Apparent Isotope Fractionation Research Articles

Identification of processes in Cu-ore heap leaching using Cu isotopes and leachate chemistry at Tschudi mine, northern Namibia

Copper isotopic fractionation (in δ65Cu) and leachate characterization were studied in the context of heap leaching at the Tschudi copper mine in northern Namibia. The leached solution is of Mg-SO4 type with high Al and Fe concentrations. The source of Mg and Al in the leachate can be from the alteration of micas such as Mg-bearing muscovite confirmed by X-ray diffraction and scanning electron microscopy; however, the source of Mg cannot be determined with certainty. The principal secondary minerals identified in the leached ore are gypsum and jarosite. The value of pH in leachate is ∼1.21 and the concentration of dissolved Cu, occurring mostly as CuSO40 and Cu2+, is about 2 g/L. In comparison with unleached ore (avg. δ65Cu −1.47 ‰), leached ore exhibits lighter isotopic values (avg. δ65Cu −6.01 ‰) with apparent isotopic fractionation Δ65Culeached ore-unleached ore of about −4.54 ‰. In contrast, there is isotopic enrichment of leachate in heavier 65Cu isotope (leachate δ65Cu 0.34 ‰) with apparent isotopic fractionation Δ65Culeachate-unleached ore value of +1.81 ‰. These results are in good agreement with Cu isotopic fractionation and depletion in heavier 65Cu isotope reported for dissolution experiments in laboratory and groundwater linked to the porphyry copper ore deposits around the world. The leaching of heaps can be considered an analogy of upper part of gossans, but here the supergene enrichment zone is missing due to extremely low pH and oxidizing conditions.

Tissue-variation of iron stable isotopes in marine fish coupled with speciation analysis using X-ray absorption fine structure

The Fe stable isotope ratio (δ56Fe) in tissues is a potential parameter for examining the Fe metabolism in marine fish. Although the effect of ferritin storage has been proposed as a possible cause of heavy isotope (56Fe) enrichment in the liver, no speciation and stable isotope ratio coupling data are available. Here, we report the δ56Fe values measured by multicollector ICP-MS and the result of Fe K-edge X-ray absorption near-edge structure (XANES) analysis of multiple tissues obtained from a skipjack tuna (Katsuwonus pelamis) and a chub mackerel (Scomber japonicus). Apparent isotopic fractionation between the liver and the muscle samples (Δ56FeL–M = δ56Feliver − δ56Femuscle) from these species was observed (0.85 ‰ and 0.57 ‰, respectively). The dominant Fe species in the muscle was heme Fe (the sum of methemoglobin, oxyhemoglobin, and deoxyhemoglobin), while ferritin was not detected according to the linear combination fitting of the XANES spectra. In the liver, ferritin contribution was ca. 28 %–54 % of the total Fe content. The apparent difference in δ56Fe between heme Fe and ferritin was estimated to range from 1.41 ‰ to 1.52 ‰ based on the tissue-specific δ56Fe values and the XANES results. These results indicate that the Fe storage as ferritin does not induce the lowering of δ56Fe in the muscle, considering the low contribution of the liver Fe to the total Fe content in the body.

Potassium isotope fractionation during granitic magmatic differentiation: Mineral-pair perspectives

To constrain the behavior of K isotopes during granitic magmatic differentiation, we present high-precision K isotope data for bulk granitoids and their K-bearing minerals from Dabie and Himalayan orogens in China. Plagioclase displays extremely large K isotopic variation, with δ41K ranging from −0.56 to 1.91‰. Other minerals also have heterogeneous K isotopic compositions, with δ41K varying from −0.56 to −0.24‰ in hornblende, from −0.72 to −0.39‰ in biotite, from −0.79 to −0.47‰ in K-feldspar, and from −0.74 to −0.61‰ in muscovite. The general trend of 41K enrichment follows the order of plagioclase ≫ hornblende > biotite ≈ K-feldspar ≥ muscovite, which can be attributed to the difference in KO bond strengths. Both δ41K of plagioclase and K isotope fractionation factors between plagioclase and other minerals are negatively correlated with K concentration in plagioclase, suggesting a compositional control on isotope fractionations. Inter-mineral K isotope fractionations among other minerals are roughly temperature-dependent. Potassium isotopic compositions of whole rocks display measurable variations (δ41K = –0.69 to −0.28‰), which are correlated with proxies for plagioclase fractionation (e.g., Sr and Rb/Sr). The correlation indicates that fractional crystallization may account for the whole-rock K isotopic variations observed in high-silica igneous rocks, which is supported by modeling of fractional crystallization using the apparent isotope fractionation factors determined in this work. Our study demonstrates the existence of large K isotope fractionation during granitic magmatism, making K isotopes a potential tool for studying differentiation of felsic magmas.

