the upper canopy to lower canopy, bark δ13C was less than xylem, fine root δ13C was less than course root. In contrast to the secondary forests, δ13C of the undecomposed litter layer was less than that of the semi-decomposed layer and decomposed litter layer in the broad-leaved Korean pine forest. Soil δ13C increased with depth. The ascending order of mean δ13C was leaf, litter, root, trunk, and soil, indicating that there is obvious fractionation among different organs of plants and among different parts of a specific organ. In addition, plant δ13C first decreased and then increased with the succession process, but soil δ13C increased with the succession processes. The different patterns of the changes of plant and soil δ13C along forest succession could be explained by the relationship between nitrogen content and carbon isotope fractionation effect, indicating that carbon isotope fractionation was affected by the change of dominant tree species and the variation of carbon turnover rate.

Silver isotope analysis of gold nuggets: An appraisal of instrumental isotope fractionation effects and potential for high-resolution tracing of placer gold

We describe a technique for high-precision analysis of 109Ag/107Ag ratios in natural and archaeological gold (1.0–120 mg/g Ag) with particular emphasis on the control of Ag yield and effects of instrumental mass bias. After dissolution of mg-sized gold samples in aqua regia and conversion to chlorides, a miniature column filled with the anion exchange resin AG@1-X8 was used for removal of Au from the sample solutions. In a second miniature column made of the Triskem TBP resin Ag was converted from the chloride to the nitrate form. Isotopic analyses on the MC-ICP-MS employed the combination of Pd-doping and standard bracketing and considering the measurements of replicate dissolutions and chromatographic separations for SRM978a and CEZAg reference solutions, the combined analytical uncertainty (2 s) of the analytical method is better than 0.016‰. A solution prepared from several gold nuggets was characterised isotopically (CEZAg with δ109Ag = 0.043 ± 0.015‰) for use as a silver-in‑gold reference material in Ag isotope studies of natural and processed gold.Results for gold nuggets from three continents indicate a wide variation in Ag concentrations (5 to 123 mg/g) and this is matched by an equally wide range in isotopic compositions (δ109Ag −0.58 to +0.83‰). This is larger than the variation found so far in in native gold from primary deposits (−0.42 to +0.5‰). Although small isotopic effects to lower δ109Ag during strong Ag loss are possible within a single grain, most analysed nuggets are internally isotopically homogeneous. This suggests preservation of the primary δ109Ag in the supergene environment and that gold was sourced from several distinct gold deposits in the catchment. Recent studies, however, indicate that even single primary deposits may be isotopically heterogeneous implying that Ag isotope fractionation is caused by numerous deposition-dissolution cycles in hydrothermal systems. Fine-grained detrital gold from three placers along the Rhine river in Germany show only small differences in δ109Ag (−0.005 ± 0.047 to +0.124 ± 0.007‰, despite being distributed along 480 km of river length. This may indicate thorough mixing of gold grain populations during transport.The small sample size required for Ag isotope work on gold opens the way for detailed micro-sampling approaches. These may be used to further examine the potential for within-grain isotopic variability related to fluviatile processing and can be used to correlate the composition of the placers with that of grains from primary sources.

High resolution, extreme isotopic variability of precipitation nitrate

Deposition of atmospheric nitrate (NO3−) in precipitation can be an important source of reactive nitrogen (N) to ecosystems, particularly in regions with high nitrogen oxide (NOx = nitric oxide (NO)+nitrogen dioxide (NO2)) emissions. However, high resolution deposition data are lacking for most systems. We conducted hourly precipitation sampling across six growing season storms in a forested area historically subjected to some of the highest levels of chronic N deposition in the United States. To characterize the influence of electricity generating unit (EGU), vehicle, and biogenic NOx emissions on NO3− deposition, we calculated the total NOx emitted from these sources within a 100 km radius of air mass back trajectories determined for Fernow Experimental Forest (West Virginia, USA). We combined these emissions estimates with established 15N isotope values for NOx sources in a three end-member mixing model to predict source-based δ15N values of deposition reaching the study site on an hourly basis. To evaluate the effect of NOx oxidation pathways on measured δ15N-NO3- values, we compared observed hourly isotope values to a coupled δ15N and Δ17O array representing N isotope exchange between atmospheric oxidized N molecules. Within individual events, δ15N, δ18O, and Δ17O values ranged by as much as 19.5‰, 28.9‰, and 13.8‰, respectively. This extreme short-term isotopic variability suggests a dynamic mix of NOx sources, oxidation pathways, and fractionation processes contributing to HNO3 formation. During every storm, precipitation δ15N-NO3- values were lower than those expected to result from predominant HNO3 formation pathways or oxidation of estimated NOx emissions along back trajectories, suggesting a systematic underestimation of NOx contributions to atmospheric HNO3 formation from isotopically depleted soil emissions. Together, these analyses represent the most comprehensive assessment to date relating high temporal resolution δ15N-NO3- observations to NOx emission sources, oxidation chemistry, and isotopic fractionation effects. We present the first observations of extreme intra-storm δ15N, δ18O, and Δ17O variability, emphasizing the need for improved constraints on soil NOx emissions, forest canopy effects, and their role in atmospheric NO3− deposition and isotope dynamics in forests.

