North and South America may have hooked up 10 million years earlier than thought.
Many scientists think that the seaway separating the two continents closed about 3 million years ago, sparking mass animal migrations and an ice age in the Northern Hemisphere. After analyzing crystals excavated from an ancient South American streambed, researchers report in the April 10 Science that this continental connection must have taken place before 13 million years ago. Although not all experts are convinced, the researchers argue that the unearthed crystals originated in Panama and flowed down a river that straddled both continents.
“The only origin for these crystal fragments we found in Colombia is in Panama,” says lead author Camilo Montes, a geologist at the Universidad de los Andes in Bogotá, Colombia. “There had to have been a river flowing over a land connection 13 million years ago.”
While interesting, the finding isn’t conclusive proof of an early connection between the two continents, says hydrologist Robert Stallard of the U.S. Geological Survey’s National Research Program in Boulder, Colo. The crystals Montes and colleagues found could have originated from somewhere other than Panama, he says. “There are plenty of opportunities to get these crystals into Colombia without closing the seaway.”
When the Pangaea supercontinent broke apart about 200 million years ago, the Americas went their separate ways and then slowly drifted back together. Around 20 million years ago, only a small strip of water called the Central American Seaway connecting the Pacific and Atlantic oceans separated the two continents. A heap of volcanic rock sitting on the Caribbean tectonic plate then slammed into northwestern South America and rose up above the sea surface, closing off the seaway.
Scientists initially dated the seaway’s closure based on its wide-scale impacts on animal populations and global climate. Animals such as mammals scurried across the newly formed land bridge and freshwater fish swam down its waterways about 3 million years ago. Around the same time, the lost passageway between the Pacific and Atlantic probably rerouted Earth’s ocean currents, creating the Gulf Stream and triggering a glaciation period in the Northern Hemisphere.
While digging around northern Colombian streambeds and drainage basins, Montes and colleagues uncovered ancient rock shards called zircon crystals embedded inside much younger rock. Zircon crystals, which form when molten rock solidifies, are relatively easy to date thanks to radioactive uranium atoms that become entombed in the crystal structure. While the surrounding rock was laid down 10 million to 13 million years ago, the zircons were as much as 60 million years old.
The zircon ages serve as a fingerprint of where the crystals came from, Montes says. In this case he traced the zircons back to Panama. He proposes that the zircons were eroded by water and wind, making their way down an ancient river system from Panama to their final resting places South America. Stallard, however, points out that volcanic rock to the east of modern-day northern Colombia may share this age fingerprint and could have supplied the zircons without the need for a closed seaway.
A much older land bridge would mean scientists have to rethink the impacts of the Central American Seaway’s closure, Montes says. For instance, the mass migration between North and South America may have been postponed by thick jungle that many animals would be unable to traverse. Around 3 million years ago, as the region’s climate cooled (for reasons unrelated to the seaway closure), the jungle could have thinned, allowing more animals to pass through, Montes says.
But even if a river did run from Panama to South America 13 million years ago, that doesn’t mean the Americas were completely connected, says geologist Anthony Coates of the Smithsonian Tropical Research Institute in Panama City. “Even if they’re absolutely right with everything having to do with the zircons, showing that something closed at one point in the extreme southern end doesn’t necessarily apply to the rest of the seaway,” he says. “There were other gaps all the way up to Mexico.”