Heavy Zn Isotopes Research Articles

Zinc isotope fractionation during grain filling of wheat and a comparison of zinc and cadmium isotope ratios in identical soil-plant systems.

Remobilization of zinc (Zn) from shoot to grain contributes significantly to Zn grain concentrations and thereby to food quality. On the other hand, strong accumulation of cadmium (Cd) in grain is detrimental for food quality. Zinc concentrations and isotope ratios were measured in wheat shoots (Triticum aestivum) at different growth stages to elucidate Zn pathways and processes in the shoot during grain filling. Zinc mass significantly decreased while heavy Zn isotopes accumulated in straw during grain filling (Δ66 Znfullmaturity-flowering =0.21-0.31‰). Three quarters of the Zn mass in the shoot moved to the grains, which were enriched in light Zn isotopes relative to the straw (Δ66 Zngrain-straw -0.21 to -0.31‰). Light Zn isotopes accumulated in phloem sinks while heavy isotopes were retained in phloem sources likely because of apoplastic retention and compartmentalization. Unlike for Zn, an accumulation of heavy Cd isotopes in grains has previously been shown. The opposing isotope fractionation of Zn and Cd might be caused by distinct affinities of Zn and Cd to oxygen, nitrogen, and sulfur ligands. Thus, combined Zn and Cd isotope analysis provides a novel tool to study biochemical processes that separate these elements in plants.

Zinc association with surface-bound iron-hydroxides on cultured marine diatoms: A zinc stable isotope perspective

Abstract Uptake of trace metals by marine phytoplankton for metabolic use exerts a fundamental control on their marine geochemical distributions. Moreover, such trace metals limit primary productivity over large areas of the surface ocean. As such, an understanding of the mechanisms and extent of phytoplankton uptake are essential components of oceanic trace metal chemistry. Efforts to quantify intra-cellular quotas of phytoplankton are complicated by the presence of metals adsorbed to external surfaces, including surface-bound Fe-hydroxides, both in nature and in culturing experiments. In the relatively new discipline focused on oceanic metal isotopes, these surface-bound metal hydroxides may be of particular importance in that they could result in isotope signatures that complicate studies that seek to understand intra-cellular signatures related to metabolic uptake. In this contribution, we assess the extent to which heavy Zn isotopes are preferentially adsorbed to surface-bound Fe-hydroxides on marine diatoms. For this purpose, the marine diatom Thalassiosira oceanica has been cultured at low versus high inorganic Fe concentrations in the medium, while two further diatoms strains have been compared at elevated Fe levels. The formation of surface bound Fe-hydroxides at elevated Fe was further stimulated by reducing the trace metal buffering capacity of the experimental medium, lowering the concentration of the used organic chelator. We also investigate an alternative procedure for quantifying intra-cellular metal quotas, the analysis of the contents of deliberately lysed cells. In good agreement with previous work, we find that biomass associated Fe/P ratios represent a good proxy for the absolute quantity of Fe-precipitates on diatom surfaces. Zn sorption to these surface-bound Fe-hydroxides can drive bulk biomass δ 66Zn compositions up. On the other hand, the loss of heavy Zn from the experimental medium causes the biomass Δ66Zn, if referred to the starting medium, to be biased in the other direction, towards more negative values. To avoid any such complications, likely to occur at high Fe or low buffer capacities, we conclude that diatoms cultured at low Fe are most likely to record the Δ66Zn signatures of Zn uptake into the phytoplankton cell.

Examining the homeostatic distribution of metals and Zn isotopes in Göttingen minipigs.

