Environmental control of the oxygen isotope composition of Porities coral microatolls

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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 delta(18)O 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 delta(18)O 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 delta(18)O values of transects that represent the same time periods is 0.03 parts per thousand, and is within measurement error for a single analysis (0.04 parts per thousand); (2) the average standard deviation for time-equivalent, near-monthly samples along the transects within the same microatoll is 0.07 parts per thousand and; (3) comparison of the average delta(18)O values of records for different microatolls from across Kiritimati Island show only a small between-coral differences of 0.04 parts per thousand and 0.11 parts per thousand for different time periods. These differences in mean delta(18)O are within the range for intra-and inter-colony differences in seasonal and interannual delta(18)O reported for dome-shaped Porites. Based on these results, a stacked microatoll delta(18)O record was constructed for the period 1978-2007 for comparison with published coral delta(18)O records for nearby dome-shaped Porites. There is a systematic offset between the two types of records, which is probably due to variations in delta(18)O seawater across Kiritimati Island. Despite the offset, all records show similar amplitudes for the seasonal-cycle of delta(18)O, and there is a strong correlation (r = -0.71) between microatoll delta(18)O and local sea surface temperature (SST). The delta(18)O-SST slope relationship for microatolls is -0.15 parts per thousand/degrees C, very similar to that reported for fast-growing domed corals (-0.18 parts per thousand to -0.22 parts per thousand/degrees C). Statistical analysis of the stacked microatoll delta(18)O 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 delta(18)O 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 delta(18)O signals that can significantly extend instrumental records of tropical climate variability. Crown copyright (C) 2011 Published by Elsevier Ltd.
Environmental effects, Oxygen, Isotopes, Corals, Tropical regions, Oscillations
McGregor H. V., Fischer M. J., Gagan M. K., Fink, D., & Woodroffe, C.D.(2011). Environmental control of the oxygen isotope composition of Porites coral microatolls. Geochimica et Cosmochimica Acta, 75(14), 3930-3944. doi:10.1016/j.gca.2011.04.017