The Measured Coral Oxygen Isotopes Result of The Superimposition of Two Fractionations

International audienceCoral oxygen isotopes (d18O) are regarded as a robust proxy of environmental variables. Despite the apparent isotopic desequilibrium, attributed to a kinetic behavior, there is a strong temperature dependence. The calibrations are usually performed by comparing monthly proxy records of a coral head with instrumental data over the last decades. However the derived temperature variations, even over only the last century, match very poorly with the observed SST record. The geochemical proxies measured from coral skeletons are governed by physical processes strongly influenced by the vital effect. The oxygen isotopes from aragonite record temperature and/or water isotopic composition (d18Osw) changes. The formulas vary according to the species and the local conditions but the isotopic deviation per 1°C is very close to the coefficient inferred for calcite by Urey. This result suggests that the isotopic desequilibrium is the same for a colony during its lifetime. The isotopic desequilibrium affects both oxygen and carbon. By comparing several coral records we demonstrate that interannual oxygen and carbon isotopic curves exhibit a common shape and it is possible to separate the O and C common signal from the measured d18O variations by using EOF analysis. Two modes are generated by the analyses, which are converted into oxygen isotopic composition; the d18O calculated from the first mode is called d18Ocommon whereas the second mode is called d18O residual. We proof that d18O residual, is the thermodynamic isotopic response and d18Ocommon is the modulation of the d18O offset, due to a kinetic process occuring during calcium carbonate deposition. d18O residual obeys to temperature and d18Osw changes. The d18Ocommon variations are temperature and salinity dependent. Since d18Osw and salinity, covary, the relationship which links d18Osw and salinity being specific at each site, d18Ocommon and d18O residual may be expressed following temperature and salinity,. Thus from each coral head it is possible to reconstruct temperature and salinity back in time from oxygen and carbon aragonite isotopes