Isotope Fractionation Factors Research Articles

Ca isotopic gradients in epeiric marine carbonates: Diagenetic origins of and significance for Ca cycle reconstructions

Marine carbonates record geochemical information on the state of Earth’s surface environment in the geological past, but recrystallization during burial can make the records difficult to interpret. Here, we report shore to basin gradients in δ44Ca values in matrix limestone and burrow dolomite across a sequence of Late Ordovician (Katian Stage) burrow-mottled carbonates in the Williston Basin, an intracratonic epeiric marine basin in North America. Both gradients are interpreted to have diagenetic origins. The gradient in limestone δ44Ca values developed in a bathymetrically controlled sediment- to fluid-buffered diagenetic system operating between the center and the edges of the basin. Two dolomitizing events formed the gradient in dolomite δ44Ca values. The sediment-buffered end of the limestone gradient preserves the δ44Ca value of the original carbonate mud (∼–1.28 ± 0.10 ‰). The fluid-buffered end of the gradient conserves the δ44Ca value of contemporaneous seawater (–0.47 ± 0.10 ‰), because: (1) pore water exchange drivers operated with greater variety and intensity in shallow water settings, (2) shallow nearshore deposits exchanged more Ca with seawater during recrystallization to diagenetic calcite than deeper offshore deposits, and (3) diagenetic calcite formed with negligible fractionation under equilibrium conditions. The gradient conserves the local value of Δsed (–0.81 ± 0.20 ‰) despite its diagenetic origin, and the magnitude of Δsed is consistent with primary calcite precipitation in a ‘calcite sea’. Field assessment of the equilibrium Ca isotope fractionation factor between calcite and dolomite of –0.41 ± 0.17 ‰ at ambient temperature is applied to correct basin-centered dolomite for fractionation, yielding –1.62 ± 0.27 ‰, which is very different from the –0.25 ± 0.07 ‰ value of Ca in evaporatively concentrated seawater from which the dolomite originally formed. Applying the same fractionation factor to correct literature data on early diagenetic dolomite in the Great Basin, North America, yields –0.69 ± 0.22 ‰ for the δ44Ca value of seawater, which is ∼0.2 ‰ lower than ‘normal marine’ waters in stratigraphically equivalent deposits in the Willison Basin, and 0.44 ‰ lower than evaporatively concentrated seawater during a brief period of basin restriction and anhydrite deposition. The nearly 0.5 ‰ variation in the δ44Ca value of Katian seawater falls within the range of scatter in the brachiopod reconstruction of seawater Ca isotopes. Here, we propose that the true ocean record of secular change lies towards the bottom of the range of scatter, and that the higher values in the record reflect local Ca cycling and circulation restriction in epeiric seas. Epeiric seas cultivate the fluid-buffered diagenetic conditions required to promote Ca isotope exchange between reactive carbonate sediments and seawater through carbonate mineral dissolution/recrystallization, crystal coarsening by way of Ostwald ripening, and replacive dolomitization. It is not the apportioning of kinetically fractionated aragonite and calcite in the carbonate deposition flux that drove secular changes in global value of Δsed between ‘calcite’ and ‘aragonite’ seas, but rather the replacement of kinetic fractionations with equilibrium fractionations during fluid-buffered diagenesis with seawater.

Experimental and theoretical constraints on lithium isotope fractionation during brine evaporation and halite precipitation

Marine evaporitic halite, precipitating from the advanced stage of extremely evaporated seawater, is the most common and abundant evaporitic mineral. The extent to which lithium (Li) is incorporated into evaporitic halite during geological periods of massive halite deposition, and the mechanisms of incorporation, are both unknown. These are each important, however, for the quantification of the isotope effect of the evaporitic sink on δ7Liseawater, and for the potential use of halite as a recorder of seawater δ7Li. Here, new experimental data are presented for the Li fractionation factor for halite from an isothermal evaporation experiment (25∘C for 100 days) using marine-derived brine. The resultant solids were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to confirm the mineralogy and crystallinity of the product. Our results show that Li isotopes are not constant during the evaporation process implying that there must be a process that fractionates Li isotopes during halite precipitation. We suggest that this results from Li ionic substitution for sodium (Na) in crystalline halite, a scenario supported by ab-initio calculations, but we also show that Li within halite is predominantly present within fluid inclusions. Thus, the Li isotopic composition of halite is controlled by a mixture of Li within the fluid inclusions and Li that is incorporated into the halite crystal. Overall, the brine becomes enriched in Li7 during evaporation due to the preferential incorporation of Li6 into chloride precipitates, and the evolution of δ7Librine is controlled by the precipitated mineral assemblage. The theoretical equilibrium fractionation factor 1000lnα(mineral−fluid) is -7.8▪ for Li-halite (LiNa26Cl27) at 25 C∘ at equilibrium, which is at the lower limit of Li isotope fractionation factors. Rayleigh fractionation models were fitted to the experimental data yielding a fractionation factor of -25 ▪ 3▪. The difference in Li isotope fractionation factor between brine and structurally bound Li may result from kinetic fractionation effects. This study presents a scenario highlighting that extensive halite deposits,formed over geological time in environments with relatively high Li concentration, could potentially impact the δ7Li values of seawater during that period, and place constraints on their utility as an archive for δ7Liseawater.

