The K_D Sr/Ca in cultured massive <i>Porites</i> 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, KD 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 KD 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 KD Sr/Ca and coral calcification rate after combining all coral genotypes to allow identification of the correct KD Sr/Ca to apply to coral records