A Tale of Two Amazons

PHILADELPHIA, Pennsylvania, October 25, 2006 (ENS) – The world's largest river, the Amazon, once flowed from the Atlantic to the Pacific, the opposite of its current direction across South America, finds new research released today.

The new study is based on the evidence left behind in the river's ancient sediments and in the form of tiny crystals of a mineral called zircon.

Zircons are long-lived and tend to be repeatedly recycled without stopping their internal uranium-lead radioisotope clocks, making them tiny windows on long-lost mountains and entire continents.

Geologist Russell Mapes, a doctoral candidate at the University of North Carolina, UNC, in Chapel Hill, boarded planes and boats over a two year period to study the 6,516 kilometer (4,049 mile) long river. He presents his findings today at the annual meeting of the Geological Society of America in Philadelphia.

Russell Mapes on a research expedition in the Amazon Basin. (Photo courtesy Russell Mapes)

The Amazon's once westward flow was caused by a highland near what today is the river's mouth. That highland, which has long since disappeared, was created by the separation of South America from Africa and the creation of the Atlantic Ocean during the Cretaceous Period, 65 to 145 million years ago.

Later, when the Andes rose up on the western side of South America, the river had no choice but to drain into the new ocean.

"It just happened in a way that the current Amazon could take advantage of where an old river and ocean basin used to sit," said Mapes.

The key to Mapes' findings reside in the zircons found in the river's sediment.

Working with his UNC faculty advisor Drew Coleman, and Brazilian colleagues Afonso Nogueira and Angela Maria Leguizamon Vega, Mapes found that the age of rocks on the South American continent differs between east and west.

Rocks as old as 2.5 billion years are found on the eastern side of the continent. On the western side, rocks are much younger because of continual geological activity in the Andes.

Russell Mapes collects samples at a sandbar on the upstream side of the Rio Negro/Solimões confluence. (Photo courtesy Russell Mapes)

The zircons in old sands studied by Mapes, stood out because they do not appear to come from the Andes at all. In fact, they date to about 1.3 to 2.1 billion years ago. So they had to have been formed in rocks that solidified in mountains that eroded away into the earliest Amazon.

Mapes said that sediments of eastern origin were washed down from a highland area that formed in the Cretaceous Period, when the South American and African tectonic plates separated from each other.

That could have tilted the river's flow westward, sending sediment as old as two billion years toward the center of the continent.

Afterwards, a relatively low ridge, called the Purus Arch, rose mid-continent, running north and south. This divided the Amazon's flow, so that one half flowed eastward toward the Atlantic and the other westward toward the Andes.

In the late Cretaceous, mineral grains younger than 500 million years old began to fill in the basin between the Andes in the west and Purus Arch.

Exposed submarine dunes on the Amazon River. Mapes notes, "Heavy mineral accumulation between beds. West on right, east on left." (Photo courtesy Russel Mapes)

Mapes and his colleague team traversed about 80 percent of the Amazon basin, gathering zircon samples.

"It was a pretty exotic trip," said Coleman. "We took planes and aluminum small boats. In the first year we took a two-and-a-half week cruise up the river from Manaus. We sampled young and old sediment outcrops on the river. We traveled about 500 kilometers (300 miles)."

"In the second year, we did three small hops - plane flights downstream then rented small boats at each stopping point," recalls Mapes.

"One night in a boat we ran out of gas in an electrical storm. Luckily, it was the one place in the basin where my cell phone worked. We called the man who rented the boat to us and he came out and got us. We just bobbed around. It was a place where the river is several miles wide."

Mapes said it was worth the experience for being able to look back in time. "When I got the actual data back," he said, "I was happy."