Researchers search for clues to date Drake Passage
Posted July 3, 2008
Earth scientists David Barbeau and Ian Dalziel both want to know exactly when the Drake Passage between South America and the Antarctic Peninsula opened, an event that may have played a key role in turning Antarctica to ice 35 million years ago.
The theory goes something like this: After the Drake Passage became deep enough and wide enough millions of years ago, the Antarctic Circumpolar Current (ACC) formed. This belt of cold water spins around Antarctica from west to east, effectively insulating the continent from the intrusion of warmer ocean water.
“It’s sort of a superhighway of cold water that moves swiftly around Antarctica,” Barbeau explained. Thanks to the formation of that coldwater highway, the continent cooled, and the ice sheets eventually formed. The Drake Passage, the hypothesis goes, helped turn Antarctica into an icehouse.
“It is a fact that the approximate date of the openings of the deep passageways and the completion of a circumpolar current coincides with ice on the Antarctic continent,” Dalziel said.
But wait a minute
Of course, nothing in science is quite that easy and tidy. The northern hemisphere cooled around the same time, according to Tom Wagner, program director of Earth Sciences at the National Science Foundation’s Office of Polar Programs . Coincidence?
And there are competing theories about why Antarctica went into a deep freeze 35 million years ago.
Photo Credit: Dann Blackwood/PolarTREC
A scientist aboard the RVIB Nathaniel B. Palmer with Ian Dalziel's group examines a rock retrieved from a dredge.
One hypothesis notes that atmospheric carbon dioxide, a key greenhouse gas in today’s warming world, was decreasing around the same time that ice sheets began to form. This theory, put forward by scientists like Matthew Huber at Purdue University (related news story ), said fossils found in sediment cores from the ocean floor south of Australia, in another key ocean passageway, indicate cold water circulated around Antarctica for millions of years before the ice sheets formed.
Huber concluded that because the ice sheets appeared very rapidly, over a period of just a few tens of thousands of years, some other factor must have caused the rapid cooling that engulfed Antarctica. The culprit, he said, was lower levels of CO2.
Another theory, Dalziel said, claims the uplift of the Tibetan Plateau may have played a role in turning Antarctica cold. In this scenario, scientists William Ruddiman and Maureen Raymo have suggested that as the Tibetan Plateau and the one in North America rose, they changed circulation and wind patterns (related news story ). The Tibetan Plateau, about a third the size of the United States, also created the regional monsoons that bring heavy rain. These heavy rains then help remove large amounts of carbon dioxide from the atmosphere.
The carbon dioxide in the rain reacts with silicate rock that is exposed as the plateau continues to rise. The rain weathers the rock, and the minerals find their way into the ocean, becoming deposited as limestone on the ocean floor.
The Drake Passage polar gateway hypothesis, which originated in the 1970s, hinges on the tectonic history that pushed Antarctica and South America apart.
“It’s by no means certain that it’s the circumpolar current,” Dalziel said. “There’s no way we’re going to completely solve the problem. We’re contributing to the solving of it.”
A viable theory
Another piece of evidence, from the teeth of fish, would seem to buttress the Drake Passage polar gateway theory. Scientists published in the April 12, 2006, issue of the journal Science that, according to fossil evidence, the Drake opened up about 41 million years ago, a scenario that would fit well with the timing of Antarctica’s glaciation and the opening of the Tasman Gateway between Antarctica and Australia (related news story ).
The scientists — Ellen Martin at the University of Florida in Gainesville and Howie Scher at the University of California Santa Cruz — used a chemical called neodymium, which accumulates in fish teeth that rest upon the ocean floor, to date the opening. The neodymium signatures for the Atlantic and the Pacific are different, allowing the scientists to determine when water from the Pacific Ocean began seeping into the Atlantic Ocean through the Drake from analyzing the fish teeth.
However, more evidence is needed to keep the Drake Passage theory viable, according to Barbeau.
“What we don’t know is if the tectonic history lines up,” Barbeau explained. “The reason the tectonic history is important is [just] because some water gets through the Drake Passage that doesn’t necessarily mean that the passageway is wide enough or deep enough for the ACC to develop.”