Thirteen thousand years ago, when the last ice age came to an end, entire sections of the Australian Great Barrier Reef came to fruition. Rising sea levels covered the world's largest accumulation of coral with sediments coming from the newly flooded land, blocking the sunlight that corals need to grow. The reef finally recovered, but it took hundreds to thousands of years. This near-death and final resurrection was not an isolated event, according to a new study revealing the shifting boundaries of the reef over geological time. It's a story that has happened five times in the last 30,000 years – and it could happen again today.
The study "contains some really important lessons" to understand how resilient corals are in the face of change and how fast they recover after catastrophic events, says Kim Cobb, a paleoclimatologist at the Georgia Institute of Technology in Atlanta who does not was involved in the work. Today's rate of sea-level rise is moderate – about 10% of the rate 13,000 years ago – but it could accelerate dramatically, she says.
To conduct the study, scientists used underwater sonar to locate locations on the seafloor, beyond the current reef, where corals could have grown in the past. Then they drilled 20 holes and extracted rock cores containing fossil corals and sediments deposited in the last 30,000 years, spanning part of the last ice age and the subsequent warm millennia.
The reef wandered up and down during this time. The team found that sea level changes were closely tracked at speeds of up to 20 meters per year. And when the sea level reached its lowest point 21,000 years ago – 118 meters below today's level, its water is trapped in massive ice caps – corals have survived on the outer terraces of the Australian continental shelf, the team reports today in Nature Geosciences
Scientists have long wondered where the Great Barrier Reef was during the last Ice Age, says Jody Webster, marine geologist at the University of Sydney, Australia, and lead author of the study. "We could find it."
But the reef could not always keep up with the changing sea level. The researchers identified five times when they died twice during the cooling of the last ice age, when the sea level was exposed to coral. and three times before 10,000 to 17,000 years ago, when the glacial melt caused sea levels to rise rapidly. "We did not drill or sample everything," Webster says, so he and his colleagues can not confirm how dying was. But they think that corals remained in place along the continental shelf during that time, so that reefs could be restored elsewhere in 2000 years.
The historical deaths are similar to "what we just see on Earth Great Barrier Reef," says Mark Eakin, a coral reef ecologist at the National Oceanic and Atmospheric Administration College Park, Maryland, who was not involved in the study , Sea level changes are not a big problem right now, but the temperatures are: Heat waves have triggered mass bleaching events, times when heat-laden coral emits symbiotic algae that live in their tissues. In 2016 alone, the world's hottest year, 67% of corals died along the northernmost 700 kilometers of reef.
The new research is "another reminder" that what we do with the ocean will have dramatic consequences, says Eakin. "Do not expect reefs to fight back fast."