NOAA/WDS Paleoclimatology - Kiritimati Island Porites Coral Microatoll d18O Data

STUDY NOTES: Porites coral microatoll oxygen isotope (d18O) data at monthly resolution, from samples collected near Kiritimati (Christmas) Island in the central Pacific. Data include individual colony d18O series, both new and previously published, plus a stacked d18O record combining data from 3 colonies. Kiritimati (Christmas) Island: 1°52'N, 157°25'W.
ABSTRACT SUPPLIED BY ORIGINATOR: 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 d18O 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 d18O 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 d18O values of transects that represent the same time periods is 0.03 per mil, and is within measurement error for a single analysis (0.04 per mil); (2) the average standard deviation for time-equivalent, near-monthly samples along the transects within the same microatoll is 0.07 per mil and; (3) comparison of the average d18O values of records for different microatolls from across Kiritimati Island show only a small between-coral differences of 0.04 per mil and 0.11 per mil for different time periods. These differences in mean d18O are within the range for intra- and inter-colony differences in seasonal and interannual d18O reported for dome-shaped Porites. Based on these results, a stacked microatoll d18O record was constructed for the period 1978-2007 for comparison with published coral d18O records for nearby dome-shaped Porites. There is a systematic offset between the two types of records, which is probably due to variations in d18O seawater across Kiritimati Island. Despite the offset, all records show similar amplitudes for the seasonal-cycle of d18O, and there is a strong correlation (r = -0.71) between microatoll d18O and local sea surface temperature (SST). The d18O-SST slope relationship for microatolls is -0.15 per mil/°C, very similar to that reported for fast-growing domed corals (-0.18 per mil to -0.22 per mil/°C). Statistical analysis of the stacked microatoll d18O record shows that it is correlated with both local and large-scale climate variables (primarily SST) at semiannual, annual and interannual timescales. Our results show that the signal reproducibility and fidelity of skeletal d18O in coral microatolls is comparable to that observed for more conventional coral growth forms. Longer-lived, and fossil, Porites microatolls, where they have grown in suitably flushed environments, are likely to contain d18O signals that can significantly extend instrumental records of tropical climate variability.