Influences of coral genotype and seawater pCO₂ on skeletal Ba/Ca and Mg/Ca in cultured massive Porites spp. corals

Coral skeletal Ba/Ca is a proxy for seawater Ba/Ca, used to infer oceanic upwelling and terrigenous runoff while [Mg²⁺] is implicated in the control of coral biomineralisation. We cultured large individuals (>12 cm diameter) of 3 genotypes of massive adult Porites spp. corals over a range of seawater pCO₂ to test how atmospheric CO₂ variations affect skeletal Ba/Ca and Mg/Ca. We identified the skeleton deposited after a 5 month acclimation period and analysed the skeletal Ba/Ca and Mg/Ca by secondary ion mass spectrometry. Skeletal Mg/Ca varies significantly between some duplicate colonies of the same coral genotype hampering identification of genotype and seawater pCO₂ effects. Coral aragonite:seawater Ba/Ca partition coefficients (K_D Ba/Ca) do not vary significantly between duplicate colonies of the same coral genotype. We observe large variations in K_D Ba/Ca between different massive Porites spp. coral genotypes irrespective of seawater pCO₂. These variations do not correlate with coral calcification, photosynthesis or respiration rates or with skeletal K_D Mg/Ca or K_D Sr/Ca. Seawater pCO₂ does not significantly affect K_D Ba/Ca in 2 genotypes but K_D Ba/Ca is significantly higher at 750 μatm seawater pCO₂ than at 180 μatm in 1 P. lutea genotype. Genotype specific variations in K_D Ba/Ca between different Porites spp. could yield large errors (~250%) in reconstructions of seawater Ba when comparing Ba/Ca between corals. Analysis of fossil coral specimens deposited at low seawater pCO₂, may underestimate past seawater Ba/Ca and ocean upwelling/freshwater inputs when compared with modern specimens but the effect is small in comparison with the observed difference between coral genotypes.