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Tasmania’s North American Ancestry

Antarctica was once linked to North America, with Tasmania tightly sandwiched between the two land masses and a considerable distance from what is now mainland Australia, according to research published in Geology.

The new findings build on a link between sedimentary rocks found in north-west Tasmania and the Belt–Purcell Supergroup in Montana, Idaho and southern British Columbia, which indicated that Tasmania and North America were geographically very close when they formed part of the Nuna supercontinent 1.4 billion years ago.

“While this new research corroborates the link between Tasmania and North America, it goes further by providing strong evidence that Antarctica was also part of that chain, with Tasmania relatively closely squeezed in between,” said PhD student Jacob Mulder of The University of Tasmania.

“Importantly, it also indicates that Tasmania was situated a substantial distance away from the land masses that later formed modern-day Australia.”

Mulder and co-workers determined the age and isotopic composition of zircon from the Rocky Cape Group in Tasmania’s north-west, and compared this with the age and isotopic composition of zircons from potential source rocks around the globe.

By using zircon to fingerprint the source rocks that eroded to form the Rocky Cape Group, the researchers were able to position Tasmania within the Nuna Supercontinent. This location was substantiated by palaeocurrent data, such as fossil wave ripples and tide channels, which indicated the direction of the tides and currents transporting the sediment, and hence its source.

While they were not surprised to confirm the link with North America, they also discovered that the zircon samples from Rocky Cape, which they had dated to the time of Nuna (1.45 billion years ago), had isotopic compositions that were remarkably similar to zircons of a comparable age found in granites beneath the ice cover in East Antarctica. This provided strong evidence that North America, Tasmania and Antarctica were once extremely close neighbours.

Furthermore, their studies revealed that similar links were not present in rocks found in other parts of the country, which enabled them to conclude that Tasmania was once considerably more remote from mainland Australia than it is today.