Zinc isotope fractionation during adsorption onto Mn oxyhydroxide at low and high ionic strength

Accepted Manuscript Zinc isotope fractionation during adsorption onto mn oxyhydroxide at low and high ionic strength Allison L. Bryan, Shuofei Dong, Elise B. Wilkes, Laura E. Wasylenki PII: DOI: Reference:

S0016-7037(15)00043-5 http://dx.doi.org/10.1016/j.gca.2015.01.026 GCA 9114

To appear in:

Geochimica et Cosmochimica Acta

Received Date: Accepted Date:

21 July 2014 19 January 2015

Please cite this article as: Bryan, A.L., Dong, S., Wilkes, E.B., Wasylenki, L.E., Zinc isotope fractionation during adsorption onto mn oxyhydroxide at low and high ionic strength, Geochimica et Cosmochimica Acta (2015), doi: http://dx.doi.org/10.1016/j.gca.2015.01.026

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ZINC ISOTOPE FRACTIONATION DURING ADSORPTION ONTO MN OXYHYDROXIDE AT LOW AND HIGH IONIC STRENGTH

Revised version submitted to Geochimica et Cosmochimica Acta December 27, 2014

Allison L. Bryan1†, Shuofei Dong1, Elise B. Wilkes2, and Laura E. Wasylenki1

1

Department of Geological Sciences, Indiana University Bloomington, Indiana 47405 USA

2

Department of Earth and Planetary Sciences Harvard University Cambridge, Massachusetts 02138 USA

Corresponding author: A. L. Bryan† [email protected] (07939-115-545) †

Current address: Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK

Abstract

Marine ferromanganese sediments represent one of the largest sinks from global seawater for Zn, a critical trace metal nutrient. These sediments are variably enriched in heavier isotopes of Zn relative to deep seawater, and some are among the heaviest natural samples analyzed to date (Maréchal et al., 2000; Little et al., 2014a). New experimental results demonstrate that adsorption of Zn to poorly crystalline Mn oxyhydroxide results in preferential association of heavier isotopes with the sorbent phase. At low ionic strength our experimental system displayed a short-lived kinetic isotope effect, with light isotopes adsorbed to birnessite (Δ66/64Znadsorbeddissolved

~ -0.2 ‰). After 100 h the sense of fractionation was opposite, such that heavier isotopes

were preferentially adsorbed at steady state, but the magnitude of Δ66/64Znadsorbed-dissolved was indistinguishable from zero (+0.05 ± 0.08 ‰). At high ionic strength, we observed preferential sorption of heavy isotopes, with a strong negative correlation between Δ66/64Znadsorbed-dissolved and the percentage of Zn on the birnessite. Values of Δ66/64Znadsorbed-dissolved ranged from nearly +3 ‰ at low surface loading to +0.16 ‰ at high surface loading. Based on the EXAFS work of Manceau et al. (2002), we infer that Zn adsorbs first as tetrahedral, inner-sphere complexes at low surface loading, with preferential incorporation of heavier isotopes relative to the octahedral Zn species predominating in solution. As surface loading increases, so does the proportion of Zn adsorbing as octahedral complexes, thus diminishing the magnitude of fractionation between the dissolved and adsorbed pools of Zn. The magnitude of fractionation at high ionic strength is also governed by aqueous speciation of Zn in synthetic seawater; a substantial fraction of Zn ions reside in chloro complexes, which preferentially incorporate light Zn isotopes (Fujii et al. 2010), and this drives the adsorbed pool to be heavier relative to the bulk solution than it was at low ionic strength. Our results explain the observation that ferromanganese sediments are enriched in heavier isotopes of Zn relative to deep seawater. This represents a step towards building a robust

mass balance model for Zn isotopes in the oceans and potentially using Zn isotopes to trace biogeochemical cycling of this important element in the modern and ancient oceans.

1. INTRODUCTION Zinc is the second most abundant transition metal in the oceans, exhibiting a nutrient depth profile similar to those of biologically active major elements like nitrogen and phosphorus (Bruland, 1989; Bruland and Lohan, 2003; Morel and Price, 2003; Milne et al., 2010; Baars et al., 2011). Zinc concentrations range from 2 to 10 nM in the deep Atlantic and Pacific Oceans, with values typically below 0.1 nM in filtered samples of surface waters (Milne et al., 2010; Aparicio-González et al., 2012). The decreased Zn concentrations in surface waters are likely a result of biological uptake and assimilation for critical cellular functions. Zinc plays an important role as a metal cofactor for essential enzymatic processes in bacteria and phytoplankton in surface waters (Sunda and Huntsman, 2005; Twining and Baines, 2013). For example, Zn is a metal center in the carbonic anhydrase enzyme that catalyzes the interconversion of bicarbonate ions and CO2 and is required for uptake and fixation of inorganic carbon in diatoms (Sunda and Huntsman, 2005). The biogeochemical cycle of Zn may influence productivity or species composition in surface waters (Gélabert et al., 2006; Varela et al., 2011), but this cycling is in and of itself poorly understood. Application of the Zn stable isotope system may aid in investigations of present and past biological cycling of Zn because of the likelihood that biological assimilation in surface waters results in the preferential uptake of light Zn isotopes (Bermin et al., 2006; John et al., 2007). Direct isotope measurements of NE Pacific and North Atlantic seawater as a function of depth

displayed heavier Zn isotope values in filtered surface waters relative to deeper waters, which the authors attributed to the removal of light Zn isotopes by biological uptake (Bermin et al., 2006; Boyle et al., 2012; Vance et al., 2012; Conway et al., 2013; John and Conway, 2014). These authors also observed distinct zones of light Zn isotopic compositions from 100 m to 600 m, ostensibly due to recycling of assimilated Zn and scavenging of heavy Zn isotopes onto particles, and homogeneous isotopic compositions in deeper waters. The magnitude of the fractionation between surface waters and deep waters ranged up to Δ66/64Znsurface-deep = +0.8 ‰, where Zn isotope compositions are given in the standard delta per mil notation: δ

/

Δ

/

Zn

Zn

(

= [(

/

/

=δ

)

/

)

Zn

− 1] × 1000 ‰ −δ

/

Zn

(1) (2)

However, in another recent study of several depth profiles in the Southern Ocean, Zhao et al. (2014) did not observe heavier isotopes in filtered surface waters, nor lighter isotopes between 50-200 m, and these authors expressed doubt that biological assimilation indeed fractionates stable isotopes of Zn. An experimental study by John et al. (2007) reported the Zn isotope compositions of intracellular Zn in cultured diatoms. Low Zn concentrations in media drove high-affinity uptake and a more negative isotope fractionation effect (Δ66/64Zncells-medium = -0.8 ‰) compared to lowaffinity transport when more Zn was available (Δ66/64Zncells-medium = -0.2 ‰). Both John et al. (2007) and Gélabert et al. (2006) observed a complication: heavier isotopes of Zn preferentially adsorbed to cell membranes with a small magnitude of fractionation. The degree to which such adsorption may have affected the isotopic compositions of filtered seawater in the studies cited above is unknown.

While the question of fractionation during biological uptake remains, the range of Zn isotope values recorded in various kinds of marine samples and sediments listed below suggests that a number of diverse biotic and abiotic reactions and processes fractionate Zn isotopes. Natural samples including marine aerosols, basalts, shales, deep-sea clay sediments from 0 to 160 cm, eolian particles, bulk plankton samples, and sediment trap material collected from 250 m to 2500 m fall within a range of δ66/64Zn of 0.13 ‰ to 0.82 ‰ (Maréchal et al., 2000; Archer and Vance, 2002). Additionally, studies have reported significant enrichment of heavy Zn isotopes in natural samples of ferromanganese nodules (δ66/64Zn = 0.5 to 1.2 ‰; Maréchal et al., 2000), ferromanganese crusts (δ66/64Zn = 0.8 to 1.2 ‰; Little et al., 2014a), biogenic opal (δ66/64Zn = 0.8 to 1.5 ‰; Andersen et al., 2011), and carbonates (δ66/64Zn = 0.3 to 1.4 ‰; Pichat et al., 2003) compared to seawater. All of the above isotope compositions were measured and reported relative to the Lyon JMC Zn ICP standard solution. The majority of the mechanisms driving observed Zn isotope variability have not been identified or quantified. Experimental studies are required to resolve the fractionation mechanisms and associated magnitudes of fractionation prior to reconstruction of the past Zn cycle using the marine sedimentary record. This study focuses on an important pathway of Zn removal from the ocean via adsorption onto authigenic sediment particles. Adsorption reactions at mineral/water interfaces in general are thought to result in mass-dependent fractionation of Zn isotopes throughout the water column (Maréchal et al., 2000; Archer and Vance, 2002; Albarède et al., 2004; Little et al., 2014a). Specifically, the heavy Zn isotope values of natural Fe-Mn crusts compared to seawater represent one of the largest Zn isotope contrasts yet measured and may well be attributable to adsorption of Zn primarily to Mn oxyhydroxides (Koschinsky and Hein, 2003; Little et al., 2014b). The goal of