Isotope fractionation of alkaline and alkaline-earth elements (Li, K, Rb, Mg, Ca, Sr, Ba) during diffusion in aqueous solutions

In this study, we used an improved “diffusion cell” method to precisely determine the diffusion-driven kinetic isotope fractionation factors of the Li, K, Rb, Mg, Ca, Sr, and Ba cations in aqueous solutions under room temperature. The obtained isotope fractionation factors (±2σ errors) are, α7/6Li = 0.996139 ± 0.000140, α41/39K = 0.998572 ± 0.000072, α87/85Rb = 0.999333 ± 0.000020, α26/24Mg = 0.999877 ± 0.000010, α44/42Ca = 0.999704 ± 0.000010, α88/86Sr = 0.999781 ± 0.000014, α138/135Ba = 0.999716 ± 0.000018. The results show that the charge of the cation and the ion-water bond length for the aquo ions are the two predominant factors affecting the mass dependence of isotope fractionation (β factor) during cation diffusion in aqueous solutions. Cations with higher charge numbers and shorter ion-water bond lengths exhibit less kinetic isotope fractionation during diffusion. Therefore, the isotope separation effect during diffusion (or β factor) in fluids is fundamentally controlled by the intensity of ion-water interaction. Weaker ion-water interaction (e.g., lower charge number, longer ion-water bond length) leads to less prominent hydrodynamic behavior for diffusing ions at the molecular level, thus more significant isotope fractionation in bulk solutions, and vice versa. Ions of larger radius would show stronger mass dependence of isotope fractionation (β factor), which can cancel the effect of decreasing relative isotope mass difference for heavier elements, thus kinetic isotope fractionation during diffusion in aqueous solutions remains prominent even for heavy elements such as Rb, Sr, and Ba. The diffusion-driven kinetic isotope fractionation factors measured in this study could provide a useful basis for interpreting specific natural isotopic variability of alkaline and alkaline-earth elements in supergene environments where chemical diffusion takes place.

Geochemical evidence for biodegradation in high-rank coals from Qinshui Basin, North China

Understanding the generation of secondary microbial methane (SMM) is important for the evaluation of natural gas resources and instructive for the stimulation of methane production. Coal seams are popular targets for extracting in situ preserved methane and studying microbe-stimulated methane yield. However, few studies have been done on overmature coals. Here we collected gas samples from coals varying from bituminous to meta-anthracite in the Qinshui Basin, North China, and analysed the molecular and stable isotopic compositions to systematically evaluate the influence of biodegradation in high-rank coals in geological settings. The stable isotope signatures (δ13C and δD) of methane are dominated by the thermal decomposition of organic matter in deep coals but inconsistent with the maturity rank of shallow burial coals. The decoupling of coal maturity with C1/C2+ ratios and δ13C-CH4 values, and positive δ13C-CO2 values (−9.2 to +24.4 ‰) suggest biodegradation of light wet gases (C2+ components) and CO2 reduction. Negative trends between δ13C-CO2 and CH4/CO2 in shallow burial coal seams reveal the mutual conversion of CH4 and CO2 and carbon isotope exchange, driven by microorganisms. The calculated isotopic temperatures (33–328 °C) based on the carbon isotope fractionation factors between CH4 and CO2 (1.024–1.069) demonstrate that carbon isotope exchange is prevalent in high-rank coals. It also reveals that the burial depth is an imperative factor in controlling microbial environments and thus the biodegradation process. This study implicates the potential of high-rank coals as the target for microbial-enhanced methane recovery and also implies that microorganisms are widely involved in reservoir carbon cycling.

Oxygen Isotope Fractionation between Carbonate Minerals and Carbonic Acid Systems and Constraints for Environmental Science and Geological Processes

