Stable Isotope Fractionation Research Articles

Re-assessing the effect of differential weathering of minerals on strontium stable isotope behavior during granodiorite weathering

The shift in Sr stable isotope ratios (88Sr/86Sr, reported as δ88/86Sr values) in weathering systems provides insights into the transformation of the binding form of Sr between the dissolved and solid phases. However, the mechanism behind the fractionation of Sr stable isotopes during chemical weathering remains poorly constrained. We present mineral and elemental compositions along with the radiogenic Sr isotope ratios (87Sr/86Sr) and δ88/86Sr values from a ∼ 1 m deep horizontal spheroidal weathering profile of granodiorite in southeast China. Strontium was markedly lost during granodiorite weathering and a large amount of plagioclase and hornblende was decomposed in saprolite relative to the parent rock. The 87Sr/86Sr ratios substantially increased from 0.708704 to 0.712532 and δ88/86Sr values sharply decreased from 0.29‰ to −0.12‰ in the corestone-near saprolite as the weathering intensity increased. However, these values were rather constant in the corestone-distant saprolite, albeit with a large increase in the abundance of kaolinite. The Sr in the different chemical fractions indicates that the silicate components dominate the Sr isotopic composition in bulk rock and saprolite samples. Differential weathering of minerals is mainly responsible for the Sr stable isotopic composition of the corestone-near saprolite, while the lower δ88/86Sr values of the corestone-distant saprolite point to the possibility of preferential incorporation of lighter Sr isotopes into the structural sites of secondary clay minerals. In contrast, δ88/86Sr values of the exchangeable and reducible fractions (0.11‰–0.48‰) were more positive than those of the corresponding bulk samples because they were able to inherit the isotopic signatures of the weathered minerals. Further, an acid leaching experiment on the granodioritic parent rock reveals that mineral weathering reactions dominated the Sr isotope composition of the dissolved phase. The substantially higher δ88/86Sr values in the leachates relative to those in the residual solids (Δ88/86Srleachate-residue = 0.14‰–0.42‰) are consistent with the fact that isotopically heavier Sr in the dissolved load of rivers is mostly set by differential weathering of minerals (such as biotite and plagioclase). Combined with classic indices of chemical weathering (such as the chemical depletion fraction (CDF) and chemical index of alteration (CIA)), we suggest that Sr stable isotopes offer a promising proxy of the intensity of silicate weathering in saprolite studies in which isotope fractionation caused by biological cycling is negligible.

Reactive transport of micropollutants in laboratory aquifers undergoing transient exposure periods

Groundwater quality is of increasing concern due to the ubiquitous occurrence of micropollutant mixtures. Stream-groundwater interactions near agricultural and urban areas represent an important entry pathway of micropollutants into shallow aquifers. Here, we evaluated the biotransformation of a micropollutant mixture (i.e., caffeine, metformin, atrazine, terbutryn, S-metolachlor and metalaxyl) during lateral stream water flow to adjacent groundwater. We used an integrative approach combining concentrations and transformation products (TPs) of the micropollutants, compound-specific isotope analysis (δ13C and δ15N), sequencing of 16S rRNA gene amplicons and reactive transport modeling. Duplicate laboratory aquifers (160 cm × 80 cm × 7 cm) were fed with stream water and subjected over 140 d to three successive periods of micropollutant exposures as pulse-like (6000 μg L−1) and constant (600 μg L−1) injections under steady-state conditions. Atrazine, terbutryn, S-metolachlor and metalaxyl persisted in both aquifers during all periods (<10 % attenuation). Metformin attenuation (up to 14 %) was only observed from 90 d onwards, suggesting enhanced degradation over time. In contrast, caffeine dissipated during all injection periods (>90 %), agreeing with fast degradation rates (t1/2 < 3 d) in parallel microcosm experiments and detection of TPs (theobromine and xanthine). Significant stable carbon isotope fractionation (Δδ13C ≥ 6.6 ‰) was observed for caffeine in both aquifers, whereas no enrichment in 15N occurred. A concentration dependence of caffeine biotransformation in the aquifers was further suggested by model simulations following Michaelis-Menten kinetics. Changes in bacterial community composition reflected long-term bacterial adaptation to micropollutant exposures. Altogether, the use of an integrative approach can help to understand the interplay of subsurface hydrochemistry, bacterial adaptations and micropollutants biotransformation during stream-groundwater interactions.