this study is to quantify fractionation experimentally and to further constrain the mechanism responsible for preferential enrichment of heavy isotopes of Zn in natural Fe-Mn crusts. In addition to adsorption onto Fe-Mn crusts, Zn is known to adsorb onto nanoparticles of oxyhydroxide minerals in surface waters (Maréchal et al., 2000). Zinc adsorption to ferromanganese oxides and other particles is potentially an important control on the amounts and isotopic composition of bioavailable Zn in surface waters and on Zn exported to deeper waters. The same processes that fractionate isotopes in seafloor Fe-Mn sediments are almost certainly active wherever Fe-Mn oxides occur throughout the water column. Specifically, the isotope fractionation induced by Zn adsorption onto available surface sites of birnessite, the dominant manganese oxide mineral in hydrogenetic Fe-Mn crusts, may result in an enrichment of heavy Zn isotopes in those sediments (Little et al., 2014a) and on particles of the same oxyhydroxide phases in surface waters. Previous laboratory experiments have shown that adsorption reactions play an important role in the isotope behavior of Zn. Pokrovsky et al. (2005) conducted pioneering work to investigate Zn isotope fractionation during adsorption onto selected mineral surfaces including birnessite. Contrary to results from analyses of natural Fe-Mn crusts relative to seawater, their experiments displayed an enrichment of light isotopes (Δ66/64Znsorbed-aqueous = -0.1 to -0.3 ‰) on the birnessite surface. A possible explanation for this enrichment is that instrumental limitation required use of Zn concentrations much higher than typically observed in natural systems. Because Zn concentrations in the experiments were high, Pokrovsky et al. (2005) used a closed system with no CO2, in order to avoid precipitation of Zn as hydrozincite, and lack of CO2 may have resulted in speciation of dissolved Zn different from that found in nature. Another potential reason is that their short duration experiments (20 h) expressed an initial kinetic isotope effect

that would have reversed with time (a point further supported in our results). A subsequent study by Juillot et al. (2008) revealed an equilibrium fractionation mechanism with an enrichment of heavy Zn isotopes during adsorption onto 2-Line ferrihydrite (Δ66/64Znsorbed–aqueous = +0.53 ‰) and goethite (Δ66/64Znsorbed–aqueous = +0.29 ‰). These studies show that Zn isotope behavior can be affected by variables ranging from structural differences in the substrate to solution chemistry. For this study we have quantified the magnitude of Zn isotope fractionation during adsorption onto synthetic birnessite (K0.5Mn2O4·1.5H2O) at low and high ionic strength and interpreted the results in the context of solution speciation and previous work on the coordination chemistry of Zn adsorbed on birnessite. We decreased Zn concentrations in our experiments compared to Pokrovsky et al. (2005), in order to avoid supersaturation with hydrozincite even in the presence of atmospheric CO2 and to achieve speciation of dissolved Zn that more closely reflects natural systems. The results, in combination with those from previous studies, yield a mechanistic understanding of why Fe-Mn crusts are isotopically heavy relative to seawater. Additionally, this work provides insight into the role of Zn adsorption onto nanoparticulate minerals within the Zn isotope system, which may be a fruitful approach to understanding the biogeochemical cycling of Zn in the ocean.

2. MATERIALS AND METHODS 2.1 Birnessite synthesis and stock solution preparation Experimental methods for preparation of stock solutions are similar to those described by Wasylenki et al. (2008) and Barling and Anbar (2004). Birnessite (K0.5Mn2O4·1.5H2O) was synthesized following the procedure described by Stroes-Gascoyne et al. (1987). Reagent grade KMnO4 was reduced with 2.4 M HCl in a 50-mL polypropylene tube for 48 h with constant

agitation on a platform shaker. The resulting precipitate was filtered through a polyethersulfone (PES) membrane with 0.2 μm pores and rinsed with 500 mL of 18.2 MΩ-cm water. The birnessite precipitate was then transferred to a PFA bottle and suspended in 18.2 MΩ-cm water. Birnessite stock suspensions were equilibrated with atmospheric CO2 for 7-10 days, and pH was adjusted with NaOH until it stabilized between 8.0 and 8.5. An aliquot of birnessite suspension collected following vigorous shaking contained 0.8 mg of birnessite per g of suspension, or 0.4 mg Mn per g (Wasylenki et al., 2008). The identity of our synthesized birnessite was confirmed as poorly crystalline potassium birnessite by X-ray diffraction at Indiana University. Additionally, the birnessite stock contained at least 3 orders of magnitude less Zn than the sorption experiments, as determined by ICP-MS analysis (Agilent 7700). Zinc stock solutions were prepared from aliquots of a Zn ICP standard solution (Alfa Aesar, Lot# 04-17077B). Batches of Zn stock contained between 152 μg and 330 μg of ICP Zn diluted in 2 L of 18.2 MΩ-cm water, for low ionic strength experiments, or the synthetic seawater solution described in Table 1, for high ionic strength experiments. The Zn stock solutions were mixed on a platform shaker with loosened caps for equilibration with atmospheric CO2 and adjusted with small amounts of 1 M NaOH and 1 M HCl to obtain a stable pH between 8.0 and 8.5. The amount of Zn in the experiments was always below saturation with hydrozincite [Zn5(CO3)2(OH)6], as computed using PHREEQC (Parkhurst, 1995). Zinc concentrations in the stock solutions were analyzed by ICP-MS and isotopic compositions by MC-ICP-MS (Nu Plasma II).

2.2 Adsorption experiments

The experiments were designed to result in adsorption of Zn onto birnessite with a range of surface coverage, in order to capture the isotope behavior of this system. Each low ionic strength experiment contained 380 mL of Zn stock solution and varying amounts of birnessite stock suspension (0.11 to 3.00 g, details in Table 2). Depending on the batch of stock solution used, experiments contained between 30.3 and 66.1 μg of ICP Zn (see Table 2). Each experiment was mixed on a platform shaker in a 500-mL Nalgene polystyrene bottle for 48 h with loosened caps to allow open atmospheric conditions. The birnessite with adsorbed Zn and remaining dissolved Zn were separated using a polystyrene, bottle-top filter unit with a PES membrane and 0.2 μm pores. The dissolved Zn fraction was collected in a 500-mL Nalgene polystyrene bottle. In an effort to achieve a clean separation of the dissolved and adsorbed Zn fractions, ~150 mL of 18.2 MΩ-cm water (pH adjusted to 8.0-8.5) was poured over the solids and passed through the membrane into a separate bottle to wash through any remaining dissolved Zn on the filter membrane (i.e., in pore spaces). The solid fraction with adsorbed Zn was then resuspended from the filter membrane with 18.2 MΩ-cm water, transferred to a PFA vial, dried down, and dissolved in 6 mL of heated, distilled 3 M HCl. A similar procedure was conducted for the high ionic strength experiments with a few modifications. Each high ionic strength experiment had a volume of 380 mL with between 48.8 and 53.0 μg of ICP Zn and varying amounts of birnessite stock suspension (0.22 to 9.90 g, see Table 4). The amount of birnessite stock suspension was increased to compensate for competition for adsorption sites by salts, and the solid fraction was not rinsed with 18.2 MΩ-cm water as it was in the low ionic strength experiments. The distribution of Zn between solid and dissolved samples was analyzed by ICP-MS for all low and high ionic strength experiments. Uncertainties on concentration measurements are estimated as ± 5 % relative.

Time series experiments were also conducted at both low and high ionic strength. For each time series, a set of identical experimental replicates was prepared with between 45.5 and 53.7 μg of ICP Zn and 0.92 and 2.03 g of birnessite stock suspension. The experiments were mixed and agitated for different amounts of time before being processed and analyzed using the same procedure applied to the other experiments. Details are given in Tables 3 and 5. Each set of low and high ionic strength experiments included a procedural blank experiment. Each blank experiment contained 200 mL of Zn-free stock solution and 1 g of birnessite stock suspension and was mixed on a platform shaker. After 48 h, the experiments were filtered and analyzed using the same procedure applied to the other experiments, except that the entire fluid and solid samples were dried down and analyzed for the total Zn blank. The amount of Zn contamination in all blank samples was checked by ICP-MS. The procedural blanks were 3 or 4 orders of magnitude lower in Zn than the sorption experiments.

