Isotopic disequilibrium of Cu in marine ferroman-ganese crusts: Evidence from ab initio predictions of Cu isotope fractionation on sorption to birnessite

摘要

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 Cu~(2+) complexes and Cu as a surface complex on birnessite. We find that isotopic fractionation between the Cu(H_2O)_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 (Δ_(sw-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 Δ_(sw-fmc) = +1.1 to2.4‰. 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.