Zircon crystals make Oklahoma's buried Ames crater nearly 100 million years younger

Until this study, Ames had been dated only with biological evidence: teeth from conodonts, eel-like animals of the Ordovician, preserved in the crater's rock ^[7]. A fossil gives a maximum age. The rock holding it cannot be older than the animal, but it can be far younger, especially when an impact churns up old sediment and buries it again. Catlos's account of Ames fits that pattern, with teeth already millions of years old mixed into the impact rocks intact ^[8].

Zircon offers a direct date. The grains came from granite the impact altered ^[5]. Danny Stockli, dean of UT's Jackson School of Geosciences and a co-author, said zircon uranium-lead dating is one of the most accurate ways to determine when events happened deep in Earth's past ^[12]. The step in the design I find most persuasive is the screening. A zircon in granite might simply record when the granite itself crystallized, so the team imaged the grains with NASA, using cathodoluminescence and electron backscatter diffraction, to find the distinctive recrystallization an impact produces ^[11]. "No matter what technique we used, it was coming back to this younger signal," Catlos said ^[6].

The revision moves the impact by about 97.5 million years, from roughly 467.5 million years ago to about 370 ^[1]. The Frasnian-Famennian extinction is placed at about 372 million years ago ^[10]. Taken at face value, the central figures put the impact about 2 million years after the die-off ^[2]. The release does not give the uncertainty on the zircon age, the number of grains dated, or what the revision means for the oil and gas field. Without error bars, this account cannot settle whether those 2 million years are a real gap. Even an exact match would show only that two events were close in time. The open question, as the release puts it, is whether extinctions were triggered mainly from space, by impacts, or from within Earth, by episodes such as massive volcanism ^[14].

Ames had been one of the craters whose shared age, about 467.5 million years, made the Middle Ordovician look crowded with impacts ^[3]. That crowding is why some researchers proposed a Saturn-like ring of asteroid debris around Earth at the time ^[4]. Removing one crater does not dispose of the ring idea. The larger effect is on method. If a fossil age at Ames was wrong by nearly 100 million years ^[1], any other cluster crater dated only by fossils is open to the same error. "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," Stockli said ^[13].

For Oklahoma, the producing field at Ames now sits in a Late Devonian crater ^[2][1]. The study was published in July in Meteoritics & Planetary Science ^[9].