Below‐cloud evaporation effect on stable water isotopes in precipitation at the eastern margin of Qinghai‐Tibet Plateau

AbstractQuantitatively assessing the evaporation‐caused stable water isotope fractionation in precipitation below the cloud base is of great significance to understand the modern hydrological circulation especially in a plateau climate. Based on the hourly meteorological data at eight meteorological stations of the Gannan Plateau at the eastern margin of Qinghai‐Tibet Plateau in 2020, we examined the spatial and temporal variations of below‐cloud evaporation effect on precipitation isotopes, and then discussed the factors influencing the below‐cloud evaporation across the Gannan Plateau. The contemporary observation of precipitation isotopes at one site in the Gannan Plateau is also applied to understand the local meteoric water line associated with the below‐cloud evaporation. From 9:00 to 21:00 Beijing Time, the impact of below‐cloud evaporation is greater than that during the rest periods. Within the study region, the below‐cloud evaporation in the central area is usually stronger than that of the surrounding area. The diurnal and monthly variability of relative humidity is highly consistent with that of the remaining raindrop mass fraction after evaporation as well as the isotope variation in raindrop below the cloud base. When the relative humidity is greater than 95%, there is a linear relationship between the remaining raindrop mass fraction and the below‐cloud deuterium‐excess variation of approximate 1.1‰/%. Global warming will increase the below‐cloud evaporation effect across the Gannan Plateau. The findings are useful to understand the below‐cloud evaporation and isotope fractionation under a plateau climate.

Stable Isotope Fractionations Involving Ca–Mg Carbonates: Evaluation of the β-Factors by the “Frozen Phonon” Method

Stable Isotope Fractionations Involving Ca–Mg Carbonates: Evaluation of the β-Factors by the “Frozen Phonon” Method

Large Fractionation in Iron Isotopes Implicates Metabolic Pathways for Iron Cycling in Boreal Shield Lakes.

Stable Fe isotopes have only recently been measured in freshwater systems, mainly in meromictic lakes. Here we report the δ56Fe of dissolved, particulate, and sediment Fe in two small dimictic boreal shield headwater lakes: manipulated eutrophic Lake 227, with annual cyanobacterial blooms, and unmanipulated oligotrophic Lake 442. Within the lakes, the range in δ56Fe is large (ca. -0.9 to +1.8‰), spanning more than half the entire range of natural Earth surface samples. Two layers in the water column with distinctive δ56Fe of dissolved (dis) and particulate (spm) Fe were observed, despite differences in trophic states. In the epilimnia of both lakes, a large Δ56Fedis-spm fractionation of 0.4-1‰ between dissolved and particulate Fe was only observed during cyanobacterial blooms in Lake 227, possibly regulated by selective biological uptake of isotopically light Fe by cyanobacteria. In the anoxic layers in both lakes, upward flux from sediments dominates the dissolved Fe pool with an apparent Δ56Fedis-spm fractionation of -2.2 to -0.6‰. Large Δ56Fedis-spm and previously published metagenome sequence data suggest active Fe cycling processes in anoxic layers, such as microaerophilic Fe(II) oxidation or photoferrotrophy, could regulate biogeochemical cycling. Large fractionation of stable Fe isotopes in these lakes provides a potential tool to probe Fe cycling and the acquisition of Fe by cyanobacteria, with relevance for understanding biogeochemical cycling of Earth's early ferruginous oceans.

