Ancient coral collapse may help save the Great Barrier Reef

Rising sea levels are a constant reminder of how climate change is reshaping reef ecosystems around the world.

New research reveals that the Great Barrier Reef might be able to keep pace with rising waters alone, but when combined with other environmental pressures, its future becomes much less certain.

Scientists from the University of Sydney have dug deep – literally – to understand how reefs have responded to dramatic changes in the past.

Using fossil reef cores extracted from beneath the Great Barrier Reef, they uncovered important clues. These cores, which serve as geological time capsules, show that rapid sea level rise alone did not lead to the downfall of an ancient version known as Reef 4.

Rising seas shape ancient reefs

The study focused on how the reef ecosystem evolved from about 13,000 to 10,000 years ago. This period includes a particularly intense phase called Meltwater Pulse 1B.

During this 350-year window, sea levels rose sharply as polar ice sheets melted under warmer global temperatures.

“This 350-year period is crucial; it covers a time when global sea levels rose very rapidly,” said Professor Jody Webster from the School of Geosciences.

“It’s a period when polar ice sheets are thought to have experienced accelerated melting due to warming temperatures,” he said. “Based on records from Barbados, we previously thought sea levels were rising by about 40 millimeters a year at this time.”

“Our research shows the rise wasn’t so large and fast. It was more likely to have been in the order of three to five millimeters a year, comparable to what we’re experiencing today.”

Learning from Reef Four

The fossil reef cores tell the story of Reef Four, the so-called proto-Great Barrier Reef. Extracted from the shelf edge at depths between 40 and 50 meters, these cores revealed that Reef Four had a structure and coral community very much like today’s reef.

“Reef Four is very exciting,” said Professor Webster. “It had a similar morphology and mix of coral reef communities to the modern Great Barrier Reef.”

“The types of algae and corals, and their growth rates, are comparable. Understanding the environmental changes that influenced it, and led to its ultimate demise, therefore offers clues on what might happen to the modern reef.”

Following its collapse, Reef Four’s ecosystem didn’t disappear altogether. Over 1,000–2,000 years, it moved landward, forming the foundation of today’s Great Barrier Reef.

Will the Great Barrier Reef survive?

Today’s Great Barrier Reef is under mounting environmental pressure. While it has endured natural changes over thousands of years, the challenges it faces today are more complex and widespread. Scientists warn that its future could look very different if current trends continue.

“The modern reef faces rising sea levels, more heat waves and extensive bleaching, along with increasing sediment and nutrient input. This combination, on top of rising sea levels, is of deep concern,” Professor Webster said.

“If the current trajectory continues, we should be concerned about whether the Great Barrier Reef will survive the next 50 to 100 years in its current state.”

“It won’t die but its characteristics may change. We will see a different collection of coral species, perhaps simpler and not as structurally complex.”

What we can learn from the past

To carry out this research, the team used radiometric dating to precisely match fossil coral samples to the period of Meltwater Pulse 1B. The cores were collected through the International Ocean Discovery Program (IODP), a collaborative marine research initiative involving 21 countries.

“These data allow us to more precisely understand how reef and coastal ecosystems have responded to rapid environmental changes, like the rises in sea level and temperature we face today,” said Professor Webster.

The value of these ancient records is hard to overstate. Unlike modern instrumental records, which span only the past century or so, these fossil cores stretch back thousands of years. They provide an irreplaceable window into how ecosystems have weathered – and sometimes failed to weather – rapid climate shifts in the past.

As the Great Barrier Reef stands at a critical point in its history, lessons from its ancient predecessor offer a sobering glimpse of what might lie ahead.

The full study was published in the journal Nature Communications.

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