The role of metals in biologic systems is manifold, and understanding their behaviour in bodily processes, especially those relating to neurodegenerative diseases, is at the forefront of medical science. The function(s) of metals - such as the transition metals - and their utility in both the diagnosis and treatment of diseases in human beings, is often examined via the characterization of their distribution in animal models, with porcine models considered exceptional proxies for human physiology. To this end, we have investigated the homeostatic distribution of numerous metals (Mg, K, Ca, Mn, Fe, Cu, Zn, Rb and Mo), the non-metal P, and Zn isotopes in the organs and blood (red blood cells, plasma) of Göttingen minipigs. These results represent the first set of data outlining the homeostatic distribution of metals and Zn isotopes in Göttingen minipigs, and indicate a relatively homogeneous distribution of alkali/alkaline earth metals and P among the organs, with generally lower levels in the blood, while indicating more heterogeneous and systematic abundance patterns for transition metals. In general, the distribution of all elements analysed is similar to that found in humans. Our elemental abundance data, together with data reported for humans in the literature, suggest that element-to-element ratios, e.g. Cu/Mg, show potential as simple diagnostics for diseases such as Alzheimer's. Isotopic data indicate a heterogeneous distribution of Zn isotopes among the organs and blood, with the liver, heart and brain being the most depleted in heavy Zn isotopes, and the blood the most enriched, consistent with observations in other animal models and humans. The Zn isotopic composition of Göttingen minipigs displays a systematic offset towards lighter δ66Zn values relative to mice and sheep models, suggesting physiology that is more closely aligned with that of humans. Cumulatively, these observations strongly suggest that Göttingen minipigs are an excellent animal model for translational research involving metals, and these data provide a strong foundation for future research.

Shell evolution beyond Z = 28 and N = 50: Spectroscopy of 81,82,83,84Zn

We report on the measurement of new low-lying states in the neutron-rich 81,82,83,84Zn nuclei via in-beam γ-ray spectroscopy. These include the 41+→21+ transition in 82Zn, the 21+→0g.s.+ and 41+→21+ transitions in 84Zn, and low-lying states in 81,83Zn were observed for the first time. The reduced E(21+) energies and increased E(41+)/E(2+1) ratios at N=52, 54 compared to those in 80Zn attest that the magicity is confined to the neutron number N=50 only. The deduced level schemes are compared to three state-of-the-art shell model calculations and a good agreement is observed with all three calculations. The newly observed 2+ and 4+ levels in 84Zn suggest the onset of deformation towards heavier Zn isotopes, which has been incorporated by taking into account the upper sdg orbitals in the Ni78-II and the PFSDG-U models.

Kinetic control on Zn isotope signatures recorded in marine diatoms

Marine diatoms dominate the oceanic cycle of the essential micronutrient zinc (Zn). The stable isotopes of zinc and other metals are increasingly used to understand trace metal micronutrient cycling in the oceans. One clear feature of the early isotope data is the heavy Zn isotope signature of the average oceanic dissolved pool relative to the inputs, potentially driven by uptake of light isotopes into phytoplankton cells and export to sediments. However, despite the fact that diatoms strip Zn from surface waters across the Antarctic polar front in the Southern Ocean, the local upper ocean is not isotopically heavy. Here we use culturing experiments to quantify the extent of Zn isotope fractionation by diatoms and to elucidate the mechanisms driving it.We have cultured two different open-ocean diatom species (T. oceanica and Chaetoceros sp.) in a series of experiments at constant medium Zn concentration but at bioavailable medium Fe ranging from limiting to replete. We find that T. oceanica can maintain high growth rates and Zn uptake rates over the full range of bioavailable iron (Fe) investigated, and that the Zn taken up has a δ66Zn that is unfractionated relative to that of the bioavailable free Zn in the medium. The studied representative of the genus Chaetoceros, on the other hand, shows more significantly reduced Zn uptake rates at low Fe and records more variable biomass δ66Zn signatures, of up to 0.85‰ heavier than the medium. We interpret the preferential uptake of heavy isotopes at extremely low Zn uptake rates as potentially due to either of the following two mechanisms. First, the release of extracellular polymeric substances (EPS), at low Fe levels, may preferentially scavenge heavy Zn isotopes. Second, the Zn uptake rate may be slow enough to establish pseudo-equilibrium conditions at the transporter site, with heavy Zn isotopes forming more stable surface complexes.Thus we find that, in our experiments, Fe-limitation exerts a key control that not only limits diatom growth, but also affects the Zn uptake physiology of diatoms. Uptake of heavy isotopes occurs under Fe-limiting conditions that drive extremely low Zn uptake rates. On the other hand, more rapid Zn uptake rates result in biomass that is indistinguishable from the external bioavailable free Zn pool. These experimental results can, in principle, explain the range of Zn isotopic compositions found in the real surface ocean, given the geographically variable interplay between Fe-limitation, Zn uptake rates, and the degree of organic complexation of oceanic Zn.

