Airbursts in the Sky with Diamonds? Shock Limits to a Younger Dryas Impact. (Invited)

Firestone, Kennett, West and others recently suggested that the Younger Dryas event, with its associated megafaunal extinctions, was precipitated by an extraterrestrial impact somewhere in North America, on or above the Laurentide ice sheet. Evidence initially put forward in favor of this hypothesis included elevated levels of iridium, microspherules and fullerenes in the “Black Mat” layer present at many Clovis cultural sites, although no impact crater has been found. While this evidence is presently strongly disputed by other investigators, the proponents of this idea currently cite the occurrence of nannodiamonds, particularly of the hexagonal Lonsdaleite variety, as definitive evidence of impact-generated shock, and suggest that no crater was formed because the impactor was a dispersed cometary object that exploded in the atmosphere. Such diamonds have been previously reported associated with terrestrial impacts, including Chicxulub, Ries Crater and Popagai. Observations at the Ries indicate that diamonds form only at shock pressures between 45 and 55 GPa, while at Popagai the range is 35 to 50 GPa. While these pressures are readily attained in the impact of a solid body on the Earth’s surface, it is much more difficult to attain them in an airburst because air is so easily compressed. Estimates from both the stagnation pressure upon entry and a planar impact computation using the measured Hugoniot parameters of air and the Murchison chondrite show that the minimum impact velocity that can yield these transformation pressures in chondritic material is more than 200 km/sec (independent of impact angle), well beyond the possible impact velocity of an object bound to the solar system. In another scenario it was suggested that the impact occurred on the ice sheet itself but did not penetrate the ca. 3 km thick ice mantle. However, this limits the size of the impactor to less than 1 km diameter. Many impacts of this size have occurred in the past (the average recurrence interval is about 1 per Myr), but no extinctions are associated with these events. Indeed, for an impact on the Laurentide ice sheet to ignite fires in Southern Arizona by thermal radiation from its fireball, approximately 5000 km from the impact site, a projectile almost 100 km in diameter would be required! Such an impact would have wiped out all life on the Earth except microbes, not just the megafauna. I thus conclude that there is no plausible scenario in which the Younger Dryas event was caused by an impact and suggest instead that the formation of nannodiamonds by non-impact processes (such as forest fires) simply needs to be better understood.