For tens of millions of years the staggeringly powerful current that encircles Antarctica has helped shape the world's climate. Without it much of the land we know today would have been buried beneath the waves, and anything above the surface might have been too warm and constant to lead to the evolution of humans. Now we have a better idea when and how this mighty phenomenon first appeared.
During the age of the dinosaurs the Antarctic circle was warm enough to support species such as Leaellynasaura. This was possible, not just because carbon dioxide levels were higher than they are today, but because South America, Australia and Antarctica were still attached as part of the supercontinent Gondwana.
Great currents flowed from the equator down the eastern slopes of Australia and South America. In spring, the warmth these brought melted coastal snows. Summer sunlight was absorbed, instead of being reflected back to space, and more snow melted. The giant glaciers that now dominate Antarctica did not build up, and the entire planet was a warmer and wetter place as a result.
Slowly the Drake passage opened up as South America pulled away and Australia moved north to create a wide gap between the continents. Nevertheless, as Howie Scher of the University of Southern Carolina reported in Nature, one small but crucial obstacle stood in the way of global change. Tasmania blocked the path of ocean currents circling Antarctica, keeping the continent, and entire planet, warm.
Eventually, the tectonic forces that drove Australia north pushed Tasmania far enough from Antarctica's shores to kick-start the current that has been dubbed “the global mixmaster.”
Current strength is determined in part by the fetch, the distance over which wind blows across open water. With Tasmania now sitting north of Antarctica, the furious fifties could circle the world uninterrupted, making infinite fetch happen. “With infinite fetch, you can have a very strong ocean current, and because this particular band of ocean connects all of the world's oceans, it transports heat and salt and nutrients all around the world,” said Scher in a statement.
Scher and his team dated the current's appearance through neodymium concentrations in fossilized fish teeth. The rocks beneath the Indian Ocean are older than those beneath the Pacific, and therefore contain less radioactive neodymium. Prior to the current becoming established the two great oceans maintained different ratios of neodymium isotopes, but when the current started it evened out the isotopic ratios.
A deep passage between Tasmanian and Antarctica opened up 35-32 million years ago. However, Scher reports that the flow did not take hold until 30 million years ago. The lag, he believes occurred because the initial gap was not far north enough to be subject to powerful westerly winds.
Today the current flows from west to east, driven by the mighty winds that flow in that direction from 30° to 60° south. However, the team were surprised to find the initial flow went the other way, driven by polar easterlies. Only when Tasmania moved far enough north that most of the gap lay in latitudes with westerly winds did the direction of the current reverse.
The gap between Tasmania and Antarctica when it first became wide enough to allow a circumpolar current. Sites where fossilized teeth were collected are marked with red and black squares. Credit: Scher et al/Nature.