Scientists reveal details of the fate of the "sinking continent"

The Timescales of Mineral Systems research team collected sediment samples from various points along Perth's coastline, waterways and even the seabed. This allowed them to track the changes that have occurred due to ocean levels rising tens of metres over thousands of years.

Study leader Dr Andreas Zametzer said divers found certain types of mineral grains off the coast, confirming the complex geological history of Rottnest Island (Wajemup in the Nungar language) and the Swan River (Derbarl Yerrigan).

Unique ancient grains off the coast of Rottnest have been dated as old as 3.6 billion years. These same grains can be traced back to the modern day Swan River Estuary and its tributary, the River Avon, where they were brought and deposited thousands of years ago.

The scientists detailed their findings in a paper, "When the river meets the sea: transport and provenance in a long-lived estuary", published in the journal Basin Research.

'Perth is on a continental margin that is gradually submerging,' Dr Zametzer noted. - Rottnest Island was the tip of a peninsula until about 6,500 years ago, similar to what we see in the Shark Bay region. Because it is still part of the submerged continental margin, the depth between Perth and Rottnest rarely exceeds 20 metres.

He added: We now know for sure that the mouth of the Swan River used to be located north of Rottnest's current position.

Dr Sametzer admitted the team was surprised to find ancient river mineral grains so far out to sea.

'We expected they would have been destroyed and washed away by the turbulent environment of waves and tides,' he said. - But the river's characteristic mineral 'footprints' are still present despite thousands of years of sea-level changes and shelf reworking. We have evidence preserved offshore, confirming the history of the coast.

Understanding how coastlines have adapted in the past is important for predicting the effects of sea level rise in the future, he said.

It's amazing to realise how fast some geological processes are occurring as sea level continues to rise," he concluded.