Ancient fish teeth pulled from the ocean floor around Tasmania have handed scientists a new way to plot changes in Antarctic currents spanning tens of millions of years.
The fossilised teeth are recovered from seabed drill cores, and each contains a signature of the seawater conditions at the time the fish died.
University of Tasmania scientists, with US colleagues, used this information and data on tectonic plate shift to map when the world's major ocean current came into effect.
It suggests the Antarctic Circumpolar Current kicked in five million years earlier than was previously thought.
"Tasmania separating from Antarctica about 35 million years ago created the Tasmanian Seaway and for a long time scientists have thought that the opening of this seaway enabled the onset of the ACC," says Joanne Whittaker, from the University of Tasmania.
"We've found out this is not the case ... opening the Tasmanian Seaway on its own wasn't enough."
The study suggests the ACC did not really get underway until Tasmania had migrated much further north, allowing westerly winds to rush uninterrupted around the south pole, about 30 million years ago.
The stability of massive ice sheets depends on the ACC as it keeps warmer waters away from the south pole.
The research is published in the journal Nature.