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 [Ca]. I consider how different co-precipitating DIC species affect aragonite B/Ca and also estimate the impact of variations in the B(OH)/co-precipitating DIC aragonite partition coefficient (K), which may be associated with changes in the DIC and Ca chemistry of the calcification fluid. The coral skeletal B/Ca versus calcification fluid pH relationships reported previously can be reproduced by estimating B(OH) and co-precipitating DIC speciation as a function of pH and assuming that K are constant i.e. unaffected by calcification fluid saturation state. Assuming that B(OH) co-precipitates with CO, then observed patterns can be reproduced by a fluid with approximately constant [DIC] i.e. increasing pH concentrates CO, as a function of DIC speciation. Assuming that B(OH) co-precipitates with HCO only or CO + HCO then the observed patterns can be reproduced if [DIC] and pH are positively related i.e. if DIC is increasingly concentrated in the calcification fluid at higher pH probably by CO diffusion into the calcification site.