Amazon River Once Flowed in Reverse

Oct. 25, 2006 — Even the most reliable geographic facts — like the direction of the world's largest river — can change dramatically over geologic time.

Millions of years ago, a new study suggests, the Amazon River flowed from east to west — a 180-degree difference from its current eastward path to the Atlantic Ocean.

The evidence for this continent-wide reversal comes from tiny, dark, virtually indestructible minerals called zircons that are perpetually recycled in the sandstones of the Amazon Basin.

"People use the zircons in ancient sands to reconstruct continents, to understand what might have been upstream," said geologist Russell Mapes, a doctoral candidate at the University of North Carolina in Chapel Hill.

Very often zircons are the only mineral remnants of long-gone mountain ranges.

Geologists know this thanks to new technology that measures the age of the minerals by the minute quantities of uranium and lead isotopes within them. The isotopes reveal how long it has been since the zircon solidified from molten rock.

In the case of the Amazon zircons, Mapes and his advisor Drew Coleman, along with their Brazilian colleagues Afonso Nogueira and Angela Maria Leguizamon Vega, discovered that the minerals don't come from the relatively young Andes at all.

Instead, they are from 1.3 to 2.1 billion years ago. That strongly suggests the zircons must have eroded out of some mountain range that bordered eastern South America back when it was still attached to Africa — before there was an Atlantic Ocean at all.

Yet the zircons were found to the west of the region where they were born, so the river must have once flowed in that direction, the researchers concluded.

"The rocks we saw on the river suggested this," said Mapes. "But when I got the actual data back, I was happy."

Ages later, about 65 million years ago, the Andes began to rise on the western edge of South America, blocking the river's passage to the Pacific and shifting its flow to the east. Today the zircon is all that remains of the South American-African Mountains.

Mapes plans to present the discovery on Oct. 25 at the meeting of the Geological Society of America in Philadelphia.

"You couldn't tell this without this new tool," said geologist Paul Link of Idaho State University, referring to the technology to sample individual layers of zircon minerals with precision, and then count the isotope atoms to gauge age.

Link is working on a similar drainage reversal in North America, even further back in time.

"I think that what it illustrates is that when a new tool is developed, then all of a sudden you can make interesting conclusions about which way rivers drained," said Link.

That, in turn, can reveal how very different the world once was.

"Even things like huge rivers are very temporary in the scheme of Earth time," said Mapes.