Caves reveal Australia wasteland’s secret past

A network of hidden caves has helped reveal the lush rainforests that thrived in one of Australia's harshest deserts between three and five million years ago.

The iconic Nullarbor Plain is a vast expanse of desert straddling South and Western Australia, which receives less than 3cm of rain annually and is nearly entirely devoid of trees.

This inhospitable landscape is generally believed to have evolved in a linear fashion, becoming increasingly more arid in response to a cooling event in the southern hemisphere, which began around 14 million years ago.

But now, scientists at the University of Melbourne say a mysterious period of rapid warming, beginning five million years ago, dramatically altered the landscape of the Nullarbor Plain, bringing substantially more rain and allowing new plant life to flourish.
"Our study shows that the warming and associated increase in rainfall happened quite abruptly by geological standards, within about 100,000 years," said palaeoclimatologist Kale Sniderman, who led the research.

"This increase in rainfall coincided with an increase in ocean temperatures, which implies that it may have been a direct response to a globally warming climate."

It triggered a two-million-year period when the Nullarbor was up to four times wetter than it is today, and low-lying shrubs were replaced with eucalypt forests, banksias and other plants that are now restricted to the east coast of Australia, he said.

This period occurred at the beginning of the Pliocene Epoch (5.3 to 2.6 million years ago), which is the last time the Earth's climate was as warm as it's predicted to get 100 years from now. As such, this period is viewed as the best indicator we have for how our planet will respond to global warming.

Caves hold the clues

The Nullarbor Plain is one of the world's largest exposed limestone plateaus and is peppered with hundreds of caves, says Sniderman.

These caves contain bountiful collections of stalagmites and stalactites — cone shaped rock formations that build up layer by layer over thousands of years as water seeps through the bedrock and drips into the cave.

Due to the presence of these caves, Sniderman says "it was always obvious that the Nullarbor had been wetter in the past — but nobody knew when that period was".

One of the team, Prof Jon Woodhead, is an expert in rock dating and had already amassed a collection of stalagmites and stalactites from the Nullarbor caves.

Prof Woodhead had dated these samples to the Pliocene Epoch, but hadn't been able to obtain any environmental information to support evidence for a wetter climate. It was Sniderman who suggested they begin searching for traces of pollen trapped in the samples, which might have entered the cave with some dust through a surface opening.

"So, by trial and error, and quite a lot of tedious work, we found that a small proportion… did have pollen in them," he says.

The discovery allowed them to reconstruct the vegetative landscape of the Nullarbor and to establish a "new record of environmental climatic change which had previously been impossible to generate".

Their new record shows that, before the rapid warming, the Nullarbor already had a dry climate, as it was dominated by Australian plants that mainly grow in dry conditions.

The team's results, which provide new insight into the ancient evolution of Australia's arid interior — and arid ecosystems across the southern hemisphere — were published on Tuesday in the Proceedings of the National Academy of Sciences.
A 'mysterious reversal'

Prof Michael Archer, a vertebrate palaeontologist at the University of New South Wales who was not involved in the study, says recovered mammal fossils indicate that there were rainforests in Australia much further west than there are today.

He says the team has "demonstrated with very high precision" that there was a brief period when the southern hemisphere went through a rapid return to a greenhouse state.

"Some people have assumed it was a steady shift toward the ice ages, but the early Pliocene represents a mysterious reversal in this global movement. All of a sudden there was a moment when it was brilliant. The trees blossomed and spread, the possums smiled, and it was a wonderful time.

"But it just didn't last. And then, as quickly as it began, the world began to return to the long-term trend, headed for the ice ages," says Prof Archer.

"The most important takeaway message here… is that we're actually beginning to get a glimpse of what we may be headed for now," he says.

"Australia has profoundly changed, very rapidly in the past, and it's likely to do so again."