Abstract
The incorporation of iodine into each of the three polymorphs of CaCO3 – calcite, aragonite and vaterite, is compared using first-principles computational simulation. In each case iodine is most easily accommodated as iodate (IO3−) onto the carbonate site. Local strain fields around the iodate solute atom are revealed in the pair distribution functions for the relaxed structures, which indicate that aragonite displays the greatest degree of local structural distortion while vaterite is relatively unaffected. The energy penalty for iodate incorporation is least significant in vaterite, and greatest in aragonite, with the implication that iodine will display significant partitioning between calcium carbonate polymorphs in the order vaterite > calcite > aragonite. Furthermore, we find that trace iodine incorporation into vaterite confers improved mechanical strength to vaterite crystals. Our results support the supposition that iodine is incorporated as iodate within biogenic carbonates, important in the application of I/Ca data in palaeoproxy studies of ocean oxygenation. Our observation that iodate is most easily accommodated into vaterite implies that the presence of vaterite in any biocalcification process, be it as an end-product or a precursor, should be taken into account when applying the I/Ca geochemical proxy.
Highlights
Understanding seawater oxygen content is important, since oxygen minimum zones (OMZs) adversely affect marine fisheries and biological productivity
That the differences in enthalpies of these three polymorphs of CaCO3 are generally similar to the distribution of estimates of enthalpy that are provided by different density functionals (Demichelis et al, 2013a) and that calcite is stabilized against the other polymorphs at ambient by entropic effects, especially associated with carbonate rotational disorder at high temperatures (Redfern et al, 1989)
Iodine itself can adopt multiple valence states, with I3+ and I5+ each forming iodite or iodate oxyanions in nature. In view of this it is quite possible that iodine incorporation and substitution for C4+ in the carbonates that we have considered is accommodated by variable oxidation state of the iodine itself
Summary
Understanding seawater oxygen content is important, since oxygen minimum zones (OMZs) adversely affect marine fisheries and biological productivity. Demands the development of a geochemical proxy for water oxygen content. Iodine has been recognised as a promising geochemical indicator for oxygen content: the variation of speciation of iodine in seawater, as either iodide (IÀ) or iodate (IO3À) anions, and the iodide/iodate redox potential lies close to that of O2/H2O (Rue et al, 1997). The speciation of iodine in aqueous solutions changes from dominantly iodate to iodide, and iodide is thermodynamically stable in anoxic waters (Chapman and Truesdale, 2011).
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