Zinc (Zn) is an essential trace element that is involved in both biotic and abiotic processes in Earth's surface environments. Over the last 20 years, advances in mass spectrometry instrumentation have enabled unprecedented high-precision Zn isotope abundance ratio determinations in natural and anthropogenic samples. Supported by a solid theoretical background inherited from the traditional non-metal stable isotopes (C, N, H, O, S), the understanding of the controlling factors of Zn isotope fractionation has rapidly evolved. In this article, we review the main applications and findings of Zn isotopes in a variety of scientific domains, including metal contamination, magmatic differentiation, plant uptake, weathering, global climate change, dietary and trophic chains, and biomedicine. The analytical aspects of Zn isotope determination in solid and water samples and the principles and mechanisms of Zn isotope fractionation are also reviewed. This work aims to provide a general yet in-depth panorama of Zn isotope chemistry and to demonstrate the versatility and potential of this isotope system for studying the biological, geological and chemical cycling of Zn.
Supplementary material: Further discussion of the origins and mechanisms of Zn isotope fractionation is available in the supporting information at https://doi.org/10.6084/m9.figshare.c.7240146
Over the last 20 years, the analysis of metal stable isotopes has found application in diverse fields, including inorganic chemistry (Veeramani et al. 2015), biomedicine (Moynier et al. 2013), environmental sciences (Araújo et al. 2021; Junqueira et al. 2023), anthropology (Jaouen et al. 2020), biology (Deng et al. 2014; Köbberich and Vance 2019; Araújo et al. 2021), climate change (Zhang et al. 2022b) and mineral exploration geochemistry (Wilkinson et al. 2005). A main factor driving this interest is the advance of mass spectrometry and sample preparation methods, now allowing for precise determination of the isotope compositions of metals in a wide variety of natural and anthropogenic materials (Johnson et al. 2004; Bullen and Walczyk 2009; Wiederhold 2015; Desaulty et al. 2017; Kyser et al. 2020).
Zinc (Zn) is among the emerging metal isotope systems of interest to epidemiological, economical, ecological, technological and environmental studies (Caldelas et al. 2011; Wiederhold 2015). Zinc is the second most abundant transition metal in organisms (Vašák and Hasler 2000), and a component of ∼3000 human proteins. It has many roles in the human body, including contributing to normal growth and development, immunity, cellular homeostasis, cell survival and biochemical functions (Vašák and Hasler 2000; Truong-Tran et al. 2001; Andreini et al. 2006; Haase and Rink 2014). Zinc also catalyzes reactions and is a cofactor or component of more than 300 metalloenzymes (Rink 2000; Vašák and Hasler 2000). It is thus an essential micronutrient for organisms, but toxic in high concentrations (Cloquet et al. 2008). Zinc is used in fertilizers, pesticides and electroplating, and can be released to the environment during mining and metallurgical operations (Sivry et al. 2008; Sonke et al. 2008; Bigalke et al. 2010; Yin et al. 2015, 2018). Zinc occurs as a trace element in different geological reservoirs and in biological systems as Zn(II). It is not naturally redox-sensitive in ambient environments but may be reduced to Zn(0) in industrial high-temperature processes or via electrolysis (Wiederhold 2015; Moynier et al. 2017). Interpreting the isotopic compositions of primary Zn reservoirs and the mass transfer between these reservoirs can enhance our comprehension of the Zn biogeochemical cycle, whether at a local or global level, and at both microscopic and macroscopic scales (Bullen and Eisenhauer 2009; Rehkämper et al. 2012; Albarède 2015; Moynier et al. 2017). Recently, reviews of Zn stable isotope systematics have covered specific fields, such as geochemistry (Moynier et al. 2017), biomedicine (Schilling et al. 2021), botany (Caldelas and Weiss 2017), atmospheric sciences (Schleicher et al. 2020) and Zn-contaminated environments (Yin et al. 2015), including soils (Wang et al. 2021) and aquatic systems (Li et al. 2019; Desaulty and Petelet-Giraud 2020). This review incorporates and discusses recent studies on Zn isotopic systematics and emerging applications of Zn fractionation processes across environments, focusing on large-scale patterns of Zn isotope ratios in biogeochemical systems. To this end, Zn isotope biogeochemistry and Zn isotope analysis in fields ranging from metal contamination, magmatic differentiation, weathering, nutrient cycling and palaeoclimate to plant uptake, dietary and trophic transfer, and biomedicine are discussed.
Biogeochemistry of zinc
General chemistry, biological importance and technological applications
Zinc is a chalcophile transition metal that occurs in nature as five stable isotopes, 64Zn, 66Zn, 67Zn, 68Zn and 70Zn, with isotopic abundances of 49.2, 27.7, 4.0, 18.4 and 0.6%, respectively. It is the 24th most abundant element in Earth's crust, ubiquitous in the environment and with important roles in diverse biotic and abiotic processes in terrestrial and aquatic ecosystems (Frassinetti et al. 2006; Cloquet et al. 2008; Hambidge et al. 2010; Chasapis et al. 2012; Maret 2013; Kaur et al. 2014). Zinc plays a vital function for living organisms, providing structural integrity to several metabolically important proteins (Frassinetti et al. 2006; Rink and Haase 2007; Cloquet et al. 2008; Hambidge et al. 2010; Chasapis et al. 2012; Maret 2013; Kaur et al. 2014). In an adult human, about 2 g of Zn are distributed throughout the body, mostly intracellularly in enzymes such as carbonic anhydrase, which interconverts carbon dioxide and bicarbonate and regulates the acid-base balance of the cytosol and superoxide dismutase, which controls reactive oxygen (Tapiero and Tew 2003). Zinc further regulates the glutathione metabolism and metallothionein expression (Cruz et al. 2015) and affects signalling pathways and the activity of transcription factors with Zn finger domains (Frassinetti et al. 2006; Cloquet et al. 2008; Hambidge et al. 2010; Chasapis et al. 2012; Maret 2013; Kaur et al. 2014).
In the Earth's (near-)surface environment, Zn primarily forms chemical bonds with sulfur and oxygen atoms (Albarède 2004). Principal sulfidic Zn ores are sphalerite and wurtzite (ZnS), of the same chemical formula but with distinct crystallographic structures, cubic and hexagonal, respectively. Other important ores are zincite (ZnO), smithsonite (ZnCO3), willemite (Zn2SiO4) and hemimorphite [Zn4Si2O7(OH)2·H2O] (Albarède and Beard 2004). In magmatic silicate rocks, Zn occurs as a trace element (typically 40–120 ppm), whereas it is likely to be concentrated in argillaceous sediments in sedimentary rocks, in the range of 80–120 ppm (Adriano 2001). In coal, Zn concentrations vary from 50 to 19 000 ppm. Mean Zn for worldwide soils is calculated as 64 ppm (Wang et al. 2021). The vast majority of Zn produced in the world is derived from sulfidic ores (Cloquet et al. 2008). In 2020, the world's Zn reserves were estimated at about 250 million tonnes.
Most of the produced refined Zn (∼60%) is used as a coating to protect iron and steel from corrosion (galvanization), as well as alloying to make bronze, brass and zinc-based die casting (Cloquet et al. 2008). Other applications of Zn include as catalysts in the chemical industry (e.g. rubber, pigments, plastic, lubricants and pesticides) and as a component in technological devices such as batteries, screen phosphors and automotive equipment (Meylan and Reck 2017).
In the ocean, Zn displays nutrient-like depth profiles (Mason 2013), reflecting effective scavenging of this element by phytoplankton and posterior regeneration at depth (Conway and John 2014; Zhao et al. 2014; Weber et al. 2018). The same is observed in pristine lakes and river surface waters (Shiller and Boyle 1985; Lum et al. 1991), as well as sediments (Ochieng et al. 2008; Xie et al. 2019). In terms of groundwater concentrations, studies of these systems have demonstrated the importance of water–rock interactions in controlling the mobility of zinc (Kent et al. 2000). However, the extent of these reactions depends on pH and biogeochemical conditions and processes, besides the geological background.
Zinc cycling in the biogeosphere
Zinc is mobilized into the surface environment from natural processes such as weathering and erosion of rocks and soils and volcanic activity. Anthropogenic sources, such as vehicle emissions, sewage sludge, road runoff, industrial emissions and mining and metallurgical wastes, contribute to Zn distribution dynamics as well, and diverse reviews analysing the isotope geochemistry of Zn have been performed in different specific fields (Sparks 2005; Brown and Calas 2011; Little et al. 2014; Table 1). Further discussion of the origins and mechanisms of Zn isotope fractionation is provided in Supporting Information S1 and S2.
