Uranium-series dating of corals from the southwest Pacific


The uranium-series technique is based on the observation that carbonates precipitating in nature, in particular corals and oolites, show an initial disequilibrium between Th²³⁰ and its parent U²³⁸. The amount of Th²³⁰ initially present in a coral is negligible in comparison to that subsequently generated by the radioactive decay of uranium. If the system remains closed then the ratio of Th²³⁰ to U²³⁸ is a simple function of time. Because there is a 15 percent excess of U²³⁴ to U²³⁸ in sea water this has to be taken into account when determining the age. Corals from New Guinea, the Loyalty Islands, and the east coast of Australia were provided for dating. The New Guinea corals, from the Huon Peninsula, had been dated previously, and so they provided a check on the reliability of the techniques used in this study. With one exception the ages from this study are within the error limits placed on the original ages. The one sample that does not agree is shown to have a high proportion-of void-filling low-Mg calcite cement. Corals from the Capricorn Group and Hayman Island within the Great Barrier Reef province show relatively young ages. One coral recovered by drilling at a depth of 17m on the Hayman Island reef indicates that the time of recolonization of the reef towards the end of the Holocene transgression is about 8,300 yr B.P. Coral samples below a marked discontinuity at a depth of about 20m are extensively recrystallized. Ages of corals from the Inner Barrier of New South Wales show that this feature 'formed during the last inter-glacial at about 120,000 yr B.P. The ages suggest that there were two periods of high sea level at about this time. Ages from reef terraces 2-6m above present sea level from three islands of the Loyalty Archipelago show the varying degrees of uplift of these islands. Corals from +2m on Beautemps-Beaupre are older than 200,000 yr B.P., while a coral at +6.5m from Ouvea gave an age of 117,000 ± 6,000 yr B.P. Ages from the +2m terrace on Lifou support the interpretation of a relatively high sea level at about 180,000 yr B.P. Ages of corals from a slightly raised fringing reef around Mud Island, Moreton Bay indicate a sea level about one metre higher than present during the interval 4,000 - 6,000 yrs B.P. This slightly higher sea level could have been the result of a changing.tidal regime within the bay

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