The equilibrium oxygen isotope fractionation factor is widely used in geological thermometry. However, under most natural conditions, the oxygen isotope exchange is rare to reach equilibrium. Especially for the complex water–rock interaction process, the contribution of the H2CO3 solution, CO32− solution, Ca(HCO3)2 solution, and CaCO3 solution to the equilibrium oxygen isotope fractionation factor of this process is poorly understood. In view of this predicament, these key parameters are obtained by ab initio calculations. The results showed that the contributions of different carbonate minerals and different aqueous solutions to the equilibrium oxygen isotope fractionation factor were different. Among all nine carbonate minerals (dolomite, calcite, aragonite, magnesite, siderite, otavite, smithsonite, ankerite, and strontianite), the minerals with the highest and lowest reduced partition function ratios (RPFR) were siderite and strontianite, respectively. At the same time, the RPFR of nitratine, which has the same structure as carbonate, was studied. The RPFRs of the three most widely distributed carbonates in nature (dolomite, calcite, and aragonite) were dolomite > calcite > aragonite. Among the H2CO3 solution, CO32− solution, Ca(HCO3)2 solution, and CaCO3 solution, the H2CO3 solution had the strongest ability to enrich 18O. In addition, the equilibrium oxygen isotope fractionation factors between aqueous solutions and gas phase species (CO2(g), H2O(g), and O2(g), etc.) were calculated systematically. The results showed that the oxygen isotope fractionation factors between solutions and gas phases were often inconsistent with the temperature change direction and that the kinetic effects played a key role. These theoretical parameters obtained in this study will provide key equilibrium oxygen isotope constraints for water-rock interaction processes.

Calcium isotope fractionation by intracellular amorphous calcium carbonate (ACC) forming cyanobacteria.

The formation of intracellular amorphous calcium carbonate (ACC) by various cyanobacteria is a widespread biomineralization process, yet its mechanism and importance in past and modern environments remain to be fully comprehended. This study explores whether calcium (Ca) isotope fractionation, linked to ACC-forming cyanobacteria, can serve as a reliable tracer for detecting these microorganisms in modern and ancient settings. Accordingly, we measured stable Ca isotope fractionation during Ca uptake by the intracellular ACC-forming cyanobacterium Cyanothece sp. PCC 7425. Our results show that Cyanothece sp. PCC 7425 cells are enriched in lighter Ca isotopes relative to the solution. This finding is consistent with the kinetic isotope effects observed in the Ca isotope fractionation during biogenic carbonate formation by marine calcifying organisms. The Ca isotope composition of Cyanothece sp. PCC 7425 was accurately modeled using a Rayleigh fractionation model, resulting in a Ca isotope fractionation factor (Δ44Ca) equal to -0.72 ± 0.05‰. Numerical modeling suggests that Ca uptake by these cyanobacteria is primarily unidirectional, with minimal back reaction observed over the duration of the experiment. Finally, we compared our Δ44Ca values with those of other biotic and abiotic carbonates, revealing similarities with organisms that form biogenic calcite. These similarities raise questions about the effectiveness of using the Ca isotope fractionation factor as a univocal tracer of ACC-forming cyanobacteria in the environment. We propose that the use of Δ44Ca in combination with other proposed tracers of ACC-forming cyanobacteria such as Ba and Sr isotope fractionation factors and/or elevated Ba/Ca and Sr/Ca ratios may provide a more reliable approach.

Antimony isotope fractionation during Sb(V) and Sb(III) adsorption on secondary Fe-minerals (schwertmannite, ferrihydrite) typical of mine waters

The application of antimony (Sb) isotopes as tracers for Sb environmental cycling is currently limited. Indeed, there is a lack of knowledge of isotope fractionation factors associated with key (bio-) geochemical processes controlling its behaviour in surface environments. This study investigated the equilibrium isotope fractionation generated by Sb(V) and Sb(III) sorption on two iron minerals typical of acid mine drainage (AMD) impacted streams, ferrihydrite and schwertmannite, under controlled conditions of pH and solid to liquid ratio. Sorption behaviour and Sb isotope fractionation were similar for the different mineral phases and Sb oxidation degrees, with fractionation factors Δ123Sbsolid-solution of −0.25 ± 0.08 ‰ for Sb(III) and −0.34 ± 0.08 ‰ for Sb(V) adsorbed on ferrihydrite and −0.36 ± 0.06 ‰ for Sb(III) and −0.27 ± 0.03 ‰ for Sb(V) adsorbed on schwertmannite. The pH and initial Fe:Sb ratio did not significantly affect the Δ123Sbsolid-solution value under the experimental conditions. The light 121Sb isotope was preferentially adsorbed on the mineral phases, following an equilibrium closed system between dissolved and adsorbed Sb species. This fractionation may be related to the apparition of iron as the second closest neighbour which distorts the Sb(OH)3 or Sb(OH)6- atomic polyhedron. This study confirms that Sb equilibrium isotope fractionation occurs during sorption of Sb(III) and Sb(V) onto secondary iron minerals and suggests that the pH and redox of Sb do not exert significant effect. pH and redox conditions are important parameters which control Sb mobility in AMD streams, therefore, the study provides data for interpreting Sb isotope signatures in mine waters.

