Western Australian coastlines reveal a complex history of sea level rise

Western Australian coastlines reveal a complex history of sea level rise
by Simon Mansfield
Sydney, Australia (SPX) Oct 23, 2024

A Curtin University study has traced the geological evolution of two iconic Western Australian landmarks-Rottnest Island and the Swan River-highlighting the transformations driven by rising sea levels over millennia. The research offers new insights into how these coastal features have changed and what the future may hold for the region.

Scientists from Curtin's Timescales of Mineral Systems Group, based in the School of Earth and Planetary Sciences, gathered sediment samples from multiple locations, including Perth's coast, waterways, and the seafloor. These samples allowed the team to track shifts that occurred as ocean levels rose by dozens of meters over thousands of years.

Lead researcher Dr. Andreas Zametzer explained that the team discovered ancient mineral grains offshore, which provided key evidence of the geological histories of Rottnest Island (known as Wadjemup in the Noongar language) and the Swan River (Derbarl Yerrigan).

The grains found off Rottnest Island were determined to be as old as 3.6 billion years. These grains, carried by rivers over millennia, were also detected in the modern Swan River estuary and the Avon River tributary, demonstrating the long history of sediment transport and deposition.

"Perth is sitting on a continental margin that is drowning," Dr. Zametzer noted. "Rottnest Island was once the tip of a peninsula, about 6,500 years ago-similar to what we see in Shark Bay today. The ocean between Perth and Rottnest remains shallow, only about 20 meters deep in most areas, as it's part of the drowned continental margin."

He added, "We now have confirmation that the Swan River's mouth was once north of where Rottnest Island is located today."

The discovery of ancient river-transported mineral grains so far out to sea surprised the team. "We would have expected these grains to have been eroded or washed away, given the turbulent, high-energy environment they were in," said Dr. Zametzer. "But the distinct mineral traces from the river system have been preserved offshore, offering a clear record of the coast's geological history."

Dr. Zametzer emphasized the importance of studying how coastlines have changed in the past to better understand the impact of future sea level rises. "It's quite shocking to realize how quickly geological processes can occur, especially with sea levels continuing to rise."

Research Report:When the river meets the sea: Transport and provenance in a long-lived estuary