Silicon Isotope Signatures of Radiolaria Reveal Taxon-Specific Differences in Isotope Fractionation

The global silicon (Si) cycle plays a critical role in regulating the biological pump and the carbon cycle in the oceans. A promising tool to reconstruct past dissolved silicic acid (DSi) concentrations is the silicon isotope signature of radiolaria (δ30Sirad), siliceous zooplankton that dwells at subsurface and intermediate water depths. However, to date, only a few studies on sediment δ30Sirad records are available. To investigate its applicability as a paleo proxy, we compare the δ30Sirad of different radiolarian taxa and mixed radiolarian samples from surface sediments off Peru to the DSi distribution and its δ30Si signatures (δ30SiDSi) along the coast between the equator and 15°S. Three different radiolarian taxa were selected according to their specific habitat depths of 0–50 m (Acrosphaera murrayana), 50–100 m (Dictyocoryne profunda/truncatum), and 200–400 m (Stylochlamydium venustum). Additionally, samples containing a mix of species from the bulk assemblage covering habitat depths of 0 to 400 m have been analyzed for comparison. We find distinct δ30Sirad mean values of +0.70 ± 0.17‰ (Acro; 2 SD), +1.61 ± 0.20 ‰ (Dictyo), +1.19 ± 0.31 ‰ (Stylo) and +1.04 ± 0.19 ‰ (mixed radiolaria). The δ30Si values of all individual taxa and the mixed radiolarian samples indicate a significant (p < 0.05) inverse relationship with DSi concentrations of their corresponding habitat depths. However, only δ30Si of A. murrayana are correlated to DSi concentrations under normally prevailing upwelling conditions. The δ30Si of Dictyocoryne sp., Stylochlamydium sp., and mixed radiolaria are significantly correlated to the lower DSi concentrations either associated with nutrient depletion or shallower habitat depths. Furthermore, we calculated the apparent Si isotope fractionation between radiolaria and DSi (Δ30Si ∼ 30ε = δ 30Sirad − δ 30SiDSi) and obtained values of −1.18 ± 0.17 ‰ (Acro), −0.05 ± 0.25 ‰ (Dictyo), −0.34 ± 0.27 ‰ (Stylo), and −0.62 ± 0.26 ‰ (mixed radiolaria). The significant differences in Δ30Si between the order of Nassellaria (A. murrayana) and Spumellaria (Dictyocoryne sp. and Stylochlamydium sp.) may be explained by order-specific Si isotope fractionation during DSi uptake, similar to species-specific fractionation observed for diatoms. Overall, our study provides information on the taxon-specific fractionation factor between radiolaria and seawater and highlights the importance of taxonomic identification and separation to interpret down-core records.

Stratospheric carbon isotope fractionation and tropospheric histories of CFC-11, CFC-12, and CFC-113 isotopologues

Abstract. We present novel measurements of the carbon isotope composition of CFC-11 (CCl3F), CFC-12 (CCl2F2), and CFC-113 (CF2ClCFCl2), three atmospheric trace gases that are important for both stratospheric ozone depletion and global warming. These measurements were carried out on air samples collected in the stratosphere – the main sink region for these gases – and on air extracted from deep polar firn snow. We quantify, for the first time, the apparent isotopic fractionation, ϵapp(13C), for these gases as they are destroyed in the high- and mid-latitude stratosphere: ϵapp(CFC-12, high-latitude) =(-20.2±4.4) ‰, and ϵapp(CFC-113, high-latitude) =(-9.4±4.4) ‰, ϵapp(CFC-12, mid-latitude) =(-30.3±10.7) ‰, and ϵapp(CFC-113, mid-latitude) =(-34.4±9.8) ‰. Our CFC-11 measurements were not sufficient to calculate ϵapp(CFC-11), so we instead used previously reported photolytic fractionation for CFC-11 and CFC-12 to scale our ϵapp(CFC-12), resulting in ϵapp(CFC-11, high-latitude) =(-7.8±1.7) ‰ and ϵapp(CFC-11, mid-latitude) =(-11.7±4.2) ‰. Measurements of firn air were used to construct histories of the tropospheric isotopic composition, δT(13C), for CFC-11 (1950s to 2009), CFC-12 (1950s to 2009), and CFC-113 (1970s to 2009), with δT(13C) increasing for each gas. We used ϵapp(high-latitude), which was derived from more data, and a constant isotopic composition of emissions, δE(13C), to model δT(13C, CFC-11), δT(13C, CFC-12), and δT(13C, CFC-113). For CFC-11 and CFC-12, modelled δT(13C) was consistent with measured δT(13C) for the entire period covered by the measurements, suggesting that no dramatic change in δE(13C, CFC-11) or δE(13C, CFC-12) has occurred since the 1950s. For CFC-113, our modelled δT(13C, CFC-113) did not agree with our measurements earlier than 1980. This discrepancy may be indicative of a change in δE(13C, CFC-113). However, this conclusion is based largely on a single sample and only just significant outside the 95 % confidence interval. Therefore more work is needed to independently verify this temporal trend in the global tropospheric 13C isotopic composition of CFC-113. Our modelling predicts increasing δT(13C, CFC-11), δT(13C, CFC-12), and δT(13C, CFC-113) into the future. We investigated the effect of recently reported new CFC-11 emissions on background δT(13C, CFC-11) by fixing model emissions after 2012 and comparing δT(13C, CFC-11) in this scenario to the model base case. The difference in δT(13C, CFC-11) between these scenarios was 1.4 ‰ in 2050. This difference is smaller than our model uncertainty envelope and would therefore require improved modelling and measurement precision as well as better quantified isotopic source compositions to detect.

