Applications of the ²³⁸U–²³⁰Th decay series to dating of fossil and modern corals using MC-ICPMS

Abstract

Methods for U-series dating of corals are described using a multi-collector (MC) ICPMS. The utility of this approach is illustrated with two examples: first, dating of Last Interglacial (∼10⁵ years) corals from southwestern Western Australian; and second, the dating of near-modern (10⁰ to 10² years) corals from the Great Barrier Reef. A previously unreported occurrence of Last Interglacial corals from the southernmost tip (Cape Leeuwin) of Western Australia is shown to have U-series ages ranging from 129 to 125 ka, indicating that relatively prolific coral growth occurred during the first half of the Last Interglacial period. This represents an extension of the southernmost limit of coral growth by almost ∼300–400 km compared with present-day limits, requiring significantly warmer (∼2°C) sea-surface temperatures, consistent with intensification of the Leeuwin Current. Furthermore, at this southernmost site, corals are preserved in growth position at heights of 2–2.5 m above present-day sea-level, confirming previous observations from along the stable coastal margin of Western Australia that sea-levels were ∼3 m higher during the first part of the Last Interglacial. The substantially improved sensitivity of MC-ICPMS is illustrated by ²³⁸U–²³⁰Th dating of modern coral samples extracted from a living Porites colony from the Great Barrier Reef. The ²³⁸U–²³⁰Th ages from annual samples are in excellent agreement with an independent chronology derived by counting of annual density bands and show that the coral grew continuously from 400 years ago to the present day. The typical precision achievable for corals ranging in age from tens to hundreds of years is shown to be 2–5 years, with the main limitation being uncertainties in the correction for initial non-radiogenic ²³⁰Th.

Key Words:

• corals
• Great Barrier Reef
• Last Interglacial
• uranium-series dating
• Western Australia

Acknowledgements

We thank Julie Trotter, Paul Hearty, Michael O'Leary and Barry House for assistance and helpful discussions on the Last Interglacial reefs from Western Australia. We are also grateful to Jianxin Zhou and an anonymous reviewer for constructive comments on the manuscript. This research has been supported by funding from the ARC Centre of Excellence on Coral Reef Studies.