2.3 Avoiding Zn contamination The results of preliminary adsorption experiments by Wilkes et al. (2010) revealed unacceptable levels of Zn contamination from lab plastics, especially polyvinyl gloves, which are commonly manufactured using zinc stearate as a mold release compound. In each of those adsorption experiments, the weighted sum of the isotopic compositions of the dissolved Zn and adsorbed Zn was expected to agree with the isotopic composition of the starting solution (the source of which was a commercial ICP solution from Ultra Scientific that happened to have a very light Zn isotopic composition). Those Zn isotopic sums, however, were consistently heavy by varying amounts (see select experimental results in Figure 1), and all solutions and solids were found to be isotopically heavier than the starting solution. Column replicates were

performed for selected samples, and the elutions had highly variable isotopic compositions, likely indicating variable amounts of contaminant Zn added during handling. In addition, nominally Zn-free blank experiments and column blanks in that previous study contained highly variable amounts of Zn, ranging from a few nanograms to hundreds of nanograms. When Zn leached from polyvinyl gloves was analyzed, it was found to be 10 ‰ heavier (in terms of δ66/64Zn) than the ICP standard used in the starting solution and similar in composition to the commonly used Lyon JMC zinc standard and many natural rock samples (e.g., Maréchal et al., 2000). When some experimental products were re-purified without the use of gloves and after pre-cleaning of all containers in dilute HCl, the measured isotopic compositions more closely matched mass balance expectations (Figure 1). Some level of contamination persisted, likely because the hood and the lab in which the work was conducted had been used by many glovewearing researchers for several years. Wilkes et al. (2010) pointed out that researchers measuring Zn isotopes in natural samples must take care; the extent of the contamination problem was obvious only because of the mass balance expectations in experiments and the chance use of a starting solution highly enriched in lighter isotopes of Zn. Because the level of contamination varies greatly from one sample to the next, monitoring of Zn levels in blanks is necessary, but not sufficient, for ensuring negligible Zn contamination. To address this problem, we employed an extensive cleaning procedure to avoid Zn contamination from zinc stearate. All experimental work was completed in a brand-new, positive-pressure clean lab in a new polypropylene, ULPA-filtered, laminar flow, exhausted hood at Indiana University in which vinyl gloves are not used. All reagents (HCl and HNO3) were distilled in-house, and all water was deionized to 18.2 MΩ-cm. In addition to our usual cleaning procedure for non-PFA plastics (soaking for one day in dilute Micro-90 detergent

solution, two days in cold 20 % HNO3, two days in cold 20 % HCl, and rinsing with water between steps), bottles and centrifuge tubes were soaked for one week in cold 20 % HNO3 and for one week in 18.2 MΩ-cm water (to leach residual acid). All PFA vessels were cleaned for one day in dilute Micro-90 detergent solution, 8 h on a 200 ºC hotplate in 50% HNO3, 8 h on the hotplate in 18.2 MΩ-cm water, with water rinses between steps. Clean plastics were handled without gloves. All our experimental and column blanks were at least 3 orders of magnitude lower than the concentrations of Zn in the experiments.

2.4 Ion Exchange Chromatography Procedure Samples were purified by ion exchange chromatography to isolate Zn from the matrix and prevent spectral interferences during isotopic analysis. To correct for procedural fractionation and instrumental mass bias, an aliquot of Zn double-spike solution mixed from 64Zn and 67Zn spikes (Isoflex USA) was added to each sample. The optimal double-spike composition and sample-spike ratio were computed using the algorithm of Rudge et al. (2009). Approximately 2 μg of Zn from each sample (both dissolved and adsorbed Zn samples) was dried down in an acid-cleaned, PFA vial and dissolved in 7 M HCl. For anion exchange chromatography, all samples were loaded onto 0.6 mL of BioRadTM AG-MP1 anion resin in 7 M HCl. Matrix was removed with 11 mL of 2 M HCl, and Zn was eluted with 8 mL of 0.1 M HCl into PFA vials and dried down. For the high ionic strength samples, the amount of 2 M HCl was increased to 30 mL to wash out the large amount of salts. One mL of 15 M HNO3 (distilled inhouse) was added to each dried sample, and these samples were heated with caps tightened for 12 h to break down organic residues from the resin. Prior to isotopic analysis, the samples were

dried down and redissolved in 1 mL of 0.32 M HNO3. The starting stock solutions were purified through the same ion exchange chemistry as the final experimental products.

2.5 Mass spectrometric methods and data validation The Zn isotopic analyses of the experiments were performed on Nu Plasma II MC-ICPMS with an Aridus II (Cetac) desolvation system at Indiana University. Signals were collected for

64

Zn,

66

Zn,

67

Zn, and

68

Zn, along with

62

Ni to correct for isobaric interference on mass 64

from 64Ni. An aliquot of each purified experimental sample was diluted with 0.32 M HNO3 to a concentration of 200 ppb. The sample and standard concentrations were chosen to ensure a high signal-to-noise ratio with ~18 V on mass 64, ~3 V on mass 66, and ~2 V on masses 67 and 68. A washout time of 100 s was used during the analyses to minimize memory effects. Signal intensity was recorded over 50 cycles of 4 s. Isotope ratios were calculated as an average of the 50 cycles. All voltages were corrected for background by subtracting the voltages measured on an acid blank. The methods applied to correct for mass discrimination included double spike and sample-standard bracketing techniques. We chose 64Zn and 67Zn as the spiked isotopes, because this choice enabled all masses from

62

Ni to

68

Zn to be measured at once with excellent peak

shapes on all masses. The selection of spike isotopes produced compositions in terms of d68/66Zn, but are reported as the more conventional notation of d66/64Zn. Note that since differ by two atomic mass units as do

66

68

Zn and

66

Zn

Zn and 64Zn, and because our samples are experimental

and are simply being compared to one another, this choice has no consequence to our results or interpretation. (Given the assumption of exponential fractionation, the difference between d68/66Zn and d66/64Zn is far smaller than analytical uncertainty). An in-house reference material,

Sesame Zn (Alfa Aesar zinc wire, Lot# C23Y008, δ66/64ZnSesame ≡ 0), double-spiked to match the

concentrations and sample-spike ratios of the experimental samples, bracketed every one or two

sample analyses. The long-term precision on the standard over several months during this study was ±0.05 ‰ (2s). An ICP standard solution, Alfa Aesar Lot# 04-17077B, and an in-house standard from the MAGIC Lab at Imperial College London (London Zn) were measured repeatedly in each analysis session, to assess instrument stability and long-term reproducibility (Arnold et al., 2010). The ICP Zn relative to the Sesame Zn measured as -5.71 ± 0.11 ‰ (δ66/64Zn; n=14, 2sd). The Lyon JMC Zn relative to the Sesame Zn measured as -4.18 ± 0.12 ‰ (δ66/64Zn; n=5, 2sd). The London Zn relative to the Sesame Zn measured as -4.37 ± 0.08 ‰ (δ66/64Zn; n=21, 2sd). The optimal sample-spike ratio from the algorithm of Rudge et al. (2009) is 0.40. Our sample-spike ratios on samples varied from 0.25 to 0.72, but tests on variably spiked Sesame Zn during method development indicate that sample-spike ratios ranging from 0.14 to 1.24 result in accurate and precise data. Data processing was performed using a routine written in MATLAB (written by S. Romaniello and described by Wasylenki et al., 2014). This routine includes automated trimming to detect any aspiration problems, an iterative multivariate outlier detection algorithm to remove spurious measurement cycles, a non-linear least-squares package for deconvolution of the double-spike equations, as well as automated quality control flags and sample-standard bracketing.

3. RESULTS 3.1 Low ionic strength adsorption experiments

The low ionic strength experiments were designed to have similar concentrations of Zn with varying amounts of birnessite to provide a range of percentages of sorbed Zn. The proportions of Zn that adsorbed onto birnessite after 48 h ranged from 11.0 % to 84.2 %, and the remaining amount of Zn in the solution varied from 5.3 to 32.9 μg (Table 2). As each Zn stock solution was divided for use in five experiments at a time, several stocks were made over the course of the study, resulting in differences in the Zn concentrations and isotopic compositions from one stock to another. The isotopic compositions of Zn stock solutions ranged from δ66/64Znstock = -5.50 ± 0.07 ‰ to -6.67 ± 0.07 ‰ (see Table 2). We do not know why the stocks, which were prepared from the same source of Zn, varied so much, but we suspect that contamination by zinc stearate despite the precautions taken is likely to blame. The validity of our experimental results is supported by smooth data trends and highly satisfactory mass balance assessment for each individual experiment (see below). To make the results from all the experiments easier to compare, they are isotopically normalized in the figures relative to a stock solution of δ66/64Znstock = 0 ± 0.05 ‰. The tables contain the original, measured isotopic compositions of all samples. Isotopically light Zn preferentially adsorbed onto the birnessite surface in all 48-hour experiments at low ionic strength. Results are detailed in Table 2 and shown graphically in Figure 2. Values of Δ66/64Znsorbed-aqueous ranged from -0.10 ± 0.08 ‰ to -0.20 ± 0.08 ‰, with an average magnitude of fractionation of Δ66/64Znsorbed-aqueous= -0.16 ± 0.08 ‰. There is no overlap of error bars, thus we observe a small, but resolvable fractionation, with approximately constant magnitude, between dissolved and sorbed Zn fractions. As mentioned in section 2.2, the solid material on the filter was rinsed with ~150 mL of pH adjusted, 18.2 MΩ-cm water, and this fluid was collected and analyzed separately from the

main dissolved and sorbed pools, to check whether a significant amount of dissolved Zn remained in the pore spaces of the solid material following filtration. The amount of Zn in the rinses averaged 1 μg, or 1.9 % of the total Zn recovered from the experiment. The maximum amount in the rinse was 4.6 % of the total recovered Zn. We considered these amounts to be sufficiently small that we did not analyze the isotopic compositions of the rinses, as their effect on mass balance would not have been resolvable. The quality of the experiments was checked by Zn recovery and isotope mass balance analysis. The sum of the Zn recovered in the combined fractions (fluid + rinse + solid) ranged from 94.8 % to 102.3 %, with differences from 100 % most likely attributable to analytical uncertainties associated with ICP-MS analysis. An isotope mass balance was calculated to verify the success of the experiments and exclude the possibility of major contamination in any experiments following preparation and use of the stock solutions. For each experiment we calculated δ66/64Znaqueous · (100 - % sorbed) + δ66/64Znsorbed · % sorbed. The offset between the calculated isotope mass balance of the experiments and the measured isotope value of the corresponding starting stock solution was computed as δ66/64Znstock – (δ66/64Znaqueous · (100 - % sorbed) + δ66/64Znsorbed · % sorbed), and these values are listed in the last column of Table 2. With the exception at 365 h at low ionic strength, each experiment’s mass balance offset is within ±0.20 ‰ of its corresponding starting stock composition.

