Environmental control of the oxygen isotope composition of Porites coral microatolls

Understanding the influence of climatic and non-climatic factors on geochemical signals in corals is critical for assessing coral-derived records of tropical climate variability. Porites microatolls form large, disk-shaped colonies constrained in their upward growth by exposure at or close to mean spring low water level, and occur on Indo-Pacific reefs. Microatolls appear suitable for paleoclimate reconstruction, however the systematics of the microatoll chemistry–climate relationship are yet to be characterized. In this study, the δ 18O signal in Porites microatolls from well-flushed reef flats on Kiritimati (Christmas) Island, central Pacific was investigated for intra-coral (growth aspect and extension rate effects) and between-coral effects, and to explore the climate signal contained within their skeletons. Samples for δ 18O analysis were taken from six individual transects from different positions within Porites microatoll XM22. The results show that: (1) the average standard deviation for the mean δ 18O values of transects that represent the same time periods is 0.03‰, and is within measurement error for a single analysis (0.04‰); (2) the average standard deviation for time-equivalent, near-monthly samples along the transects within the same microatoll is 0.07‰ and; (3) comparison of the average δ 18O values of records for different microatolls from across Kiritimati Island show only a small between-coral differences of 0.04‰ and 0.11‰ for different time periods. These differences in mean δ 18O are within the range for intra- and inter-colony differences in seasonal and interannual δ 18O reported for dome-shaped Porites. Based on these results, a stacked microatoll δ 18O record was constructed for the period 1978–2007 for comparison with published coral δ 18O records for nearby dome-shaped Porites. There is a systematic offset between the two types of records, which is probably due to variations in δ 18O seawater across Kiritimati Island. Despite the offset, all records show similar amplitudes for the seasonal-cycle of δ 18O, and there is a strong correlation ( r = −0.71) between microatoll δ 18O and local sea surface temperature (SST). The δ 18O-SST slope relationship for microatolls is −0.15‰/°C, very similar to that reported for fast-growing domed corals (−0.18‰ to −0.22‰/°C). Statistical analysis of the stacked microatoll δ 18O record shows that it is correlated with both local and large-scale climate variables (primarily SST) at semiannual, ann