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

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 (similar to 10(5) years) corals from southwestern Western Australian; and second, the dating of near-modern (10(0) to 10(2) 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 similar to 300-400km compared with present-day limits, requiring significantly warmer (similar to 2 degrees 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 2-2.5m above present-day sea-level, confirming previous observations from along the stable coastal margin of Western Australia that sea-levels were 3m higher during the first part of the Last Interglacial. The substantially improved sensitivity of MC-ICPMS is illustrated by U-238-Th-230 dating of modern coral samples extracted from a living Porites colony from the Great Barrier Reef. The U-238-Th-230 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 Th-230.