Morphology and formation of relict coral reef on the shelf around Lord Howe Island.

Coral reefs track sea level and are particularly sensitive to changes in climate. Reefs are threatened by global warming, with those in tropical waters experiencing increased incidences of bleaching. Although it has been suggested that reefs may extend poleward at their latitudinal limit, there has been little evidence to support this contention. In this paper, we report on a much more extensive coral reef that flourished around Lord Howe Island, which presently supports the southernmost coral reef in the Pacific. Multibeam swath mapping and sub-bottom profiling reveal an extensive reef that encircled the island, in the middle of the shelf, rising from water depths of around 50 m to 30 m, with isolated peaks reaching 23.5 m. Coring has indicated that this relict reef is composed of corals that grew between 9200 and 7100 years ago, and that the main phase of reef growth had terminated and the reef backstepped by 7000 years BP. Localised re-establishment of corals over the surface around 2500 and in the past few hundred years provides some indication that corals may be able to recolonise the relict reef and extend further in response to warmer temperatures anticipated later this century and beyond. However, this will depend on the availability of suitable substrate, as well as other environmental factors, as the relict reef is now in greater water depths than when it was most actively growing.National Institute of Water & Atmospheric Researc