The rapid resetting of the Ca isotopic signatures of calcite at ambient temperature during its congruent dissolution, precipitation, and at equilibrium

This study provides direct experimental evidence of the resetting of the calcium (Ca) isotope signatures of calcite in the presence of an aqueous fluid during its congruent dissolution, precipitation, and at equilibrium at ambient temperatures over week-long timescales. Batch reactor experiments were performed at 25 °C in aqueous NaCl solutions; air or CO2-gas mixtures were bubbled through this fluid to fix pH. During congruent calcite dissolution, the fluid became enriched in isotopically heavy Ca, and the Ca isotope composition continued to become heavier after the fluid attained bulk chemical equilibrium with the mineral; the δ44/42Ca composition of the fluid was up to 0.8‰ higher than the dissolving calcite at the end of the dissolution experiments. Calcite precipitation was provoked by increasing the reactor fluid pH after chemical equilibrium had been attained via dissolution. Rayleigh isotope fractionation effects were observed immediately after the pH was increased and rapid calcite precipitation occurred. However, isotopic exchange continued after the system chemically equilibrated, eradicating this Rayleigh signal. Taken together, these observations 1) confirm dynamic mineral-fluid equilibrium (i.e. dissolution and precipitation occur at equal, non-zero rates at equilibrium), and 2) indicate that isotopic compositions of calcite can readily equilibrate even when this mineral is in bulk chemical equilibrium with its coexisting fluid. This latter observation suggests the preservation of paleo-environmental isotopic signatures in calcite may require a combination of the isolation of the fluid-mineral system from external chemical input and/or the existence of a yet to be defined calcite dissolution/precipitation inhibition mechanism.

Most Earth-surface calcites precipitate out of isotopic equilibrium

Oxygen-isotope thermometry played a critical role in the rise of modern geochemistry and remains extensively used in (bio-)geoscience. Its theoretical foundations rest on the assumption that 18O/16O partitioning among water and carbonate minerals primarily reflects thermodynamic equilibrium. However, after decades of research, there is no consensus on the true equilibrium 18O/16O fractionation between calcite and water (18αcc/w). Here, we constrain the equilibrium relations linking temperature, 18αcc/w, and clumped isotopes (Δ47) based on the composition of extremely slow-growing calcites from Devils Hole and Laghetto Basso (Corchia Cave). Equilibrium 18αcc/w values are systematically ~1.5‰ greater than those in biogenic and synthetic calcite traditionally considered to approach oxygen-isotope equilibrium. We further demonstrate that subtle disequilibria also affect Δ47 in biogenic calcite. These observations provide evidence that most Earth-surface calcites fail to achieve isotopic equilibrium, highlighting the need to improve our quantitative understanding of non-equilibrium isotope fractionation effects instead of relying on phenomenological calibrations.

Carbon Isotope Fractionation in Coal and Marine Source Rocks and Implications for Exploration

Stable isotope composition of gas is widely used in hydrocarbon exploration to determine the composition and thermal maturity of source rocks. The analyses of Australian coal and marine shale samples indicate that during gas desorption both molecular and isotopic compositions change with time. Therefore, a detailed understanding of the mechanism of isotope fractionation is required to improve our ability to better characterize source rocks and fluids. δ13C of Permian coals range between -22 and -26‰ (VPDB) and that of thermogenic methane generated from these range from -25 to -40‰. δ13C of gas desorbed from coal varies with time according to molecular weight and sorption properties. For example, in a set of deep Bowen Basin coals the difference in δ13C-CH4 between early and late desorbed gas varies from 2‰ to 29‰. For higher hydrocarbons this fractionation is lower, where for ethane it is <8‰ and for propane <3‰. Similar isotope fractionation happens during desorption from marine source rocks in the Beetaloo Basin. δ13C for an immature kerogen from the Velkerri shale is -33‰. Where the Velkerri shale is gas mature, during desorption, δ13C-CH4 shows fractionation of up to 28‰. δ13C-C2H6 and δ13C-C3H6 show lower isotope fractionation of 1.6‰ and 0.9‰, respectively. Many published classification systems relating isotope composition of gas to source rock and thermal maturity do not consider the effects of such isotope fractionation. The fractionation mechanism may also have an impact on the so-called “isotope reversal” behaviour in some shale reservoirs which, at present, is poorly understood.