Fractionation of mercury stable isotopes in lichens

Bio-monitoring of mercury (Hg) in air using transplanted and in-situ lichens was conducted at three locations in Slovenia: (I) the town of Idrija in the area of the former Hg mine, where Hg contamination is well known; (II) Anhovo, a settlement with a cement production plant, which is a source of Hg contamination, and (III) Pokljuka, a part of a national park. Lichens from Pokljuka were transplanted to different sites and sampled four times—once per season, from January 2020 to February 2021. Lichens were set on tree branches, fences, and under cover, allowing them to be exposed to different environmental conditions (e.g., light and rain). The in-situ lichens were sampled at the beginning and the end of the sampling period. The highest concentrations were in the Idrija area, which was consistent with previous research. Significant mass-dependent fractionation has been observed in transplanted lichens during summer period. The δ202Hg changed from −3.0‰ in winter to −1.0‰ in summer and dropped again to the same value in winter the following year. This trend was observed in all samples, except those from the most polluted Idrija sampling site, which was in the vicinity of the former Hg ore-smelting plant. This was likely due to large amounts of Hg originating from polluted soil close to the former smelting plant with a distinct isotopic fingerprint in this local area. The Δ199Hg in transplanted lichens ranged from −0.5‰ to −0.1‰ and showed no seasonal trends. These findings imply that seasonality, particularly in summer months, may affect the isotopic fractionation of Hg and should be considered in the sampling design and data interpretation. This trend was thus described in lichens for the first time. The mechanism behind such change is not yet fully understood.

Stable Neodymium Isotope Ratios of Geological Reference Materials

Here, we report the stable Nd isotopic compositions of twenty‐seven widely available geological reference materials (RMs) including silicates, sediments, soils and carbonates. Nd was purified from geological samples using a single TODGA resin column. The Nd isotopic ratios were determined via MC‐ICP‐MS using the combined standard‐sample bracketing and internal normalisation method. The (long‐term) intermediate measurement precision of δ146/144Nd values relative to the reference material JNdi‐1 was greater than 0.030‰ (2s). Measurements of five previously analysed RMs yielded δ146/144Nd values that were consistent with those obtained using the double‐spike method. The igneous rocks showed minimal variations in their δ146/144Nd values, which ranged from ‐0.039‰ to +0.015‰. The sediments displayed large stable Nd isotopic fractionations ranging from ‐0.099‰ to +0.057‰, implying differences in their sources. The soil RMs showed a range of ‐0.093‰ to ‐0.010‰, likely owing to complex geological processes such as the degree of chemical weathering. The sedimentary rock dolomite had the heaviest δ146/144Nd values at +0.147 ± 0.032‰ (2s, n = 3). Of the analysed RMs, the δ146/144Nd values of thirteen of them are being reported for the first time. The datasets presented here should play fundamental role in quality assurance and allow for universal comparisons for stable Nd isotope systematics.

Variability and controls of stable carbon isotopic fractionation during aerobic methane oxidation in temperate lakes

The aerobic oxidation of methane (CH4) by methanotrophic bacteria (MOB) is the major sink of this highly potent greenhouse gas in freshwater environments. Yet, CH4 oxidation is one of the largest uncertain components in predicting the current and future CH4 emissions from these systems. While stable carbon isotopic mass balance is a powerful approach to estimate the extent of CH4 oxidation in situ, its applicability is constrained by the need of a reliable isotopic fractionation factor (αox), which depicts the slower reaction of the heavier stable isotope (13C) during CH4 oxidation. Here we explored the natural variability and the controls of αox across the water column of six temperate lakes using experimental incubation of unamended water samples at different temperatures. We found a large variability of αox (1.004–1.038) with a systematic increase from the surface to the deep layers of lake water columns. Moreover, αox was strongly positively coupled to the abundance of MOB in the γ-proteobacteria class (γ-MOB), which in turn correlated to the concentrations of oxygen and CH4, and to the rates of CH4 oxidation. To enable the applicability in future isotopic mass balance studies, we further developed a general model to predict αox using routinely measured limnological variables. By applying this model to δ13C-CH4 profiles obtained from the study lakes, we show that using a constant αox value in isotopic mass balances can largely misrepresent and undermine patterns of the extent of CH4 oxidation in lakes. Our αox model thus contributes towards more reliable estimations of stable carbon isotope-based quantification of CH4 oxidation and may help to elucidate large scale patterns and drivers of the oxidation-driven mitigation of CH4 emission from lakes.