3.2 Low ionic strength time series experiments The low ionic strength time series experiments aimed to determine whether the isotope systematics would evolve over the course of several weeks (1680 h). Results are tabulated in

Table 3 and plotted in Figure 3. The Zn recovery (fluid + solid + rinse) of the adsorption experiments ranged from 87.3 % to 101.1 %. Time series experiments at low ionic strength display a shift from preferential adsorption of isotopically light Zn initially to no measurable isotope fractionation outside of analytical error from 100 h on (see Table 3 and Figure 3). The isotopic compositions of the two Zn stock solutions measured as δ66/64Znstock = -5.88 ± 0.05 ‰ and -6.28 ± 0.05 ‰. In Figure 3, for clarity, data from those experiments have been normalized to δ66/64Znstock = 0 ‰ for straightforward comparison of the two sets of experiments. Values of Δ66/64Znsorbed-aqueous ranged from -0.21 ± 0.07 ‰ at 24 h to no measurable fractionation outside of analytical uncertainty at 100 h. No measurable isotopic fractionation persists from 100 h to 1680 h, although all experiments beyond 100 h suggest that adsorbed Zn is very slightly heavier than dissolved Zn, by a few hundredths of a permil.

3.3 High ionic strength adsorption experiments As at low ionic strength, Zn concentrations were varied in the 48-hour high ionic strength experiments to generate a wide range of percentages of adsorbed Zn. The results are tabulated in Table 4 and shown graphically in Figure 4. The percentage of Zn adsorbed ranged from 8.1 % to 77.1 %, and the remaining amounts of Zn in the solution varied from 15.4 to 42.8 μg Zn. The fraction of Zn sorbed does not vary perfectly monotonically with the amount of birnessite added to the experiments. We attribute this to the difficulty of drawing uniform aliquots from a suspension of birnessite nanoparticles, as well as the possibility that high ionic strength may induce a small amount of particle clumping, thus aggravating this effect. We emphasize that this has no consequences for the measured isotopic fractionations, since all dissolved and sorbed

pools of Zn were analyzed separately, and mass balance was satisfied within propagated analytical uncertainties for nearly all experiments and within ± 0.2 ‰ for all high ionic strength experiments. Isotope behavior at high ionic strength differs greatly from what we observed at low ionic strength; heavy Zn isotopes are preferentially adsorbed on the birnessite surface. There were 4 stock solutions, and their isotopic compositions ranged from δ66/64Znstock = -5.67 ± 0.05 ‰ to 6.20 ± 0.05 ‰. Fractionation magnitude decreases sharply with increasing fraction of adsorbed Zn, with Δ66/64Znsorbed-aqueous range of 2.74 ± 0.07 ‰ to 0.13 ± 0.07 ‰ (Figure 4a) and very nearly constant at ~0.16 ‰ beyond 50 % adsorbed (Figure 4b).

3.4 High ionic strength time series experiments The time series at high ionic strength comprised five identical experiments allowed to equilibrate for durations ranging from 24 h to 172 h, to determine whether isotope behavior is time-dependent, as at low ionic strength. Results are tabulated in Table 5 and plotted in Figure 5. The proportions of Zn that adsorbed over time increased from 17.3 % to 28.2 %, and the remaining amount of Zn in the solution varied from 36.5 to 44.7 μg. Zinc recovery ranged from 90.4 % to 104.6 %. The magnitude of Zn isotope fractionation remains fairly consistent as a function of time with preferential adsorption of heavy Zn isotopes on the birnessite surface (see Table 5 and Figure 5). Values of Δ66/64Znsorbed-aqueous ranged from 0.77 ± 0.07 ‰ to 0.52 ± 0.07 ‰. The isotopic compositions of the Zn stock measured as δ66/64Znstock = -5.65 ± 0.05 ‰. The isotopic compositions of the sorbed Zn fractions ranged from δ66/64Znsorbed= -5.05 ± 0.05 ‰ to -5.28 ±

0.05 ‰, whereas those of the remaining dissolved Zn fractions varied from δ66/64Znaqueous= -5.80 ± 0.05 ‰ to -5.85 ± 0.05 ‰.

4. DISCUSSION 4.1 Kinetic and equilibrium fractionation at low ionic strength At first glance, the data plotted in Figure 2 (48 h experiments) appear consistent with either a closed-system (reversible) equilibrium isotope effect leading to slight enrichment of lighter Zn isotopes on the birnessite surface or with Rayleigh trends that would indicate an opensystem (irreversible) equilibrium fractionation or a kinetic effect. However, the time series experiment (see Figure 3) demonstrates that Δ66/64Znsorbed-aqueous decreased gradually over the first 100 h, as dissolved and adsorbed Zn exchanged. This behavior clearly indicates that the small fractionation observed prior to 100 h is a short-lived, kinetic isotope effect, driven by lighter isotopes diffusing and adsorbing at a higher rate in the solution compared to the heavier isotopes. Because the kinetic isotope effect is evident on short time scales, the process of desorption must be slower than adsorption. The results of experiments run for 100 to 1680 h suggest that the system reaches steady state and perhaps equilibrium after ~100 h. The error bars for dissolved and adsorbed Zn overlap for every experiment beyond 100 h, so the magnitude of the equilibrium fractionation cannot be truly distinguished from zero given analytical uncertainties. However, as shown in Figure 3, the consistently heavier values for sorbed Zn suggest a slight preference for heavier isotopes on the birnessite surface, with a magnitude for Δ66/64Znsorbed-aqueous of approximately 0.05 ‰. In general, in the absence of changes in oxidation state, equilibrium isotope fractionation for metals at low temperatures is mainly driven by differences in the vibrational frequencies of

bonds between the element of interest and its nearest neighbors (Bigeleisen and Mayer, 1947; Schauble, 2004). When an element partitions among multiple species in a system, such as between dissolved species and adsorbed complexes on a mineral surface, the heavier isotopes preferentially concentrate in sites with stronger and stiffer bonds (Bigeleisen and Mayer, 1947; Schauble, 2004). Thus our results suggest a slightly stronger, stiffer bonding environment in the adsorbed complex(es) compared to the dominant aqueous species, Zn(H2O)62+once the observed short-lived, kinetic isotope effect causing adsorption of light Zn isotopes to the birnessite surface reverses as the system approaches equilibrium. Using extended X-ray absorption fine structure analysis (EXAFS) of synthetic birnessite with adsorbed Zn, Manceau et al. (2002) demonstrated that Zn adsorbs over vacant sites for Mn in the birnessite structure. Surface loading is quantified by Manceau et al. (2002) in terms of Zn/Mn ratios. At low surface loading, with a Zn/Mn ratio of 0.008, a single inner-sphere complex in which Zn is tetrahedrally coordinated with oxygen atoms and shares three oxygens with the birnessite {001} surface fit the EXAFS spectrum well. As surface loading with Zn increased to a Zn/Mn ratio of 0.128, EXAFS spectra were best modeled with mixtures of two inner-sphere complexes on the {001} surface: the tetrahedral surface complex described above and another in which Zn is octahedrally coordinated with oxygen and bound to three surface oxygens that are shared with three layer Mn atoms (Manceau et al., 2002; Kwon et al., 2009; 2013). As surface loading increased, so did the proportion of adsorbed Zn that was octahedrally coordinated. Other studies have made similar observations regarding Zn surface complexation in natural samples (Marcus et al., 2004; Isaure et al., 2005; Toner et al., 2006; Little et al., 2014b). Given that smaller coordination numbers generally imply stronger, stiffer bonds, we expect that any tetrahedrally coordinated Zn sorbed to the birnessite surface in our experiments is

likely enriched in heavier isotopes of Zn compared to the dominant aqueous species, Zn(H2O)62+, in which Zn is octahedrally coordinated (Bigeleisen and Mayer, 1947; Schauble, 2004). We expect little or no fractionation between octahedrally coordinated, sorbed Zn and octahedrally coordinated, dissolved Zn, although slight distortion of the octahedral coordination geometry in the sorbed complex could drive a small isotope effect, as inferred for Zn on goethite by Juillot et al. (2008), for U by Brennecka et al. (2011), and for Cd by Wasylenki et al. (2014). Our low ionic strength experiments exhibit higher Zn/Mn ratios than the samples of Manceau et al. (2002), suggesting that nearly all of the adsorbed Zn is octahedrally coordinated. The Zn/Mn ratio was only measured for experiments with 11 % to 25 % Zn adsorbed and corresponds to ratios between 0.185 and 0.204. The small isotope effect apparent in our results at low ionic strength may be driven either by a small proportion of tetrahedrally coordinated, isotopically heavy, adsorbed Zn or by a slight distortion of the octahedral coordination environment in sorbed Zn compared to the highly symmetric, octahedral species in solution, or both, but we do not currently have the means to distinguish between these possibilities.