The origin of volatile element depletion in early solar system material: Clues from Zn isotopes in chondrules

Volatile lithophile elements are depleted in the different planetary materials to various degrees, but the origin of these depletions is still debated. Stable isotopes of moderately volatile elements such as Zn can be used to understand the origin of volatile element depletions. Samples with significant volatile element depletions, including the Moon and terrestrial tektites, display heavy Zn isotope compositions (i.e. enrichment of 66Zn vs. 64Zn), consistent with kinetic Zn isotope fractionation during evaporation. However, Luck et al. (2005) found a negative correlation between δ66Zn and 1/[Zn] between CI, CM, CO, and CV chondrites, opposite to what would be expected if evaporation caused the Zn abundance variations among chondrite groups.We have analyzed the Zn isotope composition of multiple samples of the major carbonaceous chondrite classes: CI (1), CM (4), CV (2), CO (4), CB (2), CH (2), CK (4), and CK/CR (1). The bulk chondrites define a negative correlation in a plot of δ66Zn vs 1/[Zn], confirming earlier results that Zn abundance variations among carbonaceous chondrites cannot be explained by evaporation. Exceptions are CB and CH chondrites, which display Zn systematics consistent with a collisional formation mechanism that created enrichment in heavy Zn isotopes relative to the trend defined by CI–CK.We further report Zn isotope analyses of chondrite components, including chondrules from Allende (CV3) and Mokoia (CV3), as well as an aliquot of Allende matrix. All chondrules are enriched in light Zn isotopes (∼500 ppm on 66Zn/64Zn) relative to the bulk, contrary to what would be expected if Zn were depleted during evaporation, on the other hand the matrix has a complementary heavy isotope composition. We report sequential leaching experiments in un-equilibrated ordinary chondrites, which show sulfides are isotopically heavy compared to silicates and the bulk meteorite by ca. +0.65 per mil on 66Zn/64Zn. We suggest isotopically heavy sulfides were removed from either chondrules or their precursors, thereby producing the light Zn isotope enrichments in chondrules.

Zn Isotope Fractionation during Sorption onto Kaolinite.

In this study, we quantify zinc isotope fractionation during its sorption onto kaolinite, by performing experiments under various pH, ionic strength, and total Zn concentrations. A systematic enrichment in heavy Zn isotopes on the surface of kaolinite was measured, with Δ(66)Znadsorbed-solution ranging from 0.11‰ at low pH and low ionic strength to 0.49‰ at high pH and high ionic strength. Both the measured Zn concentration and its isotopic ratio are correctly described using a thermodynamic sorption model that considers two binding sites: external basal surfaces and edge sites. Based on this modeling approach, two distinct Zn isotopic fractionation factors were calculated: Δ(66)Znadsorbed-solution = 0.18 ± 0.06‰ for ion exchange onto basal sites, and Δ(66)Znadsorbed-solution = 0.49 ± 0.06‰ for specific complexation onto edge sites. These two distinct factors indicate that Zn isotope fractionation is dominantly controlled by the chemical composition of the solution (pH, ionic strength).