A variety of Zn compounds are introduced and mobilized into the environment by atmospheric input, groundwater or fluvial fluxes to estuaries and ocean systems, whereas at a geological scale, Zn may be reincorporated as a mineral component through sedimentation (Fig. 1) (Adriano 2001; Sparks 2005). The natural biogeochemical Zn cycle is partially disturbed by anthropogenic remobilization from deep reservoirs to the Earth's surface, most notably from the mining of ores, coal and crude oil, which has led to localized Zn contamination and enrichment of soils, water and air (Wong et al. 2006; Mason 2013). At the sediment–water or soil–water interface, Zn can be distributed across particulate (0.45 mm), colloidal (1 nm–0.45 mm) and dissolved metal species (<1 nm) in different chemical forms (Bianchi et al. 2007). Its distribution among these different fractions depends on the prevailing physicochemical conditions, including pH, Eh, temperature, ionic strength, as well as suspended particulate and organic matter abundance. This interface is dynamic and many reactions of complexation, adsorption, ion exchange, (co-)precipitation and redissolution of compounds occur concomitantly, with or without biological mediation (Albarède 2008). The combination of these biogeochemical reactions also controls Zn bioaccumulation routes in the trophic chain, e.g. in sediment (benthic organisms) or by uptake of the dissolved fraction in interstitial or surface waters (primary producers including plants, phytoplankton, macrophytes and bacteria) (Clearwater et al. 2002; Zhou et al. 2008; Ahlf et al. 2009; Andersen et al. 2011).
In the solid phase, Zn may exist as an adsorbed species on primary or secondary minerals (clays), oxides, carbonates, sulfides and biogenic compounds generated by biological production or decaying organic matter. In particular, the finest particles and the colloidal fractions of inorganic and biogenic materials play a pivotal role in Zn adsorption due to their significant surface area and surface charge (Turner and Millward 2000; Sparks 2005; Brown and Calas 2011). In the dissolved phase, Zn can exist as a free hydrated ion or form inorganic and organic complexes, including with F−, Cl−, SO42−, OH−, HCO3−, CO32, HPO32−, NH3 and, in anoxic waters, HS− and H2S (Gaillardet et al. 2013; Mason 2013). A Pourbaix diagram is provided in Figure S1, showing chemical species as a function of both redox standard potential and pH values. Important organic ligands include humic acids, that may lead to different fractionation proportions depending on the various functional groups (carboxylic, amino, sulfidic and phenolic) that the free Zn2+ has affinity to (Albarède 2008; Jouvin et al. 2009).
Sample collection, preparation and analysis
Preventing contamination
Exogenous contamination is a primary concern for Zn isotopic analysis, especially in samples with low Zn concentrations (i.e. samples with <500 ng Zn). Gloves can be an important source of exogenic contamination: in weak acid at room temperature, gloves released up to 17 ng of Zn (Garçon et al. 2017). To avoid contamination inside the laboratory, vinyl gloves should be used for biological and geological materials: a single contact between the glove and the labware releases an average of 6 ng of Zn and hence significantly shifts 66/64Zn outside measurement uncertainty when the amount of Zn analysed is below 500 ng (Jaouen et al. 2020). Other exogenic contaminant sources may be related to the use of sunscreen, perfume, makeup and other routine chemicals. Zinc is an abundant element, and particularly prone to contamination from care products and sampling equipment. The sampling and protocol performance should be conducted with minimum human contact to prevent contamination. Furthermore, the samples should be stored in plastic containers, and care should be taken to ensure that sampling equipment (e.g. shovels, hammers, augers) does not introduce Zn (Gill 1997). In addition, multiple procedural blanks should be performed in order to monitor the contaminant addition to the sample.
Solid samples
The determination of the Zn isotope composition of solid samples can be performed in situ using laser ablation techniques, but the conversion to the dissolved form via acid-dissolution for nebulization and introduction into spectrometers is more common (Becker 2005). For the latter, the bulk sample should be first crushed to facilitate acid dissolution, as well as homogenized (Gill 1997). Sediments and soils normally require freeze-drying (Sivry et al. 2008; Araújo et al. 2017). Rocks are crushed and ground into fine power using agate, swing or ball mill. In the case of biological tissues, water removal by freeze-drying is crucial to avoid sample deterioration (Gill 1997). After pre-preparation, the samples are dissolved totally on hot plates with mixtures of concentrated acids such as HF, HNO3 and HCl, preferentially using perfluoralkoxy polymer (PFA) labware (Chapman et al. 2006; Araújo et al. 2017; Souto-Oliveira et al. 2018). In the case of refractory samples, digestion assisted by microwave is often required. Reagents of high purity are desirable to keep blanks as low as possible during the sample preparation; sub-boiling distillation of acids is commonly used (Araújo et al. 2017). The sample preparation, as well as dissolution and Zn chromatographic separation should be conducted in an over-pressurized and air-filtered cleanroom equipped with class 100–1000 hoods (Weiss et al. 2007; Archer et al. 2017; Souto-Oliveira et al. 2019).
Aqueous samples
The sampling protocol for Zn isotope analysis of water is similar to that for standard dissolved cation analysis (Chen et al. 2009a; Conway et al. 2013; Petit et al. 2015; Guinoiseau et al. 2017). The sample is collected using plastic tubing for pumps or plastic containers for dipping and placed into plastic containers that have been rinsed several times with the water being collected (Chen et al. 2009a, b; Conway et al. 2013; Petit et al. 2015; Guinoiseau et al. 2017). Pre-cleaning of the plastic containers with nitric acid is required to assure lower blanks. The water sample needs to be filtered using membranes with a pore size of 0.45 or 0.22 µm (Chen et al. 2009a, b; Conway et al. 2013; Zhao et al. 2014; Petit et al. 2015; Guinoiseau et al. 2017). Ultra-centrifugation can be used to separate the colloidal phase (e.g. Leybourne et al. 2000). After obtaining the dissolved fraction, it should be acidified with nitric or hydrochloric acid to a pH∼2 for sample preservation (Chen et al. 2009a, b; Conway et al. 2013; Zhao et al. 2014; Petit et al. 2015; Guinoiseau et al. 2017). Knowledge of metal concentrations in the sample prior to collection for isotope analysis is essential to ensure that sufficient metal is collected to meet the requirements of the analytical technique. The minimum amount of Zn required for isotope analysis ranges from 100 to 1000 ng, depending on factors such as ionization efficiency in the mass spectrometer, instrumental settings and laboratorial blanks (Maréchal et al. 1999; Archer and Vance 2004; Souto-Oliveira et al. 2019). After collection, water samples can be dried and additionally digested with acids for posterior analyte separation and analysis (Borrok et al. 2007). In case of very diluted waters or marine waters that contain high dissolved solids, pre-concentration methods can be performed using specific resin packed on columns (such as BioRad Chelex100 and NOBIAS Chelate PA-1) (Chen et al. 2008; Conway et al. 2013; Takano et al. 2017) or through co-precipitation by addition of alkaline compounds to water samples (Bermin et al. 2006). As for solid samples as discussed above, chemical procedures of sample preparation, dissolution, pre-concentration and chromatographic separation should be conducted in a cleanroom environment.
Ion exchange separation of Zn: elimination of matrix interferences
To eliminate interferences, Zn must be separated from the sample matrix and particularly from elements that produce isobaric interferences on the target masses for analysis (Becker 2005, Supporting Table S2). Ion-exchange chromatography currently is the preferred technique for this purpose (Maréchal et al. 1999; Chapman et al. 2006; Sossi et al. 2015; Zhu et al. 2015). For Zn, the anionic exchange resin AG MP-1 has been employed in many studies (Maréchal et al. 2000; Desaulty et al. 2017; Araújo et al. 2021). Because the ion-exchange between eluted Zn and resin can fractionate Zn isotopes, a recovery close to 100% is required to avoid significant bias on original isotope compositions of the samples (Jouvin et al. 2009). Thus, a prior calibration of chromatography protocols should be performed for the different analysed matrixes to ensure matrix-dependent separation of Zn from other elements. A large number of protocols have been developed, adjusting for specific matrix requirements, or multiple-element separations (e.g. Zn, Cu, Fe, Cd, Mo; Archer and Vance 2004; Luck et al. 2005; Mason et al. 2005; Schoenberg and von Blanckenburg 2005; Bermin et al. 2006; Borrok et al. 2007). Generally, these protocols rely on the conversion of Zn (II) free ions to chloride- or bromide-complexes in hydrochloric and bromic acids (Maréchal et al. 1999; Moynier et al. 2017), and can be performed in the same procedure for other metals such as Cu or Fe (Figure S2).
Analytical challenges
Quantification of isotope ratios
Zinc isotopes can fractionate in mass spectrometers during their transmission from the ICP source to detector (Maréchal et al. 1999; Archer and Vance 2004). Therefore, raw Zn isotope composition data should be corrected for induced instrumental fractionation (Maréchal et al. 1999; Archer and Vance 2004). One of the techniques used to correct mass bias is the sample standard bracketing (SSB), which consists of analysing an unknown sample ‘bracketed' by standards that are used to interpolate and correct mass drift bias during data collection (Archer and Vance 2004; Araújo et al. 2017). External normalization, which consists of doping samples and standards with an element with a known isotope composition and similar fractionation behaviour to that of Zn, is another common technique to correct mass bias. Copper-doping is a common choice for Zn isotope analysis (Zhang et al. 2022a). The measured isotope ratio of the dopant can be compared to its known value to quantify machine-induced fractionation and the resultant correction is applied to the isotope ratio of the target element (Heumann et al. 1998; Archer and Vance 2004; Peel et al. 2008; Zhu et al. 2015). The double-spike (DS) technique has been applied for Zn isotope determination (Arnold et al. 2010), which has advantages over the standard-sample bracketing technique and external normalization as there is no requirement for quantitative yield. However, major difficulties in applying the DS technique are obtaining pure spikes, determining optimal double-spike compositions and double-spike sample mixing proportions, and calibrating the double spike (Arnold et al. 2010; Moeller et al. 2012; Costas-Rodríguez et al. 2014).