Zinc isotope composition of enameloid, bone and muscle of gilt-head seabreams (Sparus aurata) raised in pisciculture and their relation to diet

The isotope ratios of zinc (66Zn/64Zn expressed as δ66Zn), a vital nutrient, increasingly demonstrate trophic discrimination among vertebrates, making δ66Zn a valuable dietary proxy for ecological, archaeological, and palaeontological studies. Given the novelty of the methodology, tissue-diet and tissue-tissue zinc isotope fractionation factors remain poorly understood and have so far only been studied in a few terrestrial mammals. Here, we investigate δ66Zn compositions of enameloid, bone, and white muscle of seven artificially-fed pisciculture gilt-head seabreams (Sparus aurata) from offshore Israel, in comparison to the Zn isotope composition of their diet. In addition, we also analysed δ66Zn values in the same tissues of wild-caught S. aurata, bluespotted seabream (Pagrus caeruleostictus) and grey triggerfish (Balistes capriscus) caught off the coast of Israel. We determine a tissue-diet δ66Zn offset for Sparus aurata of − 0.04 ± 0.09 ‰ (2SD) for bone, − 0.29 ± 0.06 ‰ (2SD) for enameloid, and − 0.45 ± 0.07 ‰ (2SD) for white muscle. Wild-caught fish have much higher among-individual δ66Zn variability with values distinct from the pisciculture S. aurata, documenting a much more isotopically heterogeneous diet consumed by the wild individuals. Still, tissue–tissue δ66Zn differences in wild-caught individuals are close to those observed in the pisciculture ones with progressively lower δ66Zn values in the order bone > enameloid > white muscle. Our results demonstrate predictable tissue-diet and tissue-tissue δ66Zn differences among fish hard and soft tissues and can be applied to identify the δ66Zn values of dietary input, thereby informing trophic (palaeo)ecological reconstructions.

Silicon isotopes in an Archaean migmatite confirm seawater silicification of TTG sources

Unraveling ancient melting processes is key to understanding how the earliest, tonalite-trondhjemite-granodiorite (TTG)-dominated continental crust formed from partial melting of amphibolite. Application of silicon isotope analysis to ancient crust reveals that Archaean TTGs exhibit consistently high Si isotope values (δ30Si) compared to modern granitoids, attributed to seawater-derived silica introduced by either (a) partial melting of variably silicified basalts or (b) assimilation of authigenic silica-rich marine lithologies in the melt source. However, both mechanisms can introduce highly variable δ30Si, conflicting with the strikingly consistent δ30Si compositions of Archaean TTGs. This study investigates an alternative model, whereby the distinct mineralogy and chemistry of TTG melt sources impart a distinct silicon isotope composition to the melt, compared with “modern” granitoids. We measured δ30Si in component parts (melanosome and leucosome) of an Archaean (2.7 Ga) mafic migmatite and coeval amphibolites and mafic granulites from the Kapuskasing uplift, Canada, to explore how Si isotopes fractionate during incipient TTG melt formation. Our data reveal leucosome (i.e., melt) exhibits consistently high δ30Si values compared to a relatively isotopically lighter melanosome (i.e., residuum). We also derive inter-mineral silicon isotope fractionation factors for mineral separates that agree well with those of ab initio estimates for the same minerals and show that the magnitude of equilibrium fractionation between TTG source rock and melt replicates that in Phanerozoic granitoids. We conclude the effects of magmatic differentiation on δ30Si have remained consistent throughout Earth history, meaning that Archaean TTGs must require a source isotopically heavier than unaltered basalt, as reflected by our amphibolites and mafic migmatite components. The consistently heavy δ30Si of seawater through Earth history, and the high SiO2 content of amphibolites relative to coeval leucosome-free granulites in our study area, imply seawater silicification is the source of the observed high δ30Si. Thus, the consistently heavy Si isotope compositions measured in Archaean melt products define a unique aspect of ancient crust formation: that of the silicification of TTG source rock, implying the intrinsic involvement of a primeval hydrosphere.

Oxygen isotopic fractionation during dissolved oxygen consumption in the bottom layer of the Ulleung Basin, East/Japan Sea

The interpretation of decline in dissolved oxygen (DO) in the oxygenated bottom water of the Ulleung Basin (UB), southwest of the East/Japan Sea has been challenging because of the integrated influence of various DO-consuming processes. Therefore, the stable oxygen isotopic fractionation of DO was investigated to enhance our understanding of the distinct DO consumption observed in the bottom layers of the center of the UB. We explored the relationship between DO and its oxygen isotope composition (δ18ODO) using data collected at a station located in the center of the UB in 2020, 2021, and 2022. An unforeseen decrease in δ18ODO in the bottom layer (> 1800 m) where DO was depleted was discovered. The overall DO consumption in the mesopelagic water layer (300–1000 m), primarily attributed to water column respiration, exhibited an isotopic fractionation factor (α) with 0.985 ± 0.001 in the δ18ODO/[O2] relationship. The consumptive isotope fractionation factor in the bottom waters near the sediments (approximately 2146 m) showed a value slightly higher (0.988 ± 0.002) than that in the mesopelagic water layer. This isotopic signature is likely due to a smaller fractionation in the bottom waters relative to the mesopelagic water. The isotopic evidence suggests the involvement of mineral oxidation associated with excess dissolved Mn and Fe in the bottom waters because mineral oxidation exhibits a smaller fractionation effect than respiration. Our study demonstrates that DO depletion results from multiple consumption processes, including respiration, mineral oxidation, and diffusive transport, and the isotopic behavior provides evidence that mineral oxidation significantly influences DO consumption.

