Timing and duration of the Last Interglacial: evidence for a restricted interval of widespread coral reef growth

Abstract We report new mass spectrometric U-series ages for eight Last Interglacial fossil reefs along the continental margin of Western Australia. 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 corrections for 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. These data supplement our previous results for Rottnest Island and Leander Point, leading to more than seventy mass spectrometric U-series ages from which constraints can 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, 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 128 ± 1 ka, assuming reef growth started soon after sea-level approached interglacial levels. 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 and/or sea-levels were stable enough to allow prolific reef growth only during the earlier part of the Last Interglacial.

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