| The biogeochemical cycling of zinc (Zn), an important micronutrient in the ocean, may influence primary productivity and species composition within surface waters. The chemical speciation and bioavailability of Zn is governed by diverse abiotic and biotic processes. These processes include adsorption reactions at mineral/water interfaces, as nanoparticles of oxyhydroxide minerals are known to adsorb significant amounts of Zn in surface waters (and during formation of ferromanganese crusts). Investigation of Zn isotope fractionation caused by adsorption onto birnessite, the dominant manganese oxide mineral in ferromanganese crusts, may help to explain the enrichment of heavy Zn isotopes in ferromanganese crusts (Little et al., 2014a). This will provide insight into the role of adsorption of Zn to nanoparticulate minerals in surface waters and into the overall biogeochemical cycling of Zn.;This work aims to determine the mechanism and magnitude of Zn isotope fractionation during adsorption onto synthetic birnessite (K0.5Mn 2O4*1.5H2O) at low and high ionic strength. The system at low ionic strength exhibits an initial kinetic isotope fractionation effect that diminishes over time to no measurable isotope fractionation as dissolved and birnessite-sorbed Zn exchange. At high ionic strength, the results show an equilibrium isotope fraction effect with adsorption of heavy Zn isotopes onto the birnessite surface over a delta68/66Znsorbed-aqueous range of 2.74 +/- 0.06 / to 0.13 +/- 0.05.;The experimental results support that the observed enrichment of heavy Zn isotopes in natural Fe-Mn crusts in the oceans is most likely mechanistically driven by inorganic Zn complexation (e.g. Zn-chloro complexes) and Zn surface loading. The coordination numbers and bond partners between dissolved and adsorbed Zn complexes change at low and high surface loading. At low surface coverage, the coordination change from aqueous Zn(H2O) 62+ to a fourfold Zn complex most likely results in a large isotope effect. However, at high surface coverage, the isotope effect diminishes because of increasing proportions of octahedrally coordinated Zn complexes (Manceau et al., 2002). Additionally, at high ionic strength, Zn forms complexes with chloride and sulfate that preferentially incorporate light Zn isotopes (Fujii et al., 2010) leaving an isotopically heavier pool of aqueous Zn(H2O)62+ species to sorb onto the birnessite surface. |
