EFFECTS OF CALCIFICATION PATTERNS ON THE OXYGEN-ISOTOPE COMPOSITION OF THE SKELETON OF THE SCLERACTINIAN CORAL ACROPORA-FORMOSA

Oxygen isotope ratios were measured along the growth axis of branches of the scleractinian coral Acropora formosa collected at 2 and 12 metr e depths at Yonge reef (Northern Great Barrier Reef, Australia). Measu rements were made between two reference points separated by a distance corresponding to a growth period of six months, from mid-winter to mi d-summer. For each of the two reference points, information concerning the environmental parameters controlling the development of the coral colonies was collected in situ. The variability of the isotopic value s recorded from coral skeletons grown in identical physical and chemic al conditions cannot be ascribed to technical problems, but may rather be induced by calcification mechanisms. In order to define the influe nce of calcification processes on oxygen isotopic composition, a simpl e mathematical model is developed, simulating the behaviour of oxygen isotopes. According to Gladfelter's studies (1982, 1983, 1984), we inf er that the main factors affecting the isotopic ratio of skeletal arag onite in the scleractinian coral Acropora genus are on the one hand th e initial quantity of aragonite deposited at the apical part of the br anch and consequently the relative amount of primary and secondary ara gonite infilling residual pores during coral growth, and on the other hand, the duration of the secondary aragonite infilling. Comparisons b etween the measured and the calculated isotopic profiles reveal that d ifferences in calcification processes account for isotopic discrepanci es encountered in the different colonies analysed. This study stresses that care must be taken when using the oxygen isotope composition of coral skeleton as a paleoenvironmental proxy.