Geochemical modeling of carbon isotope fractionation during methane transport in tight sedimentary rocks

Methane transportation in tight sedimentary rocks results in significant and complex isotope fractionation. Existing models and mechanistic studies of isotope fractionation cannot fully explain the actual observations in gas transport processes that occur in natural reservoirs and in the laboratory. Thus, geological applications based on transport-related isotope fractionation have been lacking in substantive progress. Here, we established a multi-scale model in which seepage in fractures, diffusion and adsorption/desorption in matrix pores, and concentration diffusion in kerogen structural pores are coupled. The results show that both diffusion and adsorption/desorption lead to significant isotope fractionation, while the contribution of pressure-driven seepage, is limited. The diffusion of kerogen-dissolved gas is of considerable significance to isotope fractionation in the later stage of transport, despite its weak contribution to gas production. The sequence of controlling factors of isotope fractionation during pure diffusion is as follows: diffusion coefficient ratio (D⁎/D) > initial pressure (P0) > others, where the value of D⁎/D mainly depends on the average pore size of rocks. The isotope fractionation caused by adsorption/desorption is closely related to the Langmuir parameters of sedimentary rock. It is essentially a dynamic non-equilibrium fractionation during adsorbed gas transport, rather than thermodynamic equilibrium fractionation between adsorbed gas and free gas. The model developed herein determines the contribution of each single effect to the apparent isotope fractionation and provides a novel method for obtaining the Langmuir parameters of rocks under in-situ geological conditions. Under variable boundary conditions, this model can be used to evaluate the gas resource potential and recoverable reserves in unconventional reservoirs and demonstrates the great potential for monitoring the production status of shale gas wells.

Cadmium isotope fractionation during Cd-calcite coprecipitation: Insight from batch experiment

Cadmium (Cd) calcite coprecipitation experiments were conducted to constrain the possible Cd isotope fractionation mechanism under different conditions. The changes in Cd/Ca molar ratio, HCO3− concentration, addition of Mg2+, Na2EDTA, or ionic strength (INaNO3) did not affect the composition of the precipitation but resulted in substantial change in mineral morphology. Cd exhibited an apparent isotope fractionation in the process of coprecipitation with calcite, and the fractional coefficient αCaCO3-Cd(aq) was less than one, indicating that the solution is preferentially enriched in heavy isotopes. The Cd mainly existed in the form of Cd(H2O)62+ in the solution before the reaction and was dominated by Cd(NO3)2, CdNO3+, Cd(EDTA)2− at the end of the experiments. The different isotopic ratios of Cd between the liquid phase and the solid phase can be explained by changes in Cd speciation which have different bond lengths of CdO. Expect for the experiments with Na2EDTA and high ionic strength, the measured isotope ratios under different conditions plotted around the theoretical Rayleigh fractionation curve. This demonstrates that Cd isotopes almost reach equilibrium with Cd-calcite coprecipitation under the different experiment conditions and the Cd isotope fractionation mechanism is closely related to the thermodynamic isotope fractionation. In the experiment with Na2EDTA and high ionic strength, the formation of Cd(EDTA)2− and ion occlusion at surface sites inhibit Cd2+ entering the calcite and the isotopes of the solid and liquid phases do not reach equilibrium. As the result, obtained experiment data shift from the Rayleigh fractionation curve indicating the Cd isotope fractionation mechanism is closely related to the kinetic isotope effect. In general, the study results indicate that the Cd/Ca molar ratio, ionic strength, HCO3−, Na2EDTA, and Mg2+ concentrations of the solution can cause Cd isotope fractionation in different degrees during Cd-calcite coprecipitation. Cd isotope ratios might potentially provide important information on Cd mobilization and transportation in groundwater environment.

