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Great drying reveals clues to big wet
The ancient dry lakes of central Australia could provide clues to modern climate fluctuations, and should recalibrate our thinking on major flood events, Australian scientists say.

In a paper published in the February edition of Geology , the researchers present a new timeline for the shorelines of Australia's "inland seas" over the past 100,000 years.

Their studies indicate that Lakes Frome, Callabonna, Blanche and Gregory in central Australia were joined in a mega-lake until 45,000 years ago. But then 'Lake Mega-Frome', which was at times connected with 'Lake Mega-Eyre' to the northwest, started to dry out around the same time as humans arrived on the continent.

"Certainly this paper suggests that Australia had quite a lot of moisture on it after humans arrived, but it was much less than it had many thousands of years beforehand," says the paper's co-author, Professor Gerald Nanson of the University of Wollongong.

"The drying of the continent had been progressively occurring [when humans arrived]. The last significant period of drying appears to be a very long process from about half a million years ago, but within the context of our study the drying out became particularly acute from about 40 to 45,000 years ago to the present."

Around 50,000 years ago, temperatures were 8 to 9 degrees cooler with much higher precipitation, which contradicts the established theory that an enhanced monsoon filled the lakes, Nanson says. He believes that stronger westerlies from the Southern Ocean and trade winds from the Coral Sea brought most of the moisture onto the continent.

He says it looks like the rivers flowing into the mega-lakes from the north were the first to dry out, and in the Geology paper he and his co-authors write that Lake Eyre had dried out significantly by about 60,000 years ago.

"It looks like the more southern lakes, Lakes Frome, Callabonna and Blanche were full of water - at least at times - until about 45 to 40,000 years ago," he says.

These lakes combined after receiving floodwaters from either the tropics, the westerly weather systems, or a combination of the two Nanson and colleagues write.

They say that as a result, Lake Mega-Frome represents a long term record of continental rainfall derived from the two sources.
Dating 'the ring around a bath'

Using trench transects on the lakes' ancient shorelines, Nanson and lead author Tim Cohen used radio-carbon dating on shell and carbon material to establish the age of beach ridges going back about 40,000 years.

To date older beach ridges they used luminescence dating on quartz crystals deposited on ancient shorelines. While quartz grains are buried, they store energy from radioactive trace elements in soil. The energy is released when the quartz is exposed to sunlight.

"Once that beach ridge is buried [by more layers of sediment], the quartz grains start to accumulate a luminescence signal," Nanson says.

This signal gives an index of how long it is since the sample saw sunlight, and that indicates when those beach ridges were formed.

He says that even once the age of an ancient shoreline has been determined, it's hard to tell how long the lakes ware at a particular level.

"It's a bit like looking at the ring around a bath," he says.

"You know the bath had water up to there but you don't know how long the bath was full. That's the problem with dating these beach ridges."
Past clues to future floods

The study shows the most recent significant flooding of Lake Mega-Frome happened during a period of warming from AD 950 to about 1200, says Nanson.

"It raises some really interesting questions about modern climate because the medieval period was perhaps about as warm as it is today as a result of global warming," he says.

"What we're trying to do at this stage is work out where all this water has come from."

"We're talking about many tens of times the volume of the present lakes," Nanson says, "so there has to have been a lot more rainfall to fill those lakes, and presumably maintain that level for a number of years, perhaps decades, perhaps even centuries, to form the beach ridges."

"The biggest flood we have on record for most of Australia, and certainly for central Australia is 1974, and that put barely a metre of water into Lake Frome and yet we're talking of [ancient] floods filling up to 15 to 18 metres depth," he says.

"It would have a pretty big effect on our understanding of modern climate if we knew if every few hundred or perhaps once a thousand years we could have a flood of that magnitude."

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