Quantifying Isotopologue Reaction Networks (QIRN): A modelling tool for predicting stable isotope fractionations in complex networks

Natural-abundance stable isotope compositions are powerful tools for understanding complex processes across myriad scientific disciplines. However, quantitative interpretation of these signals often requires equally complex models. Previous stable isotope models have treated isotopic compositions as intrinsic properties of molecules or atoms (e.g. δ13C, 13R, etc.). This has proven to be a computationally efficient but inflexible approach. Here, we present a new isotope modelling software tool that combines computational strategies used in metabolic modeling with an understanding of natural isotope fractionations from the geosciences, called Quantifying Isotopologue Reaction Networks (QIRN, “churn”). QIRN treats isotopic properties as distributions of discrete isotopologues, i.e. molecules with different numbers and distributions of isotopic substitutions. This approach is remarkably generalizable and computationally tractable, enabling models of reaction networks with unprecedented complexity. QIRN parameterizes reactions as rate law equations with distinct isotopologues as the reactants and products. Isotope effects are implemented as small changes to the relevant isotopologues’ rate constants. Running this model forward in time gives the numerical solution for steady state isotopologue abundances. Different subsets of the isotopologue population can then be sampled to quantify numerous isotopic proprieties simultaneously (i.e. compound-specific, site-specific, and multiply-substituted isotope compositions). Furthermore, QIRN can model any physical, chemical or biological process as reversible or irreversible. As such, it incorporates both kinetic and equilibrium isotope effects. It can be readily applied to any isotope system (i.e. C, N, O, etc.), though at present can only track two isotopes of one element at a time. Given its generalizability, QIRN has a diverse range of applications. To demonstrate the flexibility and efficiency of QIRN, we reconstructed previous (intrinsic-property) models of sulfate reduction, abiotic amino acid synthesis, lipid biosynthesis, and photosynthesis. In these examples, QIRN consistently reproduced outputs from prior models and predicted isotopic anomalies that have been measured in nature. With its new approach to isotope modelling, QIRN will expand the potential complexity of modelled reaction networks, help predict isotopic signals that can direct experimental efforts, and provide a more efficient means of modeling emerging isotopic properties such as ‘clumped isotopes’.

Cr stable isotope fractionation by evaporation from silicate melts

We present new experimental results of Cr isotope fractionation during degassing of silicate melts. Our experiments at oxidizing conditions (in air) at 1 bar total pressure show that evaporative loss of Cr is substantial and depends on run duration and on temperature (between 1350 and 1500 °C). The stable Cr (δ53Cr) isotope compositions of the evaporation residues were analyzed by thermal ionization mass spectrometry using the double spike method. Our results indicate that during degassing the lighter isotope 52Cr is enriched in the gas phase and consequently the residual melt becomes enriched in the heavier 53Cr isotope in a mass-dependent fashion. The kinetic isotopic fractionation factor, α, derived from our experimental data is 0.9995 ± 0.0001, which is significantly different from the fractionation factor predicted for an ideal gas composed of CrO3, αkin = 0.995. Our experimentally-determined value of α is consistent with diffusive transport of CrO3 (g) away from the melt surface, proportional to the inverse cube-root of the reduced masses of the two isotopologues. We hypothesize that Cr loss by evaporation may be recorded by Cr isotopes in terrestrial magmas with sufficiently high oxygen fugacities erupting near the surface, which are plausibly found in lava lakes or erupting arc magmas.

Large contribution of fossil-derived components to aqueous secondary organic aerosols in China

Incomplete understanding of the sources of secondary organic aerosol (SOA) leads to large uncertainty in both air quality management and in climate change assessment. Chemical reactions occurring in the atmospheric aqueous phase represent an important source of SOA mass, yet, the effects of anthropogenic emissions on the aqueous SOA (aqSOA) are not well constrained. Here we use compound-specific dual-carbon isotopic fingerprints (δ13C and Δ14C) of dominant aqSOA molecules, such as oxalic acid, to track the precursor sources and formation mechanisms of aqSOA. Substantial stable carbon isotope fractionation of aqSOA molecules provides robust evidence for extensive aqueous-phase processing. Contrary to the paradigm that these aqSOA compounds are largely biogenic, radiocarbon-based source apportionments show that fossil precursors produced over one-half of the aqSOA molecules. Large fractions of fossil-derived aqSOA contribute substantially to the total water-soluble organic aerosol load and hence impact projections of both air quality and anthropogenic radiative forcing. Our findings reveal the importance of fossil emissions for aqSOA with effects on climate and air quality.