Theoretical estimates of equilibrium carbon and hydrogen isotope effects in microbial methane production and anaerobic oxidation of methane

Microbial production and consumption of methane are widespread in natural and artificial environments, with important economic and climatic implications. Attempts to use the isotopic composition of methane to identify its sources are complicated by incomplete understanding of the mechanisms of variation in methane’s isotopic composition. Knowledge of the equilibrium isotope fractionations among the large organic intracellular intermediates in the microbial pathways of methane production and consumption must form the basis of any exploration of the mechanisms of isotopic variation, but estimates of these equilibrium isotope fractionations are currently unavailable. To address this gap, we calculated the equilibrium isotopic fractionation of carbon (13C/12C) and hydrogen (D/H) isotopes among compounds in the anaerobic methane metabolisms, as well as the abundance of double isotope substitutions (“clumping,” i.e., a single 13C–D bond or two 12C–D bonds) in these compounds. The isotope fractionation factors were calculated using density functional theory at the M06-L/def2-TZVP level of theory with the SMD implicit solvation model, which we recently tested against measured equilibrium isotope fractionations. The computed 13β and 2β values decrease with decreasing oxidation state of the carbon atom in the molecules, resulting in a preference for enrichment in 13C and D in the molecules with more oxidized carbon. Using the computed β values, we calculated the equilibrium isotope fractionation factors in the prominent methanogenesis pathways (hydrogenotrophic, methylotrophic and acetoclastic) and in the pathway for anaerobic oxidation of methane (AOM) over a temperature range of 0–700 °C. Our calculated equilibrium fractionation factors compare favorably with experimental constraints, where available, and were then used to investigate the relation between the apparent isotope fractionation during methanogenesis or AOM and the thermodynamic drive for these reactions. We show that a detailed map of the equilibrium fractionation factors along these metabolic pathways allows for an evaluation of the contribution of equilibrium and kinetic isotope effects to apparent isotope fractionations observed in laboratory, natural and artificial settings. The comprehensive set of equilibrium isotope fractionation factors calculated in this study provides a firm basis for future explorations of isotope effects in methane metabolism.

Silicon isotope variations in hydrothermal systems at Yellowstone National Park, Wyoming, U.S.A.

Silicon (Si) isotopic compositions of authigenic Si-bearing materials can be powerful tracers of geological Si cycling and reconstruction of Earth’s climate. Interpretations of Si isotope data, however, are complicated by an inadequate understanding of natural Si isotopic fractionation pathways. This study investigated Si isotope fractionation between hydrothermal fluids and amorphous silica sinters of Cistern Spring and Deerbone Spring at Yellowstone National Park, USA, with a particular focus on spatial variations along spring flowpaths. Modeling of geochemical data indicate that the fluids at Cistern Spring were supersaturated with respect to amorphous silica whereas the fluids at Deerbone Spring were undersaturated due to lower SiO2 content and a more alkaline pH. The δ30Si values of the spring waters for both sites exhibit small variations, whereas the associated amorphous silica sinters displayed a considerable range and generally decreased downflow (from − 2.25 to −5.04‰ for Cistern and from −0.11 to −0.81‰ for Deerbone). The magnitude of apparent isotopic fractionation between solid and aqueous silica (Δ30Sisolid-fluid) was significantly larger for the samples from Cistern Spring than for Deerbone Spring or similar hot spring systems at Geysir geothermal field in Iceland. The Δ30Sisolid-fluid values for Cistern Spring ranged from −2.40 to −5.49‰ along the flowpath and correlated inversely with the rate of silica precipitation, indicating a process dominated by kinetic isotope fractionation. In contrast, the magnitude of Si isotopic fractionation was significantly smaller at Deerbone Spring (+0.21 to −0.64‰). A portion of the opaline sinters at Deerbone may have formed cryogenically under freezing winter conditions that altered silica saturation and formation pathway. Rapid isotope exchange with silica undersaturated hydrothermal fluids may have erased initial kinetic isotope effects. Re-equilibration between opal-A and fluids was strongly affected by isotopic fractionation between aqueous silica species and fluid ionic strengths. These results document both kinetic and equilibrium Si isotope fractionation from field observations and highlight the pivotal role of fluid-rock interaction on the preservation of Si isotope signatures in the rock record.

