K’gari (Fraser Island) is known for its iconic beaches that stretch as far as the eye can see, freshwater lakes and ancient dunes.
- Scientists excavated the sands of K’gari to provide the first snapshot of the island’s age
- The dates suggest that the island was formed due to dramatic swings in sea level rise caused by a mysterious change in the Earth’s climate
- Scientists say K’gari Formation allowed coral to settle in the southern part of the Great Barrier Reef
Jutting out from the southeast coast of Queensland in Butchulla Country, it is the largest sand island in the world.
Over the years, people have speculated about when and how this UNESCO-listed island was formed.
“We knew the island’s dunes were old…but we had no idea when [they] format,” said Jamie Shulmeister of the University of Canterbury.
Now, a team of scientists led by Stockholm University’s Daniel Ellerton 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 Cooloola National Park.
“Early Ages suggest K’gari is nearly 900,000 years old,” Dr. Ellerton said.
K’gari’s rise appears to have coincided with sea level changes caused by a dramatic change in climate about 1 million years ago, scientists report in Nature Geoscience.
The findings also suggest that the island played a key role in the creation of another iconic but little-known wonder: the Great Barrier Reef (GBR).
East Coast Sand Conveyor
Approximately 500,000 cubic meters of sand is transported along the east coast of Australia each year by rivers such as the Hawkesbury and Hunter.
“It’s why we have beautiful sandy beaches in New South Wales and Queensland,” said Dr. Ellerton.
Blown north by trade winds and southeast swell, it forms around Stradbroke and Moreton Islands and eventually reaches K’gari.
The island sits on a pivotal point on Queensland’s southeast coast, near at the edge of the continental shelf.
When the sand hits the northern point of the island, it slides off the edge of the shelf into deep water.
But if K’gari hadn’t existed, the researchers argue, the sand would have continued further north and stopped coral growth on what are now the southern and central parts of the reef.
“What [K’gari] it’s essentially forming a seawall,” said Dr Ellerton.
“The sand would have been carried northwest [by the trade winds]but now it’s actually going northeast because of K’gari.”
Despite a favorable climate for millions of years, there were no coral reefs in the southern part of the GBR until around 650,000 to 700,000 years ago, Professor Shulmeister said.
“Just before they appear, we have 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 begin, the researchers used a technique known as optically stimulated luminescence, which dates the time when individual grains of sand last saw light.
The ancient sands along Australia’s east coast are perfect for this technique, said Professor Shulmeister.
“When you get to southeast Queensland, there’s nothing in there except some heavy minerals and these beautiful grains of quartz.”
Normally, the technique gives ages up to 200,000 years old, but samples taken from Rainbow Beach — just south of K’gari — and the Fraser Island Cliffs have yielded dates concentrated between 700,000 and 800,000 years old, with the oldest around 1.2 million.
To make sure the dates were accurate, they turned to another technique known as paleomagnetism, which analyzes the alignment of iron nanoparticles in relation to the Earth’s magnetic poles.
About 773,000 years ago, Earth’s magnetic poles flipped so that particles excavated from layers below what’s known as the Matuyama/Brunhes boundary are in a different direction from those above.
This type of analysis is very difficult to do with friable sand, but the cliffs of Rainbow Beach also contain a harder substance called ferricrete, iron that has leached out of the sand and set like cement.
While samples from the upper two-thirds of the reefs had normal magnetization, the particles in the lower third were reversed.
“So we knew 773,000 years was real. There were one or two samples that weren’t perfect, but most of the [samples] they work really well,” said Professor Shulmeister.
How Dramatic Climate Change Has Shifted the Sand
Strata in the cliffs, particularly those around Rainbow Beach that were the same age and part of the same system as those on K’gari, reveal how the island developed.
“Rainbow Beach is very unique,” said Dr Ellerton, who sampled the area for his PhD at the University of Queensland.
“You can see the older material at the bottom to the younger material that’s at the top and you can just stand there and see it the entire length of the beach.”
While the cliff bottom is dominated by vast plates of rippling sand known as transverse dunes, the upper two-thirds contain large parabola-shaped dunes like those on the island today.
Until about 1 million years ago, the Earth went through glacial periods every 40,000 years in line with changes in its orbit, causing sea levels to rise and fall by 60-80 metres.
But suddenly at a point known as the Middle Pleistocene transition, the glacial cycle passed 100,000 years and sea level fluctuations doubled to more than 120 meters.
“This is the same climate system that we currently know and the one we experience in the modern period,” said Dr Ellerton.
The team believes this unexplained transition may have triggered the formation of K’gari.
According to their hypothesis, the sudden drop in sea level exposed sand stored on the continental shelf which was then bulldozed into huge slabs as the sea rose again.
‘You have this giant sand storage system on the outer parts of the shelf, and then you basically lower the sea level not by 75 metres, but by 120 metres,’ said Professor Shulmeister.
“Erosion caused by sea level rise will release all that sand that has been stored and huge volumes of sand will be pushed back onto the coast.”
The island grew in pulses over hundreds of thousands of years as sea levels returned to modern levels after each ice age.
As time went on, there was less sand on the continental shelf to lift, submerging everything in its path.
“That’s when we start seeing the parabolic dunes,” said Dr. Ellerton.
“The parabolic dunes tell you that the landscape must have been at least a little vegetated.”
The old dunes have been eroded and blown inland through fissures in the vegetation, producing the parabolic shape.
“If you look at K’gari today, some of the parabolic dunes are 10 kilometers long,” Dr. Ellerton said.
“They are eroding and displacing the old material later in time, unlike what we see at the base of the island and Rainbow Beach.”
Major changes to Queensland
Oceanographer Helen Bostock has claimed the rise of large sand islands along the east coast it was fascinating.
‘This is a relatively recent feature of eastern Australia,’ said Dr. Bostock, who was not involved in the research.
“It amazes me that these were formed from very ancient sand…that has been constantly reworked.”
While the rework means the dates in the study jump off a bit, Dr. Bostock said the research was truly impressive and provided more insight into how the coastline of our tectonically tranquil continent was shaped 1 million years ago by global climate change.
“This time period has not been well studied in Australia,” said Dr. Bostock, who is involved in the million-year ice core project in Antarctica to understand why the climate has changed so dramatically.
“It’s actually very difficult to get continuous records going back 1 million years.”
Although sand islands don’t provide a consistent record like ice cores, Dr. Bostock said they still provided an interesting insight into the period.
“It’s great to see how Australia has responded to this climate transition, which has clearly been significant enough to cause some pretty major changes in Queensland.”
But, he said, more work is needed to understand when and how different parts of the Great Barrier Reef formed.
“We need to find evidence that the sand made it further north before K’gari became this wall to block it.“
Dr Bostock is currently analyzing coral cores to establish the age of the northern and central parts of the reef.
“If sand had been the main thing preventing it from developing further south, we could have developed a Great Barrier Reef further north,” he said.
“There’s still a lot of work to be done, but I think this is an interesting hypothesis that we can test in the future.”