During 1970s, coral skeleton oxygen isotope composition (δ18O) was regarded as the isotopic thermometer following thermodynamic rules. Recently, coral aragonite oxygen isotopic fractionation could appear to be controlled by biology, its rate being accelerated by an enzyme (carbonic anhydrase or CA). Such a new concept results of an original approach involving coral culture in controlled conditions. Environmental factors, temperature and also light have been tested on macrosize scale samples (some mg), and δ18O revealed vital effects, anomalies compared with chemical and isotopic equilibrium, related to metabolic activity. δ18O analyses at microsize scale (some μm), using ion microprobe, could display the mechanism of crystallisation, δ18O fractionation responding to biological kinetic effects. The understanding of coral aragonite δ18O is the absolute prerequisite to develop the first model of a coral proxy.
Part of the book: Isotopes Applications in Earth Sciences