Sulfate and phosphate oxyanions alter B/Ca and d11B 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 delta B-11, 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 mu mol phosphate into experimental solutions of otherwise unaltered condition caused comparable declines in calcite precipitation rates R, increases in B/Ca, and decreases in calcite .511B towards the calculated delta B-11 of B(OH)4.(- )The pattern of changes in B/Ca and delta B-11 as a function of R observed here is at odds with normal kinetic effects confirmed in previous studies, where B/Ca and delta B-11 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 delta B-11 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 delta B-11, 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 delta B-11. 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.

Automated summary

We conducted inorganic experiments to investigate the effects of various oxyanions on the concentration and isotopic composition of boron in calcite. Sulfate and phosphate were found to have significant impacts on the incorporation of boron and isotopic fractionation during calcite precipitation. The observed kinetic trends of boron in the presence of sulfate and phosphate differ from previous studies, suggesting structural deformations of the calcite lattice due to the substitution of CO3. These findings have important implications for paleo-reconstructions of ocean chemistry.