Isotopic evidence for alteration of nitrous oxide emissions and producing pathways' contribution under nitrifying conditions

Abstract. Nitrous oxide (N2O) emissions from a nitrifying biofilm reactor were investigated with N2O isotopocules. The nitrogen isotopomer site preference of N2O (15N-SP) indicated the contribution of producing and consuming pathways in response to changes in oxygenation level (from 0 % to 21 % O2 in the gas mix), temperature (from 13.5 to 22.3 ∘C) and ammonium concentrations (from 6.2 to 62.1 mg N L−1). Nitrite reduction, either nitrifier denitrification or heterotrophic denitrification, was the main N2O-producing pathway under the tested conditions. Difference between oxidative and reductive rates of nitrite consumption was discussed in relation to NO2- concentrations and N2O emissions. Hence, nitrite oxidation rates seem to decrease as compared to ammonium oxidation rates at temperatures above 20 ∘C and under oxygen-depleted atmosphere, increasing N2O production by the nitrite reduction pathway. Below 20 ∘C, a difference in temperature sensitivity between hydroxylamine and ammonium oxidation rates is most likely responsible for an increase in N2O production via the hydroxylamine oxidation pathway (nitrification). A negative correlation between the reaction kinetics and the apparent isotope fractionation was additionally shown from the variations of δ15N and δ18O values of N2O produced from ammonium. The approach and results obtained here, for a nitrifying biofilm reactor under variable environmental conditions, should allow for application and extrapolation of N2O emissions from other systems such as lakes, soils and sediments.

Evaluation of bacterial glycerol dialkyl glycerol tetraether and <sup>2</sup>H–<sup>18</sup>O biomarker proxies along a central European topsoil transect

Abstract. Molecular fossils, like bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs), and the stable isotopic composition of biomarkers, such as δ2H of leaf wax-derived n-alkanes (δ2Hn-alkane) or δ18O of hemicellulose-derived sugars (δ18Osugar), are increasingly used for the reconstruction of past climate and environmental conditions. Plant-derived δ2Hn-alkane and δ18Osugar values record the isotopic composition of plant source water (δ2Hsource-water and δ18Osource-water), which usually reflects mean annual precipitation (δ2Hprecipiation and δ18Oprecipiation), modulated by evapotranspirative leaf water enrichment and biosynthetic fractionation (εbio). Accuracy and precision of respective proxies should be ideally evaluated at a regional scale. For this study, we analysed topsoils below coniferous and deciduous forests as well as grassland soils along a central European transect in order to investigate the variability and robustness of various proxies and to identify effects related to vegetation. Soil pH values derived from brGDGTs correlate reasonably well with measured soil pH values but are systematically overestimated (ΔpH = 0.6±0.6). The branched vs. isoprenoid tetraether index (BIT) can give some indication whether the pH reconstruction is reliable. Temperatures derived from brGDGTs overestimate mean annual air temperatures slightly (ΔTMA=0.5 ∘C ± 2.4). Apparent isotopic fractionation (εn-alkane/precipitation and εsugar∕precipitation) is lower for grassland sites than for forest sites due to signal damping; i.e. grass biomarkers do not record the full evapotranspirative leaf water enrichment. Coupling δ2Hn-alkane with δ18Osugar allows us to reconstruct the stable isotopic composition of the source water more accurately than without the coupled approach (Δδ2H = ∼-21 ‰ ± 22 ‰ and Δδ18O = ∼-2.9 ‰ ± 2.8 ‰). Similarly, relative humidity during daytime and the vegetation period (RHMDV) can be reconstructed using the coupled isotope approach (ΔRHMDV=∼-17±12). Especially for coniferous sites, reconstructed RHMDV values as well as source water isotope composition underestimate the measured values. This can likely be explained by understorey grass vegetation at the coniferous sites contributing significantly to the n-alkane pool but only marginally to the sugar pool in the topsoils. Vegetation-dependent variable signal damping and εbio (regarding 2H between n-alkanes and leaf water) along our European transect are difficult to quantify but likely contribute to the observed underestimation in the source water isotope composition and RH reconstructions. Microclimate variability could cause the rather large uncertainties. Vegetation-related effects do, by contrast, not affect the brGDGT-derived reconstructions. Overall, GDGTs and the coupled δ2Hn-alkane–δ18Osugar approach have great potential for more quantitative paleoclimate reconstructions.

Nitrogen and Oxygen Isotope Signatures of Nitrogen Compounds during Anammox in the Laboratory and a Wastewater Treatment Plant.

Isotopic fractionation factors against 15N and 18O during anammox (anaerobic ammonia oxidization by nitrite) are critical for evaluating the importance of this process in natural environments. We performed batch incubation experiments with an anammox-dominated biomass to investigate nitrogen (N) and oxygen (O) isotopic fractionation factors during anammox and also examined apparent isotope fractionation factors during anammox in an actual wastewater treatment plant. We conducted one incubation experiment with high δ18O of water to investigate the effects of water δ18O. The N isotopic fractionation factors estimated from incubation experiments and the wastewater treatment plant were similar to previous values. We also found that the N isotopic effect (15εNXR of –77.8 to –65.9‰ and 15ΔNXR of –31.3 to –30.4‰) and possibly O isotopic effect (18εNXR of –20.6‰) for anaerobic nitrite oxidation to nitrate were inverse. We applied the estimated isotopic fractionation factors to the ordinary differential equation model to clarify whether anammox induces deviations in the δ18O vs δ15N of nitrate from a linear trajectory of 1, similar to heterotrophic denitrification. Although this deviation has been attributed to nitrite oxidation, the O isotopic fractionation factor for anammox is crucial for obtaining a more detailed understanding of the mechanisms controlling this deviation. In our model, anammox induced the trajectory of the δ18O vs δ15N of nitrate during denitrification to less than one, which strongly indicates that this deviation is evidence of nitrite oxidation by anammox under denitrifying conditions.