Data reporting and analytical quality control
Certified reference materials
Because isotope and elemental standards are synthetic solutions, laboratories commonly analyse certified reference materials such as soils, sediments, plants and rocks to improve quality control (Cloquet et al. 2008; Araújo et al. 2017; Moynier et al. 2017). A compilation of reference materials of geological, environmental, and biological matrixes is presented in Table S1. These reference materials cover much of the variability of 66/64Zn in ‘natural' Earth surface materials, ranging from −1.00 to 1.00‰. Isotopic compositions of materials with matrix types for more recent applications of Zn isotopes (soil, biological, aqueous) are also provided in Table S1. However, these require further confirmation. Despite numerous studies and applications of Zn isotopes, well-characterized reference materials for more complex environmental samples are noticeably lacking. To unlock additional hydrogeochemical applications of Zn isotopes, reference materials for surface water samples and soil samples are particularly needed.
Controlling factors of Zn isotope variability
Biotic processes
In biotic processes, Zn isotopes are fractionated when they are transferred between different enzymes to which Zn is bound. This concept has been elucidated in studies conducted by Moynier et al. (2013) and Fujii et al. (2014). Zinc distribution, as well as its fractionation, within living organisms is partially controlled by the supplied concentration and by the type of cellular Zn transporters (Caldelas et al. 2011; Deng et al. 2014; Weiss et al. 2021).
Studies on Zn isotope fractionation have been performed to elucidate the mechanisms of Zn transport in soils and plants (Weiss et al. 2005; Bigalke et al. 2010), metabolism changes (Albarède et al. 2017), biological productivity (Andersen et al. 2011) or fractionation during uptake by microorganisms (Balistrieri et al. 2008; Jouvin et al. 2009).
In plants, there is a notable fractionation of Zn during transport from the roots to the shoots and its transfer from vegetative to reproductive tissues (Caldelas et al. 2011; Arnold and Wade 2015; Wiggenhauser et al. 2018). Heavy Zn tends to form chelates, and in the shoot, it is retained through apoplastic absorption (Caldelas et al. 2011; Aucour et al. 2017; Wiggenhauser et al. 2018): Zn isotopic fractionation in soil–plant systems can vary depending on the availability of Zn, which in turn is influenced by soil properties (Fig. 2). Therefore, the Zn isotope compositions can serve as a valuable tool for detecting changes in biogeochemical processes that govern the distribution of zinc in soil–plant systems in response to alterations in soil environments.
Whereas many studies are related to Zn fractionation during plant uptake, Zn isotopic fractionation during internal plant processes and transport remain more enigmatic (Caldelas and Weiss 2017; Wiggenhauser et al. 2018). Wiggenhauser et al. (2018) studied Zn isotope ratios in wheat shoots at different growth stages in order to understand the main pathways and processes during grain filling, and concluded that Zn mass is inversely proportional to the heavy isotope accumulation: 75% of the total Zn mass in the shoot moved to the grains, which are enriched in light isotopes relative to the straw (66/64Zn grain–straw = −0.31 to −0.21‰). This corroborates previous studies showing that light Zn isotopes are accumulated in phloem sinks and heavy isotopes are retained in phloem sources.
Numerous investigations over the past few decades have focused on Zn fractionation in diatoms (e.g. Gélabert et al. 2006; John et al. 2007) and bacteria (e.g. Wanty et al. 2013). Additionally, Peel et al. (2009) and Andersen et al. (2011) have demonstrated that Zn isotopes found in particulate matter and diatom frustules can be used as indicators for tracing the biochemical cycling of Zn. Despite this attention, the relative importance of surface complexation v. intracellular incorporation on Zn isotope fractionation remains debated. For instance, Gélabert et al. (2006) demonstrated that in sorption and uptake processes, both freshwater and seawater diatoms exhibited a preference for heavier Zn, with 66/64Zn cell-solution values of 0.35 and 0.27‰, respectively. In contrast, an experimental study by John et al. (2007) reported that low Zn concentrations in media drove high-affinity uptake and a more negative isotope fractionation effect (66/64Zncells-medium = −0.8‰) compared to low affinity transport when more Zn is available (66/64Zncells-medium = −0.2‰) (John et al. 2007). Furthermore, Kafantaris and Borrok (2014) conducted experiments under a variety of conditions (e.g. pH, time or growth phase), and concluded that bacterial adsorption preferentially incorporates the heavier isotope, but the extension of the uptake, and then fractionation, is strongly related to the presence of dissolved organic molecules and free Zn2+ in the environment of growth.
There remains uncertainty regarding the extent of Zn isotopic fractionation among different bacterial species and under varying conditions of Zn stress. Furthermore, there is a lack of knowledge regarding the roles of cell surface adsorption and speciation of aqueous Zn on the extent of isotopic fractionation. Although Zn isotopes hold promise as tools to elucidate the metal-binding pathways linked to microorganisms in natural environments, the interpretation of the isotopic compositions is complicated by the presence of competing surface interactions, metal speciation, and differing impacts of various microbial species.
Zn isotope fractionation in cosmochemistry
Investigations of the Zn isotope composition of meteorites showed 66/64Zn values spread over a range of +0.3‰ for carbonaceous chondrites and 2‰ for ordinary chondrites (Bridgestock et al. 2014; Moynier et al. 2017). All meteoritic and terrestrial samples plot on the same mass-dependent fractionation line, implying that Zn from the inner solar system formed from a single isotopically homogeneous reservoir (Luck et al. 2005). For CI chondrites, 66/64Zn values of +0.46 ± 0.08‰ have been used as a best estimate for the primordial Zn isotope composition of the Solar system (Moynier et al. 2017). The large range of Zn isotope compositions observed in the most part of primitive meteorites seems related to the variable mixing of isotopically distinct components in the case of the carbonaceous chondrites (Luck et al. 2005), and in the case of the ordinary and EL enstatite chondrites, related to thermal reworking on parent bodies (Moynier et al. 2017). In turn, ureilite samples showed a range of 66/64Zn values ∼0.7‰ and a negative correlation between the abundance of Zn and 66/64Zn values, which suggests isotope fractionation occurred by evaporation during the heating event on the ureilite parent body generated by possible impacts (Moynier et al. 2010).
Zinc isotope variation in silicate rocks (including igneous, metamorphic and sedimentary) is small and tends to cluster within a narrow range of +0.20 to +0.40‰ (Chapman et al. 2006; Cloquet et al. 2008; Herzog et al. 2009; Moynier et al. 2009a; Chen et al. 2013). Investigations on suites of volcanic rocks have shown that high-temperature igneous processes induce fractionations no larger than 0.10‰, and therefore Zn isotopes seem limited in revealing planetary formation and differentiation processes. Based on those findings and assuming a homogeneous Earth's mantle and that Zn is not partitioned into the core, the Zn isotope composition for the bulk Earth is estimated at +0.28 ± 0.05‰ (2) (Chen et al. 2013). The overall homogeneous Zn isotope composition of the Earth is most likely the result of large-scale melting, mixing and differentiation events that occurred in the terrestrial mantle during and post-accretion (Chen et al. 2013). Despite this apparent global homogeneity, some geological settings with contrasting 66/64Zn values have been reported in the Taiwan orogeny and in granites from the Lachlan Fold Belt (Australia) (Telus et al. 2012). Continental basalts in eastern China showed a large range of 66/64Zn values, from +0.30 to +0.63‰, a possible result of the recycling of isotopically heavy marine carbonates (66/64Zn average of +0.91‰) during marine plate subduction processes (Liu et al. 2016).
Because Zn is a moderately volatile element (condensation at ∼730 K), Zn isotope signatures have the potential to reveal evaporation–condensation processes in actively degassing volcanoes, planetary bodies such as lunar samples, and shocked rocks from terrestrial impact craters such as tektites. At Merapi Volcano degassing, the gas cooling from 590 to 297°C is accompanied by a decrease in Zn concentrations of the gaseous phase, which becomes gradually enriched in the light Zn isotope, while the condensates become progressively enriched in the heavier isotopes (Toutain et al. 2008). Gas and condensate isotope compositions could be explained by a Rayleigh condensation mode (Toutain et al. 2008). The 66/64Zn of lunar basalt is ∼1‰ heavier than terrestrial basalt, suggesting that this fractionation occurred during a large-scale evaporation event, likely related to the giant impact that originated the Moon (Paniello et al. 2012). Tektite samples also showed broader 66/64Zn values than the terrestrial crust, encompassing lighter values of +0.3‰ to anomalous values of +2.49‰ (Moynier et al. 2009a). All these were negatively correlated with Zn abundance, implying that isotope fractionation occurred by evaporation during the heating event upon tektite formation (Moynier et al. 2017).