Influence of Sub-Cloud Secondary Evaporation and Moisture Sources on Stable Isotopes of Precipitation in Shiyang River Basin, Northwest China

Fractionation of stable isotopes in precipitation runs through the water cycle, and deuterium excess is a second-order parameter linking water-stable oxygen and hydrogen isotopes. It is strongly influenced by under-cloud evaporation in unsaturated air, especially in arid climates. Based on the improved Stewart model, this study used 670 precipitation stable isotope data and measured meteorological element data from 11 sampling points from January 2018 to September 2019 to verify the existence of sub-cloud secondary evaporation in the Shiyang river basin and quantitatively calculate the intensity of sub-cloud secondary evaporation and its influence on precipitation stable isotopes. The study used the vapor flux and the improved Lagrangian model to track the moisture source of precipitation and analyze the influence of the moisture source of different paths on the stable isotopes of precipitation. Therefore, this study is helpful to understand the evapotranspiration loss mechanism and recharge mechanism of moisture in the watershed. The results showed that there is sub-cloud secondary evaporation in the Shiyang River Basin, and from the seasonal scale, the sub-cloud secondary evaporation is stronger in spring and summer, but weaker in autumn and winter, which makes heavy isotopes enriched in spring and summer and depleted in autumn and winter. From the perspective of spatial distribution, the sub-cloud secondary evaporation is stronger in the midstream and downstream of the Shiyang river, resulting in more enrichment of heavy isotopes. In the vertical direction, the sub-cloud secondary evaporation at 850–700 hPa is the strongest, which enriches the heavy isotope in this layer and reduces the deuterium excess. In addition, the main moisture source of precipitation in the Shiyang River Basin is the westerly air mass, and the mid and high-latitude land sources contribute more moisture to the precipitation. However, the supply of the sea source is very limited, which makes the deuterium excess of precipitation higher and does not show regional consistency and seasonality well.

Stable carbon and hydrogen isotope fractionation of volatile organic compounds caused by vapor-liquid equilibrium

Several types of laboratory experiments were conducted to evaluate isotope fractionation caused by phase transfer process for a selection of common environmental contaminants. Carbon and hydrogen isotope fractionation caused by vaporization of non-aqueous phase liquid (NAPL), by volatilization from water and by dissolution into an organic solvent (tetraethylene glycol dimethylether or TGDE) under equilibrium conditions was investigated with closed system experimental setups to isolate the air-liquid partitioning process. A selection of aromatic, aliphatic and chlorinated compounds along with one fuel oxygenate (methyl tert-butyl ether or MTBE) were evaluated to determine isotope enrichment factor related to respective phase transfer process. During NAPL vaporization, the residual mass of aromatic compounds, aliphatic compounds and MTBE became progressively depleted in heavy carbon and hydrogen isotopes. In contrast, during volatilization from water, the residual mass of aromatic compounds and MTBE dissolved in the water became progressively enriched in heavy hydrogen isotopes, whereas no significant change in carbon isotope was observed, except for MTBE showing a significant depletion. For the air-TGDE partitioning process, most of the aromatic compounds tested led to no significant carbon (except ethylbenzene) or hydrogen (except toluene and o-xylene) isotope fractionation. In contrast, significant carbon isotope fractionation was observed for aliphatic and chlorinated compounds and hydrogen isotope fractionation for aliphatic compounds, and are comparable to progressive NAPL vaporization in direction and magnitude. The isotope fractionation factors determined in this study are key for interpreting the change in isotope ratios when assessing the fate of gas-phase VOCs present in the soil air or when gas-phase VOCs are sampled using TGDE as the sink matrix. The results of this study contribute to expand the list of common environmental contaminants that can be assessed by the compound-specific isotope analysis (CSIA) method deployed in the frame of gas-phase studies.

Isotopic Composition and Fractionation of Stable Magnesium Isotopes in Relation to Fruits Growing in Different Regions of Russia

Background: Present study on determining the region of origin of raw materials, the authenticity of organic food products is relevant. The determination of the natural content of stable isotopes are not able to answer these questions. The solution can be obtained by studying the ratios and fractionation of stable isotopes, which provide information on the transformations of chemical elements in biological, ecological and geochemical studies. Methods: During 2020-2021 obtained data on the content of magnesium isotopes in apple fruits of 14 varieties from three growing regions of the Russian Federation. Previously, over 10 years, a database was formed of more than 500 samples on the level of potassium, calcium, magnesium, phosphorus and nitrogen cations, depending on the variety and phase of fruit development. Result: Research presents the results of determining the natural content of stable magnesium isotopes in apples, the isotopic composition and their fixation, depending on different regions of the growth of raw materials. The isotopic composition of apples (26Mg/24Mg) varies from 0.1669047 to 0.1782649; the deviation of the magnesium isotopic composition from the conventional standard varies within 25.52-43.52; their chemical fractionation has value: in Central Zone of Russia-185.2-212.3; South of Russia-248.0-311.5; North Caucasus-182.7-214.8.