4.2 Isotope effect influenced by complex speciation at high ionic strength Results at high ionic strength contrast strongly with those at low ionic strength, and thus a different explanation for the observed isotopic fractionation is necessary. In all experiments at high ionic strength, heavier isotopes are preferentially adsorbed. A plot of isotopic compositions for dissolved and adsorbed Zn versus fraction of Zn adsorbed (Figure 4) displays neither parallel, linear trends or Rayleigh trends as might be expected. Instead, the magnitude of fractionation is very large at low fraction adsorbed (Δ66/64Znsorbed-aqueous ~1 ‰ at 12.1 % adsorbed), then decreases sharply as the fraction of Zn adsorbed increases, and stabilizes at Δ66/64Znsorbed-aqueous =

0.16 ‰ when the fraction adsorbed is greater than ~30 %. Because the trends do not reflect simple reversible equilibrium or Rayleigh behavior, we must look to complexities in Zn speciation both on the birnessite surface and within the solution to explain the observed isotope systematics. The observation by Manceau et al. (2002) that tetrahedrally coordinated Zn adsorbs to birnessite at low surface loading, with an increasing proportion of octahedral adsorbed complexes as surface loading increases, is critical to explaining our results, as is the expectation based on Schauble (2004) that Zn in tetrahedral adsorbed complexes will be isotopically heavier than Zn in octahedral complexes, because of the stronger, stiffer bonds that result from the lower coordination number. We infer that Zn adsorbed to birnessite in our experimental samples represents mixtures of two adsorbed species, one tetrahedral complex that preferentially incorporates heavier isotopes relative to the octahedral species present in solution, and one octahedral complex that fractionates very little or not at all from the octahedrally coordinated Zn in solution. The trend observed in isotope fractionation as a function of the fraction adsorbed reflects the changing proportions of the two adsorbed complexes. The non-linearity of the trend suggests that the proportions of the two adsorbed species do not vary linearly with surface loading, which is an observation that would be difficult to make in any other way, even by EXAFS, given the uncertainty in coordination numbers associated with that technique. We note that the average fractionation observed for experiments with ≥30 % of Zn adsorbed (Δ66/64Znsorbed-aqueous = 0.16 ±0.08 ‰, propagated 2s) is somewhat larger than the isotope effect observed at low ionic strength for experiments longer than 100 h and with ≥30 % of Zn adsorbed (Δ66/64Znsorbed-aqueous = 0.05 ±0.08 ‰, propagated 2s). At low ionic strength, we attributed the very small fractionation to either the presence of a small proportion of tetrahedrally

coordinated, adsorbed Zn and/or to distortion of the octahedral coordination geometry for adsorbed Zn. We surmise that the same explanation applies at high ionic strength, and we look to more complex aqueous speciation of Zn at high ionic strength to explain the slightly larger magnitude of fractionation. Equilibrium speciation of Zn in our low ionic strength stock and in the synthetic seawater stock, which includes the six most abundant salts in the ocean (Table 1), was modeled at relevant conditions (pH = 8.2, T = 25 ºC, P = 1 bar) using PHREEQC (Parkhurst, 1995), and results are given in Table 6. At low ionic strength, essentially all Zn2+ is present as Zn2+, ZnOH+, and Zn(HCO3)+. Expressed more completely, these species are Zn(H2O)62+ and ZnOH(H2O)5+ (Fujii et al., 2010), such that all aqueous Zn is in octahedral coordination with oxygen atoms and is likely very nearly isotopically homogeneous in our low ionic strength solutions. In contrast, at high ionic strength, these species represent only slightly more than half of all Zn2+ (~54 %), and the complement comprises various chloro complexes (~17 % of all Zn), Zn carbonate ions (~16 %), and a minor ZnSO40 (~8 %). We therefore expect that intraspecific fractionation occurred within the solutions in our high ionic strength experiments, as suggested by Little et al. (2014b), and in turn affected the fractionation we observed between solutions and solids. Previous work indicates that Zn chloride species should preferentially incorporate lighter isotopes of Zn at equilibrium relative to hydrated Zn2+ (Fujii et al., 2010; Black et al., 2011). Chloride ligands complex with Zn to form a series of mono-, di-, tri-, and tetrachloro-Zn complexes (Fedorov et al., 1970; Pye et al., 2006). Using ab initio calculations, Pye et al. (2006) reported an increase in Zn-O and Zn-Cl bond distances with the successive replacement of oxygen ligands by chloride ligands, which should also increase bond vibrational frequencies and decrease reduced partition function ratios. Fujii et al. (2010) and Black et al. (2011) employed

theoretical calculations to predict reduced partition functions for various aqueous species of Zn. Both studies determined that chloro complexes of Zn should be enriched in lighter isotopes relative to hydrated Zn2+. The results of Black et al. (2011) for Zn(H2O)62+ versus ZnCl42- are equivalent to a fractionation very near 1 ‰ (in terms of Δ66/64Znhexaquo-tetrachloro), with small variations depending on choice of second-shell solvation model and theoretical approach (unrestricted Hartree-Fock vs density functional theory). Fujii et al. (2010) performed calculations for a series of chloro complexes and reported reduced partition functions that decrease as the number of Cl- ligands in the complex increases. The magnitudes of fractionation from Zn(H2O)62+ for chloro complexes translate to values of Δ66/64Znhexaquo-chloro ranging from 0.13 ‰ for ZnCl(H2O)5+ to 1.40 ‰ for ZnCl42-. We hypothesize that equilibrium fractionation between all aqueous and adsorbed species occurs in our high ionic strength experiments, and what we have measured and plotted in our figures reflects the differences in isotopic composition between the weighted average of all aqueous species and the weighted average of all adsorbed species, for each experiment. If we further hypothesize that it is primarily free Zn2+ that adsorbs to the negatively charged birnessite surface, then it is not surprising that we observe a larger fractionation at high ionic strength, since chloro complexes can be expected to tie up a pool of isotopically light, aqueous Zn, making the Zn2+ pool slightly enriched in heavier isotopes compared to the whole solution. Using the proportions of Zn chloride species and hydrated Zn2+ species in Table 6 and the isotope information from Fujii et al. (2010) for the various chloride species, we can attempt a mass balance calculation to determine the isotopic composition of free Zn2+, or Zn(H2O)62+, in a solution with our starting stock composition of d66/64Znstock = 0 ‰ with this equation:

0 ‰ =

∑

/

∑

∆

/

,

(3)

where f is the fraction of a species i in the solution from Table 6, and D66/64ZnZn2+-i is equal to 1000 times the difference in reduced partition functions (ln b) for the relevant species from Fujii et al. (2010) and Black et al. (2011). An important note is that we have grouped together the three species in which Zn is coordinated only by OH- or H2O and the two species that have one Cl- ligand each and assumed them to be isotopically the same. Once the fi and isotope fractionations are entered, the one unknown is d66/64ZnZn2+, and solving for this yields a value of 0.10 ‰, i.e., 0.10 ‰ higher compared to the Zn stock solution. If we assume the isotopic offset between aqueous hydrated Zn species and adsorbed Zn is the same as at low ionic strength (Δ66/64Znsorbed-aqueous = 0.05 ‰), then we should expect a fractionation between the aqueous and adsorbed Zn fractions at high ionic strength of approximately 0.10‰ + 0.05 ‰, or 0.15 ‰, which agrees well with the observed 0.16 ‰ fractionation. This calculation is necessarily rough, and current uncertainties do not allow any unequivocal conclusions, but nonetheless this result is at least entirely consistent with the inference that aqueous speciation at high ionic strength accounts for the difference in isotopic behavior that we observe at relatively high surface loading compared to low ionic strength. We note also that the kinetic effect observed at low ionic strength in experiments shorter than 100 h is not evident at high ionic strength; we see heavier isotopes associated with the birnessite even in the 48 h experiments shown in Figure 4. It is difficult to predict the effect of dissolved salts on the kinetics of isotope exchange between solution and surface, but, in a previous study of Cd adsorption to calcite, Horner et al. (2011) inferred that the presence of abundant major cations at high ionic strength slowed the uptake and isotopic exchange with the

calcite surface, due to competition for adsorption sites for Cd with the major cations. Wasylenki et al. (2014) also observed what was likely a kinetic isotope effect for Cd adsorption on birnessite at high ionic strength and suggested the same mechanism. It would therefore be unexpected to observe a kinetic effect in this Zn system at low, but not at high, ionic strength. The small magnitude of the kinetic effect observed at low ionic strength, however, makes it possible that a kinetic effect is occurring in short-duration, high ionic strength experiments, but it is overwhelmed by the large magnitude of thermodynamically driven fractionation and is thus not apparent. The kinetic effect, if it exists, would slightly diminish the magnitude of fractionation in the short-duration experiments. A look at the time series at high ionic strength does not allow us to move beyond this speculation; the slight variations in magnitude of fractionation there appear to be governed principally by the fraction of Zn sorbed, rather than the duration of the experiment. As far as implications for natural samples are concerned, any kinetic effect lasting less than 100 hours is likely unimportant, given the long timescales involved in accumulation of ferromanganese sediments (Koschinsky and Hein, 2003).

