Background
Noting that "coral reefs track sea level and are particularly sensitive to changes in climate," the authors wondered whether "warmer sea surface temperatures might enable reef expansion into mid latitudes."

What was done
To explore this possibility, Woodroffe et al. employed sonar mapping of the seabed topography around Lord Howe Island (31°30'S) -- which is fringed by the southernmost reef in the Pacific Ocean -- supplemented with single-beam echosounder and Laser Airborne Depth Sounder (LADS) data, while bottom sediments were examined using an acoustic sub-bottom profiler and a grab sampler aided by radiocarbon and uranium-series dating.

What was learned
The nine researchers discovered an extensive relict coral reef around Lord Howe Island in water depths of 25-50 meters, which flourished in early Holocene times about 9000 to 7000 years ago, which they describe as "immense," as it was "more than twenty times the area" of the modern reef at that site.

What it means
Woodroffe et al. say their finding "demonstrates that reefs were much more extensive 9000 years ago than they are at present at this latitudinal limit to reef growth," and they conclude that the "relict reef, with localized re-establishment of corals in the past three millennia, could become a substrate for reef expansion in response to warmer temperatures, anticipated later this century and beyond." And to indicate that this situation is not unique, they report that "shelf-edge reefs are common throughout the Caribbean (Hubbard et al., 2008), and backstepped to modern reef locations 7000-6500 years ago," while noting that "complex early Holocene shelf reefs flourished 9000-7000 years ago in southeast Florida, at the northern latitudinal limit to reef growth, ceasing growth before 6000 calendar years before present (Toscano and Lundberg, 1998; Banks et al., 2008)." Hence, they opine that these and other similar sites "may represent important refugia from increases in sea surface temperature," citing additionally in this regard the work of Riegl and Piller (2003).

References
Banks, K.W., Riegl, B.M., Richards, V.P., Walker, B.K., Helme, K.P., Jordan, L.K.B., Phipps, M., Shivji, M.S., Speiler, R.E. and Dodge, R.E. 2008. The reef tract of continental southeast Florida (Miami-Dade, Broward and Palm Beach counties, USA). In: Riegl, B.M. and Dodge, R.E. (Eds.) Coral Reefs of the USA, Springer, Dordrecht, Netherlands, pp. 175-220.

Hubbard, D.K., Burke, R.B., Gill, I.P., Ramirez, W.R. and Sherman, C. 2008. Coral-reef geology: Puerto Rico and the US Virgin Islands. In: Riegl, B.M. and Dodge, R.E. (Eds.) Coral Reefs of the USA, Springer, Dordrecht, Netherlands, pp. 263-302.

Riegl, B. and Piller, W.E. 2003. Possible refugia for reefs in times of environmental stress. International Journal of Earth Sciences 92: 520-531.

Toscano, M.A. and Lundberg, J. 1998. Early Holocene sea-level record from submerged fossil reefs on the southeast Florida margin. Geology 26: 255-258.