Geologists: Ancient Fish Teeth Provide Clues to Beginning of Antarctic Cooling

Gainsville, Florida (Apr 20, 2006 15:42 EST) Ancient fish teeth are yielding clues about when Antarctica became the icy continent it is today, highlighting how ocean currents affect climate change.

University of Florida geologists have used a rare element found in tiny fish teeth gathered from miles below the ocean surface to date the opening of a passage at the bottom of the globe between the Atlantic and Pacific. The opening, which occurred millions of years ago in a much warmer era, allowed the formation of an ocean current around the pole. That event preceded – and may even have brought about -- Antarctica's transformation from a forested continent to an icy moonscape.

"We're saying we now have a date for the opening of the Drake Passage that looks like it's early enough that it may have contributed to the cooling," said Ellen Eckels Martin, a UF associate professor of geology.

Scientists have long puzzled over the rapid cooling that seemed to sweep over Antarctica more than 30 million years ago, replacing boreal pine forests with ice and snow. The cooling occurred in a very warm era when levels of carbon dioxide, the gas responsible for the greenhouse warming effect, were three to four times today's levels.

Theorists had suggested the plummeting temperatures could be related to the opening of the Drake Passage, a connection between the Atlantic and Pacific named after Sir Francis Drake, the English captain who circumnavigated the globe in the 16th century. But there has been a longstanding debate over when that passage opened. That's a key point because Antarctica is known to have been covered with ice by about 33.6 million years ago, meaning the circumpolar current would have had to be established before that event if it could be considered a cause of the cooling.

Estimates for the passage's opening have ranged from 15 million years to 49 million years ago. Martin and Scher's research confirms the older dates.

The scientists' source: neodymium isotopes retrieved from fish teeth the size of grains of sand – teeth themselves retrieved from sediment cores recovered from the deep ocean bottom more than two miles beneath the surface.