Ca-isotopes as a robust tracer of magmatic differentiation

The large mass difference (∼10%) between the two most abundant isotopes of calcium, 40Ca and 44Ca, gives Ca great potential in tracking mass-dependent fractionation during magmatic processes. Resolvable Ca-isotope fractionation during fractional crystallization of magma, particularly by feldspar in evolved melts, has been theoretically inferred but not robustly tested in nature. To further explore the effects of magmatic differentiation on Ca-isotope systematics, we studied the late-Permian alkaline igneous suite of the Øyangen Caldera, Oslo Rift, Norway, consisting of volcanic and intrusive units ranging from basanitic to rhyolitic compositions. Major and trace element variations and modeling demonstrate that the main series of samples (N=21), including basanites, ring-dyke syenites, and central-dome syenites, likely documents a co-genetic and closed-system fractional crystallization sequence. Our data show minimal δ44/40Ca variation (<0.05‰) in the intermediate magma and a marked increase in δ44/40Ca in the felsic magma of the Øyangen Caldera (from 0.62±0.02‰ to 1.15±0.03‰ relative to Ca standard, SRM915a). The systematic increase is best explained by equilibrium isotopic fractionation dominated by alkali feldspar in the fractionating mineral assemblage. This is further supported by strong correlations between δ44/40Ca, CaO, and Eu/Eu* in the main-series samples. Implementing a Monte Carlo approach, isotopic modeling of the liquid line of descent using Rayleigh fractionation is highly consistent with the observed Ca-isotope evolution. For the first time, we confirm prominent Ca stable isotope fractionation in felsic-stage differentiation of alkaline magma and constrain the isotope fractionation factors of plagioclase and K-feldspar. Integrated with extant estimations on mineral fractionation factors from the literature, our results suggest increasing fractionation effects of rock-forming minerals with decreasing Ca content. The affirmation of significant Ca-isotope fractionation in alkaline magma by feldspar empowers the application of Ca as a versatile tracer of crustal evolution, allowing further tests in other magmatic conditions across various planetary objects.

Responses of Tree Growth and Intrinsic Water Use Efficiency to Environmental Factors in Central and Northern China in the Context of Global Warming

The Loess Plateau is a fragile ecological zone that is sensitive to climate change. The response, adaptation, and feedback of tree growth in forest ecosystems to global warming and CO2 enrichment are urgent scientific issues. Intrinsic water use efficiency (iWUE) is an important indicator for understanding forest ecosystem adaptability to climate change and CO2 enrichment. In this study, tree-ring width, tree-ring stable carbon isotope ratio (δ13C), and iWUE of P. tabulaeformis Carr. were established. Climate response analysis showed that temperature was the main limiting factor affecting radial tree growth and that relative humidity significantly affected the stable carbon isotope fractionation of tree rings. During 1645–2011, the iWUE increased by 27.1%. The responses of iWUE to climate factors and atmospheric CO2 concentrations (Ca) showed that the long-term variation in iWUE was affected by Ca, which could explain 69% of iWUE variation, and temperature was the main factor causing iWUE interannual variation. The ecosystem of P. tabulaeformis showed a positive response to rising Ca, as its carbon sequestration capacity increased. In response to global warming and CO2 enrichment, rising Ca promoted increases in iWUE but ultimately failed to offset the negative impact of warming on tree growth in the study area.