Terrestrial rocks and volcanic systems
Understanding the controls on Zn isotopic variability in rocks and minerals can provide insights into the geological processes that have influenced the formation and evolution of Earth materials. The potential of the Zn isotope system as a tracer of geological processes happening on Earth has been extensively studied in the past decade (Johnson et al. 2004; Cloquet et al. 2008; Moynier et al. 2017; Mathur et al. 2021; Wilkinson 2023) due to the significantly fractionation during continental weathering, carbonate and sulfide precipitation, and fluid–rock interactions.
Within Earth's interior, at high temperature and pressure, there are four possible mechanisms that can lead to heavy Zn isotopic compositions in basalts, including: (1) crystal-melt differentiation, which occurs when there is crystallization of olivine and/or Fe–Ti oxides at the very end of the differentiation sequence (Chen et al. 2013; Sossi et al. 2015); (2) magmatic degassing, which shows significant Zn isotope fractionation, with gas condensates enriched in the heavy isotopes (Chen et al. 2013); (3) crustal contamination, for example with sedimentary carbonates while it ascends through the crust (Liu et al. 2016); and/or (4) isotopic heterogeneity caused by recycled materials in the mantle sources, via plate subduction (Zhao et al. 2014; Liu et al. 2016). Zinc isotopes can be used to constrain the evolution of the Earth and processes involved, besides being useful as a proxy for planetary differentiation and volatile history (Chen et al. 2013; Liu et al. 2016; Wang et al. 2021).
Zinc isotope variation in silicate rocks (including igneous, metamorphic and sedimentary) is small and values tend to cluster within a narrow range of 0.20–0.40‰ (Chapman et al. 2006; Cloquet et al. 2008; Herzog et al. 2009; Moynier et al. 2009a, b; Chen et al. 2013). Investigations on suites of volcanic rocks have shown that high-temperature igneous processes induce fractionations no larger than 0.10‰ and, therefore, Zn isotopes may not be very useful to assess geological differentiation processes. The overall homogeneous Zn isotope composition of the Earth is most likely the result of large-scale melting, mixing and differentiation events that occurred in the mantle during and post-accretion (Chen et al. 2013). In spite of this apparent global homogeneity, some cases of geological settings with contrasting 66/64Zn values have been reported in the Taiwan orogeny and in granites from Lachlan Fold Belt (Australia) (Telus et al. 2012). In some cases, the isotopic composition of Zn in rocks may be influenced by the mixing of different Zn sources with different isotopic compositions. For example, the mixing of isotopically distinct Zn sources during magma genesis can result in isotopic variation in volcanic rocks (Chen et al. 2013).
The 66/64Zn values of clay minerals from sedimentary rocks (e.g. shales, sapropels, aeolian dust, and Pacific and Atlantic sediments) display a range of 0.17–0.35‰ (Maréchal et al. 2000; Albarède 2004) and show little variability with respect to their precursor rocks. Besides the extensive studies on Zn isotopes in sedimentary rocks, the relationship of bulk carbonate 66/64Zn to seawater is poorly understood (Müsing et al. 2022). Whereas inorganic carbonate records heavier 66/64Zn values compared with the aqueous phase (e.g. incorporation into and adsorption on to inorganic calcite; Mavromatis et al. 2019), biogenic carbonates from modern cold-water corals and foraminifera record the ambient seawater 66/64Zn, and indicate negligible fractionation (Little et al. 2014). Consequently, there appear to be different fractionation patterns for Zn in carbonates, depending on the precipitation pathway (Müsing et al. 2022). The potential of contamination from different phases (i.e. ferromanganese-oxide coatings, as well as detrital silicate and organic matter), as well as the relationship between 66/64Zn from modern seawater and those incorporated into carbonate, is poorly understood (Müsing et al. 2022). Therefore, for palaeoceanography applications, the pathways of carbonate precipitation and associated Zn fractionation processes should be further explored.
Soils
Soils show a large range of Zn isotope compositions in relation to parental rock (0.18–0.55‰ in unpolluted soils; Fekiacova et al. 2015; Fig. 2), which is attributed to the combination of natural abiotic and biological processes involved in the redistribution of Zn into different soil horizons during pedogenesis (Viers et al. 2007). Literature compilations demonstrate a general tendency of 66/64Zn values centred on 0.1‰ and on 0.2‰ in the B and O horizons, respectively (Fekiacova et al. 2015), but establishing general principles of Zn isotope fractionation in soils is challenging due to the combined roles of aqueous chemistry, weathering, mineralogy, amorphous solids and organic matter in Zn isotope fractionation within different soil horizons (Vance et al. 2016; Moynier et al. 2017). Soils with a low degree of weathering from Central Siberia presented almost constant 66Zn values (∼0.2‰) close to basalt parental rocks (Viers et al. 2015). Zinc isotope data for soil profiles from Hawaii and Scotland, covering both granitoid and basaltic substrates, showed little isotope variations (<0.2‰), and are associated with material removal by weathering, effects of redox transitions on sorption substrates and reactions with organic ligands (Vance et al. 2016). The partitioning of Zn isotopes between aqueous organic complexes and particulate oxyhydroxide surfaces is likely the key factor for Zn isotope fractionation in soils (Vance et al. 2016).
Furthermore, the Zn isotope geochemistry of soils is affected by biological activity, which has an important role in recycling metals through the biogeosphere (Caldelas and Weiss 2017). Previous studies have demonstrated significant Zn isotope fractionation on the continuum soil–plant (Viers et al. 2007) and large isotope variations during uptake, translocation and distribution of Zn between the internal compartments of plants (Moynier et al. 2009b; Weinstein et al. 2011; Jouvin et al. 2012; Couder et al. 2015; Caldelas and Weiss 2017; Martín et al. 2018). The magnitude and direction of fractionation are related to the interspecies characteristics, nutrient uptake strategies and solution concentrations, elemental speciation, ligands, transpiration flow and height, amongst others (Moynier et al. 2009b; Weinstein et al. 2011; Jouvin et al. 2012; Couder et al. 2015; Caldelas and Weiss 2017; Martín et al. 2018). Models of the Zn transference across the root cell membrane have been based on kinetic fractionation with preferential incorporation of the light isotope via diffusion in low-affinity plants; or with preferential heavy isotope incorporation via Zn complexation in carrier-mediated transport (e.g. zinc/iron-regulated protein, ZIP) in high-affinity pathways (Jouvin et al. 2012). The preferential adsorption of the heavy isotope on the cell wall of roots may also be responsible for lighter Zn isotope composition in solution. The pattern of Zn isotope compositions in plants may also vary depending on the toxic conditions of the grown medium, where preferential uptake of the heavy isotope of Zn in the roots results in the absorption of the light isotope in the aerial compartments (Couder et al. 2015). In mangroves close to Rio de Janeiro (Sepetiba Bay), leaves of Laguncularia racemose showed different 66/64Zn values between highly Zn-contaminated (0.08‰) and low Zn (0.23‰) mangroves (Araújo et al. 2018).
Finally, the soil mineralogy can have a significant impact on the isotopic composition of Zn. Different minerals may have different isotopic signatures, and the weathering of minerals can lead to fractionation of Zn isotopes (Liu et al. 2020). Soil pH can affect the availability and mobility of Zn in soils, which can in turn influence the isotopic composition of Zn. For example, in acidic soils, Zn may be more mobile and may undergo more isotopic fractionation than in alkaline soils (Wisawapipat et al. 2017). The presence of organic matter in soils can also affect the isotopic composition of Zn. Organic matter can influence the adsorption and desorption of Zn, which can affect its isotopic composition (Gu et al. 2022). Microbial activity in soils can also affect the isotopic composition of Zn: microbes can alter the speciation and availability of Zn in soils, leading to isotopic fractionation (Liu et al. 2023).
Aqueous environment
A variety of processes control metal fractionation in the aqueous environment, including equilibrium isotope distribution between dissolved aqueous species and equilibrium/kinetic effects due to processes as adsorption, precipitation, dissolution, and uptake from live organisms (Desaulty et al. 2017). The main physical and chemical processes governing the isotope compositions of Zn in the aqueous environment are: Zn uptake by microorganisms and plants, adsorption on to mineral particles and biological surfaces; complexation with organic matter, mineral phase precipitation and dissolution (Fig. 2) (Pokrovsky et al. 2005; Weiss et al. 2005; Viers et al. 2007; Balistrieri et al. 2008; Fernandez and Borrok 2009; Jouvin et al. 2009, 2012; Caldelas et al. 2011; Veeramani et al. 2015; Guinoiseau et al. 2016).