4.3 Implications for interpreting Zn isotope fractionation in natural Fe-Mn crusts Precipitation of Fe and Mn oxyhydroxides from seawater to form ferromanganese (FeMn) crusts and nodules occurs in all the world’s major ocean basins. The majority of Fe-Mn sediments are characterized by high porosity, high surface area, extremely slow growth rate (1-6 mm/Ma), and a high capacity for sorption of trace metals like Zn (Koschinsky and Hein, 2003). Natural Fe-Mn nodules and crusts are variably enriched in heavy Zn isotopes relative to seawater. The current best estimate for dissolved Zn isotope composition of global seawater below 200 m is d66/64Zn = 0.52 ‰ (Bermin et al., 2006; Boyle et al., 2012; Vance et al. 2012;

Conway et al., 2013; Zhao et al., 2014). The surface layers of Fe-Mn nodules from around the world analyzed by Maréchal et al. (2000) ranged from 0.53 to 1.16 ‰ and averaged 0.90 ‰. Little et al. (2014a) found that nearly all values from depth profiles into three hydrogenetic crusts, one from each major ocean basin, fell between d66/64Zn = +0.8 ‰ and 1.2 ‰ (all reported relative to the JMC-Lyon standard). Because Fe-Mn sediments represent a substantial proportion of the sink for Zn from global oceans, the fractionation that occurs between seawater and these sediments has a profound effect on the global marine isotope budget for Zn. However, except for one very recent study by Little et al. (2014b) that documented tetrahedrally coordinated Zn in crusts using EXAFS, the reason for enrichment in heavier isotopes of Fe-Mn sediments and the cause(s) of the observed variability among crusts and nodule samples have not been investigated. The experimental results reported here indicate that fractionation during adsorption of Zn to Mn oxyhydroxides can readily explain the heavy Zn isotopic compositions of Fe-Mn crusts, including the very heaviest ever analyzed, which differ from average seawater by nearly 0.7 ‰ (Little et al., 2014a). This study is particularly relevant to hydrogenetic crusts (those that precipitate from cold waters, as opposed to those strongly affected by hydrothermal and/or diagenetic processes), since birnessite (or d-MnO2) is the dominant Mn phase in those rocks (Peacock and Sherman, 2007; Koschinsky and Halbach, 1995). Little et al. (2014b) recently demonstrated with micro X-ray fluorescence mapping of natural crusts that Zn is predominantly associated with Mn-oxyhydroxides, rather than with Fe-oxyhydroxides, thus confirming the similar conclusion of Koschinsky and Hein (2003), and also showed that Zn in the natural crusts is in tetrahedral coordination. Although we did not directly determine the coordination number of Zn adsorbed to birnessite in our low surface loading experiments, our results taken together with those of previous studies provide a strong argument that adsorption of Zn into tetrahedral

complexes on the birnessite surface indeed drives an isotope effect with correct sense and sufficient magnitude to explain the observed fractionation between Zn in seawater and Zn in FeMn crusts. In fact, the fractionation between average seawater and the heaviest natural crusts analyzed thus far (~0.7 ‰) is considerably smaller than the fractionation measured in our lowest surface loading experiments (>1 ‰) and far smaller than the fractionation that could be expected if our experimental results were extrapolated to the degree of surface loading expressed in natural crusts (>3 ‰). This discrepancy raises two important questions: (1) why are the fractionations observed in nature smaller than in our lowest surface loading experiments, and (2) what governs the Zn isotope variability among natural Fe-Mn crusts? Zn/Mn ratios in natural crusts are much lower than in our experimental solids; the Zn/Mn ratios in birnessite from our high ionic strength experiments ranged from 0.039 to 0.299, while the mean Zn/Mn value for natural Fe-Mn crusts is only 0.004 (Manheim and Lane-Bostwick, 1991). The low Zn/Mn ratios and observed tetrahedral complexes in natural samples (Little et al., 2014a; Marcus et al., 2004) mean Zn isotope fractionation from seawater should at least be as large as the fractionations observed in our experiments with lowest surface loading. Given the slow accumulation rate of natural sediments, a kinetic effect whereby slow exchange between solution and solid surface dampens the fractionation is highly unlikely, so here we will consider hypotheses for how equilibrium fractionation could be dampened in nature relative to our experimental system. One possible explanation is that there are crucial differences between our experimental system and natural conditions that affect isotope systematics. Our stock solutions had much higher concentrations of Zn than seawater, of course, but equilibrium speciation calculations

indicate that the relative proportions of inorganic Zn species should be approximately the same. Perhaps more importantly, our experimental system lacked organic ligands, even though a large proportion of Zn is known to be chelated by organic molecules in seawater (Bruland, 1989; Donat and Bruland, 1990; Bruland and Lohan, 2003). A study by Bruland (1989) in the North Pacific reported that organic Zn species are restricted to ocean depths shallower than 400 m, due to breakdown of these molecules at greater depths. Conversely, Ellwood and Van den Berg (2000) reported that zinc-binding ligands exist uniformly from the surface to maximum depth measured at 2000 m in the Atlantic Ocean. Metal-binding ligands present at 400-4000 m depth during Fe-Mn formation (Hein et al., 1997; Koschinsky and Hein, 2003) would likely have heavy isotopic compositions, particularly Zn-citrate complexes (Fujii et al., 2010; Jouvin et al., 2009; Black et al., 2011), and thus they could be responsible for dampening the magnitude of fractionation seen in our experimental system, in exactly the same way chloride ligands increase the magnitude of fractionation, but with opposite sense. The lack of other mineral surfaces in our experimental system may also be important. The two major phases in Fe-Mn oxyhydroxide crusts are birnessite and poorly crystalline FeOOH (Hein et al., 1997; Koschinsky and Hein, 2003). Using a sequential leaching procedure, Koschinsky and Hein (2003) determined that, while most Zn is associated with the Mn oxyhydroxide, between 10 % and 35 % of Zn is associated with the FeOOH phase in hydrogenetic Fe-Mn crusts. Little et al. (2014b) confirmed the predominant Zn-Mn association using micro X-ray fluorescence mapping and EXAFS, but also noted that some Zn is associated with Fe. Adsorption of Zn to Fe oxides results in smaller fractionations, according to the experiments of Juillot et al. (2008), with Δ66/64Znsorbed-aqueous = 0.29 ‰ for goethite and Δ66/64Znsorbed-aqueous = 0.53 ‰ for 2-Line ferrihydrite, and thus adsorption of some proportion of

Zn to FeOOH, rather than birnessite, could alter the observed Zn isotopic compositions of Fe-Mn crusts. A third possibility is that some nodules and crusts may contain some Zn that was not incorporated by adsorption of dissolved Zn to authigenic precipitates; for example, some samples may have incorporated a detrital Zn component, with isotopic composition similar to average continental rocks (d66/64Zn = 0.31 ±0.11 ‰, 1s, see supplementary information of Little et al., 2014a). Little et al. (2014a) noted that a Fe-Mn crust sample from the Indian Ocean had lower values of Zn/Al than those from the Atlantic and Pacific Oceans, suggesting lower authigenic enrichment, and this sample also had more Zn isotope variability in a depth profile. Another possibility is that the Zn isotope variability among Fe-Mn sediments reflects differences in the isotopic compositions of dissolved Zn in the water masses from which the crusts precipitate, as this would suggest tremendous potential value of Zn isotopic signatures in crusts as a proxy for spatial or temporal changes in Zn isotope ratios in seawater. Based on analysis of 40 nodule samples, Maréchal et al. (2000) noted that samples from high latitudes were generally slightly lighter in d66/64Zn than samples at low or mid-latitudes, suggesting possible spatial variations in seawater Zn isotopic compositions governed by gradients in biological productivity or movement of isotopically distinct water masses. However, the deep seawater samples analyzed thus far do not suggest much inhomogeneity below the zone of remineralization of biologically assimilated Zn (Bermin et al., 2006; Boyle et al., 2012; Vance et al., 2012; Conway et al., 2013; Zhao et al., 2014), and thus we do not prefer this explanation. The causes of Zn isotope variations among Fe-Mn sediments and the usefulness of those variations as paleoproxies for important ocean variables await further investigation. In the future, more analyses of crusts with careful attention to correlations between d66/64Zn and other variables

such as Fe/Mn and Zn/Al, as well as further analyses of deep seawater, especially near where FeMn crusts form, will help distinguish between the hypotheses just discussed and will help to establish the potential value of Zn isotope signatures in these sediments.

