The K_D Sr/Ca in cultured massive Porites spp. corals are reduced at low seawater pCO₂

Coral skeletal Sr/Ca has valuable potential as a proxy of sea surface temperatures (SSTs). However seawater pCO₂ can influence skeletal Sr incorporation and Sr/Ca-SST calibrations derived from present day corals may not be applicable to ancient specimens or older sections of modern corals deposited under lower seawater pCO₂ than the present day. In this study we analysed skeletal Sr/Ca in multiple genotypes of massive Porites spp. cultured over a range of seawater pCO₂ (from 180 to 750 μatm) and temperature (25°C and 28°C). Multiple linear regression analysis indicates that the Sr/Ca aragonite partition coefficient, K_D Sr/Ca is inversely related to seawater temperature and positively related to seawater pCO₂ (equivalent to changes in skeletal Sr/Ca of 0.046 mmol mol^-1 °C^-1 and 0.0002 mmol mol^–1 µatm^-1 respectively). Applying present day Sr/Ca-SST equations to older coral skeletons growing at lower pCO₂ could underestimate seawater temperatures. However K_D Sr/Ca vary significantly between some coral genotypes cultured at the same seawater pCO₂ indicating that other unidentified processes also influence skeletal Sr/Ca and it is unknown how these processes varied when ancient corals were deposited. We do not observe a significant relationship between K_D Sr/Ca and coral calcification rate after combining all coral genotypes to allow identification of the correct K_D Sr/Ca to apply to coral records.