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.
Highlights
The boron geochemistry of coral skeletons offers a potential method to reconstruct the dissolved inorganic carbon (DIC) chemistry of the coral calcification fluid (Allison et al, 2014) and to determine how it responds to environmental change
In the case that B(OH)4− co-precipitates with CO32− and KD is variable, aragonite B/Ca and pHCF are always positively correlated (Fig. 8g,j,m)
In the case that B(OH)4− co-precipitates with CO32−, approximately constant skeletal B/Ca can be generated over a wide pHCF range by a calcification fluid with approximately constant [DIC], as in the low DIC scenario modelled here
Summary
The boron geochemistry of coral skeletons offers a potential method to reconstruct the dissolved inorganic carbon (DIC) chemistry of the coral calcification fluid (Allison et al, 2014) and to determine how it responds to environmental change. Increasing calcification fluid pH shifts the fluid DIC equilibrium in favour of carbonate (CO32−) at the expense of CO2 and bicarbonate (HCO3−) and creates a concentration gradient facilitating the diffusion of CO2 from the overlying coral tissue into the fluid (Erez, 1978). Recent inorganic aragonite precipitation studies indicate that the borate: aragonite partition coefficient can be highly variable (Mavromatis et al, 2015; Holcomb et al, 2016) and is probably affected by the saturation state of the precipitating fluid (Holcomb et al, 2016) This complicates the interpretation of coral skeletal [B]. Most coral datasets do not exhibit significant correlations between calcification fluid pH (inferred from δ11B) and skeletal B/Ca (Fig. 1) and it is timely to consider why this is It is unclear which dissolved inorganic carbon (DIC) species is/are involved in aragonite precipitation. I consider how different co-precipitating DIC species affect aragonite B/Ca and estimate the impact of variations in the B(OH)4−/co-precipitating DIC aragonite partition coefficient, which may be associated with changes in the DIC and Ca2+ chemistry of the calcification fluid
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