Copper and zinc isotope fractionation during deposition and weathering of highly metalliferous black shales in central China

Black shales represent one of the main reservoirs of metals released to hydrosphere via chemical weathering and play an important role in geochemical cycling of metals in the ocean. The stable isotope systematics of transitional metals (e.g., Cu and Zn) may be used as a proxy for evaluating their geochemical cycling. To investigate the behaviors of Cu and Zn isotopes during metal enrichment of black shales and the migration during weathering, in this study we reported Cu and Zn concentration and isotope data for unweathered and weathered metalliferous shales and siliceous interbeds from the Maokou Formation in central China. The unweathered shales and cherts have moderately enriched Cu and Zn concentrations with silicate-like δ65Cu (+0.14±0.09‰, 1σ) but heavy δ66Zn (0.51±0.11‰, 1σ). The elevated δ66Zn values reflect an important contribution from seawater via sulfide precipitation and/or organic matter (OM) adsorption. The Zn isotopic compositions of these metalliferous shales are different from those of the ‘normal’ shales, highlighting the potential of Zn isotopes as a tracer for metal enrichment in natural systems.The weathered shales and cherts have an extreme δ65Cu range from −6.42‰ to +19.73‰ and a modest δ66Zn range of +0.25‰ to +0.78‰. The strongly weathered samples have lower Cu and Zn concentrations and lighter isotopic compositions compared to the weakly weathered samples. The leaching of Cu- and Zn-rich sulfides in shallow depths and their downward transport and refixation by Fe-sulfide account for the Cu and Zn isotope fractionation, with the huge Cu isotope variation generated by multistage redox leaching. In general, δ66Zn values of the weathered shales shift towards light values compared to the unweathered protoliths, suggesting that shale weathering releases Zn which is isotopically heavier than igneous rocks and the global riverine average (+0.33‰). Our results therefore indicate that Cu isotopes can be extremely fractionated during weathering of Cu-rich shales and both heavy Cu and Zn isotopes are preferentially released into fluids during shale weathering. These results should be considered when evaluating geochemical cycling of Cu and Zn in the modern or past oceans.

Extensive volatile loss during formation and differentiation of the Moon.

Low estimated lunar volatile contents, compared with Earth, are a fundamental observation for Earth–Moon system formation and lunar evolution. Here we present zinc isotope and abundance data for lunar crustal rocks to constrain the abundance of volatiles during the final stages of lunar differentiation. We find that ferroan anorthosites are isotopically heterogeneous, with some samples exhibiting high δ66Zn, along with alkali and magnesian suite samples. Since the plutonic samples were formed in the lunar crust, they were not subjected to degassing into vacuum. Instead, their compositions are consistent with enrichment of the silicate portions of the Moon in the heavier Zn isotopes. Because of the difference in δ66Zn between bulk silicate Earth and lunar basalts and crustal rocks, the volatile loss likely occurred in two stages: during the proto-lunar disk stage, where a fraction of lunar volatiles accreted onto Earth, and from degassing of a differentiating lunar magma ocean, implying the possibility of isolated, volatile-rich regions in the Moon's interior.

Zn isotope fractionation in a pristine larch forest on permafrost-dominated soils in Central Siberia

Stable Zn isotopes fractionation was studied in main biogeochemical compartments of a pristine larch forest of Central Siberia developed over continuous permafrost basalt rocks. Two north- and south-oriented watershed slopes having distinctly different vegetation biomass and active layer depth were used as natural proxy for predicting possible future climate changes occurring in this region. In addition, peat bog zone exhibiting totally different vegetation, hydrology and soil temperature regime has been studied.The isotopic composition of soil profile from Central Siberia is rather constant with a δ66Zn value around 0.2‰ close to the value of various basalts. Zn isotopic composition in mosses (Sphagnum fuscum and Pleurozium schreberi) exhibits differences between surface layers presenting values from 0.14 to 0.2‰ and bottom layers presenting significantly higher values (0.5 – 0.7‰) than the underlain mineral surface. The humification of both dead moss and larch needles leads to retain the fraction where Zn bound most strongly thus releasing the lighter isotopes in solution and preserving the heavy isotopes in the humification products, in general accord with previous experimental and modeling works [GCA 75:7632–7643, 2011].The larch (Larix gmelinii) from North and South-facing slopes is enriched in heavy isotopes compared to soil reservoir while larch from Sphagnum peatbog is enriched in light isotopes. This difference may result from stronger complexation of Zn by organic ligands and humification products in the peat bog compared to mineral surfaces in North- and South-facing slope.During the course of the growing period, Zn followed the behavior of macronutrients with a decrease of concentration from June to September. During this period, an enrichment of larch needles by heavier Zn isotopes is observed in the various habitats. We suggest that the increase of the depth of rooting zone, and the decrease of DOC and Zn concentration in soil solution from the root uptake zone with progressively thawing soil could provoke heavy isotopes to become more available for the larch roots at the end of the vegetative season compared to the beginning of the season, because the decrease of DOC will facilitate the uptake of heavy isotope as it will be less retained in strong organic complexes.

