The Interpretation of Geochemical Data from Coral Skeleton Need to Be Revisited

The aragonite skeleton of the long-live massive corals, developed all around the tropical belt, are unique archives to supplement the instrumental records. It is generally admitted that the oxygen isotopic composition is governed by the classical isotopic thermometer. Recently ion microprobe investigations proved that at micrometre scale, during a year, the signal is not linear relative to time and the isotopic fractionation is mainly due to a kinetic process. Nevertheless, a quasi-equilibrium formula is derived from the mean annual oxygen isotopic composition for two coral genera, from annual data series published by Weber and Woodhead (1972). The kinetic process, revealed by ion probe data, is confirmed by analysing corals cultured in 5 controlled temperature conditions: it affects both oxygen and carbon isotopic partition, the oxygen showing also the quasi-equilibrium fractionation. By performing a stastistical treatment (an empirical orthogonal function analysis), on annual oxygen and carbon isotope time series over at least one hundred years, we separate the two fractionations from the oxygen signal. The trend showed by the quasi-equilibrium signal from the beginning of the XXth century is mainly due SST increase and it appears that when the SST changes linked with an El Nino event are reduced, the interannual fluctuations are essentially related to precipitation changes, revealing an unexpected abrupt shift of El Nino consequence between 1924 and 1931.