Biologic Influence on Annual and Seasonal Oxygen Isotopic Ratio of Scleractinian Coral Skeleton

The oxygen isotope ratio from coral skeleton is the more commonly used seasurface temperature (SST) proxy. Thus, it is admitted that this isotopic composition is only dependent on external factors, SST and the water isotopic ratio. Even by considering ultra-structures, deep corals and tropical ones developed in constant conditions show identical crystal types, characterized by similar oxygen isotopic signature and variability. However, the calibration calculated from cultured Acropora over temperature ranging between 21 and 29°C, indicated a higher absolute slope (in isotopic deviation per 1°C) than that derived from Weber and Woodhead data (1972). The latter data series provided statistical mean annual values lacking of vital effect (isotopic variations due to biologic activity) and the obtained calibration, taking into account seawater isotopic ratio, was similar with Epstein relationship, though isotopic values are lower. Cultured Acropora submitted to light change equivalent to seasonal one showed a systematic isotopic increase. This variation was always associated with an increase of aragonite deposition and a decrease of the linear extension, illustrating the strong relationship between the algae metabolism and the skeleton formation. Temperature and light lead to opposite oxygen isotopic biology-mediated responses. Compared with the temperature, the light influence is lower but it does not mean that light effect is negligible in regard to seasonal isotopic amplitude. Is it specific to cultured Acropora? In the field, it is difficult to separate temperature and light effect, however the link between light, growth properties and oxygen isotopes has been yet demonstrated on Porites. In order to estimate the isotopic influence of a seasonal factor we compared data from literature concerning Porites colonies developed in the same site. Although influenced by identical seawater and temperature, calibrations from different species, showed high variability between colonies and also between different sampling of a single head. It can be only explained by another seasonal biologic effect. Light and temperature being the main changes recorded between winter and summer, we conclude that both temperature and light, in addition to the physical SST influence, affect the oxygen isotopic composition through metabolism. It could explain biases observed from temperature and/or salinity reconstructions using tropical corals. Based on these evidences we developed a new method using multi-proxies (see PP18).