Ancient landmass emerged and drowned over 70,000 years

The sea off the north-west of Australia used to host islands and even a huge landmass, big enough to support half a million people, according to new archaeological research.

A study published in Quaternary Science Review has mapped a world that appeared and disappeared with changing sea levels over the past 70,000 years.

It’s estimated that people migrated to the continent 45,000-65,000 years ago.

The region was part of the palaeocontinent Sahul, which connected Australia with New Guinea. The drowning of this world may have caused big cultural and demographic shifts in northern Australia.

“It’s this incredible landscape that we really don’t have any analogy for in Australia today,” lead author Kasih Norman, a research fellow at Griffith University, tells Cosmos.

Two maps showing landmass north of australia
During lower sea levels a vast archipelago formed on the Australian northwest continental shelf (top). A modern-day example of an archipelago on a submerged continental shelf is the Åland Islands near Finland (bottom). Credit: US Geological Survey, Geoscience Australia

The researchers used bathymetric data from Geoscience Australia to build a 3D map of the undersea area. This data is collected by ships in marine surveys, using sonar and other mapping technologies.

“We can see huge ravines; massive river channels that are still preserved on the ocean floor; we can see where lakes might have been,” says Norman.

“Then we can start to do things like project past sea levels onto that three-dimensional landscape surface.”

This allowed Norman and colleagues to see what the area might have looked like at different points in the past.

“We worked out that there was a massive archipelago that extended right out towards Indonesia, between 70,000-61,000 years ago, and which was stable for 9,000 years,” says Norman.

This archipelago connected the modern-day Kimberley, in northern WA, with Arnhem Land at the top of the NT. Then, sea levels rose and began to fluctuate more through a period arhaeologists call Marine Isotope Stage 3.

“As you come into the last Ice Age, sea levels drop really dramatically. They drop right down to negative 120 meters compared to today’s levels.

“At that point, that huge area of land between the Kimberley and Arnhem Land is completely exposed.”

This area was about 390,000km2: 1.5 times the size of New Zealand, or slightly larger than Japan.

“In the middle of that was a gigantic inland sea. It was connected to the Indian Ocean through the Malita Valley, which was this 200km long canyon that was tidal at its mouth,” says Norman.

Demographic modellers in the research team estimate that at different points the archipelago and landmass could have supported between 50,000 and 500,000 people.

But this vast landscape had a startling downfall as the ice began to melt and sea levels rose, between 14,000-9,000 years ago. The change was sharpest during a point called Meltwater Pulse A.

“100,000 kilometres of that northwest shelf region went underwater in 400 years,” says Norman.

“It changed from being about a metre per 100 years of sea level rise to four metres, sometimes higher.

“People that were living in this region would have seen the landscape going underwater, almost before their eyes. It would have changed incredibly dramatically in the space of their lifespan.”

These abrupt rises match other archaeological records from the regions.

“We looked at published archaeological data from the Kimberley and Arnhem Land. And we could see, coinciding with those really rapid periods of sea level rise, a big increase in the number of stone tools,” says Norman.

“We’re interpreting that to mean there’s suddenly a lot more rubbish being thrown out of these occupation sites. That normally means that there’s a lot more people suddenly in that area.”

These people likely came from populations getting pushed off the vanishing shelf.

Another clue is art.

“There’s been a lot of work done on precisely dating the rock art in both the Kimberley and Arnhem Land in the last 5 years. They’re also pinning down the period of these new rock art styles that just suddenly appear in those 2 regions. They coincide with those periods of really rapid sea level rise.

“So there’s quite a bit of speculation out there [among] archaeologists and rock art experts that probably what we’re seeing is people bringing in those styles from off the shelf into these regions.”

The findings add to a growing narrative of population changes over the history of northern Australia.

“Within Australia, at least, it was always kind of assumed that those massive continental shelves that used to be above sea level weren’t very much used by Aboriginal Australian people, even though there was plenty of evidence from around the world that they were used by lots of people in the past,” says Norman.

“We actually do have a number of cultural signatures now that indicate people were out there, and they were getting pushed back inland.”

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