Sonar mapping revealed five new limestone columns hidden underwater at Australia's iconic tourist attraction, the Twelve Apostles. Researchers from the University of Melbourne spotted the never-before-seen sea stacks, subsequently dubbed the "Drowned Apostles," six kilometers offshore from the Great Ocean Road and 50 meters beneath the water's surface.
The Twelve Apostles are a series of limestone stacks located off the shore of the Port Campbell National Park in Victoria, Australia. Compared to their more famous cousins, the Drowned Apostles are smaller in both size and number.
However, the new-found pillars appear to have resisted normal erosion rates and are still in remarkable shape. Researchers say this is the first time such limestone stacks have been found preserved in the ocean.
"Sea stacks are always eroding, as we saw with the one that collapsed in 2005, so it is hugely surprising that any could be preserved at that depth of water," said David Kennedy, an associate professor from the University of Melbourne's School of Geography. "They should have collapsed and eroded as the sea level rose."
Ph.D. student Rhiannon Bezore discovered the Drowned Apostles while looking at sonar images taken underwater. She noticed five unusual features located next to a submerged ancient coastal cliff. With further analysis, researchers discovered the features were once part of a larger limestone sea cliff, much like the limestone stacks that exist on the surface.
Bezore worked alongside Kennedy and Deakin University's Daniel Ierodiaconou, who provided the high-resolution sonar data. Researchers estimate the Drowned Apostles date back 60,000 years.
"We had to check what we were seeing because no one has seen stacks submerged at this sea level before," Bezore added.
Currently, only eight of the original Twelve Apostles are left standing. They used to be part of the cliffs that line the coast, but over time the soft limestone eroded away, leaving behind caves that eventually dwindled down to weak arches.
When the arches finally collapsed, limestone stacks standing upwards of 45 meters in height were all the remained. While the Drowned Apostles formed in the same way, researchers say they were submerged by rising sea levels about 20,000 years ago, at the end of the last Ice Age.
"Sea levels probably rose at the end of the last ice age so quickly that the sea just ran across the top of these things without knocking them over. It is amazing that they survived," Kennedy added. "When we go into an ice age it is a very slow freezing process, but when it decides to melt it generally does it quite quickly."
This means that there was not enough time for the pillars to be eroded by the sea as it rose.
The sonar data that made this discovery possible is actually part of a much larger project, aimed at mapping potential reef habitats in Victoria.
"We are only just starting to understand the biodiversity value of these deep reefs which harbor diverse invertebrate communities, many unknown to science," Ierodiaconou explained.
Their findings were recently published in the Journal of Coastal Research. Bezore will present their study at the International Coastal Symposium in Coogee, Sydney.
