Beneath the turquoise waters of the Great Barrier Reef lies a hidden geological archive: fossilised coral reefs that once flourished along the same submerged shelf where the reef thrives today. In a landmark study published in Nature Communications, scientists have tapped into this deep-time record, revealing that while the reef has withstood rapid sea level rise in the past, its future under modern climate stress may not be so assured.
The research, led by Professor Jody Webster from the University of Sydney’s School of Geosciences, analysed 15- to 20-metre fossil reef cores that were extracted by a drilling ship from the shelf edge at depths of 40 to 50 metres. The cores were obtained through the International Ocean Drilling Program (IODP), a multinational effort involving 21 countries. These cores, rich with fossil coral, algae, and sediments, offer a “geological time capsule” documenting the reef’s response to sea level and environmental change between 13,000 and 10,000 years ago.
Reef 4: A Window into the Past
The study focuses on an ancient predecessor to the modern reef, known as Reef 4 or the “proto–Great Barrier Reef.” Contrary to assumptions that rapid sea level rise alone caused its demise, the findings reveal that Reef 4 survived even significant marine transgressions. Instead, it was the combination of rising seas, warming temperatures, and declining water quality that led to its collapse around 10,000 years ago, towards the end of the last ice age.
“This research shows us a healthy, active barrier reef can grow well in response to quite fast sea level rises,” Professor Webster said. “It’s the combination of additional environmental stressors, on top of rapid sea level rise, that lead to its demise.”
Following its collapse, Reef 4’s ecosystem did not disappear altogether. Over the course of 1,000 to 2,000 years, it migrated landward and re-established itself as the Great Barrier Reef we know today.
This continuity highlights that Reef 4 was not lost entirely, but evolved into the modern reef system. It stands as the direct predecessor of today’s Great Barrier Reef — evidence of both resilience and vulnerability.
A Crucial Climate Episode: Meltwater Pulse 1B
Of particular interest to the team was the period known as Meltwater Pulse 1B, a 350-year episode between 11,450 and 11,100 years ago, when global sea levels were believed to have risen dramatically, potentially due to accelerated polar ice sheet melt.
“This 350-year period is crucial; it covers a time when global sea levels rose very rapidly,” said Professor Webster. “It’s a period when polar ice sheets are thought to have experienced accelerated melting due to warming temperatures. Based on records from Barbados, we previously thought sea levels were rising by about 40 millimetres a year at this time.”
However, the new evidence from the Great Barrier Reef tells a different story.
“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 millimetres a year, comparable to what we’re experiencing today.”
Using uranium-thorium and radiocarbon dating of 154 samples, the researchers reconstructed a more refined sea level timeline. Their analysis suggests a more moderate but still rapid rise of 23 to 30 mm per year, challenging the older Barbados record, which may have been skewed by tectonic uplift. This finding not only refines our understanding of past meltwater pulses but also draws direct parallels to current sea level rise under anthropogenic warming.
In total, sea levels rose approximately 7.7 to 10.2 metres during this 350-year episode — a reminder that even moderate annual rise rates can drive dramatic cumulative change.
Beyond the reef’s ecological transitions, the study offers critical refinements to global sea level reconstructions during Meltwater Pulse 1B. Using high-resolution uranium-thorium and radiocarbon dating across fossil coral and algal samples, the researchers reconstructed relative sea level with unprecedented accuracy. Contrary to earlier estimates suggesting extreme rates of sea level rise up to 40 mm per year, the new data show a more moderate rise of 23 to 30 mm annually over a 350-year period. This difference underscores the value of region-specific sea level reconstructions and suggests that previous estimates may have been skewed by tectonic uplift at the Barbados site. The improved timeline not only sharpens our understanding of past meltwater events but also provides a more relevant analogue for current and projected sea level rise, reinforcing the study’s core message: it is not sea level rise alone, but its combination with other climate stressors, that poses the greatest threat to reefs.
Lessons for the Future
Reef 4’s structural complexity, mix of coral communities, and ecological characteristics closely mirror today’s Great Barrier Reef.
“Reef 4 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.”
And the implications are sobering.
“The modern reef faces rising sea levels, more heat waves and extensive bleaching, along with increasing sediment and nutrient input,” said Professor Webster. “This combination, on top of rising sea levels, is of deep concern. 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.”
A Global Effort for Deep-Time Insight
The fossil reef cores analysed in this study were obtained through the International Ocean Drilling Program (IODP), a 21-nation scientific collaboration that enables marine geoscientific research by drilling deep beneath the seabed. These records go back far beyond the 50–100 years of instrumental data, allowing researchers to reconstruct how Earth’s ocean systems have responded to past climate events.
“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 study was conducted in collaboration with researchers from the University of Tokyo, Australian National University, Nagoya University, University of Grenada, and Aix- Marseille University. It was supported by funding from theAustralian Research Council, the Hanse Wissenschaftskolleg Institute for Advanced Study in Germany, and JST CREST. As the Great Barrier Reef approaches an increasingly precarious future, this research provides both a warning and a scientific tool. While reefs have endured past sea level rise, their survival now depends on more than just ocean heights. Without meaningful mitigation of climate change and local stressors, the Great Barrier Reef may endure in name, but not in nature, shifting from a vibrant underwater metropolis to a shadow of its former self. Sea level rise alone did not drown Reef 4; it was the gathering storm of heat, sediment, and stress that brought it down — a cautionary echo for coral systems today.
