Geotimes - October 2007 - Current not responsible for Antarctica's ice?NEWS NOTES
Paleoclimate Current not responsible for Antarctica's ice?
Until about 34 million years ago, when a thick layer of ice began to encase the continent, Antarctica used to be a fairly warm and lush place. Why its permanent ice sheet formed has been a matter of debate among scientists, with one long-held theory positing that the onset of the Antarctic Circumpolar Current (ACC) triggered the glaciation by deflecting warmer, subtropical waters away from the continent. But a new study suggests that a strong ACC didn’t start until about 10 million years after the Antarctic ice cap formed, challenging the notion that the current was the culprit behind the sudden chilling of the continent.
The ACC formed after Antarctica’s neighboring continents had drifted away (see Geotimes, March 2006). One step in this process was the opening of the Tasmanian Gateway, a shallow channel that widened as Australia slowly moved away from Antarctica about 32 million years ago. A second event, the opening of the Drake Passage between South America and Antarctica, left an open passageway around the continent that allowed water to encircle it unhindered. Geoscientists have long debated the timing of the Drake Passage opening, placing it anywhere from about 45 million years ago to as recently as the early Miocene, between 23 million and 17 million years ago (see Geotimes, June 2006). “But if the Drake Passage opened in the early Miocene, the ACC could not have caused extensive cooling 10 million years prior because it wouldn’t have existed then,” says David Rea, an oceanographer at the University of Michigan in Ann Arbor.
To shed light on the problem, Rea and his colleagues extracted a core from a spot at the bottom of the South Pacific that would have been in the path of the ACC between 40 million and 20 million years ago. The ACC is such a strong current that it causes erosion at the seafloor, which is then recorded in sediments. The team found that erosion didn’t begin until after 25 million years ago, separating the time of Antarctic glaciation and the beginning of the ACC by nearly 10 million years.
“That means that there needs to be another reason why Antarctica suddenly got cold,” Rea says, adding that one possibility is a sudden drop in greenhouse gases. Modeling studies have suggested that a decline in carbon dioxide around that time could have caused temperatures to plummet, turning Antarctica into the frigid continent it is today.
At first glance, the new findings may appear to conflict with previous studies based on isotope data that have placed the Drake Passage opening much earlier than the Miocene. But Rea and his colleagues, who reported their findings in the August Geology, say the discrepancy can be reconciled. The deep-water channels that allowed the strong ACC flow were established through a long, complex process that began with a shallow opening, he explains. Although a shallow gap lets lots of water through, it doesn’t allow the establishment of a deep and coherent flow. Indeed, “just because you had an open Drake Passage doesn’t mean that you had a vigorous ACC,” says Lawrence Lawver of the University of Texas at Austin.
Only after a deep, wide channel was formed could the flow have become strong. “It’s like pulling down a zipper — the top opens first and after awhile the bottom opens,” Rea says. “Isotope studies that found evidence of water from the Pacific pouring into the Atlantic earlier than the Miocene are likely seeing the beginning of the process while we are seeing the end of it.”