The Great Barrier Reef in Australia is the largest coral reef on earth. But when and how it came into being was not clear until now. Scientists may now have clarified this question. Accordingly, the formation of the reef was probably closely related to the formation of Fraser Island, a hook-shaped protruding sandy island off the east coast of Australia. Dating reveals that this island formed 1.2 to 0.7 million years ago as a result of sea level fluctuations and altered sediment flows. This in turn created a barrier that kept the sea area north of the island free from new alluvial sand - and thus made it possible for corals to settle.
The Great Barrier Reef off the east coast of Australia stretches over 2300 kilometers in length and is therefore larger than any other coral reef area on earth. Because of the unique biodiversity and beauty of this reef landscape, it is a UNESCO World Heritage Site. But as famous as the Great Barrier Reef is, its origins remain obscure. "The formation of the reef and the mechanism responsible for it have not yet been elucidated," write Daniel Ellerton of Stockholm University and his colleagues. One of the reasons for this are discrepancies in the possible time of formation: The geological conditions for the coral reef have existed off eastern Australia for around 25 million years. At that time, Australia had moved far enough north and the necessary warm water temperatures for tropical corals had been around for around five million years at the latest. Despite this, dating of samples from the reef suggests that it formed much later, around 450,000 years ago.
Sand transport in focus
To clarify this contradiction, Ellerton and his team have investigated another possible prerequisite for reef formation: sand transport on the Australian shelf. Because if too much sand is washed in again and again, this disturbs the settlement of the corals and inhibits the formation of new reefs. The scientists therefore investigated what role the formation of Fraser Island, the largest sand island in the world, played in sand transport in the Great Barrier Reef area. This hook-shaped protruding island off the coast of eastern Australia is part of a vast zone of sandbars and dunes in south-east Queensland and part of one of the largest alluvial deposits on earth. According to the researchers, the prevailing ocean currents move around 500,000 cubic meters of sand per year.
Most of the sand washed up on Australia's east coast is shipped north. However, on the east side of outwardly projecting Fraser Island, this alluvial sand is diverted away from shore and transported to the shelf edge. This makes the island act like a kind of screen that keeps most of the alluvial sand away from the Great Barrier Reef further north. "The formation of Fraser Island prevented further sand transport up the coast north - and thus into the area where the Great Barrier Reef now lies," explains co-author Tammy Rittenour of Utah State University.
To find out how long Fraser Island has existed, she and her colleagues took sediment cores from the island and dated them using the optically stimulated luminescence (OSL) method. This method makes it possible to determine when a grain of sand was last exposed to light. In this case it shows when the sand was washed up and piled up.
Island formation cleared the way for corals
"We found that the sand island and dune fields started to form between 1.2 and 0.7 million years ago," reports the research team. At that time, the earth was going through a period of varying climatic conditions, with growing and shrinking glaciers of the early ice ages causing sea levels to fluctuate sharply. These fluctuations also affected ocean currents and sand transport off the coast of eastern Australia. "The sea-level fluctuations caused a redistribution of the sediment on the continental shelf," explains Rittenour. As a result, increasingly higher masses of sediment piled up near the coast, which over time formed Fraser Island and the surrounding sandbars and dunes.
According to the scientists, this was also the decisive impetus for the formation of the Great Barrier Reef. Because of this newly formed barrier, sand transport to the north slowed down and the area of today's reef now offered corals more favorable conditions for settlement. "The formation of Fraser Island was a necessary step in enabling the formation of the southern and central sections of the Great Barrier Reef," Ellerton and his colleagues state. This could also explain why this coral reef formed so much later than would have been possible from a purely geological and climatic point of view. "These significant findings change our view of coastal sediment systems," says Rittenour.
Source: Daniel Ellerton (Stockholm University) et al., Nature Geoscience, doi: 10.1038/s41561-022-01062-6