4.4 Further applications to the Zn biogeochemical cycle The Zn stable isotope system may provide important insight into the biogeochemical Zn cycle in the ocean. Sources, sinks, and fluxes for this element remain poorly understood, but isotopes provide an additional dimension of constraints on the marine mass balance for this important element. Zinc isotopes vary in nature among the sources and sinks, and, just as the elemental source flux must balance the sink flux, the weighted average isotopic composition of the sources must square with the weighted average isotopic composition of the sinks, unless the Zn cycle deviates significantly from steady state. Little et al. (2014a) identified the problem with closing a mass balance model for Zn isotopes by compiling the measured isotopic compositions of known sources and sinks of Zn in the marine realm and showing that the weighted sum of known sinks is heavier than the weighted sum for sources. An accurate mass balance model for Zn isotopes will remain incomplete until an isotopically light sink or an isotopically heavy source is identified or else flux estimates can be substantially revised to address the current discrepancy. Our study has identified the mechanism responsible for the largest known fractionation between seawater and a major sink for Zn from the global oceans, which represents an important step towards resolution. Adsorption of Zn to Fe-Mn oxyhydroxide particles and concomitant isotope fractionation may also help to explain the recorded isotopic compositions of dissolved Zn in the surface waters of the oceans, where diatoms and other marine organisms assimilate this nutrient, as recently

suggested by John and Conway (2014). John et al. (2007) determined in culture experiments that diatoms have two uptake mechanisms for Zn, depending on the concentration of Zn available, and these two mechanisms both favor uptake of lighter isotopes of Zn, with fractionations of D66/64Znassimilated-dissolved = -0.2 or -0.8 ‰. Peel et al. (2009) also observed that organic-rich particles settling in a lake had light isotopic compositions consistent with a kinetic fractionation of similar magnitude driven by biological uptake. If such fractionation occurs in the surface waters of the oceans, one would expect the remaining dissolved Zn in seawater to be residually enriched in heavier isotopes, especially since assimilation-driven fractionation can be expected to follow Rayleigh trends. Using the observed 20-fold depletion in Zn concentrations of surface waters relative to deeper waters of the Southern Ocean, Zhao et al. (2014) calculated that a fractionation of -0.2 ‰ driven by diatom assimilation should result in an 0.6 ‰ enrichment in d66/64Zn relative to deeper waters, but no such contrast between surface waters and deep waters is evident in Southern Ocean depth profiles, while it was evident in NE Pacific and North Atlantic profiles analyzed in earlier studies (Bermin et al., 2006; Boyle et al., 2012; Vance et al., 2012; Conway et al., 2013; Conway and John, 2014). Zhao et al. (2014) suggest that perhaps diatom uptake does not fractionate Zn isotopes. An alternative explanation presented by John and Conway (2014) for the lack of heavy isotopic signatures in the dissolved component of surface waters is that some isotopically heavy Zn is adsorbed onto particles including Fe-Mn oxyhydroxides and cell membranes in surface waters, and this serves to obscure the true isotopic contrast between dissolved Zn and the Zn assimilated by diatoms. Tovar-Sanchez et al. (2003) designed a procedure for separation of adsorbed Fe from the interior pool of Fe in phytoplankton. If a similar procedure could be devised for separation of Zn adsorbed to Fe-Mn particles and other solid substrates, including cell membranes, from biologically assimilated Zn when treating

filtration residues, it might be possible to resolve this question by analyzing the adsorbed and assimilated Zn separately. In addition, using a similar approach, it might be possible to better understand export mechanisms and fluxes from surface to deeper waters by tracking the isotopic compositions and Fe-Mn contents of settling particles below the photic zone.

5. SUMMARY We conducted experiments to quantify Zn isotope fractionation during adsorption to synthetic birnessite and to constrain the fractionation mechanisms for any observed isotope effects. At low ionic strength the experimental system exhibited an initial kinetic isotope fractionation effect (lighter isotopes adsorbed, D66/64Zn ~ -0.2 ‰) that diminished over ~100 h to little or no isotope fractionation (0.05 ±0.08 ‰) as dissolved and birnessite-adsorbed Zn exchanged. In contrast, preferential adsorption of heavy Zn isotopes onto synthetic birnessite at high ionic strength (D66/64Znadsorbed-dissolved up to +2.7 ‰) reflects equilibrium isotope fractionation and the effects of chloro complexation of Zn in seawater. At low surface loading, Zn likely forms an adsorbed complex in which Zn is tetrahedrally coordinated, and this complex preferentially hosts the heavier isotopes of Zn relative to the mostly octahedral Zn species in solution. As surface loading increases, the adsorbed Zn inventory likely comprises increasing proportions of octahedrally coordinated Zn, and thus isotopic contrast with Zn in solution is diminished. The magnitude of fractionation we observe is also governed in part by equilibrium fractionation among multiple aqueous Zn species, with preferential adsorption of free Zn2+ onto the birnessite. The results reported here provide a mechanistic explanation for the isotopically heavy compositions of Zn in natural ferromanganese sediments relative to deep seawater, especially for

hydrogenetic crusts, in which birnessite is the predominant Mn phase. Since Zn surface loading is considerably lower in natural Fe-Mn sediments than in our experimental solids, our results are consistent with recent observations of tetrahedrally coordinated Zn in natural samples (Marcus et al., 2004; Little et al., 2014a). However, an explanation must still be found for why the isotopic signatures of natural Fe-Mn sediments are not even heavier and for what variables govern the variability among crusts and nodule samples analyzed thus far. An important aim for any future work will be to determine to what extent the variability is governed by differences in the isotopic composition of the water mass from which the crusts or nodules are precipitating, as opposed to other possibilities such as the presence of Fe oxyhydroxides as adsorbents, complexation of some dissolved Zn by organic molecules, and incorporation of detrital or other non-authigenic Zn. The heavy isotopic composition of Zn adsorbed to birnessite particles also suggests a possible resolution of the dilemma in recent literature regarding whether or not diatoms fractionate Zn during assimilation in surface waters of the ocean.

ACKNOWLEDGMENTS The authors wish to thank S. Romaniello for assistance with the double-spike MATLAB code and D. Weiss for providing the London Zn standard. We also thank S. Little, J. Black, and an anonymous reviewer for thorough and helpful comments on the original submitted manuscript. This project was supported by a grant from the US National Science Foundation, NSF-OCE 1143984, to LEW.

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Figure Captions

Figure 1: δ66/64Zn of adsorbed and dissolved Zn pools for select experiments processed with and without gloves to demonstrate contamination of samples by gloves (figure modified from Wilkes et al., 2010). Zinc in all experiments came from an Ultra Scientific ICP solution that was also used as the running standard for analysis (δ66/64Zn = 0 ‰). Products from experiments numbered 28, 20, 32, and 36 were put through ion exchange chemistry with Oak Technical gloves on, and mass balance was not satisfied. For a few samples, multiple aliquots were separately put through columns, and the plot demonstrates that these were variably contaminated. Products of experiments numbered 12, 13, and 18 were processed with bare hands. Mass balance was still not satisfied, but came much closer, with sorbed Zn now isotopically lighter than the starting stock. The compositions of another ICP standard solution (Alfa Aesar) and zinc leached from two brands of polyvinyl gloves (Oak Technical and Kimtech) are shown (dashed lines, see legend), and the gloves are believed to be the source of isotopically heavy contaminant Zn in the experimental samples. The current study employed extra measures to eliminate this problem (see Section 2.3).

Figure 2: Isotopic compositions of dissolved Zn (open squares) and sorbed Zn (filled squares) from 48 h low ionic strength experiments versus percentage of Zn sorbed, all reported relative to the Sesame Zn standard. The horizontal line at δ66/64Znsorbed-aqueous = 0 ± 0.05 ‰ represents the normalized, bulk starting composition, and the grey shading shows the error. The isotopic compositions of the starting stock solutions varied among experiments, so values in the figures have been normalized to 0 ‰ for clarity. Error bars represent the larger of 2 sd on three to four

replicate analyses of each sample or the long-term 2 sd on our standard (0.05 ‰), whichever is larger. The solid lines represent the best visual fit for closed-system equilibrium fractionation with Δ66/64Znsorbed-aqueous = -0.16 ‰. The dashed lines are Rayleigh fractionation curves with a fractionation of Δ66/64Znsorbed-aqueous = -0.16 ‰ that would imply a kinetic effect or an equilibrium-driven effect with irreversible adsorption.

Figure 3: Isotopic compositions of dissolved Zn (open squares) and sorbed Zn (filled squares) from 24 h to 1680 h at low ionic strength, all reported relative to the Sesame Zn standard. The horizontal line at δ66/64Zn sorbed-aqueous = 0 ± 0.05 ‰ represents the normalized, bulk starting composition, and the grey shading shows the error. The isotopic compositions of the starting stock solutions varied among experiments, so values in the figures have been normalized to 0 ‰ for clarity. Error bars represent the larger of 2 sd on three to four replicate analyses of each sample or the long-term 2 sd (0.05 ‰), whichever is larger.

Figure 4: a) Isotopic compositions of dissolved Zn (open squares) and sorbed Zn (filled squares) from 48 h high ionic strength experiments versus percentage of Zn sorbed, all reported relative to the Sesame Zn standard. b) Expanded view of Figure 4a at high surface coverage of Zn sorbed onto mineral surface. The horizontal line at δ66/64Zn sorbed-aqueous = 0 ± 0.05 ‰ represents the normalized, bulk starting composition, and the grey shading shows the error. The isotopic compositions of the starting stock solutions varied, so values in the figures have been normalized to 0 ‰ for clarity. Error bars represent the larger of 2 sd on three to four replicate analyses of each sample or the long-term 2 sd (0.05 ‰), whichever is larger.

Figure 5: Isotopic compositions of dissolved Zn (open squares) and sorbed Zn (filled squares) from 24 h to 172 h at high ionic strength, all reported relative to the Sesame Zn standard. The horizontal line at δ66/64Zn sorbed-aqueous = 0 ± 0.05 ‰ represents the normalized, bulk starting composition, and the grey shading shows the error. Error bars represent the larger of 2 sd on three to four replicate analyses of each sample or the long-term 2 sd (0.05 ‰), whichever is larger.

Figure 1.