Although there are various studies on Zn isotope systematics in rivers, many of these are in or near contaminated sites and consequently show isotopic variations between natural and anthropogenic Zn (Araújo et al. 2019; Ding et al. 2024). For Zn geogenic sources, similarities between siliciclastic rocks and marine detrital sediments are observed, hindering the interpretation of the distinguished geological sources (Cloquet et al. 2008; Petit et al. 2015). In rivers that are unaffected by anthropogenic activities, Zn concentrations (in solution) are generally between 1 and 2 g l−1 (Gaillardet et al. 2013; Desaulty et al. 2017), and the 66/64Zn composition can vary between 0.23 and 0.55‰ (Ding et al. 2024). However, 66/64Zn values can show greater variability resulting from natural processes in the aqueous environment. An example in an estuary in France showed progressive enrichment in light isotopes due to mixing (Araújo et al. 2017), which were similar in range to the temporal variations of 66/64Zn values in particulate matter from a temperate Swiss lake (Lake Greifen) (Peel et al. 2009). These samples showed distinct variations from −0.66 to 0.21‰ with the highest negative values observed in the summer, during the period of high primary productivity, and most positive values from May to July. The observed temporal pattern is consistent with the preferential uptake of the lighter Zn isotopes during photosynthesis and incorporation of Zn into the algal material (John et al. 2007), which results in the depletion of Zn in the surface waters and at the same time the enrichment of the heavier Zn isotopes in the residual dissolved and particulate Zn pool. At the end of the productive period, the algal detrital material enriched in the light isotope contribute to bulk particulate matter, leading the particulate Zn pool to lighter isotope compositions. Although Zn isotopes can be successfully applied to aqueous analysis, these measurements can be challenging due to the low Zn concentration in most pristine environments, resulting in difficult purification processes during Zn chromatography.
Fluvial and marine sediments
In oligotrophic systems, both fluvial and terrestrial sediments seem to inherit the lithogenic Zn isotope composition (Little et al. 2014; Fig. 2). In turn, the Zn dissolved pool of rivers and estuaries shows a large range of 66/64Zn values associated with the multitude of biogeochemical processes involved in Zn transport in waters (Petit et al. 2015; Szynkiewicz and Borrok 2016). Amongst them and of pivotal importance are adsorption processes. Various works have investigated, using controlled experimentation, the isotope effects related to aqueous–solid interactions in variable pH values and ionic strength conditions. Overall, the adsorption of Zn on solid surfaces (such as mineral particles and membranes) generally lowers the 66/64Zn of dissolved Zn in solution because of the preferential incorporation of heavier Zn isotopes during adsorption (Pokrovsky et al. 2005; Balistrieri et al. 2008; Juillot et al. 2008; Dekov et al. 2014; Kafantaris and Borrok 2014; Dong and Wasylenki 2016; Guinoiseau et al. 2016). At the surface of kaolinite, the fractionation between the adsorbed pool and the solution (66/64Znadsorbed-solution) ranged from 0.11‰ at low pH and low ionic strength to 0.49‰ at high pH and high ionic strength (Guinoiseau et al. 2016). The Zn isotope fractionation at the surface of Fe-oxides with respect to the coexisting solution is reported, with 66/64Znsolid-solution of 0.29 ± 0.07‰ and 0.53 ± 0.07‰ (ferrihydrite; Balistrieri et al. 2008; Juillot et al. 2008). For Zn adsorption on birnessite (MnO2·nH2O), Zn isotopes showed limited fractionation at low ionic strength, while heavy Zn isotopes are sorbed preferentially at high ionic strength (66/64Znsolid-solution from 0.52 to 0.77‰) (Bryan et al. 2015). Regarding the organic matter, Zn binds to the phenolic sites of purified humic acid with a 66/64Znsolid-solution = 0.24 ± 0.06‰ (Jouvin et al. 2009).
In agreement with the laboratory results, a field study on Rio Grande (USA) performed by Szynkiewicz and Borrok (2016) reported that an increase in fluid pH from 7.5 to 8.5 led to a considerable decrease in the 66/64Zn of dissolved Zn and to isotope fractionation between the adsorbed and dissolved pool (66/64Znadsorbed-dissolved) of 0.31‰. However, in contrast, in the Gironde estuary (France), the dissolved Zn showed an enrichment in heavy isotopes relative to the fluid due to the preferential sorption of the light Zn isotope on to suspended particulate matter (Petit et al. 2015; Zimmermann et al. 2020). The sorption of the lighter isotope may be associated with kinetically driven adsorption caused by the strongly increasing of sorption sites (i.e. removal on to suspended particulate matter) in the water mixing zone of the estuary. The same is observed in the mixing zone of the Solimões River and Negro River (Amazonia, Brazil), where Zn isotope composition of the dissolved pool is fractionated during adsorption on to the suspended sediments and suspended particulate matter (Guinoiseau et al. 2018). Other laboratory experiments have shown significant Zn isotope fractionation during chemical diffusion, mineral precipitation, and biological incorporation by the phytoplankton. Veeramani et al. (2015) showed an increasing trend to heavier values of 66/64Zn in solution during sphalerite (ZnS) formation but decreasing trend in 66/64Zn during the precipitation of hydrozincite (Zn5(CO3)2(OH)6) and hopeite (Zn3(PO4)2·4H2O).
Data from the Southern Ocean, North Atlantic and North Pacific Ocean are strikingly homogeneous, with an average 66/64Zn = 0.53 ± 0.14‰ (Conway and John 2014; Little et al. 2014; Zhao et al. 2014; Moynier et al. 2017; Weber et al. 2018). In contrast, the upper ocean exhibits more variability, with 66/64Zn ranging from 0.07 to +0.80‰ (Zhao et al. 2014). A thin horizon at 40–80 m with lighter 66/64Zn values close to 0.30‰ has been attributed to the preferential biological uptake of the light isotope (Little et al. 2014; Zhao et al. 2014; Moynier et al. 2017). Primary productivity and the preferential removal of the light isotope could explain the heavier isotope compositions of authigenic marine sediment, including Fe–Mn nodules (66/64Zn = 0.90 ± 0.28‰; Fig. 2; Maréchal et al. 2000), biogenic carbonates (66/64Zn = 0.91 ± 0.24‰; Pichat et al. 2003) and biogenic opal (66/64Zn = 0.76–1.47‰; Andersen et al. 2011). However, estimates on mass balance between inputs and outputs of Zn isotopes on the oceans do not match, implying that a missing light sink is still to be located (Little et al. 2014). Although the number of seawater samples presented in papers related to Zn isotopes has increased in the past decade (14 in 2010, 2000 in 2020; Conway et al. 2021), there remains a need for studies on the processes influencing Zn fractionation in high-latitude regions, as these regions present a key knowledge gap for understanding the global biogeochemical Zn cycle (Conway et al. 2020). Other studies on 66/64Zn in aquatic environments have revealed the importance of ferromanganese crusts as potential palaeoceanographic archives of Cu and Zn. For instance, He et al. (2023) show that, in contrast to Cu, the 66/64Zn values of Fe–Mn crusts are isotopically heavy when compared with deep seawater (0.94 ± 0.21‰ v. 0.51 ± 0.14‰) (Fig. 1).
Applications of zinc isotopes in acid mine drainage
In various studies of Zn in acid mine drainage (AMD), the Zn isotope signatures of the dissolved phase have appeared similar to the original ores (sphalerites, for example) and often correlated with geochemical parameters (pH, ionic strength and temperature) (Borrok et al. 2008; Fernandez and Borrok 2009). This conservative behaviour indicates that Zn isotopes are more likely to be tracers of sources rather than attenuation processes in the AMD context (Skierszkan et al. 2016). However, studies conducted in an alkaline-pH river (pH∼8) with an elevated dissolved Zn load from mining activities showed that some Zn is removed during precipitation of Zn-carbonates and amorphous Zn-silicate minerals, which had 66/64Zn values heavier than the dissolved Zn pool (Wanty et al. 2013). Further controlled laboratorial experiments confirmed that carbonate precipitation removes heavy Zn isotopes from solution (Veeramani et al. 2015).
Zn isotopes applied to geochemical exploration
Zinc isotopes fractionate during igneous processes, fluid exsolution, mineral precipitation and supergene processes. Thus, different types of Zn deposits have different Zn isotopic characteristics and variability: Zn isotopes have the potential to trace metal source and provide information into ore-forming processes (Fig. 2). Zinc isotope studies have been conducted in ore deposits, such as on the Alexandrinka volcanic-hosted massive sulfide deposit (VHMS) (Mason et al. 2005), the carbonate-hosted Zn–Pb deposits of the Irish Midlands ore field (Wilkinson et al. 2005), the Red Dog shale-hosted base metal deposit (SEDEX) (Kelley et al. 2009), and modern submarine hydrothermal systems (John et al. 2008). The above have shown that the Zn isotope composition of sphalerite is controlled by the composition of the source-rock(s), and temperature-related and kinetic fractionation during precipitation of sulfides (Pašava et al. 2014). Based on a compilation of datasets published in the literature for Zn ores sulfides, Sonke et al. (2008) calculated an average of 66/64Zn of 0.16 ± 0.20‰ (2s, n = 10 mines, n = 61 analyses). Zinc isotope fractionation in secondary Zn minerals formed during low-temperature hydrothermal and/or supergene oxidation of primary sulfide deposits collected from several Zn deposits in Ireland, Belgium, Poland, Namibia, Peru, Yemen and Zambia presented values ranging from −0.42 to 1.39‰ (Mondillo et al. 2018). Coal and fuel mixtures present 66/64Zn values from 0.13 to 0.88‰ (Borrok et al. 2010; Ochoa Gonzalez and Weiss 2015). This range is suggested to be related to different Zn speciation and diagenetic processes in the precursor material of the coal (Ochoa Gonzalez and Weiss 2015).
