Origin of the Antartic Circumpolar Current
The Antarctic Circumpolar Current originated around 30 million years ago, according to new estimates emanating from the University of South Carolina. The current, which encircles Antarctica with a constant eastward flow in the Southern Ocean, formed after the tectonic opening of a deep-water channel in the Tasmanian gateway.
A cornerstone of the Earth’s current climate system, the Antarctic Circumpolar Current (ACC) is akin to the Gulf Stream that gives the nations of western Europe a more temperate climate than would be expected from their latitude. There’s a notable difference however: the ACC is much bigger.
“It’s the largest ocean current today, and it’s the only one that connects all the ocean basins,” says research author associate professor Howie Scher. “The Atlantic, Pacific and the Indian are huge oceans, but they’re all bounded by continents; they have firm boundaries. The Southern Ocean, around Antarctica, is the only band of latitude where there’s an ocean that goes continuously around the globe. Because of that, the winds that blow over the Southern Ocean are unimpeded by continental barriers.
Circumpolar current largest today
“So the fetch – the distance that the wind can blow over the ocean – is infinite. And fetch is one of the things that determines how high the waves are, how much mixing goes on in the oceans, and ultimately what drives surface ocean currents. 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.”
In a paper recently published in the journal Nature, Scher and his team make the case for just when this massive ocean current first started flowing. One straightforward obstacle in the distant past was the arrangement of continental masses. Antarctica and Australia were part of a single super-continent, Gondwana, and began to separate about 83 million years ago, so the Pacific and Indian Oceans couldn’t have been in contact near the South Pole before then.
It was much later than the initial separation of Australia and Antarctica that deep ocean currents could flow between the two continents, though. Paleo-oceanographers have identified a transition, the opening of the Tasmanian gateway, a deep-water channel between Tasmania and Antarctica, as being a necessary part of any large-scale, sustained flow on the order of the ACC.
Using novel information about the separation of Antarctica and Australia, Scher and his team developed a tectonic model that showed that the Tasmanian gateway first developed at least 500m of depth between 35-32 million years ago.
From geochemical analyses of sediment core, however, they concluded that the channel opening to that depth wasn’t enough to get the ACC flowing. The Pacific Ocean is in contact with much younger rock than the Indian Ocean, Scher says, which leads to a distinguishing concentration in each ocean of one isotope of neodymium that has a half-life longer than that of the solar system.
By measuring neodymium isotope compositions incorporated into fish teeth fossils in core samples, the team was able to establish that eastward current flow between the Pacific and Indian Oceans didn’t begin until about 30 million years ago, some 2-5 million years after the Tasmanian gateway opened.
Taking both geophysical and geochemical data into account, they conclude that although the Tasmanian gateway was wide enough to accommodate a deep current, the gateway was located too far south to be in contact with the mid-latitude trade winds, which are the driving force for today’s eastward-flowing ACC.
Instead, when the gateway first opened, water initially flowed westward, the opposite of that today, in keeping with the prevailing polar winds located at the more southern latitudes.
Only as both continents, and the gateway between the two, drifted northward on their tectonic plates over the next several million years did alignment with the trade winds come about. That reversed the current flow, to the east, and the ACC was born.
Circumpolar current the global mix master of the oceans
“It’s the global mix-master of the oceans—that’s a quote from Wally Broecker [of Columbia University’s Lamont-Doherty Earth Observatory], and that’s what it’s been called by oceanographers for 50 years now,” Scher says. “The Antarctic Circumpolar Current is the world’s largest current today, it influences heat exchange and carbon exchange, and we really didn’t know for how long it’s been operating, which I call a major gap in our command of Earth history. It was a cool outcome.”