Younger Dryas Impact Hypothesis

The Younger Dryas impact hypothesis proposes that a cosmic impact event, or a series of airbursts from fragments of a comet or asteroid, struck North America around 12,900 years ago and triggered the abrupt onset of the Younger Dryas, a roughly 1,200-year-long return to near-glacial cold that interrupted the general warming trend at the end of the last ice age, while also contributing to the extinction of North American megafauna and the decline of the Clovis archaeological culture. Geochemist Richard Firestone led the team that first proposed the hypothesis formally in a 2007 paper, citing a claimed layer of impact-related markers, including nanodiamonds, magnetic spherules and elevated iridium concentrations, found across numerous sites dated to the same period. The hypothesis has remained highly controversial within the scientific community since its publication, since numerous subsequent studies attempting to independently replicate the original impact markers at the same or comparable sites have reported conflicting results, and several proposed alternative, non-impact explanations exist for both the Younger Dryas cooling itself and for the megafaunal extinctions. The debate over the hypothesis remains actively unresolved, with proponents and critics continuing to publish competing analyses of the same body of evidence.