Impact of root-induced mobilization of zinc on stable Zn isotope variation in the soil-plant system.

Stable Zn isotopes are increasingly used to trace the source of metal pollution in the environment and to gain a better understanding of the biogeochemical cycle of Zn. In this work, we investigated the effect of plants on Zn isotope fractionation in the soil-plant system of the surface horizon of two Zn-rich Technosols (pH 6.73-7.51, total Zn concentration = 9470-56600 mg kg(-1)). In a column experiment, the presence of Agrostis capillaris L. significantly increased the mobilization of Zn from soil to leachate, predominantly as a result of root-induced soil acidification. The zinc isotope compositions of plants and leachates indicated that the Zn uptake by A. capillaris did not fractionate Zn isotopes as compared to the leachates. Within the plant, heavier Zn isotopes were preferentially retained in roots (Δ66Znroot - shoot=+0.24 to +0.40 ‰). More importantly, the Zn released in leachates due to root-induced mobilization was isotopically heavier than the Zn released in the absence of plants (Δ66Zn=+0.16 to +0.18 ‰). This indicates that the rhizosphere activity of A. capillaris mobilized Zn from another pool than the one that spontaneously releases Zn upon contact with the percolating solution. Mobilization of Zn by the roots might thus exert a stronger influence on the Zn isotope composition in the soil solution than the Zn uptake by the plant. This study highlights the key role of the rhizosphere activity in Zn release in soil and demonstrates that stable Zn isotopes provide a useful proxy for the detection of Zn mobilization in soil-plant systems.

Zn isotope compositions of the thermal spring waters of La Soufrière volcano, Guadeloupe Island

To trace the sources and pathways of Zn in hydrothermal systems, the Zn isotope compositions of seventeen water samples from eight thermal springs and six gas samples from two fumaroles from La Soufrière, an active volcano on Guadeloupe Island (French West Indies, FWI), were analyzed using a method adapted for purifying Zn from Fe- and SO4-enriched thermal solutions. The fumaroles are enriched in Zn 100 to 8000 times compared to the local bedrock and have isotopic compositions (δ66Zn values from +0.21‰ to +0.35‰) similar to or slightly higher than fresh andesite (+0.21‰). The enrichment of Zn in the thermal springs compared with the surface waters shows that Zn behaves as a soluble element during hydrothermal alteration but is significantly less mobile than Na. The δ66Zn values of most of the spring waters are relatively constant (approximately 0.70‰), indicating that the thermal springs from La Soufrière are enriched in heavy isotopes (i.e., 66Zn) compared to the host rocks (from −0.14‰ to +0.42‰). Only three thermal springs have lower δ66Zn values (as low as −0.43%). While the Zn in the fumaroles is essentially derived from magma degassing, which is consistent with a previous study on Merapi volcano (Toutain et al., 2008), we show that the Zn in the thermal springs is mainly derived from water–rock interactions. The 66Zn-enriched isotopic signature in most of the spring waters can be explained qualitatively by the precipitation at depth of sulfide minerals that preferentially incorporate the light isotopes. This agrees with the isotopic fractionation that was recently calculated for aqueous complexes of Zn. The few thermal springs with lower δ66Zn values also have low Zn concentrations, indicating the preferential scavenging of heavy Zn isotopes in the hydrothermal conduits.This study shows that unlike chemical weathering under surface conditions, hydrothermal alteration at high temperatures significantly fractionates Zn isotopes and enriches thermal waters in heavy Zn isotopes (e.g., 66Zn). Continental hydrothermal systems therefore constitute a source of heavy Zn isotopes to the oceans; this should be taken into account in the global oceanic budget of Zn.

