By Libby Roerig
Indiana State University
If you’ve ever wondered why the Antarctic is cold, scientists say you can thank the powerful Antarctic Circumpolar Current.
The ACC, as it’s known, is a unique surface-to-bottom current that flows eastwardly around Antarctica and keeps it cold and dry. How it came to be, however, has stumped paleoceanographers for decades.
“To establish ACC, you have to completely change continental geography — you have to open the Drake Passage between South America and Antarctica, have to open the Tasmanian Gateway between Australia and Antarctica,” said Jennifer Latimer, associate professor of geology at Indiana State University. “Then, it’s even more complicated, because it has to be deep and you have to have the right kind of winds to start the current. So, the timing of all of this has been really difficult to nail down.”
Latimer is part of a team of researchers who say they’ve unlocked this secret of the current’s origin by studying markers in sediment cores from the ocean floor and collaborating with tectonic plate researchers from the University of Tasmania and University of Sydney. Their findings were published in the July edition of the scientific journal “Nature.”
Scientists have determined the Drake Passage opened anywhere from 45 to 20 million years ago and the Tasmanian Gateway, which connects the Indian and Pacific oceans, opened 35 million years ago.
Those conclusions were clues, but “just opening the gateway isn’t enough to start the ACC,” she said. “This (research) helped to show the gateway was initially open and water flowed from the west, and then it later, after this whole-scale reorganization, started to flow towards the east, which we said was the initiation of the ACC 30 million years ago.”
Latimer was a graduate student under the tutelage of famed paleoceanographer James Kennett as a shipboard scientist on Ocean Drilling Program Leg 189 expedition in 2000. Latimer later brought with her to Indiana State samples from sediment core extracted during Leg 189, and while at the International Conference of Paleoceanography in 2010, she talked to University of South Carolina colleague Howie Scher about the data from these cores. Their collaboration revealed ratio shifts in the neodymium isotopes — so-called chemical fingerprints — of the fossils in these sediment samples and indicated the formation of the current.
“It’s another piece of the story that helps us better understand the development of the climate system since about 30 million years ago,” she said. “We know what’s happened over the past 30 million years in terms of how the ice sheets have grown or retreated, and we know what’s happened in terms of (carbon dioxide). But not knowing when the ACC started makes it harder to interpret some of those past changes in climate. Now that we can nail down when that happened, it helps us understand climate change since then.”
In the 1970s, Kennett theorized why east Antarctica has ice sheets. Latimer says their recent findings support this 45-year-old theory, which has been the source of some scientific debate.
“Right after we sailed on Leg 189, there was a lot of controversy, because there was this evidence of westward-flowing current, and we couldn’t figure out how to reconcile that with Kennett’s original ideas,” she said. “This (new research) shows you can have both — you can have this initial west-flowing current that turns around and switches.”
The change is a consequence of being at the exact right spot — both in terms of latitude to have a deep connection between the Indian and Pacific and in terms of the winds, Latimer said.
“The opening of the Tasmanian Gateway and the establishment of the ACC coincides with the development of permanent ice sheets on east Antarctica,” she said.
Part of the difficulty determining the current’s origin is getting sediment cores from the right locations and from the right times isn’t an easy task. From the time of writing a proposal for an expedition to actually boarding the ship can take longer than a decade, Latimer said.
As a graduate student, Latimer also researched the Drake Passage, saying it opened about 30 million years ago. It turns out she was on to something.
“When all this came out, my advisor called me and said, ‘We were right! We were absolutely right!’ So, that was kind of fun.”
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