Examining the I/Ca paleo-proxy on a molecular level: A quantum-mechanical approach to understanding the thermodynamics of iodine incorporation in marine calcite

Abstract:

Trace element to calcium ratios in marine calcite are often employed to reconstruct past environments. These ratios rely on the co-calcification of certain ions in the place of either a calcium cation (Ca²⁺) or a carbonate anion (CO₃^2-) under different environmental conditions, informing paleoceanographers about ambient seawater chemistry at the time of biomineralization. The iodine-to-calcium ratio (I/Ca) in marine calcite has been used as an indicator of paleo-hypoxia due to iodine’s unique redox chemistry in aqueous environments. Inorganic iodine exists in one of two thermodynamically stable forms in the ocean, iodate (IO₃^-) and iodide (I^-), and laboratory precipitation experiments support the preferential substitution of IO₃^- into the calcite lattice. Computational models of ion substitutions can simulate conditions of calcite formation in natural systems through a framework of parameters that mimic natural conditions, fortifying our understanding of trace element incorporation thermodynamics at a molecular and atomic scale. Here, we use a quantum-mechanical approach to quantify the thermodynamics (enthalpies and Gibbs free energies of incorporation at 298 K) of coupled monovalent cation (Li⁺, Na⁺, K⁺) and anion (IO₃^-, I^-) substitutions into a calcite supercell by replacing one CaCO₃ cluster by a proximal pair of previously hydrated Li⁺/Na⁺/K⁺_aq cations and IO₃^-/I^-_aq anions. We find that for IO_3^- incorporation, the most favorable (least positive) incorporation is with Na⁺_aq, whereas with I^- incorporation, the most favorable is with K⁺_aq. We also tested iodine incorporation without a coupled cation substitution by displacing one Ca²⁺ and two CO₃^2- in the calcite supercell, resulting in a positive charge excess which was brought to charge neutral by a substitution of two IO₃^-_aq or two I^-_aq in the resulting vacancy. This process cannot energetically compete with the coupled-substitution described previously, further supporting a coupled substitution of a common monatomic cation and iodate/iodide as the most likely iodine incorporation mechanism. However, the modeled sodium-iodate incorporation is less favorable than observations from laboratory precipitation experiments suggest, highlighting a deficiency in our understanding of iodine’s incorporation mechanism into the calcite lattice.