Minerals extracted from the dry sands of the Nullarbor Plain may reveal when it became the parched, treeless landscape we know today.
A team of scientists, led by Maximilian Dröllner of Curtin University, analysed grains of iron oxide from ancient rocks to see how conditions changed over time.
The analysis, published today in the journal Geophysical Research Letters, indicates the area dramatically dried up between 2.7 and 2.4 million years ago.
"Our study offers the first direct age constraint on the transition to aridity on the Nullarbor Plain," Dr Dröllner said.
"We knew it must have been somewhere in this [time frame], but now we can use this as an anchor point to better understand evolution processes in Australia."
From lush to dust
The Nullarbor Plain is an ancient sea floor that gradually emerged from the ocean.
The saltbush plain, which stretches across 200,000 square kilometres of South and Western Australia, is devoid of any surface water.
But the Nullarbor wasn't always an arid area.
Between 5 and 3.5 million years ago, it was wet enough to sustain lush forests, according to previous analysis of pollen trapped in stalagmites and stalactites from caves that lie beneath its surface.
Then the world went through a climate upheaval, moving from the humid conditions in the Pliocene to a more arid state at the beginning of the Pleistocene around 2.5 million years ago.
The Arctic began to freeze in the north, and rainfall and sea surface temperatures decreased in Australia.
"When the Nullarbor Plain changed to its current state, there must have been a dramatic change for the biology of Australia," Dr Dröllner said.
As the lush forests disappeared, the ecological connection between east and west of the continent was severed.
The common ancestors of today's plants and animals were separated and started to evolve independently of each other in south-west and south-east Australia.
Eucalypts and black cockatoos are a good example of this transition.
"In south-east Australia, the yellow-tailed cockatoo has yellow cheeks. In south-west Australia, we have Carnaby's cockatoo with white cheeks," Dr Dröllner said.
"These had a common ancestor that once probably roamed across Australia."
Iron and water
To pin down exactly when the Nullarbor dried during the Pliocene-Pleistocene transition, Dr Dröllner and colleagues analysed an iron mineral called goethite.
Goethite, which is basically rust, forms when iron-rich water comes into contact with oxygen.
"As the climate [dries], groundwater levels drop, bringing in oxygen from the air," Dr Dröllner said.
"Dissolved iron in [water] reacts with oxygen, forming ferric iron (Fe3+), which ... crystallises and forms goethite."
The amount of helium trapped inside these minerals due to the decay of uranium and thorium tell us how old the deposits are.
"The more helium accumulated, the older the mineral."
The dating indicated the deposits formed some 2.7 to 2.4 million years ago.
During this time, groundwater levels dropped between 10 and 20 metres.
"We know how the rock formed and we know it's related to a groundwater decline," Dr Dröllner said.
A novel approach to a tricky question
Using iron oxide to determine when the Nullarbor dried up was a very novel technique, said Tim Cohen, a geomorphologist at the University of Wollongong.
"It's pretty unusual. This is the first time I've ever seen it.
"Normally, we're trying to date the age of the landform, not the age of a weathering product."
But, Dr Cohen said, pinning down the age of deserts was very difficult.
"How old are the deserts? We don't know.
"This is just another example of somebody trying something new to try and get it down.
"I think it's super interesting."
John Webb, an environmental geoscientist at La Trobe University, said the study's findings back up evidence from previous research dating declines in the growth of stalagmites and stalagmites in the Nullarbor's caves.
"I think what they have said is pretty reasonable," Professor Webb said.
But, as a standalone technique, more research from more sites was needed to confirm the dates across the Nullarbor, he said.
The mineral was extracted from the Jacinth-Ambrosia mine, about 200 kilometres north west of Ceduna, which sits in an area that was once an ancient shoreline.
"It's quite common to find iron oxide deposits where groundwater comes to the surface in springs or seeps," Professor Webb said.
This can happen when more rainfall flushes through a system.
"[We don't] have enough information ... to know whether or not the mine was close to an area where groundwater was coming through and flowing up," Professor Webb said.
"They need that extra evidence to confirm their interpretation of how the iron oxide precipitated as marking an arid event rather than a humid event."
