Effect of skeletal growth and lack of species effects in the skeletal oxygen isotope climate signal within the coral genus Porites

We investigated variations in skeletal δ18O within the scleractinian coral species Porites lutea, P. australiensis and P. murrayensis, looking at the intra-colony, inter-colony and between-species level. This provides unambiguous information to test the existence of species-specific effects and other non-environmental sources of variation affecting the oxygen isotope climate signal. Comparing replicate isochronic δ18O time series revealed that there was no species-specific effect on skeletal δ18O. Non-environmental variation could be attributed to methodological constraints in sampling and to variations in annual linear skeletal extension (LSE). Annual LSE was on average 10 mm±2.9 S.D., and ranged from 4.5 to 18 mm. Correlation analyses between mean annual δ18O and annual LSE showed weak but significant and negative correlations for all years. When the slope of δ18O vs. LSE of the respective years was used to correct monthly δ18O for variations due to LSE, the average standard deviation (S.D.) between the 12 replicate monthly δ18O values was reduced by 18%. Only after removing the effect of LSE on coral δ18O, it was possible to identify in the coral δ18O time series the severe drought in Indonesia that preceded the 1982–1983 El Niño event.