Fractionation of Stable Zinc Isotopes in the Field-Grown Zinc Hyperaccumulator Noccaea caerulescens and the Zinc-Tolerant Plant Silene vulgaris

Stable Zn isotope signatures offer a potential tool for tracing Zn uptake and transfer mechanisms within plant-soil systems. Zinc isotopic compositions were determined in the Zn hyperaccumulator Noccaea caerulescens collected at a Zn-contaminated site (Viviez), a serpentine site (Vosges), and a noncontaminated site (Sainte Eulalie) in France. Meanwhile, a Zn-tolerant plant ( Silene vulgaris ) was also collected at Viviez for comparison. While δ(66)Zn was substantially differentiated among N. caerulescens from the three localities, they all exhibited an enrichment in heavy Zn isotopes of 0.40-0.72‰ from soil to root, followed by a depletion in heavy Zn from root to shoot (-0.10 to -0.50‰). The enrichment of heavy Zn in roots is ascribed to the transport systems responsible for Zn absorption into root symplast and root-to-shoot translocation, while the depletion in heavy Zn in shoots is likely to be mediated by a diffusive process and an efficient translocation driven by energy-required transporters (e.g., NcHMA4). The mass balance yielded a bulk Zn isotopic composition between plant and soil (Δ(66)Zn(plant-soil)) of -0.01‰ to 0.63‰ in N. caerulescens , indicative of high- and/or low-affinity transport systems operating in the three ecotypes. In S. vulgaris , however, there was no significant isotope fractionation between whole plant and rhizosphere soil and between root and shoot, suggesting that this species appears to have a particular Zn homeostasis. We confirm that quantifying stable Zn isotopes is useful for understanding Zn accumulation mechanisms in plants.

Ab Initio Calculation of the Zn Isotope Effect in Phosphates, Citrates, and Malates and Applications to Plants and Soil

Stable Zn isotopes are fractionated in roots and leaves of plants. Analyses demonstrate that the heavy Zn isotopes are enriched in the root system of plants with respect to shoots and leaves as well as the host soil, but the fractionation mechanisms remain unclear. Here we show that the origin of this isotope fractionation is due to a chemical isotope effect upon complexation by Zn malates and citrates in the aerial parts and by phosphates in the roots. We calculated the Zn isotope effect in aqueous citrates, malates, and phosphates by ab initio methods. For pH<5, the Zn isotopic compositions of the various parts of the plants are expected to be similar to those of groundwater. In the neutral to alkaline region, the calculations correctly predict that 66Zn is enriched over 64Zn in roots, which concentrate phosphates, with respect to leaves, which concentrate malates and citrates, by about one permil. It is proposed that Zn isotope fractionation represents a useful tracer of Zn availability and mobility in soils.

Historical variations in the isotopic composition of atmospheric zinc deposition from a zinc smelter

