On the dry, barren Nullarbor Plain in southern Australia, a group of researchers has uncovered a remnant of the continent’s deep past. A massive, fossilized dome of coral sits atop the vast limestone plateau, just one of many structures from millions of years ago still visible on the plain.
The dome is likely a remnant of an ancient reef that once stretched across this landscape, a vestige of a time when shallow seas invaded what is now mainland Australia. Imprinted inside the dome are countless corals and other marine invertebrates, which give researchers valuable insights into how ancient reefs waxed and waned and could offer a more robust understanding of Australia’s Great Barrier Reef today.
The ancient coral formation, which appears something like a massive bull’s-eye in satellite images, is just one of many structures from the Miocene, which stretched from 23 million to 5 million years ago, still visible on the Nullarbor.
“You can find landforms that are really old,” said Matej Lipar, head of the Department of Physical Geography at the Research Centre of the Slovenian Academy of Sciences and Arts and lead author of a new study describing the reef published in Earth Surface Processes and Landforms. “This is spectacular because in normal environments you’ve got glaciers and a lot of rain [that leave] all of these geomorphological landforms washed away or denuded.”
On the Nullarbor, untouched by glaciers and tectonic activity, researchers are able to see the signs of 10 million years of history.
But on the Nullarbor, untouched by glaciers and tectonic activity, researchers are able to see 10 million or more years of history. The courses of rivers, dry now for millions of years, can be found on the plain, along with evidence of ancient sand dunes that long ago were fossilized into limestone.
“Nothing much happened to it,” said Lipar, who also noted a lack of rain in the region. “You can really see what was going on back in the past.”
From Ocean to Desert
Though the Nullarbor today is arid and bare, it was once a shallow, thriving sea. The plain’s bedrock, which covers almost 200,000 square kilometers and can be more than a hundred meters thick, was created bit by bit as ancient sea creatures died and left their remains on the floor of the ocean. Over time, the shells and skeletons were bonded together into limestone, which reacted with water to form a karst landscape, riddled with holes and cave systems.
“It’s a dangerously remote area,” said Michael Archer, a paleontologist at the University of New South Wales who was not involved in the study. “You can be beetling along in your four-wheel drive and suddenly disappear into a hole.”
But thanks to new technologies, more details of the Nullarbor are beginning to emerge. Using ground-mapping radar data from the TanDEM-X satellite operated by the German Aerospace Center, Lipar and his coauthors were able to create detailed elevation maps of the remote plain, revealing forms invisible from the ground.
One of those newfound details was a mysterious structure consisting of a large dome more than 1,200 meters across, surrounded by a slight trough and a raised ring.
After discarding a number of other theories (including a meteorite impact, volcanism, and tectonic activity), the researchers came upon an explanation for the feature rooted not in geology, but biology. The formation is probably a fragment of a massive reef, they thought, that once stretched across the Nullarbor.
Although coral fossils previously have been found in the region, this is the first evidence for an actual reef on the Nullarbor, said John Webb, an environmental geoscientist at La Trobe University in Australia who wasn’t involved in the study.
He said the dome looks similar to other so-called bioherms, hills of calcium carbonate found on the ocean floor where coral reefs exist.
Studying the structure could yield new insights into the biology of coral reefs during the Miocene, Archer said. “There could be fossil fish deposits there that would tell us a lot about what the marine life—not just the invertebrates, but the vertebrates that were in those shallow marine seas—was doing.”
Those insights in turn could prove useful to scientists studying the Great Barrier Reef today. The Miocene saw a period of warming similar in scale to what contemporary climate change may bring to the region; looking to how ecosystems reacted more than 10 million years ago might help us understand how modern reefs might fare.
The find also revealed that the ocean that once covered the Nullarbor was relatively warm, Webb said. This discovery may indicate that the Great Australian Current that wraps around the western and southern coasts of Australia was stronger back in the Miocene.
Millions of Years of Data
The dome feature is not the only clue to the Miocene the Nullarbor has for scientists. Inscribed into the entire landscape are records of typically ephemeral processes. For instance, in some places, the limestone is rippled, a feature that records the presence of millennia-old dunes sculpted by wind.
“You basically have the shape of the dune imprinted into that rock,” Lipar said.
The dunes reveal wind patterns that look very different from those experienced on the plain today, Webb said. Paired with other clues left on the surface of the Nullarbor, discoveries like this may shape a far more detailed picture of landscapes that are long vanished but not entirely forgotten.
—Nathaniel Scharping (@nathanielscharp), Science Writer