Holocene climate regulates multiple sulfur isotope compositions of pyrite in the East China sea via sedimentation rate

Marine authigenic pyrite sulfur isotope (δ34Spyr) is one of the most important proxies for reconstructing remarkable changes of Earth's surface in deep time. Recent observations of unexpectedly large δ34Spyr variations in Quaternary sediments complicate the application of this isotopic proxy, and a mechanistic understanding of such variabilities is needed. In this study, we measured multiple sulfur isotopic compositions of pyrite in well dated sediments spanning the last 7500 years from the East China Sea (ECS) inner shelf to study the mechanism how Holocene climate controls pyrite sulfur isotopic compositions. We found that variabilities of pyrite multiple sulfur isotopes, δ34Spyr and Δ33Spyr (ranging from −38.3‰ to 2.3‰ and from −0.01‰ to 0.21‰, respectively) are significantly correlated with large fluctuations of sedimentation rates (0.03–2.09 cm/yr). In tandem with box model calculations and compiled data, the large δ34Spyr variations up to 40‰ are primarily ascribed to fluctuated anaerobic oxidation of methane coupled sulfate reduction (AOM-SR). Further analyses suggest that heightened distillation of interstitial sulfate, likely propelled by high methane flux coupled with increased sedimentation rates, can produce such negatively correlated δ34Spyr-Δ33Spyr pattern and large variations in δ34Spyr. Possible changes in isotope fractionation factors caused by varied cell-specific sulfate reduction rates result in a maximum δ34Spyr variation of <20‰ and therefore play a minor role in the large variation in δ34Spyr (>40‰). The major and minor isotopic compositions of pyrite align with sulfide produced through pure sulfate reduction, indicating little evidence of additional oxidative sulfur cycling. Sedimentation rates in the ECS are controlled by the East Asian winter monsoon that influences the sediment transport capacity from longshore currents on a multicentennial timescale, and also by the coupled East Asian summer monsoon that affects the sediment availability from the Yangtze River on a millennial timescale. Holocene climate changes alter sediment supplies at the ECS and therefore play a vital role in controlling multiple sulfur isotope compositions of sedimentary pyrite in this region. This study has the potential to provide new insights into δ34S excursions in seawater sulfate that occurred during periods of notable acceleration in sedimentation rates throughout the geological history.

A dolomite-based record of seawater calcium isotope composition over the Neogene

The calcium isotope composition of seawater (δ44/40Casw) records important information on the evolution of the calcium and carbon cycles over geologic time. Over the past two decades, many attempts have been made to reconstruct the Neogene δ44/40Casw variation using various calcium isotope records of seawater or seawater-like pore fluids. Nonetheless, large discrepancies still exist among these Neogene δ44/40Casw records. Here we report δ44/40Ca values in a suite of Neogene island dolomites (20.8–3.4 Ma, n = 18) from the South China Sea. Combining multiple geochemical proxies (δ44/40Ca, δ13C, and δ18O, this study; Sr/Ca, ∑REY, δ7Li, δ26Mg, 87Sr/86Sr, and Δ47, previous studies) and evidence from petrography, fluid inclusions, and magnetostratigraphy, we suggest that the dolomitization for these dolomites was buffered by seawater-like fluids in the shallow marine burial domain or even marine diagenetic domain (<180 m) over a considerable period of time (<2 Myr). Then, we apply the equilibrium calcium isotope fractionation factor between dolomite and seawater of −0.52 ± 0.20 ‰ (2SD, n = 13) estimated from the literature to these fluid-buffered dolomites and reconstruct a new Neogene δ44/40Casw record. After using the Locally Weighted Regression (LOWESS) curve fitting method, our dolomite-based δ44/40Casw record is consistent with the reconciled seawater record from other calcium isotope archives, including shark teeth, corals, foraminifera, brachiopods, barites, phosphates, and bulk pelagic carbonates. Our results support the use of fabric-retentive early marine diagenetic dolomites to trace the evolution of δ44/40Casw through Earth’s history.

