Factors influencing the stable carbon and oxygen isotopic composition ofPorites lutea coral skeletons from Phuket, South Thailand

Abstract

We determined the δ18O and δ13C composition of the same fixed growth increment in severalPorites lutea coral skeletons from Phuket, South Thailand. Skeletal growth rate and δ18O are inversely related. We explain this in terms of McConnaughey's kinetic isotopic disequilibria model. Annual trends in δ18O cannot be solely explained by observed variations in seawater temperature or salinity and may also reflect seasonal variations in calcification rate. Coral tissue chlorophylla content and δ13C of the underlying 1 mm of skeleton are positively related, suggesting that algal modification of the dissolved inorganic carbonate pool is the main control on skeletal δ13C. However, in corals that bleached during a period of exceptionally high seawater temperatures in the summer of 1991, δ13C of the outer 1 mm of skeleton and skeletal growth rate (over 9 months up to and including the bleaching event) are inversely related. Seasonal variations in °13C may reflect variations in calcification rate, zooxanthellae photosynthesis or in seawater δ13C composition. Bleached corals had reduced calcification over the 9-month period up to and including the bleaching event and over the event they deposited carbonate enriched in13C and18O compared with unaffected corals. However, calcification during the event was limited and insufficient material was deposited to influence significantly the isotopic signature of the larger seasonal profile samples. In profile, overall decreases in δ18O and δ13C were observed, supporting evidence that positive temperature anomalies caused the bleaching event and reflecting the loss of zooxanthellae photosynthesis.