K’gari (Fraser Island) is known for its iconic beaches that stretch as far as the eye can see, freshwater lakes and ancient dunes.
Jutting out from the south-east Queensland coast on Butchulla country, it is the largest sand island in the world.
Over the years, people have speculated about when, and how, this UNESCO World Heritage-listed island was formed.
“We knew the dunes on the island were old … but we had no idea when [they] formed,” said Jamie Shulmeister of the University of Canterbury.
Now, a team of scientists led by Daniel Ellerton of Stockholm University and Professor Shulmeister has provided the first snapshot of the island’s geological age.
Working closely with the Butchulla community, they dated sand dunes on the east coast of K’gari and nearby Rainbow Beach in the Cooloola National Park.
“The earliest ages suggest K’gari is almost 900,000 years old,” Dr Ellerton said.
K’gari’s rise appears to have coincided with changes in sea levels caused by a dramatic shift in the climate about 1 million years ago, the scientists report in Nature Geoscience.
The findings also suggest the island played a key role in the establishment of another iconic, but poorly understood, wonder: the Great Barrier Reef (GBR).
East coast sand conveyer belt
Every year, around 500,000 cubic metres of sand is transported up the east coast of Australia from rivers such as the Hawkesbury and Hunter.
“It’s the reason why we have beautiful sandy beaches in New South Wales and Queensland,” Dr Ellerton said.
As it is pushed north by south-easterly trade winds and waves, it builds around Stradbroke and Moreton islands and eventually reaches K’gari.
The island sits on a pivotal point on the south-eastern Queensland coast, close to the edge of the continental shelf.
As the sand hits the island’s northerly point, it slips off the edge of the shelf into deep water.
But if K’gari didn’t exist, the researchers argue, the sand would have continued further north and stopped coral growing on what is now the southern and central parts of the reef.
“What [K’gari] is doing is essentially forming a seawall,” Dr Ellerton said.
“The sand would have been transported to the north-west [by the trade winds], but now it is actually going to the north-east because of K’gari.”
Despite a favourable climate for millions of years, there were no reefs at all in the southern part of the GBR until about 650,000 to 700,000 years ago, Professor Shulmeister said.
“Just before they appear, we’ve got the formation of K’gari.”
How do they know how old the island is?
Establishing the age of ancient landscapes, especially those made of sand that contains very little carbon, is very challenging.
To start with, the researchers used a technique known as optically stimulated luminescence, which dates the time that individual grains of sand last saw light.
