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

Abstract

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 pHCF and assuming that KD 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 pHCF 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 pHCF are positively related i.e. if DIC is increasingly concentrated in the calcification fluid at higher pHCF probably by CO2 diffusion into the calcification site.