Mean oxygen-isotope signatures in Porites spp. corals:: inter-colony variability and correction for extension-rate effects

The assessment of inter-colony variability in the mean skeletal delta(18)O signatures of modern Porites spp. corals is a prerequisite for the estimation of past mean climate conditions based on fossil colonies. Here we show that the mean delta(18)O signatures of Porites spp. corals from the northern end of the Gulf of Aqaba (Red Sea) with mean extension rates between 0.2 and 1.5 cm/year can have an inter-colony variability as large as 1.28parts per thousand. At extension rates of less than 0.6 cm/year the mean coral delta(18)O values of the individual colonies are strongly dependent on the mean extension rate, with increasingly higher delta(18)O values corresponding to decreasing extension rate. This suggests that extension-rate-related kinetic isotope disequilibrium effects are responsible for a large proportion of the inter-colony differences in the mean coral delta(18)O signatures. A correction procedure for these effects based on the relationship between mean delta(18)O values and mean extension rate reduces the variability of mean coral delta(18)O values among the individual colonies to 0.43parts per thousand. Although certainly not perfect, the correction procedure enables a better assessment of mid-Holocene climate conditions at this location based on Porites spp. with mean extension rates of less than 0.6 cm/year.