In a review by Mathur and Wang (2019), the application of Zn isotopes in geochemical exploration is explored in different deposit types, including VHMS, SEDEX and carbonate hosted Zn-deposits. For instance, for the VHMS deposits, a Rayleigh distillation model is proposed by several authors to explain the increasing trend in 66/64Zn (Maréchal et al. 2000; Archer and Vance 2004; Borrok et al. 2008; John et al. 2008), suggesting that sulfides are enriched in 64Zn during early phase diagenesis, which cause the heavier 66/64Zn values in the residual fluids. There is a gradual increase of this signature from early to late stages in this type of deposit (i.e. from the sphalerite core to rim; Mason et al. 2005; Mathur and Wang 2019).
Emerging applications
Climate change
According to Mahowald et al. (2006), mineral dust aerosols have the potential to affect the global climate by influencing the radiative balance of the atmosphere and the supply of micronutrients to the ocean. From this premise, Zhang et al. (2022b) applied Zn isotopes to mineral dusts from the Chinese Loess Plateau in order to elucidate the main processes leading to Zn fractionation and affirmed that 66/64Zn variations between loess and palaeosol are not sensitive to glacial–interglacial source fluctuations or wind sorting but could result from loess pedogenesis.
Biomonitoring
Zinc isotope composition of mollusc tissues have been proposed as a novel tool potentially capable of integrating sources, fates and bioavailability information of Zn contamination (Shiel et al. 2012, 2013; Petit et al. 2015; Araújo et al. 2021; Fig. 2). Metal concentrations in mollusc tissues is traditionally used in ‘Mussel Watch' programmes in several countries to monitor the levels of contamination in coastal and marine ecosystems (Schöne and Krause 2016). Studies conducted with bivalves on the west coast in Canada showed homogeneous Zn isotope signatures that overlapped those of unpolluted soils, sediments and marine water (Shiel et al. 2012). However, in the case of the Gironde estuary (a site historically impacted by metallurgy), the Zn isotopic signatures in the dissolved pool changes along the fluvial-estuarine due to successive adsorption processes in sediment particles along the Maximum Turbid Zone (MTZ) (Petit et al. 2015). In this case, the isotope records of oysters are close to isotope compositions of the dissolved Zn, besides indicating that particulate Zn is the main source of Zn to the oysters (Petit et al. 2015). Another recent study exposed oysters to waters with different isotopic signatures and concluded that Zn isotope ratios measured in oysters could be used for Zn source tracking (Ma et al. 2020). In contrast, bivalve molluscs collected in severely contaminated areas by electroplating wastes are not correlated to the sources identified in sediment samples, indicating that the isotope record of zinc in bivalve tissues may be controlled by additional factors such as speciation and bioavailability and bioaccumulation pathways via solution or particulate matter (Araújo et al. 2017).
Ma et al. (2020) measured Zn isotopes in molluscs at a hydrothermal vent to understand how the isotopic compositions can change in these organisms in an extreme environment where metal-enriched fluids are in contact with chemosymbiotic animals. Ma et al. (2020) did not observe a significant difference in 66/64Zn among tissues of the molluscs analysed, suggesting the absence of Zn isotope fractionation during internal Zn transport. In addition, they concluded that different species exhibited different mechanisms of Zn uptake, related to different nutrient acquisition and different anatomical differences, which can be tracked by Zn isotopic compositions.
Zn isotopes may record this biological activity based on the preferential uptake of lighter Zn isotopes by phytoplankton/bacteria (John and Conway 2014). The uptake of different Zn isotopes by these microorganisms depends on the concentration of free ions in the nutrient solution (Desaulty et al. 2017). The uptake of heavy isotopes is favoured in low concentration solution. Furthermore, when the mechanism is diffuse transport across the membrane cell, the planktons tend to take up the lighter isotopes (John et al. 2007). Although numerous studies have explored the extent of Zn isotopic fractionations in controlled laboratory settings, only a limited number of field studies have utilized these findings to restrict the understanding of transfer mechanisms for these elements in real environmental conditions, and because the interactions between metal and organisms are numerous and complex, there remain large gaps in understanding Zn transfer within the planktonic food web.
Zinc isotope system in dietary and metabolic processes
The isotopic composition of Zn in trophic levels is influenced by a combination of dietary sources, environmental conditions and physiological processes (Jaouen et al. 2013a; Fig. 2). By understanding how these factors can affect Zn isotopes, researchers can gain insights into the movement of Zn through food webs and the biogeochemical processes that underlie ecosystem functioning. The physiological processes of an organism can also influence the isotopic composition of Zn in its tissues. For example, different physiological processes such as absorption, assimilation and excretion can lead to isotopic fractionation. Additionally, the metabolic rate of an organism can also influence the isotopic composition of Zn in its tissues, as organisms with higher metabolic rates may have different Zn isotopic compositions than organisms with lower metabolic rates (Jaouen et al. 2016a, b).
The isotopic composition of Zn in an organism is largely determined by the isotopic composition of the food that it consumes. Different dietary sources may have different isotopic signatures, which can influence the isotopic composition of Zn in the organism. For example, plant-based diets may have different Zn isotopic compositions than diets consisting of animal products. Because of the large differences of 66/64Zn values observed in the blood of vegetarians and compared with omnivores, the effects of the diet can be tracked in each group (Costas-Rodríguez et al. 2014; McCormack et al. 2022) In general, vegetables, cereals and derived products showed an enrichment of the heavier Zn isotopes, whereas a depletion is observed in products of animal origin (meat, fish, egg and semi-skimmed milk), relative to human blood samples. Thus, the lower 66/64Zn values found in food products of animal origin appear to be reflected in the lower 66/64Zn value observed in blood from an omnivorous population compared to that from a vegetarian population (Van Heghe et al. 2013; Costas-Rodríguez et al. 2014; McCormack et al. 2022). Beyond the dietary, correlations between Zn isotope compositions of blood with age are also found in the isolated population of Yakut (Jaouen et al. 2013a). This same sampling showed a discrepancy between the average 66/64Zn value of blood between European and Japanese populations, which can be associated with different metabolic rates (Jaouen et al. 2013a).
The lighter 66/64Zn values in the food of animal origin seems to reflect the isotope fractionation that takes place along the trophic chains. In African terrestrial food webs, 66/64Zn values in both bone and dental enamel of carnivore animals are lighter by approximately 0.4–0.5‰ in comparison to bones and dental enamel of herbivores (Jaouen et al. 2013b). In the analysis of marine mammal bones (walruses, bearded seals, ringed seals and polar bears), the 66/64Zn values of marine mammal bones also showed a trend toward lighter isotopic values with the increasing of trophic level (Jaouen et al. 2016b). However, different Zn isotope patterns are found in the terrestrial trophic chain of South Africa, where no significant Zn isotope fractionation is observed between bones of herbivores and bones of carnivores (including lion and hyena) (Jaouen et al. 2016a).
More studies regarding internal Zn isotope fractionation within animal bodies and the relationship between diet and bioavailability are necessary to understand how Zn isotopes fractionate from low to high trophic levels. For example, the tendency of heavier Zn isotope compositions of herbivores has been attributed to the precipitation of Zn with phytates in the intestine, inhibiting Zn absorption. This precipitation favours the binding of light Zn isotopes to the phytates, which implies that heavy Zn isotopes are more bioavailable (Jaouen et al. 2013b). In controlled feeding studies, it has been demonstrated that for modern mammals, muscle tissue and bones have a different Zn isotope composition, where muscle tissue had 66Zn-depleted values compared to the food (66/64Znmuscles-food ≈ −0.2‰) and bone had 66Zn-enriched values (66/64Znbones-food ≈ 0.1–0.4‰) (Jaouen et al. 2013b).
Investigations in vivo of Zn isotope ratios in mayflies (Neocloeon triangulifer) from larval to adult stages using biodynamic isotope models demonstrated significant isotope fractionation through the life stages of the insect. Upon metamorphosis to subimago, Zn losses are above 70%, accompanied by enrichment of heavier isotopes in the Zn pool remaining in the subimago bodies, whereas the Zn excess pool excreted along the time is isotopically light (Wanty et al. 2017). These results indicate the potential to use Zn isotope ratio information to infer internal distribution mechanisms and pathways in the cells and tissues of invertebrate organisms.
Over the past three years, 66/64Zn values were used, combined with other stable isotopes (i.e. 15N, 13C; McCormack et al. 2021), in different mammal species from multiple Arctic archaeological sites in order to investigate trophic interactions between predator and prey over a broad geographical area, and it is concluded that 66/64Zn analysis is the best tool to compare diets between spatially and temporally distinct locations. McCormack et al. (2022) demonstrated the use of Zn isotopes to assess the trophic level in diverse extant and extinct sharks, concluding that the 66Zn signature remains preserved in animal teeth, and furthermore, the differences between this signature among different species can be useful to elucidate the extinction cause of sharks.
