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Comment by jcranmer | Hacker News Reader

There's a theory that the Younger Dryas was caused by an impact of a bolide on a North American glacier, usually of one that fragmented considerably on entry.

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There was certainly a bolide strike roughly coincident with the beginning of the 1300-year Younger Dryas cold spell.

Whether the strike caused the Younger Dryas cold spell is a matter of speculation. We can anyway be confident that the cold spell did not cause the bolide strike.

> There was certainly a bolide strike.

There was not certainly a bolide strike. No one has identified an unambiguous sign of such a strike; a purported impact crater was identified in Greenland a few years ago, but it is not yet confirmed as an impact crater, nor is it well-constrained to occur around the Younger Dryas. (There's apparently a paper suggesting that it's off a tad, maybe 50 million years too early).

The ice core record is unambiguous, corroborated by soil samples from literally dozens of sites as far away as Syria, South Africa, and southern Chile. South African lake sediment identifies an exact year.

The smoking gun is the layer of massively enriched platinum dust without the excess sulfur an eruption would emit, and melt spherules. Anyone doubting a bolide strike needs to account for it all in some other way, a tall order.

A crater would be convenient, but is not necessary: we know about airbursts now. E.g., there is no Tunguska crater. Demanding a crater is not scrupulous.

What's the citation for the ice core stuff? I found two papers that investigated this and neither found evidence to support the impact theory, so it seems ambiguous to me, but this isn't my field so obviously I might just be failing to find a more recent result.

If you wanna read em:

Paquay, François S., et al. “Absence of Geochemical Evidence for an Impact Event at the Bølling-Allerød/Younger Dryas Transition.” Proceedings of the National Academy of Sciences of the United States of America, vol. 106, no. 51, 2009, pp. 21505–10

Surovell, Todd A., et al. “An Independent Evaluation of the Younger Dryas Extraterrestrial Impact Hypothesis.” Proceedings of the National Academy of Sciences of the United States of America, vol. 106, no. 43, 2009, pp. 18155–58.

Update: Found (some?) ice core evidence, it is indeed more recent, but I'm hopelessly out of my wheelhouse at this point so I'll just cite it for anyone also interested and suggest that these more recent publications do seem pretty confident in their conclusion that the impact did happen. So it seems user moloch-hai is correct:

Petaev, Michail I., et al. “Large Pt Anomaly in the Greenland Ice Core Points to a Cataclysm at the Onset of Younger Dryas.” Proceedings of the National Academy of Sciences of the United States of America, vol. 110, no. 32, 2013, pp. 12917–20.

Pretty cool stuff, thanks for sharing and prompting me to learn more about it.

Those are both from 2009, ten years before the smoking gun cited.

They are studiously silent now. [Edit: Thank you for Petaev. I had not seen it.]

> Those are both from 2009, ten years before the smoking gun cited.

Just so you're aware, you haven't actually cited the study yet in this thread.

(Kjær, Kurt H., et al. "A large impact crater beneath Hiawatha Glacier in northwest Greenland." Science advances 4.11 (2018) is the citation, for future reference)

(FWIW, Kenny, Gavin G., et al. "A Late Paleocene age for Greenland’s Hiawatha impact structure." Science advances 8.10 (2022) disputes the age given for that impact crater.)

As already noted, any connection to Hiawatha crater was only ever conjecture. Evidence that Hiawatha was not coincident with the YDB obviously tells us nothing at all about YDB events.

A recent survey article is Sweatman,

The Younger Dryas impact hypothesis: Review of the impact evidence

https://www.sciencedirect.com/science/article/abs/pii/S00128...

The remaining controversy is how much the strike contributed to the Younger Dryas development and other coincident events, particularly extinctions. To me, it is ludicrous to blame those on Clovis people, or on a climate hiccup smaller than they had weathered easily many times before. Now that we know people had already been there for 10+ millennia, the sudden wipeout idea gets much sillier.

The idea that the strike triggered the YD is appealing, but the record shows several other such cooling periods with very similar profile and duration in the past 100ky. It seems unlikely they were all triggered by comet strikes. But they could have been: we now know our records of comet strikes are woefully incomplete, having detected a very recent one only by enormous effort.