Impact of atrazine concentration on bioavailability and apparent isotope fractionation in Gram-negative Rhizobium sp. CX-Z

Compound-specific stable isotope analysis of micropollutants has become an established method for the qualitative and quantitative assessment of biodegradation in the field. However, many of environmental factors may have an influence on the observed isotope fractionation. Herein, we investigate the impact of substrate concentration on the observed enrichment factor derived from Rayleigh plot of batch laboratory experiments conducted to measure the atrazine carbon isotope fractionation of Rhizobium sp. CX-Z subjected to the different initial concentration level of atrazine. The Rayleigh plot (changes in bulk concentration vs. isotopic composition) derived from batch experiments shown divergence from the linear relation towards the end of degradation, confirming bioavailability of atrazine changed along with the decay of substrate concentration, consequently, influenced the isotope fractionation and lowered the observed enrichment factor. When microbial degradation is coupled to a mass transfer step limiting the bioavailability of substrate, the observed enrichment factor displays a dependence on initial atrazine concentration. Observed enrichment factors (ε) (absolute value) derived from the low concentration (i.e. 9.5 μM) are below 3.5‰ to the value of −5.4‰ determined at high bioavailability (membrane-free cells). The observed enrichment factor depended significantly on the atrazine concentration, indicating the concentration level and the bioavailability of a substrate in realistic environments should be considered during the assessment of microbial degradation or in situ bioremediation based on compound-specific stable isotope analysis (CSIA) method.

A novel framework for interpreting pyrite-based Fe isotope records of the past

Abstract The variation in the iron isotopic composition (δ56Fe) of sedimentary pyrite is often interpreted to reflect the degree of Fe redox cycling in modern and ancient environments. However, the degree to which precipitation pathways, isotopic exchange, and precipitation rates can affect the isotopic fractionation associated with pyrite precipitation from aqueous Fe(II) (Fe(II)aq) is poorly understood. In this study, pyrite is precipitated at 80 °C in batch reactors through the H2S and polysulfide pathways, in which the precipitation rates and the concurrent formation of a greigite (Fe3S4) phase is modulated by the amount of initially added elemental sulfur and aqueous molybdenum. Our results indicate an average apparent isotopic fractionation (δ56Fepyrite - δ56FeFeSx, where FeSx includes FeS, Fe(II)aq, and greigite) of −0.51 ± 0.22‰ throughout the experiments irrespective of precipitation pathways and greigite formation. Early-stage precipitation is associated with ∼0.3‰ larger isotopic fractionation than late-stage precipitation, possibly indicating either a rate-dependent kinetic isotope effect (KIE) or a different isotopic fractionation factor for early-stage pyrite nucleation compared to later-stage growth. Overall, the magnitude of the apparent isotopic fractionation is significantly smaller than the

Calcium isotopic fractionation during adsorption onto and desorption from soil phyllosilicates (kaolinite, montmorillonite and muscovite)

Abstract Mineral soil constituents play an important role in the storage/release of nutrients due to their ability to adsorb and desorb cations. Phyllosilicate minerals are particularly reactive due to their small size and high specific surface area (up to ∼800 m2·g−1). Previous work has highlighted the potential of Ca isotopes (δ44/40Ca) to identify secondary processes occurring within soils. Nevertheless, the mechanisms of isotopic fractionation (amplitude and nature) associated with Ca adsorption onto and desorption from different phyllosilicate minerals that are commonly found in soil remain poorly understood. This step is fundamental to improve our understanding of the Ca biogeochemical cycle at the water-soil-plant interface. Consequently, the study of the possible Ca isotopic fractionation during adsorption/desorption phenomena was approached experimentally using three “model” substrates representative of the phyllosilicate minerals frequently encountered within soils (KGa-2 kaolinite, Swy-2 montmorillonite and Tuftane muscovite). The experiments were carried out under abiotic conditions in “water-mineral” batch systems by precisely controlling the physico-chemical conditions: pH, solid/solution ratio, initial dissolved Ca concentration, particle size distribution, reaction time and other cations concentrations. Our results show no significant isotopic fractionation associated with Ca adsorption or desorption during KGa-2 and coarse size fraction of Tuftane muscovite (50–200 µm) experiments regardless of the physico-chemical parameters used. During Ca adsorption onto Swy-2 and fine size fraction of Tuftane muscovite (0.1–1 µm), light isotopes (e.g., 40Ca) are preferentially adsorbed to the clay mineral, and a positive apparent isotopic fractionation between 0.10‰ and 0.28‰ is measured in the supernatant recovered after adsorption experiments. Kinetic and thermodynamic isotopic fractionation are observed during the Swy-2 experiments while only a thermodynamic fractionation took place during the Tuftane muscovite (0.1–1 µm) experiments. The results obtained for the Ca desorption experiments performed with a chloride hexaamine cobalt solution suggest that Ca adsorption and the associated isotopic fractionation are fully reversible. Our results show that the Ca isotopic fractionation intensity during adsorption phenomena onto clay minerals is controlled by the layer charge and specific surface area of the considered phyllosilicate mineral, as well as the presence of an interlayer space open to cationic solute.