Biomedicine
Interest in applying high-precision stable isotopic analysis in a biomedical context has increased significantly over the last 20 years (Mahan et al. 2020; Vanhaecke and Costas-Rodríguez 2021). The potential of metal stable isotopic analysis for understanding human diseases can be demonstrated in vitro using cell lines, in vivo using animal models, or in studies involving human subjects (Krayenbuehl et al. 2005; Aramendía et al. 2013; Resano et al. 2013; Sullivan et al. 2023). In these studies, samples are typically analysed that are minimally invasive to obtain (e.g. urine and blood fractions), with the hypothesis that changes in metal homeostasis that are associated with the disease pathology can cause differences in stable isotopic compositions elsewhere in the body that are detectable when compared with a suitably matched control group (Larner et al. 2015; Jaouen et al. 2019).
Due to the implication of Zn in numerous processes in the human body, an important focus has been revealing the influence of various lifestyle and physiological factors on Zn isotopic fractionation (Sullivan et al. 2023). Enhancing our knowledge of physiological and lifestyle factors that induce Zn isotopic fractionation in the human body allows for the appropriate control of confounding factors and is essential to resolving Zn isotopic signatures that are related to a particular disease pathology. In order to explore the applicability of Zn isotopes on understanding metal homeostasis and as a biomarker for diseases, Moynier et al. (2013) assessed the distribution of Zn isotopes in various mouse tissues and found that that most organs are isotopically distinct (Fig. 2). Moynier et al. (2013) observed that red blood cells and bones are enriched by ∼0.5‰ in 66Zn relative to 64Zn when compared with serum, and around ∼1‰ when compared with brain and liver, which is explained by the equilibrium isotopes between different bonding environments of Zn in different organs. Furthermore, differences in gender and genetic background did not affect the isotopic distribution of Zn.
There is strong evidence from animal model and human subject experiments that sex does not influence Zn isotopic compositions, and no difference has been found between the blood Zn isotopic composition of menstruating and non-menstruating populations (Albarède et al. 2011; Jaouen et al. 2012, 2019; Moynier et al. 2013; Van Heghe et al. 2014; Mahan et al. 2018; Schilling et al. 2021). There is conflicting evidence on the influence of age, with some studies showing an increase in 66/64Zn values with age and others showing none (Jaouen et al. 2013a; Van Heghe et al. 2014; Schilling et al. 2021; Sullivan et al. 2023). Studies of modern terrestrial food webs, experimental animals, and ancient and modern humans reveal that 64Zn tends to be enriched up the trophic chain (Balter et al. 2010; Jaouen et al. 2013b, 2016a, b, 2017, 2020, 2022; Moynier et al. 2013). Human dietary components have also been shown to have distinct Zn isotopic signatures, with food from plant origin having a 66/64Zn of 0.40 ± 0.42‰ (standard deviation, 1s) and food from animal origin having a 66/64Zn of −0.34 ± 0.42‰ (1s) (Costas-Rodríguez et al. 2014). This is reflected in a difference between the 66Zn value of blood from omnivores (0.12 ± 0.07‰ (1), and that of lacto-ovo vegetarians (0.26 ± 0.04‰ (1) (Van Heghe et al. 2012; Costas-Rodríguez et al. 2014). A similar pattern is also found between the urine 66/64Zn values of omnivores (0.19 ± 0.21‰ (1)) and both vegetarians (0.49 ± 0.27‰ (1) and dairy-free healthy humans (0.39 ± 0.19‰ (1) (Moore et al. 2019).
Several other physiological and lifestyle factors, including blood type, alcohol and caffeine consumption, smoking, postprandial effects and body mass index, have been investigated to ascertain whether they influence Zn isotopic compositions in the human body, but no effects have been demonstrated (Albarède et al. 2011; Van Heghe 2013; Moore et al. 2019; Sullivan et al. 2020). A number of factors remain to be investigated for their influence on bodily Zn isotopic compositions (e.g. vitamin and nutrient supplementation, basal metabolic rate, physical activity, wound healing and inflammation, pregnancy events, and natural and anthropogenic Zn exposure) (Sullivan et al. 2023).
When it comes to the study of Zn isotopic fractionation in the context of human disease, most investigations have focused on cancer (Larner et al. 2015; Hastuti et al. 2020b; Schilling et al. 2020, 2021, 2022; Sullivan et al. 2021), although several have studied neurological conditions, such as amyotrophic lateral sclerosis (ALS) and Alzheimer's disease (AD) (Moynier et al. 2017, 2020; Sauzéat et al. 2018). Zinc isotopes have also been applied to monitor homeostatic changes following bariatric surgery, with the difference between the 66/64Zn values in serum and whole blood becoming larger over the post-operative time (from 3 to 6 to 12 months), and this is suggested to reflect disrupted Zn homeostasis, impaired Zn status and/or reduced Zn absorption capacity in bariatric surgery patients (Hastuti et al. 2020a). Another study also investigated whether Zn status can be assessed using the Zn isotope composition of urine and found that samples with lower Zn concentrations are systematically enriched in 66Zn (Moore et al. 2019). In the context of neurological disorders, no difference was found between the Zn isotopic composition of cerebrospinal fluid (CSF) from individuals with ALS and controls, but CSF from individuals with AD tends to have slightly lower 66/64Zn values than controls (Sauzéat et al. 2018). This may be explained by results from two other studies that found the accumulation of the heavy isotope of Zn in the brains of humans with AD and APPswe/PSEN1dE9 (model of AD) mice compared with the brains of controls (Moynier et al. 2017, 2020).
In the context of cancer, most focus has been spent on studying the potential of Zn isotopic compositions as a diagnostic marker in blood fractions and urine (Larner et al. 2015; Hastuti et al. 2020b; Schilling et al. 2020, 2021). The Zn isotopic composition of the tumours that would likely influence any Zn isotopic fractionation in blood and urine has only been determined in two studies to date (Larner et al. 2015; Sullivan et al. 2021). Interestingly, despite a statistically significant decrease in 66/64Zn being found in breast cancer tumours compared with histologically normal tissue adjacent to tumours (NAT), this is accompanied by no change in 66/64Zn values in the urine of individuals with breast cancer relative to controls (Schilling et al. 2021; Sullivan et al. 2021). Paradoxically, a similar statistically significant decrease in 66/64Zn that is found in benign breast tumours compared to NAT is accompanied by a statistically significant decrease in the urine 66/64Zn values of individuals with benign breast disease relative to controls (Schilling et al. 2021; Sullivan et al. 2021). Urine from prostate and pancreatic cancer is also characterized by the relative enrichment of 64Zn compared with controls (Schilling et al. 2020, 2021). In the case of haematological malignancy (HM), Zn concentrations and Zn isotopic compositions determined in the blood plasma of individuals with HM are significantly different from controls (Hastuti et al. 2020b). Interestingly, this is the only instance of a blood fraction or urine having a heavier Zn isotopic composition than in controls. The prognostic potential of Zn isotopic compositions was demonstrated using time-dependent receiver operating characteristic curves, with plasma 66/64Zn values showing a similar area under the curve to creatinine, a prognostic marker of HM (Hastuti et al. 2020b).
One key area requiring additional attention for the advancement of Zn isotopic analysis in a biomedical context is the understanding of Zn isotopic fractionation mechanisms. Much of our current understanding of Zn isotopic fractionation in a biomedical context comes from density functional theory estimation of Zn isotopic fractionation that generally predicts the increasing enrichment of 66Zn when binding to increasingly electronegative ligands in amino acids (Fujii et al. 2014). For example, Zn bound to cysteine (Zn–S bonds) is expected to be more enriched in 64Zn than in Zn bound to the histidine (Cu–N bonds). However, it is not yet possible to predict Zn isotopic fractionation in more complex molecules, such as proteins. The further characterization of Zn isotopic compositions of cancerous tumours will also aid the understanding of fractionation mechanisms. To that end, a recent study performed Zn uptake-efflux experiments with a human breast cancer cell line (model for triple receptor negative breast cancer, MDA-MB-231) using a drip-flow reactor (Schilling et al. 2022). Zinc isotopic fractionation is in the opposite direction to what is expected from the measurement of breast cancer tumours, with cell uptake showing an enrichment of 66Zn compared to cell efflux (Schilling et al. 2022). This research highlights the differences that can occur between in vitro and in vivo results and demonstrates that much work is needed to understand Zn homeostasis and isotopic fractionation in the context of cancer (Schilling et al. 2022).
Source tracking
Anthropogenic sources
Given the evidence of variable Zn isotope ratios in environmental compartments (i.e. low-temperature biogeochemical processes, geogenic processes and in ore-refining industries), Zn isotopic signature differences provide a way to discern the sources, pathways and sinks of pollutants (natural and anthropogenic), as well as transfer mechanisms between abiotic and biotic compartments (Tonhá et al. 2020). In the past decade, Zn isotopes have been proven useful as an environmental tool to track contaminant sources in contaminated soils (Fekiacova et al. 2015), sediment cores (Thapalia et al. 2015), coastal environments (Tonhá et al. 2020), bivalves (Araújo et al. 2017) and river surface waters (Chen et al. 2008).
