Last updated May 28, 2018 at 4:34 pm
While reef is adaptable, it will struggle to cope with the pace of current change.
The Great Barrier Reef has moved back and forth across the ocean floor in response to changes in sea-levels during the past 30,000 years, according to new research.
Over the past 30 millenia sea level has varied dramatically as continental ice sheets expanded and waned. At the Last Glacial Maximum, about 20,000 years ago when the ice sheets were at their largest, the sea level was about 118 metres lower than it is today.
And that, according to the study, drove the reef to migrate to stay covered by water. Over the past 30,000 years, the researchers from the University of Sydney found there had been five large-scale deaths accompanied by the reef migrating to ether stay underwater, or move more landward following rising sea levels.
However, even though it has shown adaptability before, the researchers are still very concerned the Great Barrier Reef’s chances of survival, given the pace of changes caused by human activities.
Multiple deaths and moves
The research team examined drill cores collected near the current-day Great Barrier Reef – these cores of sediment, fossils and more provide a diary of information about the conditions of the past, which allowed them to assess how the reef system responded to challenges.
They found there were two widespread reef die-offs about 30,000 years and 22,000 years ago. These were caused by sea levels dropping as glaciers grew, leading to the reef being exposed to air, known as subaerial exposure.
To try to avoid dying completely, the reef moved seaward to try to keep pace with the falling sea levels and remain underwater.
These sea levels hit their lowest mark during the Last Glacial Maximum around 20,000 years ago.
As the glaciers began to melt after the Maximum, there were two more reef-death events at about 17,000 and 13,000 years ago caused by the rapid sea level rise. These were accompanied by the reef moving landward, trying to keep pace with rising seas.
Analysis of the core samples show these reef-deaths from sea-level rise were likely associated with large increases of sediment in the water.
After each death event, the reef was able to re-establish itself at a more suitable depth within hundreds to thousands of years.
The final reef-death was about 10,000 years ago, but did not match up with any known abrupt sea-level rise or “meltwater pulse” during glacier melting. There did, however, appear to be a massive sediment increase and reduction in water quality at the time.
That most recent death resulted in the Great Barrier Reef establishing its modern-day location around 9000 years ago.
“Our study shows the reef has been able to bounce back from past death events during the last glaciation and deglaciation,” said Associate Professor Jody Webster, who led the research at the University of Sydney.
“However, we found it is also highly sensitive to increased sediment input, which is of concern given current land-use practices.”
Bleak future for the Reef
Based on the reef-death events found in the cores, the researchers think the reef is able to migrate at between 0.2 and 1.5 metres a year.
However, Webster said it was unlikely that this rate would be enough to survive current rates of sea surface temperature rises, sharp declines in coral coverage, year-on-year coral bleaching, or decreases in water quality and increased sediment from human activities.
“I have grave concerns about the ability of the reef in its current form to survive the pace of change caused by the many current stresses and those projected into the near future,” he said.
Previous studies have established that sea temperature changes in the past were in the order of a couple of degrees over 10,000 years. However, current forecasts of sea surface temperature change are around 0.7 degrees in a century.
In addition to the faster temperature changes, it now also comes with the double whammy of sediment levels in the water.
“Our study shows that as well as responding to sea-level changes, the reef has been particularly sensitive to sediment fluxes in the past and that means, in the current period, we need to understand how practices from primary industry are affecting sediment input and water quality on the reef,” Webster said.
The research has been published in Nature Geoscience