Biological control of internal pH in scleractinian corals: Implications on paleo-pH and paleo-temperature reconstructions

Elemental ratios and isotopic compositions of scleractinian corals are often used as proxies for the pH and the temperature of seawater from which they calcified. Nevertheless, these ratios recorded in the coral skeleton are offset from the ratios expected for equilibrium, due to vital effects. Ion microprobe δ11B measurements were performed in a modern tropical coral, Porites lutea, and in a Mediterranean coral, Cladocora caespitosa, grown under two different pCO2. We show that the δ11B variations measured at micrometer scale are principally controlled by biology via modification of the pH near the calcification sites. The range of calculated pH allows to reproduce the range of δ18O measured in corals via a kinetic model of oxygen isotopic equilibrium between water and DIC. Thus we show that temperature does not seem to be the primary control of the δ18O variations in coral, though the constants of reaction and the isotopic fractionation values obviously depend on temperature.