Gallium isotope fractionation during precipitation of α-GaOOH from aqueous solution

Gallium (Ga) has two naturally-occurring stable isotopes: 69Ga (60.1%) and 71Ga (39.9%). Their isotopic ratios were employed as a potential proxy to trace the geochemical behavior of Ga and its analogue Al. Ga is also an important component of many semiconductors. When exposed to aqueous solutions, it may be dissolved and then precipitate as Ga-bearing crystals (e.g., GaOOH). Thus, the exploration of Ga isotope fractionation during α-GaOOH precipitation could be helpful for understanding geochemical behaviors of Ga and other metals (including Al and Fe) during oxyhydroxide precipitation. In this study, three series of precipitation experiments with variable initial aqueous Ga concentrations (about 9.2, 2.0, and 1.8 ppm) were carried out to determine Ga isotope fractionation factor during α-GaOOH precipitation at ∼20 °C and neutral pH as a function of time. The experimental results could be modeled using a Rayleigh precipitation process, in which fractionation factors between crystals and solution (Δ71Gasolid-solution) are between −0.28 and − 0.37‰, depending on the initial Ga concentration and type of matrix anion (Cl− vs. NO3−). This isotope fractionation is significantly smaller in magnitude than the values reported for Ga adsorption on goethite (−0.89‰) and calcite (−1.27‰). The isotope fractionation is mainly caused by the increase in GaO bond length from ∼1.84 Å in aqueous GaOH4— to ∼1.991 Å in α-GaOOH solid, with a simultaneous change of Ga coordination number from 4 to 6, although kinetic isotope fractionation may play an important role. Our study provides the first experimental evidence of significant Ga isotope fractionation during kinetic oxyhydroxide precipitation, and suggests that the measurable Ga isotope fractionation occurred at water-solid interface and during precipitations can be potentially used for tracing global geochemical cycling of Ga and its analogue Al and Fe.

Distinct trophic ecologies of zooplankton size classes are maintained throughout the seasonal cycle.

Marine food webs are strongly size-structured and size-based analysis of communities is a useful approach to evaluate food webs in a way that can be compared across systems. Fatty acid analysis is commonly used to identify diet sources of species, offering a powerful complement to stable isotopes, but is rarely applied to size-structured communities. In this study, we used fatty acids and stable isotopes to characterize size-based variation in prey resources and trophic pathways over a nine-month temperate coastal ocean time series of seven plankton size classes, from > 0.7-μm particulate organic matter through > 2000-μm zooplankton. Zooplankton size classes were generally distinguishable by their dietary fatty acids, while stable isotopes revealed more seasonal variability. Fatty acids of zooplankton were correlated with those of their prey (particulate organic matter and smaller zooplankton) and identified trophic pathways, including widespread ties to the microbial food web. Diatom fatty acids also contributed to zooplankton but fall blooms were more important than spring. Concurrent isotope-based trophic position estimates and fatty acid markers of carnivory showed that some indicators (18:1ω9/18:1ω7) are not consistent across size classes, while others (DHA:EPA) are relatively reliable. Both analysis methods provided distinct information to build a more robust understanding of resource use. For example, fatty acid markers showed that trophic position was likely underestimated in 250-μm zooplankton, probably due to their consumption of protists with low isotopic fractionation factors. Applying fatty acid analysis to a size-structured framework provides more insight into trophic pathways than isotopes alone.

Estimates of chlorine isotope fractionation factors using density functional theory: Applications to ore-forming systems

Chloride is the most important anion in ore-forming hydrothermal fluids, and chlorine isotopes are therefore potentially sensitive tracers of the origin and evolution of ore-forming fluids. However, they remain a relatively under-utilized tool in ore deposit geochemistry due to the lack of knowledge of chlorine isotope fractionation during ore-forming processes. Using first-principles density functional theory, this study estimates chlorine isotope fractionation factors for various ore-forming processes. All metal-Cl complexes in hydrothermal fluids are enriched in 37Cl compared to chloride ions, but the change in δ37Cl of the fluid trapped in ore minerals is small after the destabilization of metal-chloride complexes during ore mineral deposition. In the precipitation of evaporite minerals from brine, the sequence of enrichment of the heavy Cl isotope (37Cl) is halite > carnallite > aqueous chloride > kainite > sylvite > bischofite. The results of this study agree with the experimental observations that progressive precipitation of halite from brine lowers the δ37Cl value of the residual fluid until the formation of K-Mg chlorides. In low-temperature deposits, δ37Cl values for fluid inclusions and minerals reflect those of the hydrothermal fluid provenance and the mixing of this fluids with saltwater or basinal brines. In high-temperature magmatic-hydrothermal ore deposits that undergo liquid–vapour phase separation, chlorine isotopes fractionate among phases of silicate melt, vapour and chloride-rich liquid. The considerable range in δ37Cl in fluid inclusions may also reflect fluid mixing and hydrothermal alteration. At ambient temperature, the δ37Cl values may reflect evaporative processes and, in the case of chemical weathering of metallic mineral deposits, may record the supergene enrichment of the metals. This study highlights the use of chlorine isotopes as a new tool for interpreting ore-forming processes.