Effect of continuous light on leaf wax isotope ratios in Betula nana and Eriophorum vaginatum: implications for Arctic paleoclimate reconstructions

Reconstructions of climate using leaf wax D/H ratios (δDwax) require accounting for the apparent isotopic fractionation (εapp) between plant source water and waxes. There have been conflicting publications on whether plants in the Arctic growing under 24-hour continuous light, fractionate less than temperate and tropical plants. In this study, we examine the effect of diurnal light (DL) versus 24-hour continuous light (CL) on the isotopic composition of leaf n-alkanes and n-acids in greenhouse experiments using two common Arctic plants (Eriophorum vaginatum, or tussock cottongrass and Betula nana, or dwarf birch). For E. vaginatum, the δDwax values of various wax homologues were 5–11‰ more positive for CL plants relative to their DL counterparts, whereas for B. nana, CL waxes were 3–24‰ more negative, suggesting that daylight length is not a unifying control on leaf wax D/H ratios of Arctic plants. The δ13Cwax of B. nana was more negative for plants grown in continuous light compared to diurnal light, reflecting lower water-use efficiency, associated with prolonged stomatal opening in the CL treatment. We modeled the impact of increasing stomatal conductance and effective flow path lengths (mimicking variable leaf morphologies) on the isotopic composition of leaf waters (δDlw) and find that variations in leaf-water enrichment may explain the variable δDwax responses seen between E. vaginatum and B. nana. We suggest that between-species differences in the δDlw response to light, and differences in the utilization of stored carbohydrates, were important for governing δDwax. Our greenhouse results suggest that Arctic plant leaf waxes do not consistently display reduced εapp values as a result of 24-hour day light, providing additional support for field observations.

Using isotopes to trace freshly applied cadmium through mineral phosphorus fertilization in soil-fertilizer-plant systems

Applications of mineral phosphorus (P) fertilizer can lead to cadmium (Cd) accumulation in soils and can increase Cd concentrations in edible crop parts. To determine the fate of freshly applied Cd, a Cd source tracing experiment was conducted in three soil-fertilizer-wheat systems by using a mineral P fertilizer labeled with the radio isotope 109Cd and by exploiting natural differences in Cd stable isotope compositions (δ114/110Cd). Source tracing with stable isotopes overestimated the proportion of Cd in plants derived from the P fertilizer, because the isotope ratios of the sources were not sufficiently distinct from those of the soils. Despite indistinguishable extractable Cd pools between control and treatments, the addition of P fertilizer resulted in a more negative apparent isotope fractionation between soil and wheat. Overall, the radio isotope approach provided more robust results and revealed that 6.5 to 15% of the Cd in the shoot derived from the fertilizer. From the introduced Cd, a maximum of 2.2% reached the wheat shoots, whilst 97.8% remained in the roots and soils. The low recoveries of the fertilizer derived Cd suggest that continuous P fertilizer application in the past decades can lead to a build-up of a residual Cd pool in soils.

Conductive Particles Enable Syntrophic Acetate Oxidation between Geobacter and Methanosarcina from Coastal Sediments

