The Western Australian coast line, from Perth to Ningaloo is intermittently ringed by fossil coral platforms. At places these are substantial structures, 3 m to 5 m above present sea-level, densely packed by favide species corals and also by large (>0.5 m) porites heads. At Ningaloo, the fossil platforms were once part of a barrier reef and lagoon structure very similar to what exists there currently. Behind the fossil platforms, in the old lagoon, are there are mounds of coral colonies and also groups of very large (>2 m) porites heads standing in growth position. All of these structures date back to the Last Interglacial period about 125,000 years ago.

We have dated corals from eight of the Last Interglacial fossil reefs using mass spectrometric U-series techniques. The corals were selected in growth position from localities that are characterized by apparently low levels of diagenesis and relative tectonic stability so that the fossil reefs provide critical information on Last Interglacial sea levels without requiring models of tectonic movements. In addition, we have improved the constraint on the timing of onset of reef growth by recovering drill core coral from the base of the reefs. Uranium and thorium isotopes were measured with high levels of precision, leading to improvements in age resolution and allowing samples which have undergone diagenetic exchange of uranium and thorium to be more easily identified and discarded. With more than seventy mass spectrometric U-series ages, constraints could be placed on the timing, duration and character of the Last Interglacial sea level highstand. Reliable ages show that reef growth started contemporaneously at 128 ± 1 ka along the entire Western Australian coastline, based on a sharply defined boundary in the data, while relative sea levels were at least 3 m above the present level. Because Western Australia is located far from the former Penultimate Glacial Maximum ice sheets and are not significantly effected by glacial unloading, these data constrain the timing of onset of the Last Interglacial period to shortly (<1 ky) before 128 ± 1 ka. A unique regressive reef sequence at Mangrove Bay constrains the timing of termination of the Last Interglacial period to 116 ± 1 ka. The major episode of reef building, however, both globally and locally along the Western Australian coast, is restricted to a very narrow interval occurring from ~128 ka and ~121 ka, suggesting that global ocean surface temperatures were warm enough and/or sea-levels were stable enough for prolific reef growth only during the earlier part of the Last Interglacial.