Is ebullition or diffusion more important as methane emission pathway in a shallow subsaline lake?

Methane (CH4) emissions via ebullition contribute significantly to greenhouse gas emissions from freshwater bodies. According to the literature, the ebullition pathway may even be the most important pathway in some cases, particularly in shallow lakes. Ebullition rates are not often estimated because of the high uncertainty associated with episodic releases, leading to difficulties in their determination. This study provides an estimate of such emissions in a large, shallow, subsaline lake in eastern Austria, Lake Neusiedl, and compares them to the diffusion pathway. Ebullition gas sampling was conducted every 5–10 days over a period of 107 days from late March to mid-July 2021, using ebullition traps placed in three distinct locations: Reed belt, Channel and Open water/Lake. The aim was to study the temporal and spatial heterogeneity of ebullition and its contribution to total emissions. At the same time, several water quality and other environmental parameters were measured and then tested against the CH4 ebullition rates to explore them as potential drivers for this pathway. The carbon isotope fractionation factor (αC) of the measured CH4 ebullition gas, ranging from 1.03 to 1.06, indicates a dominance of the acetoclastic methanogenesis in the sediments of Lake Neusiedl, regardless of the location. The Reed belt location showed the highest mean CH4 ebullition rate (17 ± 28 mg CH4 m−2 d−1), which is >340-fold higher than the mean of the other two locations, and demonstrated also a strong temperature dependency. In all locations at Lake Neusiedl, the median CH4 fluxes via diffusion are significantly higher than via ebullition. Our analyses do not confirm the dominance of the ebullition pathway in any of the studied locations. Whereas at the Reed belt, ebullition accounts for 48 % of the CH4 emissions, in the other two locations, is responsible only for about 1 %.

Nickel isotope fractionation factors between silicate minerals and melt

Quantitative knowledge of inter-mineral and mineral–melt isotopic fractionation factors is of crucial importance to interpret variability of mass-dependent isotope ratios in terrestrial rocks and meteorites. The isotopic composition of Ni shows great promise as a tracer of planetary differentiation, but the lack of solid constraints on the sign and magnitude of equilibrium Ni isotope fractionation factors hampers its further development. We address this issue by studying Ni isotope fractionation between silicate minerals and melt using a combination of carefully selected natural samples and olivine crystallisation experiments.Mineral separates from xenoliths are used to determine the Ni isotope fractionation behaviour of pyroxenes, spinel, and garnet relative to olivine. Nickel isotope fractionation between olivine and orthopyroxene is negligible, whereas clinopyroxene shows large variability that does not reflect equilibrium. Spinel has a clear affinity for the heavier isotopes of Ni relative to olivine; garnet has the lightest Ni isotope composition at equilibrium. The key parameter that dictates the sign and magnitude of Ni isotope fractionation during partial melting and crystallisation, however, is the olivine–melt fractionation factor. We present data for a natural basalt that contains olivine phenocrysts in chemical equilibrium with the host glass, and furthermore employ olivine crystallisation experiments in a 1-atmopshere furnace. Although the experiments invariably suffer from Ni loss to the Pt wire used to suspend the experiments in the furnace, these effects were carefully considered and quantified. The experiments limit the olivine–melt Ni isotope fractionation factor to lie between zero and slightly negative, which agrees very well with the natural datum, and we derive a best estimate value for Δ60/58NiOl–melt of −0.142 ± 0.031 ‰ at 1000 K. Hence, olivine has a subtle but clearly resolvable affinity for the lighter isotopes of Ni relative to basaltic melt. The implication is that equilibrium partial melts of mantle peridotite should have a slightly heavier Ni isotope composition than their source. Available Ni isotope data for mid-ocean ridge basalts show some overlap with modelled partial melts but extend to notably lighter Ni isotope compositions, potentially reflecting source heterogeneity or diffusive fractionation during melt transport.

Heavy Li isotopic compositions of the terminal Ediacaran dolostones in the Tarim Basin and their possible link to dolomitizations