Redox-dependent Ti stable isotope fractionation on the Moon: implications for current lunar magma ocean models

In terrestrial magmas titanium is predominantly tetravalent (Ti4+), in contrast, lunar magmas are more reduced (IW-1) and hence approximately 10% of their bulk Ti content is trivalent (Ti3+). Changes in oxidation state and coordination number are both important parameters that can serve to drive Ti stable isotope fractionation. As such, mineral–mineral and mineral-melt Ti stable isotope fractionation factors determined for terrestrial samples may not be appropriate for lunar samples that formed under more reducing conditions. To address this issue, several experiments were carried out in gas mixing furnaces over a range of fO2 (air to IW-1) to determine Ti stable isotope fractionation factors for minerals, such as ilmenite, clinopyroxene and rutile that are highly abundant on the Moon. Results show that the extent of Ti stable isotope fractionation significantly increases with decreasing fO2. For example, the isotopic difference between ilmenite and residual melt (Δ49Tiilmenite-melt) is resolvably lower by ~ 0.44 ‰ from terrestrial-like FMQ-0.5 to lunar-like IW-1 at an intermediate precision of ± 0.003 ‰ (95% c.i. OL–Ti). This confirms that fractionation factors determined for terrestrial conditions are indeed not applicable to lunar settings. Our new fractionation factors for ilmenite, clinopyroxene and silicate melt are mostly consistent with those previously determined by ab initio modelling based on density-functional theory. Using our new experimental data in conjunction with previously published high-precision HFSE data and Ti stable isotope data of lunar basalts, we modelled the solidification of the Lunar Magma Ocean (LMO). The model for LMO solidification included fractionation of Ti stable isotopes not only by Ti-oxides, but also by typical lunar silicate minerals as pyroxene or olivine. The resulting δ49Ti for urKREEP and ilmenite-bearing cumulates are within error of previous estimates, but also indicate that ilmenite-bearing cumulates must have contained around 15% ilmenite.

Physiological control on carbon isotope fractionation in marine phytoplankton

Abstract. One of the great challenges in biogeochemical research over the past half a century has been to quantify and understand the mechanisms underlying stable carbon isotope fractionation (εp) in phytoplankton in response to changing CO2 concentrations. This interest is partly grounded in the use of fossil photosynthetic organism remains as a proxy for past atmospheric CO2 levels. Phytoplankton organic carbon is depleted in 13C compared to its source because of kinetic fractionation by the enzyme RubisCO during photosynthetic carbon fixation, as well as through physiological pathways upstream of RubisCO. Moreover, other factors such as nutrient limitation, variations in light regime as well as phytoplankton culturing systems and inorganic carbon manipulation approaches may confound the influence of aquatic CO2 concentrations [CO2] on εp. Here, based on experimental data compiled from the literature, we assess which underlying physiological processes cause the observed differences in εp for various phytoplankton groups in response to C-demand/C-supply, i.e., particulate organic carbon (POC) production / [CO2]) and test potential confounding factors. Culturing approaches and methods of carbonate chemistry manipulation were found to best explain the differences in εp between studies, although day length was an important predictor for εp in haptophytes. Extrapolating results from culturing experiments to natural environments and for proxy applications therefore require caution, and it should be carefully considered whether culture methods and experimental conditions are representative of natural environments.

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

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

Insights into amino acid fractionation and incorporation by compound-specific carbon isotope analysis of three-spined sticklebacks

Interpretation of stable isotope data is of upmost importance in ecology to build sound models for the study of animal diets, migration patterns and physiology. However, our understanding of stable isotope fractionation and incorporation into consumer tissues is still limited. We therefore measured the δ13C values of individual amino acids over time from muscle and liver tissue of three-spined sticklebacks (Gasterosteus aculeatus) on a high protein diet. The δ13C values of amino acids in the liver quickly responded to small shifts of under ± 2.0‰ in dietary stable isotope compositions on 30-day intervals. We found on average no trophic fractionation in pooled essential (muscle, liver) and non-essential (muscle) amino acids. Negative Δδ13C values of − 0.7 ± 1.3‰ were observed for pooled non-essential (liver) amino acids and might indicate biosynthesis from small amounts of dietary lipids. Trophic fractionation of individual amino acids is reported and discussed, including unusual Δδ13C values of over + 4.9 ± 1.4‰ for histidine. Arginine and lysine showed the lowest trophic fractionation on individual sampling days and might be useful proxies for dietary sources on short time scales. We suggest further investigations using isotopically enriched materials to facilitate the correct interpretation of ecological field data.