WHY FERROMANGANESE CRUSTS ARE ENRICHED IN HEAVY ZN ISOTOPES

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. Here we report results of experiments demonstrating that heavier isotopes of Zn are preferentially adsorbed to Mn oxyhydroxide. We observed a short-lived kinetic isotope effect at low ionic strength, with lighter isotopes adsorbed to birnessite (Δ^68/66Zn_{adsorbed-dissolved} ~ -0.2 ‰). Beyond 100 hours, the sense of fractionation was opposite, but the magnitude of Δ^68/66Zn_{adsorbed-dissolved} at steady state was indistinguishable from zero (+0.05 ±0.08 ‰). At high ionic strength, we observed heavier isotopes adsorbed, with a strong negative correlation between Δ^68/66Zn_{adsorbed-dissolved} and the fraction of Zn adsorbed on birnessite. Values of Δ^68/66Zn_{adsorbed-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, GCA 66, 2639), we infer that Zn adsorbs first as tetrahedral, inner-sphere complexes at low surface loading. Since the predominant aqueous species of Zn is octahedral, the sorbed pool is enriched in heavier isotopes. 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 affected by aqueous speciation of Zn, as nearly a third of the Zn is complexed with chloride ions in our simplified, synthetic seawater. Based on theoretical predictions of Fujii et al. (2010, JPhysChemA 114, 2543), the chloro complexes can be expected to tie up a pool of isotopically light Zn in solution, driving the adsorbed pool to be heavier relative to 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.