The Great Barrier Reef’s mysterious origins can finally be explained

Australia’s Great Barrier Reef might never have happened without the formation of a vast island that relies mostly on sand.

Kgari, also known as Fraser Island, has the honor of being the largest sand island in the world, covering an area of ​​about 640 square miles (nearly 1,700 square kilometers) off the southeastern coast of Queensland.

Together with the nearby Cooloola Sand massif, the dune massif and forested beaches form the informal base of the vast coral reef that lies to the north.

If the terrestrial ‘launching pad’ had never formed, researchers believe that masses of sand carried north along the coast by ocean currents would have landed exactly where the reef is now.

Quartz-rich sand has a way of absorbing carbonate-rich sediments that are essential for coral development.

Experts argue that without the K’gari on the way to channel sediment from the continental shelf into the depths, conditions would not have been right for the formation of the world’s largest coral reef.

The Great Barrier Reef has a bewildering origin story. It only formed half a million years ago, long after conditions were favorable for coral growth.

K’gari may be the missing puzzle piece the researchers have been looking for. Analysis and dating of sand from the dunes on the 123-kilometer (76-mile) Long Island indicates that the land mass formed between 1.2 and 0.7 million years ago, hundreds of thousands of years before the emergence of the Great Barrier Reef.

The researchers explain that the island’s presence may have skewed the currents northward, providing the southern and central parts of the Great Barrier Reef the respite they needed to begin farming thousands of kilometers of coral reefs.

The coast of Queensland, Australia, shows sediment dispersion before and after (left) and after (right) the K’gari and Cooloola Formation. (Ellerton et al., Nature Geoscience, 2022)

K’gari and Cooloola themselves originated from the accumulation of sand and sediment from the south.

Amidst periods of ice formation and fluctuating sea levels, researchers suspect that sediments around the world have “suddenly” become exposed. In successive periods of melting ice and rising oceans, these sediments get suspended in the currents.

Along the east coast of Australia, this probably meant a long, northward treadmill of soil and sand that traced the continental shelf.

The slope off the south coast of Queensland, however, makes for an ideal place for sediment to accumulate, and this is right where K’gari and Cooloola are located.

Just south of the sandbanks, coral reefs are conspicuously disappearing.

If the researchers are right, this is probably because the northward currents here are very strong. K’gari and Cooloola break up long-distance dispersal, preventing the quartz-rich sands from smothering developing coral reefs.

“Before Fraser Island developed, northward land transport was interfering with coral reef development in the south and centre [Great Barrier Reef]write the researchers.

Sediment records from the southern Great Barrier Reef support this idea. About 700,000 years ago, there appears to have been a rise in the carbonate content of the sediments in this area.

There now needs to be a search for corals to the north as well, but it appears that at least two-thirds of the Great Barrier Reef owes its existence to a wall of sand to the south.

The authors argue: “The development of Fraser Island has greatly reduced sediment supplies to the continental shelf north of the island.”

“This facilitated large-scale coral reef formation in the southern and central Great Barrier Reef and was a necessary precondition for its evolution.”

The study has been published in Natural Earth Sciences.

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