Fresh geological data extracted from the Great Barrier Reef suggest that Earth's largest coral system is remarkably youthful. In the June issue of Geology, an international team reports that the reef has existed in its modern form for less than a million years--a fraction of the age of many coral reefs and atolls. The rapid formation of the vast reef, which covers some 250,000 square kilometers off the northeast coast of Australia, may have been a result of prehistoric changes in climate that affected reefs around the globe.
Until now, scientists had little direct evidence of the Great Barrier Reef's age and evolution, but data from seismic profiles and drilling projects nearby suggested it might not have formed until as recently as 500,000 years ago. This would mean that, compared to other living coral systems, such as the Pacific atolls of Enewetak and Bikini, which have accumulated over more than 45 million years, the Great Barrier Reef is an infant in geological terms.
To get a better estimate, the consortium of 17 researchers from Europe and Australia carefully drilled two holes, one 86 meters deep into the reef's inner shelf, the other 210 meters into a narrow reef skirting the margin of the outer shelf. The team removed cores from both sites and analyzed their composition. Strontium isotope ratios in the outer shelf core date its oldest coral remains to between 490,000 and 1.05 million years old. That estimate is backed up by paleomagnetic data, which show that this portion of the reef must have formed after Earth's magnetic poles last reversed, some 790,000 years ago. The researchers estimate that the precursor to the modern reef, complete with its outer fringe, first established itself about 600,000 years ago.
That date, says Don McNeill, a geologist at the University of Miami who has studied much smaller reefs in the Florida Keys and the Bahamas, "is fairly similar to what we see in the western Atlantic." In that context, the finding suggests that major long-lasting fluctuations in global climate and sea level, known to have occurred around that time, may have spawned simultaneous growth in reefs across the globe, McNeill says.