The Great Barrier Reef survived five “death events” over the past 30,000 years, but might not be resilient enough to bounce back from current climate pressures, according to a new study.
By drilling into and extracting fossilised coral at the edge of the continental shelf, a team of scientists reconstructed how the reef shifted and evolved over the past 30,000 years.
In the journal Nature Geoscience today, they report that the reef migrated out to sea and landward again as sea levels rose and fell with changes in glaciation.
During these swings, the reef faced death events, but survived the ups and downs by shifting and growing at different depths.
Today, though, with a faster changing climate, the Great Barrier Reef simply might not be able to keep up.
Coral reefs might seem like stable, immobile systems, but they can shuffle around when conditions change (albeit slowly).
During ice ages, sea levels gradually fall. As the shoreline recedes, the reef follows suit, according to study co-author Jody Webster, a geoscientist at the University of Sydney.
“As the sea level fell in the last ice age, that exposed and killed the reef in some locations,” Dr Webster said.
“What was underwater became the land, the shoreline migrated to the east, and a new reef started growing.”
Dr Webster and his colleagues found the reef faced two of these exposure death events in the past 30,000 years.
One struck 30,000 years ago, and another 8,000 years later, right before the last glacial maximum, which is marked by the maximum extent of ice and the lowest sea level around the world.
On the Great Barrier Reef, the sea level was at its lowest about 21,000 years ago â€” 118 metres below the present sea level.
During exposure from falling sea levels, the reef migrated up to 1 kilometre seaward to stay underwater.
The other type of reef death event is caused by sea level rise. This occurred around 17,000 and 13,000 years ago as the world’s ice sheets started to melt â€” a period known as the “deglaciation”.
“The sea level started rising rapidly back across the shelf, and in a couple of instances the combination of sea level rise and we think increased sediment flux actually led to the decline of the reef,” Dr Webster said.
By looking at the different corals and algae in the fossil cores, the researchers worked out that there had been a decline in reef growth rates.
“We could see the growth slowed to the point where the [coral] community changed and switched completely from shallow water fast-growing forms to now deeper water forms,” Dr Webster said.
The most recent death event took place around 10,000 years ago and made way for what scientists call the “Holocene reef” â€” the modern Great Barrier Reef.
It wasn’t just the sea level rise, but the potential influx of sediments from the land as the shelf flooded that led to the demise of this reef, Dr Webster said.
University of Wollongong marine palaeoecologist Tara Clark was not involved in the study, but said that it was a significant and comprehensive look at how the reef developed over millennia â€” not only how the reef responded to changes in sea level, but also to the associated environmental changes.
“Looking at water quality, coral community changes and growth rates is an added benefit to show how these reefs have changed through time,” she said.
“It’s one of the first descriptions of this, which is crucial at this point in time with all the environmental changes and changes in land use that are taking place at the moment around the Great Barrier Reef.”
This study is the first to piece together what happened to the Great Barrier Reef during times of low sea level over the past 30,000 years, Dr Webster said.
By sonar mapping the seabed at the edge of the continental shelf, he and his team saw ridges and structures along the sea floor which they thought could be the remnants of drowned reef systems.
“We took the drill ship out to the edge of the continental shelf in front of the modern reef, and then drilled a whole series of cores through the seabed beneath which is the record of where the reef once grew,” he said.
Once they collected fossil reef cores, the team scrutinised them for organisms to “paint a picture of what the environment was like when the coral reef was growing”, Dr Webster said.
The coral and algae embedded in the cores were carbon-dated, and their chemical signatures let the team reconstruct the sea surface temperature and the water depth at the time they were alive.
Dr Webster said that understanding the reef’s survival threshold over the past 30,000 years is important for predicting and protecting the reef’s health down the track.
“There’s strong evidence that since European colonisation there has been a massive increase in sediment and nutrient flux to the Great Barrier Reef lagoon, so that is an ongoing issue of concern,” he said.
Dr Clark hopes that large baseline studies spanning huge spatial and time scales, like Dr Webster’s research, will lead to improvements in reef management.
“The fact that they’ve shown a decline in coral, not only attributable to changes in sea level but also to poor water quality, could hopefully give management an idea of where they should target improvements,” Dr Clark said.
While the Great Barrier Reef has shown remarkable resilience to respond as an ecosystem to sea level changes and sea surface temperature rise, the changes the reef is currently facing are more extreme and faster than ever before, Dr Webster warned.
“From the last ice age to about 10,000 years ago, the temperature rose a couple of degrees over tens of thousands of years, and if we compare that with the sorts of rates we see now, the rate of temperature rise is much faster now,” he said.
Dr Clark agrees that the past ability of the reef to bounce back cannot be taken as a sign it will recover from its current woes.
“The reefs may have been resilient in the past, but going into the future with the number of bleaching events we’ve had and the land-use change that’s going on,” she said.