In a series of studies conducted in Sepetiba Bay (Rio de Janeiro, Brazil), a site historically impacted by metallurgy activity, spatial and temporal analysis of Zn isotope compositions of sediment and rock samples fits well in a model of mixing between three main end-members: (i) continental materials brought via rivers; (ii) marine sources; and (iii) a major anthropogenic source associated with the old electroplating wastes released into the bay (Araújo et al. 2017, 2018). Sediment cores collected in the mud flat showed a high correlation between Zn isotope compositions and Zn enrichment factors, suggesting (i) good preservation of isotope records of natural and anthropogenic sources and (ii) no significant isotope fractionation during sediment transport or as a result of diagenetic reactions post-deposition in the estuarine system. The sediment core sampled from a mangrove located in the zone impacted by the old electroplating activity presented levels of Zn up to 4% (dry weight) and preserved the isotope signatures of electroplating wastes. However, there is evidence that biogeochemical processes triggered by hydrodynamics, bioturbation or rhizosphere processes slightly changed the isotope signatures in some layers.
Desaulty et al. (2017) collected surface water and sediments from a French river close to a wastewater treatment plant (WWTP) in order to establish the influence of this anthropogenic source on the river catchment area. The results showed that even several kilometres downstream from the WWTP, Zn could still be determined relative to this anthropogenic source. Desaulty et al. (2017) also established a correlation between the low clay mineral content and the low adsorption on sediments relative to the distance from the source. Zinc isotopes are also used in different environments to establish source relationships. Xia et al. (2020) revealed Zn sources and transport processes in a karst region by analysing tailings, river waters, sediment, dust and soil in southwestern China. The study was successful in tracking the pollution sources and pathways of heavy metals in this complicated hydrosystem, showing distinct Zn isotope compositions among tailings, agriculture and geogenic soil.
We conclude that besides being a successful tool to analyse different sources, Zn isotopic interpretation may be challenging due to the numerous sources of Zn in the environment. Combined isotopic systems can take us further on understanding the behaviour of trace metals in the different compartments of a river basin by tracking different physical and chemical processes governing the distribution of solid and dissolved phases.
Urban sources: road dust and traffic-related emissions
Providing isotope data from various anthropogenic sources remains an essential task for source identification in environmental forensic fields and estimating their contributions. For example, non-exhaust emission (NEE) sources (e.g. brake pads, tires, road paints, asphalt and curbs) are considered as significant anthropogenic sources in the urban environment with their distinct isotope signatures (Dong et al. 2017; Jeong 2022; Jeong et al. 2022). Isotope compositions of traffic-related NEE sources covered from –0.4 to 0.6‰ (66/64Zn) (Jeong 2022; Jeong et al. 2022), and of road dust ranged from –0.1 to 0.4‰ (Dong et al. 2017; Souto-Oliveira et al. 2018; Jeong and Ra 2021) (converted value by adding 0.30‰ from IRMM-3702 to JMC-Lyon; Moynier et al. 2017). Ultimately, it is imperative to prioritize the effective management of traffic-related pollutants, as these contaminants can traverse from terrestrial sources to the vast expanses of the ocean, where they can exert deleterious effects on marine life. As humans ingest marine food resources, the ramifications of this contamination have the potential to extend to human health. Hence, a concerted effort towards mitigating and preventing such pollutants is critical for safeguarding the sustainability of marine ecosystems and the welfare of humankind.
Antifouling paints
Antifouling paints are widely employed to protect submerged surfaces against biofouling, which is the attachment and growth of marine organisms on submerged surfaces such as boat hulls and pipelines, leading to detrimental effects on economic and operational aspects. Following the ban of tributyltin (TBT), Cu-based antifouling paints have emerged as viable alternatives for preventing biofouling. However, concerns regarding their potential environmental impact have been raised due to their extremely high concentrations of Cu and Zn (Jeong et al. 2023). The Zn isotopic composition (66/64Zn) of antifouling paints ranged from –0.04 to 0.33‰ (mean: 0.17 ± 0.20‰, 2, n = 25; Jeong et al. 2023) (converted value by adding 0.30‰ from IRMM-3702 to JMC-Lyon; Moynier et al. 2017), and their chemical compositions indicate that they are distinguished from the other urban sources.
Metallurgical waste
The isotope signatures of feed ores and coals have important implications to final products and byproducts produced in metallurgic industry and coal-fired power plants. High-temperature processes employed during sulfide ore smelting and coal combustion induce significant isotope fractionation, with lighter Zn isotopes entering the vapour phase and leaving the residual material (e.g. fly ash particles, slags) enriched with the heavier isotopes by Rayleigh fractionation processes (Sivry et al. 2008; Borrok et al. 2010; Ochoa Gonzalez and Weiss 2015; Yin et al. 2015). In the case of ore refining, additional electroplating steps also seem to affect Zn isotope fractionation, as demonstrated in controlled electrochemical experiments (Kavner et al. 2008; Black et al. 2011) and in the analysis of contaminated sediment by electroplating slags. Despite these gaps concerning the electrochemistry effects on industrial plants, a general trend has been observed in Zn extraction of sulfide ores: the production of air emissions with the lightest signatures (66/64Zn = −0.66‰), slags with the heaviest signatures (66/64Zn = 1.49‰) and final products (refined Zn electroplated materials) with isotope compositions close to the ores (66/64Zn = 0.22‰) (Yin et al. 2015). The significant differences in the isotope compositions derived of anthropogenic sources compared to the natural background (soils and rocks, for example) have allowed the use of Zn isotopes as a powerful tracer of sources capable of discriminating and quantifying several pollutant sources concerning mining and metallurgy byproducts (Dolgopolova et al. 2006; Weiss et al. 2007; Sivry et al. 2008; Mattielli et al. 2009), and others such as vehicle emissions and tire wear (Gioia et al. 2008; Thapalia et al. 2015; Souto-Oliveira et al. 2018, 2019), and wastewaters and sewage (Chen et al. 2008, 2009a, b).
Conclusions
The versatility and usefulness of Zn isotopes is proved by the many applications in different scientific fields. The current increase of research groups on metal isotope biogeochemistry will contribute to the continuous developing of the Zn isotope biogeochemistry, leading to new methodologies and advances in the theoretical understanding about the factors that control its isotope fractionation. The isotope determinations combined with Zn speciation are an important step forward to the understanding of biogeochemical and environmental cycles, and can open new frontiers in biomedicine applications, including the establishment of the medical isotope metallomics field.
Future studies are required to link Zn speciation and bioavailability and mechanisms of isotope fractionation during its incorporation in biota or partitioning within higher organisms or the human body. The development of selective or sequential extraction for isotope analysis of different phases of soils and sediments could be a helpful auxiliary tool for comprehending the isotope fractionation that occurs during bioaccumulation. However, more robust and reproducible protocols, beyond the verification of possible artefacts associated with the chemical procedure, are also required.
New efforts to increase the assessment of Zn isotopic signatures in environmental compartments can contribute to identify sources and sinks of anthropogenic Zn and its cycling through the biogeosphere, in either the local or global scale. Of special concern for aquatic systems is the analysis of the pore waters, which has important implications for understanding diagenetic reactions and bioavailability of Zn, mainly in coastal zones and wetlands, where the large gradients of redox conditions promote periodic releasing and trapping of metal in sediments.
In biomonitoring studies, more tests using other species must be encouraged to verify the feasibility of Zn isotopes as a monitor of metal contamination. To date, their use in bivalve molluscs has been hampered by the lack of knowledge about the physiological influences on the Zn bio-isotope record. Finally, new controlled experiments regarding the effects of chemistry coordination of Zn with different ligands will be useful to interpret and predict the isotope fractionation in the environment and biological systems, encompassing stratigraphic Zn profiles along the geological time to cellular metabolism processes. Although Zn isotopes as source trackers have been intensively studied in the past years, most studies have mainly focused on tracing point-source pollutions in the nearby environment instead of studying it on a large scale, i.e. in the case of aquatic environments, a combination of hydrodynamical processes (i.e. discharge, mixing between tributaries). The understanding of the metal behaviour demands long-term and high-resolution monitoring of metal loads.
This review gathers information on different applications of Zn isotopes in different types of materials and environments. Understanding and implementing these isotopic signatures will enhance our capacity to interpret findings and conduct routine, highly accurate and precise measurements of the Zn isotope ratio in diverse types of samples. This advancement opens avenues for new and exciting applications.
Author contributions
TPJ: conceptualization (lead), data curation (lead), formal analysis (lead), investigation (lead), methodology (lead), validation (lead), writing – original draft (lead); BV: data curation (lead), formal analysis (lead), funding acquisition (lead), investigation (lead), methodology (lead), writing – original draft (lead); MIL: conceptualization (lead), investigation (lead), supervision (lead), validation (lead), writing – original draft (lead); ALH: conceptualization (lead), data curation (lead), investigation (lead), methodology (lead), supervision (lead), validation (lead), writing – original draft (lead); KVS: conceptualization (lead), investigation (equal), validation (equal), writing – original draft (lead); HJ: conceptualization (equal), investigation (equal), validation (equal), writing – original draft (equal); DFA: data curation (lead), formal analysis (lead), investigation (lead), methodology (lead), writing – original draft (lead).
Funding
This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.