An ancient crater buried underneath the Oklahoma town of Ames has been discovered to be almost 100 million years younger than thought, consequently connecting it to one of the big five mass extinction events in Earth's history.
The crater, which spans about 10 miles (16 kilometers) in diameter, is buried under 9,000 feet (2.7 kilometers) of sedimentary rock and was not discovered until seismic readings revealed it in 1991.
Initial estimates of the crater's age came from the discovery of the tooth in a rock found within the impact site. It was identified as belonging to an ancient eel-like creature called a conodont, which existed during the middle of a geological period known as the Ordovician, in a rock found within the impact site. This period was marked by a dramatic bombardment of asteroids known as the Ordovician Meteor Event 467.5 million years ago, which left nearly two-dozen craters spread around the world, all within 30 degrees of the equator. This equatorial pattern suggests there may have been a ring of material from a broken-up asteroid encircling the Earth's equator.
However, a team led by Elizabeth Catlos of the University of Texas at Austin has arrived at a different conclusion, finding that the crater is 370 million years old instead. So rather than being part of the Ordovician Meteor Event, this new dating places it around the time of the Late Devonian mass extinction, also referred to as the Frasnian–Famennian boundary extinction.
"With this research, we're basically taking a major pawn out of the Ordovician Meteor Event and dumping it into the Frasnian–Famennian event and saying, 'this is where this impact belongs'," said Catlos in a statement.
The new evidence for the age of the Ames crater comes from zircon crystals. Zircon is a silicate mineral, and zircon crystals can be dated by analyzing the amount of uranium and lead contained within them. When zircon crystals form they contain uranium and thorium, but the formation process pushes out lead atoms. A newly formed zircon crystal therefore contains no lead, but over time uranium radioisotopes decay into lead atoms, so the more lead a zircon crystal contains, the older it is.
The pressure of the impact that gouged out the Ames crater would have formed new zircons. Working with a team at NASA's Johnson Space Center, Catlos' group were able to image the zircon crystals using cathodoluminescence and electron backscatter diffraction. Cathodoluminescence bombards materials with electrons, which causes materials to emit photons at different wavelengths depending upon the material's composition. Electron backscatter diffraction, on the other hand, is a technique that uses a scanning electron microscope to study how electrons are diffracted at different angles by different atoms.
"No matter what technique we used, it was coming back to this younger signal," said Catlos.
So what about the fossil conodont teeth? The teeth were probably in the rock a long time before the impact, and just got jumbled in the violence that formed the crater.
While the impact alone was not big enough to cause a mass extinction, it may have contributed. Other possible causes of the extinction include a reduction in oxygen, super-volcanism or a nearby supernova, or possibly all these events operating in tandem.
When the impact occurred 370 million years ago, what is now Oklahoma was submerged beneath a shallow sea, which is how the 1.6 miles (2.7 kilometers) of sedimentary rock that today covers the crater was laid down. The impact also fractured the Earth's crust beneath it, and these fractures have allowed oil and natural gas to seep up. Wells struck oil three kilometers down inside the crater and since 1991 it has produced 17.4 million barrels of oil and a volume of 79.5 billion cubic feet (2.25 billion cubic meters) of natural gas.
Danny Stockli, who is the Dean of the Jackson School of Geosciences at the University of Texas at Austin and a co-author of the research, is effusive in his praise of using zircon crystals to date the ages of craters.
"These small crystals allow us to go back in time and learn about the major changes to Earth's ancient landscapes," said Stockli. "It would be great to do this for more of the meteor impact sites across the continent so we could get a more accurate timeline for these major events."
The findings were published in July in the journal Meteoritics and Planetary Science.
