Sulfate and phosphate oxyanions alter B/Ca and δ11B in inorganic calcite at constant pH: Crystallographic controls outweigh normal kinetic effects

We report new results from inorganic experiments in which the effects of several oxyanions on the concentration and isotopic composition of boron (B/Ca and δ11B, respectively) in calcite were explored. The oxyanions examined here (sulfate, phosphate, nitrate, and nitrite) differ in their size, charge, and geometric configuration, but they are all common dissolved constituents in natural aqueous solutions including seawater. Of those, sulfate and phosphate which are known to inhibit calcite nucleation/precipitation had pronounced impacts on the B incorporation and isotope fractionation during calcite precipitation. Additions of up to 5 mmol sulfate and up to 2 μmol phosphate into experimental solutions of otherwise unaltered condition caused comparable declines in calcite precipitation rates R, increases in B/Ca, and decreases in calcite δ11B towards the calculated δ11B of B(OH)4−. The pattern of changes in B/Ca and δ11B as a function of R observed here is at odds with normal kinetic effects confirmed in previous studies, where B/Ca and δ11B were shown to increase and decrease, respectively, with an increase in degrees of calcite saturation in solutions and thus R. We argue that the paradoxical and apparently reversed kinetic trends in B/Ca and δ11B in the presence of sulfate and phosphate can be attributed to deformations of the calcite lattice structure due to their substitutions for lattice CO3, which in turn enhances the retainment and eventual incorporation of B(OH)4−. Our new findings could have important implications for paleo-reconstructions of ocean carbonate chemistry using B/Ca and δ11B, if the mechanism proposed above is similarly in effect during natural precipitation of biogenic calcite (e.g., foraminiferal tests) in seawater. Though this needs further testing, if true, the gradual but sizable increase in seawater sulfate over the Cenozoic could have biased foraminiferal B/Ca and δ11B. Our experimental data additionally indicate that Na+ has the capability of providing charge compensations for substitutions of CO3 by B(OH)4−. However, our data also demonstrate that changes in Na+ concentration do not control the degrees of B incorporation.