Did a comet hit at the end of the ice age?

A 2007 paper reported a layer at the onset of the Younger Dryas containing magnetic grains with iridium, microspherules, charcoal, soot and nanodiamonds, at sites across North America.

The authors proposed one or more extraterrestrial objects exploding over northern North America, partially destabilising the ice sheet and triggering the cooling, with the shock wave, the heat and the fires that followed contributing to the megafaunal extinctions and to changes in how people lived at the end of the Clovis period.

A 2023 review in Earth-Science Reviews examined the claimed markers and reported irreproducible observations, that very few sites with high levels of any claimed marker are dated accurately and precisely to 12,900 years ago, and that most of the markers are associated with terrestrial processes not unique to impacts.

It also reported no support for the basic premise that human populations were diminished, or that individual megafaunal species went extinct, because of a catastrophe at that moment.

Its title is a refutation rather than a review, which is itself a statement about how the exchange has gone.

The hypothesis explains several striking things at once, which is attractive and is also the warning sign. It has a committed group of proponents who continue to publish new sites and new markers.

They replied to the review in the same journal in 2024, rejecting it, and its authors replied in turn. In 2026 PLOS One retracted two of the group’s 2025 papers, citing problems with their dating and sampling; most of the authors who answered did not agree with the retraction.

Sustained disagreement in a literature does not always mean a question is open in the way a reader would assume. It can also mean a claim that keeps being restated after its evidence has been examined.

The leading account remains a large release of meltwater from the retreating Laurentide Ice Sheet, weakening the Atlantic overturning circulation. Convincing geological evidence for it was long elusive, and the route has been argued about: south down the Mississippi, east through the Great Lakes, or north-west through the Mackenzie into the Arctic.

Flood gravels along the Mackenzie, reported in 2010, and a meltwater signal in Beaufort Sea sediments beginning about 12,940 years ago have favoured the north-west, though a 2021 study of that outlet could not say whether its flood came near the start of the cold.

So the mainstream explanation has its own unresolved part, which is a fair thing to state alongside the case against the alternative.