Abstract
In the oceans, Cu is strongly scavenged by ferromanganese (Fe-Mn) crusts. The isotopic fractionation of Cu between seawater and crusts provides insight into the mechanisms of trace metal cycling in the oceans. Dissolved Cu in seawater is isotopically heavy (+0.66±0.19‰) relative to Cu in crusts (+0.31±0.24‰). The primary mineral phase sorbing divalent trace metals in Fe-Mn crusts is birnessite. Recent laboratory measurements show that isotopically light Cu is preferentially sorbed on birnessite, with a fractionation factor of 0.45±0.18‰. Here, we use first-principles (quantum mechanical) calculations to predict the isotopic fractionation between aqueous Cu2+ complexes and Cu as a surface complex on birnessite. We find that isotopic fractionation between the Cu(H2O)5+ complex and sorbed Cu should be 0.49‰ (at 25°C), in close agreement with experiments, confirming that these experimental results reflects equilibrium fractionation. We then predict the isotopic fractionation between dissolved inorganic Cu in seawater and birnessite given the thermodynamic speciation of dissolved Cu at pH 8. We find dissolved inorganic Cu should be 0.94‰ (at 5°C) heavier than Cu sorbed to birnessite. This value is substantially greater than the observed fractionation between seawater and Fe-Mn crusts (Δsw-fmc≈+0.35‰). Moreover, it is well established that dissolved Cu in seawater is strongly complexed by organic ligands. Based on model Cu complexes and published experimental data, we estimate that fractionation of Cu by organic ligands should increase the equilibrium fractionation between seawater and Fe-Mn crusts by 0.2 to 1.5‰ to yield Δsw-fmc = +1.1 to 2.4‰. We conclude that Cu in marine Fe-Mn crusts in not in isotopic equilibrium with dissolved Cu in seawater, and consider the possible explanations of this surprising finding.
Highlights
Copper is both an essential micronutrient and a toxicant in aquatic ecosystems (e.g., Bruland et al, 2013)
A combination of EXAFS and density functional theory (DFT) calculations indicated that Cu2+ sorbs predominantly via the formation of III-IV fold coordinated complexes over vacancy sites (Sherman and Peacock, 2010)
The results confirm the interpretation of the EXAFS of Cu sorbed to synthetic birnessite (Sherman and Peacock, 2010) and Cu associated with natural marine ferromanganese crusts (Little et al, 2014a)
Summary
Copper is both an essential micronutrient and a toxicant in aquatic ecosystems (e.g., Bruland et al, 2013). In aquatic environments, dissolved Cu will complex with a range of organic (and inorganic) ligands (e.g., Coale and Bruland, 1988; Moffett and Dupont, 2007) Each of these processes (biological uptake, sorption, organic complexation) is associated with Cu isotope fractionation (e.g., Bigalke et al, 2010; Navarrete et al, 2011; Pokrovsky et al, 2008; Ryan et al, 2014; Sherman, 2013). Sorption by complexation over the vacancy sites of birnessite (assuming III-IV fold coordination) was expected to favour the heavy isotope (Little et al, 2014a). We predict the equilibrium isotopic fractionation of birnessite-sorbed Cu via the statistical mechanical partition function from the relevant vibrational modes of finite clusters used to approximate the surface complexes using first-principles (ab initio) simulations. Comparison of the theoretical equilibrium fractionation with experimental values and that observed in the oceans will be used to understand fractionation process in the marine environment
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