In order to test the usefulness of stable zinc isotopes as an atmospheric source tracer, we analyzed the zinc isotopic composition of two sediment cores, taken at 1 km distance of the former zinc smelter in Lommel, Belgium. The peat bog lake sediments accumulate mainly atmospheric particles, have high organic matter contents (12–60 wt.%), are anoxic and highly contaminated with heavy metals (up to 4.7 wt.% Zn, and 1.1 wt.% Pb) with a sulfide mineralogical control on mobility. Down core variations in δ 66Zn (relative to the JMC 3-0749L standard) were small, ranging from + 0.07‰ to + 0.39‰, but are nevertheless eight times the external reproducibility of 0.04‰. Good agreement was found between the two cores, and despite 30% Zn mobilization to deeper layers, no evidence of associated diagenetic Zn isotope fractionation was found. Sediments deposited in the early 20th century have δ 66Zn of + 0.30‰ ± 0.05‰ (2SD, n = 5) and a shift takes place between 1945 and 1950 to δ 66Zn values of + 0.14‰ ± 0.09‰ (2SD, n = 7) in the 2nd half of the century. In order to understand this shift in δ 66Zn we analyzed 32 ore-grade sphalerite (ZnS) samples from African, Australian and European origin. Together with 29 published δ 66Zn values for ore-grade ZnS, we find remarkably homogeneous isotopic compositions, which when averaged for mining location gives δ 66Zn of + 0.16‰ ± 0.20‰ (2SD, n = 10 mines, n = 61 analyses). Early 20th century Zn deposition with δ 66Zn of +0.30‰ is significantly different (p < 0.001) from average ZnS δ 66Zn of +0.16‰. We suggest that this reflects the presence of an atmospheric smelting residue (slag) component, enriched in the heavy Zn isotopes due to Rayleigh type fractionation during Zn refining.

Evidence of Zn isotopic fractionation in a soil–plant system of a pristine tropical watershed (Nsimi, Cameroon)

The objective of this study is to determine for the first time the zinc (Zn) isotopic composition of different soil horizons, parent rocks, litter, and plants from a single pristine watershed. Three soil profiles from Nsimi–Zoétélé site (South Cameroon, Africa) have been investigated. The δ 66Zn (δ 66Zn = [( 66Zn/ 64Zn) sample / ( 66Zn/ 64Zn) JMC 3–0749L) − 1] ⁎ 10 3) measured in the soils and rocks range from − 0.05 to 0.64‰. In a typical hillslope soil profile developed in situ from a granitic parent rock, we observe two important and systematic trends: i) a weathering regime in the saprolite horizon that involves strong Zn depletion, with no or slight fractionation compared to the fresh igneous rock, and ii) a shallower weathering regime with significant depletion in heavy Zn isotopes in the most superficial soil horizons. In contrast, the soil profile of the swamp zone does not exhibit isotopic fractionation between the soil and the parent rock. Six plants and two litter samples were analyzed for their Zn isotopic compositions. δ 66Zn varied from 0.75 to − 0.91‰ among all samples. Plant roots and shoots are generally enriched in heavy isotopes (δ 66Zn of + 0.42 to + 0.76‰) relative to the litter layer (0.12 to 0.25‰) and most superficial soils. Similar to the controlled plant growth study of Weiss et al. [Weiss, D.J., Mason, T.F.D., Zhao, F.J., Kirk, G.J.D., Coles, B.J. and Horstwood, M.S.A. (2005) Isotopic discrimination of zinc in higher plants. New Phytologist 165, 703-710.], negative δ 66Zn values were obtained for tree leaves (the most aerial part of the tree). Based on biomass data for the tropical forest, vegetation is on average enriched in light isotopes compared to the litter and the superficial soil horizons which are the nutrients reservoirs. Due to the age of these soils and the complexity of pedogenetic processes, it is difficult to explain the isotopic composition of the soils at present. However, the transformation of the saprolite horizon into a ferruginous horizon is associated with an enrichment in light Zn isotopes. This observation may be due to the adsorption of Zn onto clays surfaces. The uptake of Zn by roots in the soils leads to an enrichment in heavy isotopes. Once the Zn is in the xylem, it will be transported within the plant. During this transport, both diffusion and active uptake of heavy isotopes by cells out of the xylem favour the mobility of light isotopes to the most aerial parts of the plants. The difference in Zn isotopic composition between the deep horizons and the superficial horizons of soils suggests that Zn isotopes can be used to discriminate the source of Zn and other metals leaving the catchment by river water.