The Great Barrier Reef is a master at cheating death. It has resurrected itself five times in the last 30,000 years after being wiped out by dramatic environmental shifts.
at the University of Sydney and his colleagues studied聽samples collected from today鈥檚 reef. They collected 30- to 40-metre-long cores taken from 16 different sites, which revealed how the reef has changed over the last 30 millennia.
The researchers found evidence of five mass reef deaths triggered by sudden changes in sea level.
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Two of these deaths occurred between 30,000 and 22,000 years ago as Earth鈥檚 climate headed towards the last glacial maximum. As the climate cooled, more water became bound up in ice sheets and the sea level dropped by about 120 metres. 鈥淭he retreating shoreline would have exposed the reef and caused it to die,鈥� says Webster.
Rise and fall
Nevertheless, both times the reef managed to re-establish itself over the next several thousand years by moving further out to sea. Tiny amounts of surviving coral may have been able to replenish the new sites with larvae, says Webster.
As the planet began to warm again, the reef encountered new problems. Melting ice sheets caused sea levels to rise, leaving coral too deeply submerged to get enough sunshine. Associated coastal flooding聽would also have released massive amounts of sediments that could have harmed the coral, says Webster.
The gradually rising seas聽caused the reef to die three times between 17,000 and 10,000 years ago. However, each time it managed to bounce back about a thousand years later by moving to shallower waters near shore, the cores revealed.
The findings suggest that the Great Barrier Reef has greater long-term resilience than previously thought, but its short-term health is harder to predict, says Webster. 鈥淲ith the year-on-year mass bleaching that we鈥檝e been seeing and the , I wouldn鈥檛 hold this study up as a great beacon of hope for the reef,鈥� he says.
Ocean temperatures are probably rising faster now that at any time in the last 30,000 years, meaning we have no reference point to compare against, says Webster. 鈥淭he reef will probably die again in the next few thousand years if it follows its past geological patterns because we鈥檙e due for another glacial period, but whether human-induced climate change will hasten that death remains to be seen.鈥�
Nature Geoscience,
