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.
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, 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. 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.
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.”
Barbeau and Dalziel share a similar goal of dating the opening of the Drake to help settle the matter, but the separately funded National Science Foundation projects diverge on their methodologies.
Barbeau, an assistant professor at the University of South Carolina, led an island-hopping science cruise aboard the ARSV Laurence M. Gould in November and December of last year to the Antarctic Peninsula to collect samples from rock outcrops. He and his team then spent another month in January traveling around Argentine Tierra Del Fuego, chipping off similar fist-sized chunks of rock for analysis.
Once upon a time, before the Drake Passage formed, a range of mountains, the Andes, connected Antarctica and South America. Barbeau believes that by matching and dating rock samples on both sides of the Drake, he can better determine when the two sides parted ways, which would help date the opening of the passageway.
“They have very similar geology on both sides and they fit together fairly nicely as a puzzle, like South America and Africa fit together nicely for that Pangaean reconstruction that people talk about,” Barbeau said, referring to the supercontinent of Pangaea that existed about 250 million years ago before the continents began to separate into their present-day form.
“When South America and Antarctica stopped having the same sort of sediments being deposited on them would be a separate indicator of when Antarctica was separated from South America, and therefore when the Drake Passage could open and the ACC could develop,” he explained.
A senior research scientist with the Institute for Geophysics at the University of Texas in Austin, Dalziel led a science cruise aboard the RVIB Nathaniel B. Palmer from mid-April through the end of May. The vessel plied the central Scotia Sea, located partly in the Southern and Atlantic oceans, to refine the age of the seafloor in an area that had presented puzzling anomalies in the past.
The survey involved using geophysical instruments, such as a seismic reflection system that uses sound to image the seafloor, and dredging for volcanic basalt. “The low-tech way to do it is to hang a bucket behind the ship and see if we can collect some rocks,” Dalziel quipped.
It may be low-tech, but finding the right rocks isn’t easy. The seafloor, Dalziel explained, is littered with drop stones — material transported by icebergs as they drift north and drop rocks trapped in the ice to the seafloor as they melt.
“A lot of time what you’re picking up are rocks that have come from some unknown place,” he said. “They don’t help at all; they just get in the way.”
Still, the dredging program was successful. “We got a lot of drop stones, but we did manage to get a lot of material that we are confident is from the oceanic basement underneath the sediments,” he said.
Geochemical analyses of the samples will take the better part of a year, Dalziel added, because colleagues must separate individual mineral grains from the big chunks of rocks. Meanwhile, Dalziel and his team will study the geophysical data.
Both projects are part of an International Polar Year-affiliated program called Plates and Gates, a loose collection of projects studying polar ocean basins and gateways like the Drake Passage. “Those gaps turn out to be pretty important for ocean circulation, and the pattern of water moving around the global ocean,” Barbeau said.
And both scientists — Barbeau on his first trip to the Antarctic and Dalziel a 35-year veteran of polar science — say they made some provocative discoveries during their expeditions.
In 2005, Barbeau conducted an exploratory project in the southern Argentine Andes. Data from sedimentary samples in a basin near the mountains showed significant changes in the southern Andes’ composition about 30 to 45 million years ago. That means something tectonically big was under way.
The samples he and his team collected in the same region in January confirm the results from three years ago, and appear to narrow the event to about 35 to 40 million years ago. “What’s more exciting is that it fits in perfectly with the timeline we’re talking about.”
The Antarctic Peninsula rocks, however, don’t show evidence of this tectonic event. But Barbeau is not discouraged. “It suggests that the tectonic activity that led to the opening of the Drake Passage was focused more on the South American side than the Antarctica side,” he said. “It’s not a deal breaker.”
For Dalziel, the Palmer cruise created even more questions, because the Scotia Sea where his team sampled turns out not to be straightforward lithosphere, or ocean crust. “It’s distinctly different,” he said. “It turns out that the central Scotia Sea is very different in its character. It does not consist of normal mid-ocean ridge basalt.”
He said it will require a lot of time “under the microscope” and geochemical analyses for age dating to determine exactly what the scientists have discovered. The revelation further complicates the study of the Drake Passage theory. “It’s not going to make it easy to understand exactly when a deep ocean passageway formed,” Dalziel said.
“It’s a much more complicated question than we thought it was in the 1970s,” he said.
NSF-funded research in this story: David Barbeau, University of South Carolina; and Ian Dalziel, Institute for Geophysics at the University of Texas in Austin.
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