Dolostones are promising archives for recording paleo-seawater Li isotope compositions, and are especially important for the Cambrian and Precambrian periods when limestones are scarce. However, the effect of dolomitization on carbonate Li isotopes is unclear, limiting their application in tracking (Pre-)Cambrian seawater Li isotopes. Research on Li isotopes in dolostone sections may resolve this issue. We present the Li isotope compositions of dolostones from the terminal Ediacaran Chigebrak Formation (NW Tarim Basin). The dolostone Li isotopes are ∼17 ± 5 ‰ heavier than contemporaneous seawater, indicating significant Li isotope fractionations during dolomite formation. Given that the formation of clay minerals may enrich the porewater with heavier Li isotopes and promote the dolomite formation, we argue that the clay-catalyzed dolomite formation would probably result in heavier Li isotope compositions of dolomites. Moreover, assuming a fixed fractionation factor (∼15 ‰) for the clay-catalyzed dolomitizations, we obtain the seawater δ7Li variations in the terminal Ediacaran, which show similar declining trends with the carbonate 87Sr/86Sr record. Using a simple ocean Li-Sr box model, we propose a reduced weathering scenario during the terminal Ediacaran (∼549 Ma) to explain the seawater Li-Sr isotope covariations. Our study suggests considerable fractionations of Li isotopes (∼17 ± 5 ‰) during clay-catalyzed dolomite formation and a reduced-weathering scenario during the terminal Ediacaran. This large Li isotope fractionation factor indicates the Li isotope fractionation factors during dolomitizations may vary widely and should be considered based on the type of dolomitization. More comparative studies on Li isotopes among coeval carbonates are expected to further explore the potential of dolostones in recording paleo-seawater Li isotopes.

Li–Na interdiffusion and diffusion-driven lithium isotope fractionation in pegmatitic melts

Abstract. In this study, we investigate the diffusion of Li and its stable isotopes (6Li and 7Li) in flux-rich (1.8 % Li2O, 2.6 % B2O3, 2.3 % P2O5 and 3 % F) pegmatitic melts in order to contribute to the understanding of Li enrichment in such systems. Two glasses were synthesized with a model pegmatitic composition, one of which is highly enriched in Li (&gt; 1 wt %, PEG2-blue) and the other one essentially Li-free (PEG2-Li-free). Diffusion couple experiments were performed to determine the chemical diffusivity of Li in dry pegmatitic melts. Experiments were conducted using rapid-heat and rapid-quench cold-seal pressure vessels in a temperature range of 650–940 ∘C at 100 MPa with Ar as the pressure medium. We observed rapidly formed diffusion profiles, driven by an interdiffusive exchange of the monovalent alkalis Li and Na, while the other elements are immobile on the timescale of experiments (1–30 min). From these experiments, activation energies for Li–Na interdiffusion were determined as 99 ± 7 kJ mol−1 with a pre-exponential factor of log D0 = −5.05 ± 0.33 (D0 in m2 s−1). Li and Na partitioning between the stronger depolymerized PEG2-blue and the less depolymerized PEG2-Li-free leads to a concentration jump at the interface; i.e. Na is enriched in the more depolymerized PEG2-blue. Li–Na interdiffusion coefficients in the studied melt composition are in a similar range as Li and Na tracer diffusivities in other dry aluminosilicate melts, confirming little to no effect of aluminosilicate melt composition on Li diffusivity. Thus, added fluxes do not enhance the Li diffusivity in the same way as observed for H2O (Holycross et al., 2018; Spallanzani et al., 2022). Using melt viscosity as a proxy for the polymerization of the melt shows that water has a stronger potential to depolymerize a melt compared to other fluxing elements. Faster diffusion of 6Li compared to 7Li leads to a strong Li isotope fractionation along the diffusion profile, resulting in δ7Li as low as −80 ‰ relative to the diffusion-unaffected regions. This diffusive isotope fractionation can be quantified with an empirical isotope fractionation factor (β) of 0.20 ± 0.04, similar to previously observed β values for Li diffusion in melts. This suggests in accordance with previously published data that a β value of ca. 0.2 seems to be universally applicable to diffusive Li isotope fractionation in aluminosilicate melts.

Abiotic aerobic oxidation pathways of stibnite revealed by oxygen and sulfur isotope systematics of sulfate

The environmental threat posed by stibnite is an important geoenvironmental issue of current concern. To better understand stibnite oxidation pathways, aerobic abiotic batch experiments were conducted in aqueous solution with varying δ18OH2O value at initial neutral pH for different lengths of time (15-300 days). The sulfate oxygen and sulfur isotope compositions as well as concentrations of sulfur and antimony species were determined. The sulfur isotope fractionation factor (Δ34SSO4-stibnite) values decreased from 0.8‰ to -2.1‰ during the first 90 days, and increased to 2.6‰ at the 180 days, indicating the dominated intermediate sulfur species such as S2O32−, S0, and H2S (g) involved in Sb2S3 oxidation processes. The incorporation of O into sulfate derived from O2 (∼100%) indicated that the dissociated O2 was only directly adsorbed on the stibnite-S sites in the initial stage (0-90 days). The proportion of O incorporation into sulfate from water (27%-52%) increased in the late stage (90-300 days), which suggested the oxidation mechanism changed to hydroxyl attack on stibnite-S sites promoted by nearby adsorbed O2 on stibnite-Sb sites. The exchange of oxygen between sulfite and water may also contributed to the increase of water derived O into SO42−. The new insight of stibnite oxidation pathway contributes to the understanding of sulfide oxidation mechanism and helps to interpret field data.