No gloves

Gloves

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Table 1: Salt species in synthetic seawater stock solutions Composition of synthetic seawater (ionic strength of 0.7 g/kg) modified from Stumm and Morgan (1981) Salt Concentration (g/kg) KCl 0.75 NaHCO3 0.19 MgSO4 5.96 CaCl2 1.45 MgCl2 4.90 NaCl 26.16

Table 2: Low ionic strength parameters and results Each experiment is maintained at ambient conditions and a pH ~8.2 and duration of 48 h. Birnessite Mass balance Zn in expt. Remaining Zn Adsorbed 2σ δ66/64Znstock** 2σ δ66/64Zn MnOx* 2σ δ66/64Zn sol’n* Suspension in offset*** (μg) in sol’n (μg) Zn (%) expt. (g) 0.33 30.3 19.9 27.2 -6.44 0.05 -6.55 0.05 -6.67 0.07 -0.18 0.11 36.8 32.9 11.0 -6.13 0.05 -6.23 0.05 -6.12 0.07 0.03 0.17 36.8 32.5 12.1 -6.11 0.05 -6.21 0.05 -6.12 0.07 0.02 0.29 36.8 29.1 22.3 -6.15 0.05 -6.28 0.05 -6.12 0.07 0.06 0.44 36.8 24.7 36.2 -6.11 0.05 -6.26 0.05 -6.12 0.07 0.04 0.47 36.8 27.9 25.0 -6.12 0.05 -6.28 0.05 -6.12 0.07 0.04 0.70 66.1 24.1 53.6 -5.91 0.05 -6.08 0.05 -6.05 0.05 -0.05 0.94 30.3 8.8 65.5 -6.56 0.05 -6.70 0.05 -6.67 0.07 -0.02 1.25 66.1 20.1 59.6 -5.94 0.05 -6.14 0.05 -6.05 0.05 0.01 1.40 30.3 5.3 84.2 -6.50 0.05 -6.66 0.05 -6.67 0.07 -0.03 1.50 51.4 27.3 38.1 -6.29 0.05 -6.47 0.05 -6.40 0.06 -0.04 2.00 49.4 10.4 62.4 -5.43 0.05 -5.61 0.05 -5.50 0.07 0.04 2.50 49.4 7.4 78.8 -5.43 0.05 -5.62 0.05 -5.50 0.07 0.08 3.00 51.4 11.7 68.5 -6.29 0.05 -6.48 0.05 -6.40 0.06 0.02 * Each sample is an average of three or four measurements on a Nu Plasma II with the 2σ values reported to reflect internal precision on replicates or our long-term 2σ on the Sesame Zn standard, whichever is larger. ** The table presents the measured isotope values. In Figure 2, all values have been normalized such that stock = 0 ‰ for clarity. *** Mass balance offset = δ66/64Znstock – (δ66/64Znaqueous · (100 - % sorbed) + δ66/64Znsorbed · % sorbed)

Table 3: Low ionic strength time series parameters and results Each experiment is maintained at ambient conditions and a pH ~8.2. Birnessite Mass balance Duration of Zn in expt. Remaining Zn Adsorbed 2σ δ66/64Znstock** 2σ δ66/64Zn MnOx* 2σ δ66/64Zn sol’n* Suspension offset*** expt. (h) (μg) in sol’n (μg) Zn (%) in expt. (g) 24 0.94 45.5 22.8 47.9 -5.86 0.05 -6.07 0.05 -5.88 0.05 0.07 72 1.10 45.5 24.1 42.0 -5.91 0.06 -6.04 0.05 -5.88 0.05 0.08 100 0.95 45.5 22.4 49.7 -5.83 0.05 -5.86 0.05 -5.88 0.05 -0.04 124 1.06 45.5 28.4 30.9 -5.94 0.05 -5.88 0.05 -5.88 0.05 0.05 172 0.96 45.5 24.4 43.5 -5.96 0.05 -5.90 0.05 -5.88 0.05 0.06 336 1.96 45.7 2.14 96.4 -6.06 0.06 -6.03 0.05 -6.28 0.05 -0.25 504 2.03 45.7 2.69 91.4 -6.28 0.05 -6.20 0.05 -6.28 0.05 -0.07 672 1.98 45.7 3.44 92.4 -6.18 0.06 -6.13 0.05 -6.28 0.05 -0.15 1680 1.99 45.7 1.96 86.7 -6.27 0.05 -6.25 0.05 -6.28 0.05 -0.03 * Each sample is an average of three or four measurements on a Nu Plasma II with the 2σ values reported to reflect internal precision on replicates or our long-term 2σ on the Sesame Zn standard, whichever is larger. ** The table is displaying the measured isotope values. In Figure 3, all values have been normalized such that stock = 0 ‰ for clarity. *** Mass balance offset = δ66/64Znstock – (δ66/64Znaqueous · (100 - % sorbed) + δ66/64Znsorbed · % sorbed)

Table 4: High ionic strength parameters and results Each experiment is maintained at ambient conditions and a pH ~8.2 and duration of 48 h. Birnessite Mass balance Zn in expt. Remaining Zn Adsorbed 2σ δ66/64Znstock** 2σ δ66/64Zn MnOx* 2σ δ66/64Zn sol’n* Suspension in offset*** (μg) in sol’n (μg) Zn (%) expt. (g) 0.22 48.9 42.0 12.1 -6.06 0.05 -5.04 0.08 -5.98 0.05 -0.04 0.50 48.8 33.9 8.1 -6.61 0.05 -3.87 0.06 -6.20 0.05 0.19 0.51 48.9 41.8 16.5 -5.92 0.05 -5.21 0.05 -5.98 0.05 -0.18 0.86 52.7 42.8 18.9 -5.91 0.05 -5.73 0.05 -5.84 0.05 0.03 0.90 48.8 31.6 12.0 -6.43 0.06 -4.58 0.05 -6.20 0.05 0.01 1.83 52.7 35.5 32.7 -5.93 0.05 -5.79 0.05 -5.84 0.05 0.04 1.95 48.9 36.4 25.1 -6.01 0.06 -5.80 0.05 -5.98 0.05 -0.02 2.85 53.0 23.1 62.8 -5.80 0.06 -5.64 0.05 -5.67 0.05 0.03 3.27 53.0 20.8 57.5 -5.73 0.05 -5.60 0.05 -5.67 0.05 -0.02 3.76 52.7 27.1 48.6 -5.98 0.05 -5.82 0.05 -5.84 0.05 0.06 5.75 52.7 18.0 65.9 -5.99 0.05 -5.80 0.05 -5.84 0.05 0.04 7.59 52.7 15.4 70.7 -5.97 0.06 -5.81 0.05 -5.84 0.05 0.02 9.90 48.9 20.0 77.1 -5.93 0.05 -5.77 0.05 -5.98 0.05 -0.17 * Each sample is an average of three or four measurements on a Nu Plasma II with the 2σ values reported to reflect internal precision on replicates or our long-term 2σ on the Sesame Zn standard, whichever is larger. ** The table is displaying the measured isotope values. In Figure 4, all values have been normalized such that stock = 0 ‰ for clarity. *** Mass balance offset = δ66/64Znstock – (δ66/64Znaqueous · (100 - % sorbed) + δ66/64Znsorbed · % sorbed)

Table 5: High ionic strength time series parameters and results Each experiment is maintained at ambient conditions and a pH ~8.2. Birnessite Mass balance Duration of Zn in expt. Remaining Zn Adsorbed 2σ δ66/64Znstock** 2σ δ66/64Zn MnOx* 2σ δ66/64Zn sol’n* Suspension offset*** expt. (h) (μg) in sol’n (μg) Zn (%) in expt. (g) 24 0.92 53.7 40.1 17.3 -5.82 0.05 -5.05 0.05 -5.65 0.05 0.03 72 0.94 53.7 40.4 19.9 -5.82 0.05 -5.17 0.05 -5.65 0.05 0.04 100 0.97 53.7 44.7 20.9 -5.80 0.05 -5.28 0.05 -5.65 0.05 0.04 124 0.97 53.7 43.1 21.4 -5.82 0.05 -5.24 0.05 -5.65 0.05 0.05 172 0.93 53.7 36.5 28.2 -5.85 0.05 -5.25 0.06 -5.65 0.05 0.03 * Each sample is an average of three or four measurements on a Nu Plasma II with the 2σ values reported to reflect internal precision on replicates or our long-term 2σ on the Sesame Zn standard, whichever is larger. ** The table is displaying the measured isotope values. In Figure 5, all values have been normalized such that stock = 0 ‰ for clarity. *** Mass balance offset = δ66/64Znstock – (δ66/64Znaqueous · (100 - % sorbed) + δ66/64Znsorbed · % sorbed)

Table 6: Equilibrium Zn speciation in synthetic seawater stock compared to low ionic strength. Calculated using PHREEQC with pH ~8.2, T = 25 ºC, and P = 1 bar. Aqueous Species Low Ionic Strength High Ionic Strength RPFR for 66/64Zn* 2+ Zn 99.90% 54.23% 3.348 ZnCl+ 0% 11.67% 3.223 ZnCO30 0% 10.08% ZnSO40 0% 7.52% Zn(CO3)220% 4.65% ZnCl20 0% 2.88% 3.034 Zn(OH)2 0% 2.25% ZnCl30% 1.88% 2.459 ZnOH+ 0.05% 1.68% Zn(HCO3)+ 0.04% 1.52% Zn(SO4)220% 0.87% ZnCl420% 0.78% 2.457 *Reduced partition function ratios (RPFR, ln (β)•1000) for 66/64Zn at 294 K from Fujii et al. (2010).

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