Reconstructing coral calcification fluid dissolved inorganic carbon chemistry from skeletal boron: an exploration of potential controls on coral aragonite B/Ca

The boron geochemistry of coral skeletons reflects the dissolved inorganic carbon (DIC) chemistry of the calcification fluid from which the skeletons precipitates and may be a valuable tool to investigate the effects of climate change on coral calcification. In this paper I calculate the predicted B/Ca of aragonite precipitating from seawater based fluids as a function of pH, [DIC] and [Ca²⁺]. I consider how different co-precipitating DIC species affect aragonite B/Ca and also estimate the impact of variations in the B(OH)₄⁻/co-precipitating DIC aragonite partition coefficient (KD), which may be associated with changes in the DIC and Ca²⁺ chemistry of the calcification fluid. The coral skeletal B/Ca versus calcification fluid pH relationships reported previously can be reproduced by estimating B(OH)₄⁻ and co-precipitating DIC speciation as a function of pH_CF and assuming that K_D are constant i.e. unaffected by calcification fluid saturation state. Assuming that B(OH)₄⁻ co-precipitates with CO₃²⁻, then observed patterns can be reproduced by a fluid with approximately constant [DIC] i.e. increasing pH_CF concentrates CO₃²⁻, as a function of DIC speciation. Assuming that B(OH)₄⁻ co-precipitates with HCO₃⁻ only or CO₃²⁻ + HCO₃⁻ then the observed patterns can be reproduced if [DIC]_CF and pH_CF are positively related i.e. if DIC is increasingly concentrated in the calcification fluid at higher pH_CF probably by CO₂ diffusion into the calcification site.