ABSTRACTCoastal sediments are rich in conductive particles, possibly affecting microbial processes for which acetate is a central intermediate. In the methanogenic zone, acetate is consumed by methanogens and/or syntrophic acetate-oxidizing (SAO) consortia. SAO consortia live under extreme thermodynamic pressure, and their survival depends on successful partnership. Here, we demonstrate that conductive particles enable the partnership between SAO bacteria (i.e., Geobacter spp.) and methanogens (Methanosarcina spp.) from the coastal sediments of the Bothnian Bay of the Baltic Sea. Baltic methanogenic sediments were rich in conductive minerals, had an apparent isotopic fractionation characteristic of CO2-reductive methanogenesis, and were inhabited by Geobacter and Methanosarcina. As long as conductive particles were delivered, Geobacter and Methanosarcina persisted, whereas exclusion of conductive particles led to the extinction of Geobacter. Baltic Geobacter did not establish a direct electric contact with Methanosarcina, necessitating conductive particles as electrical conduits. Within SAO consortia, Geobacter was an efficient [13C]acetate utilizer, accounting for 82% of the assimilation and 27% of the breakdown of acetate. Geobacter benefits from the association with the methanogen, because in the absence of an electron acceptor it can use Methanosarcina as a terminal electron sink. Consequently, inhibition of methanogenesis constrained the SAO activity of Geobacter as well. A potential benefit for Methanosarcina partnering with Geobacter is that together they competitively exclude acetoclastic methanogens like Methanothrix from an environment rich in conductive particles. Conductive particle-mediated SAO could explain the abundance of acetate oxidizers like Geobacter in the methanogenic zone of sediments where no electron acceptors other than CO2 are available.

Drought Sensitivity of the Carbon Isotope Composition of Leaf Dark-Respired CO2 in C3 (Leymus chinensis) and C4 (Chloris virgata and Hemarthria altissima) Grasses in Northeast China.

Whether photosynthetic pathway differences exist in the amplitude of nighttime variations in the carbon isotope composition of leaf dark-respired CO2 (δ13Cl) and respiratory apparent isotope fractionation relative to biomass (ΔR,biomass) in response to drought stress is unclear. These differences, if present, would be important for the partitioning of C3-C4 mixed ecosystem C fluxes. We measured δ13Cl, the δ13C of biomass and of potential respiratory substrates and leaf gas exchange in one C3 (Leymus chinensis) and two C4 (Chloris virgata and Hemarthria altissima) grasses during a manipulated drought period. For all studied grasses, δ13Cl decreased from 21:00 to 03:00 h. The magnitude of the nighttime shift in δ13Cl decreased with increasing drought stress. The δ13Cl values were correlated with the δ13C of respiratory substrates, whereas the magnitude of the nighttime shift in δ13Cl strongly depended on the daytime carbon assimilation rate and the range of nighttime variations in the respiratory substrate content. The ΔR,biomass in the C3 and C4 grasses varied in opposite directions with the intensification of the drought stress. The contribution of C4 plant-associated carbon flux is likely to be overestimated if carbon isotope signatures are used for the partitioning of ecosystem carbon exchange and the δ13C of biomass is used as a substitute for leaf dark-respired CO2. The detected drought sensitivities in δ13Cl and differences in respiratory apparent isotope fractionation between C3 and C4 grasses have marked implications for isotope partitioning studies at the ecosystem level.

Lithium isotopes and implications on chemical weathering in the catchment of Lake Donggi Cona, northeastern Tibetan Plateau

This study presents lithium (Li) isotope ratios (δ7Li) for rocks, sediments, suspended particulate material, and dissolved Li from the Lake Donggi Cona catchment, located on the northeastern Tibetan Plateau, China. The average δ7Li=+1.9‰ of the bedrocks is estimated from local loess. δ7Li values decrease progressively within the sediment cascade from loess, to river and lake floor sediments. The lake floor sediments average at −0.7‰. The difference between bedrock and lake sediments reflects the preferential fractionation of dissolved 6Li into clay minerals (mostly illite) in the weathering zone and grain-size sorting during fluvial sediment transport. The δ7Li values of stream and lake water samples range from +13.6 to +20.8‰, whereas thermal waters fall between +5.9 and +11.6‰. The δ7Li values of lake water samples are close to +17‰ and reflect mixing of waters from two perennial inflows and thermal waters. Dissolved Li in streams represents an integrated isotopic signal derived from soil solutions in the weathering zone. An apparent isotopic fractionation of −17.8±1.6‰ (αsec-sol∼0.982) between secondary minerals and solution was determined. An inflow that drains a sub-catchment in the north carries a high proportion of thermal waters. Despite of the high proportion of admixed thermal waters with high Li concentrations and low δ7Li, this stream has the highest δ7Li values of about +21‰. This is consistent with admixing of thermal waters to solutions in the weathering zone and subsequent fractionation by preferential uptake of isotopically light dissolved Li into secondary phases. Based on Li isotope ratios of the dissolved and solid export flux from the weathering zone we calculated that around five times more Li is exported in particles than dissolved in streams. An average δ7Li value of about +17‰ of most streams and the lake is reflecting a low weathering intensity and chemical weathering rate of about 4t/km2/a. Low weathering rates and an erosion dominated weathering system are consistent with moderate precipitations, the cold climate, and the high relief of the study area.