The Younger-Dryas period was an episode of extreme cooling right at the tail end of the last ice age. In 2007, a team of researchers published a shocking find. From around 50 archaeological sites across North America, all dating to the Clovis period (~13,000 BP), there seemed to be evidence for a massive, catastrophic event. They proposed that
“one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.” (Firestone et al. 2007:16016)
Just like that, the Younger Dryas Impact Hypothesis was born. This sent shockwaves through the archaeological community. If true, it would resolve several open questions all at once. The Younger-Dryas itself would be explained, but so would the extinction of many genera of ice age megafauna. It would also explain the disappearance of the Clovis culture. It’s a great theory. A lot of people were really excited. But then scientists tried to take the next step, and test the hypothesis further. Many tried to replicate elements of Firestone et al.’s study. The original authors said that in the layers of sediment from the Younger Dryas period which they studied, they found
“a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium”
So that’s what others set out to replicate. Unfortunately, these attempts have been consistently unsuccessful. For example, one study focused on the elevated levels of iridium the original paper had reported, but they found “no evidence of an extraterrestrial (ET)-PGE enrichment anomaly” (Paquay et al. 2009:21505) in any of their samples. Another team, with the first author being my Master’s advisor Todd Surovell, looked for the magnetic particles reported in the original paper. They also came up short, finding “no distinct peak in magnetic grains or microspherules uniquely associated with the YD” and “no support for an extraterrestrial cause of the YD event and New World Pleistocene extinctions” (Surovell et al. 2009:18155).
Firestone et al. described the ET impact layer from many of these sites as a distinctive “black mat.” Their results suggested a clean sequence: Clovis and megafauna are doing their thing, there’s an abrupt deposition of this anomalous layer at the same moment everywhere, and then no Clovis or megafauna on the other side. That sounds pretty convincing. A study several years later, in 2015, also supported this idea with a bayesian analysis of radiocarbon dates from Younger Dryas boundary layers at sites on four continents, showing that it really happened everywhere at the same time (Kennett et al. 2015). But… that wasn’t really an independent study. It includes many of the authors from the original paper. So another team looked into their radiocarbon dates more closely. Here’s what they did:
“we first aggregate 14C measurements from Northern Hemisphere YDB sites. We also aggregate 14C measurements associated with a known synchronous event, the Laacher See volcanic eruption. We then use a Monte Carlo simulation to evaluate the magnitude of variability expected in a 14C dataset associated with a synchronous event. The simulation accounts for measurement error, calibration uncertainty, “old wood” effects, and laboratory measurement biases.” (Jorgeson et al. 2019:123)
And… their results did not support the idea that these layers were deposited at the same time, suggesting that however they were deposited, it wasn’t from an ET impact. Conveniently, there are some other explanations for the “black mats” of the Younger Dryas. For example, one study found that soils in wetlands can accrue many of the same materials found by Firestone et al., so some of the layers at these Clovis sites might be showing us environmental changes, rather than an ET impact. (Pigati et al. 2012)
Another article points out that while many of the materials Firestone et al. reported make sense individually, they shouldn’t be occurring together in a single ET object. “Any one of these might be a credible extraterrestrial source, but together they are a Frankenstein monster, incompatible with any single impactor or any know impact event.” (Pinter and Ishman 2008:37). Meanwhile, another team found no evidence for a population decline at the end of the Younger Dryas (Buchanan et al. 2008).
For all of these reasons and more, researchers have been turning against the idea that such an impact ever took place. Despite what certain pseudo-archaeologists with recent hit shows on Netflix who appeared on the Joe Rogan Experience claim, the Younger-Dryas Impact Hypothesis has not delivered convincing evidence. Even though it would be pretty cool.
Buchanan, B., Collard, M., & Edinborough, K. (2008). Paleoindian demography and the extraterrestrial impact hypothesis. Proceedings of the National Academy of Sciences - PNAS, 105(33), 11651-11654. https://doi.org/10.1073/pnas.0803762105
Firestone, R. B., West, A., Kennett, J. P., Becker, L., Bunch, T. E., Revay, Z. S., Schultz, P. H., Belgya, T., Kennett, D. J., Erlandson, J. M., Dickenson, O. J., Goodyear, A. C., Harris, R. S., Howard, G. A., Kloosterman, J. B., Lechler, P., Mayewski, P. A., Montgomery, J., Poreda, R., . . . Wolbach, W. S. (2007). Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the younger dryas cooling. Proceedings of the National Academy of Sciences - PNAS, 104(41), 16016-16021. https://doi.org/10.1073/pnas.0706977104
Jorgeson, I. A., Breslawski, R. P., & Fisher, A. E. (2020). Radiocarbon simulation fails to support the temporal synchroneity requirement of the younger dryas impact hypothesis. Quaternary Research, 96, 123-139. https://doi.org/10.1017/qua.2019.83
Kennett, J. P., Kennett, D. J., Culleton, B. J., Tortosa, J. E. A., Bischoff, J. L., Bunch, T. E., Daniel, I. R., Erlandson, J. M., Ferraro, D., Firestone, R. B., Goodyear, A. C., Israde-Alcántara, I., Johnson, J. R., Pardo, J. F. J., Kimbel, D. R., LeCompte, M. A., Lopinot, N. H., Mahaney, W. C., Moore, A. M. T., . . . West, A. (2015). Bayesian chronological analyses consistent with synchronous age of 12,835–12,735 cal B.P. for younger dryas boundary on four continents. Proceedings of the National Academy of Sciences - PNAS, 112(32), E4344-E4353. https://doi.org/10.1073/pnas.1507146112
Paquay, F. S., Goderis, S., Ravizza, G., Vanhaeck, F., Boyd, M., Surovell, T. A., Holliday, V. T., Haynes, C. V. J., & Claeys, P. (2009). Absence of geochemical evidence for an impact event at the bølling-Allerød/Younger dryas transition. Proceedings of the National Academy of Sciences - PNAS, 106(51), 21505-21510. https://doi.org/10.1073/pnas.0908874106
Pigati, J. S., Latorre, C., Rech, J. A., Betancourt, J. L., Martínez, K. E., & Budahn, J. R. (2012). Accumulation of impact markers in desert wetlands and implications for the younger dryas impact hypothesis. Proceedings of the National Academy of Sciences - PNAS, 109(19), 7208-7212. https://doi.org/10.1073/pnas.1200296109
Pinter, N., & Ishman, S. E. (2008). Impacts, mega-tsunami, and other extraordinary claims. GSA Today, 18(1), 37. https://doi.org/10.1130/GSAT01801GW.1
Surovell, T. A., Holliday, V. T., Gingerich, J. A. M., Ketron, C., Haynes, C. V., Hilman, I., Wagner, D. P., Johnson, E., & Claeys, P. (2009). An independent evaluation of the younger dryas extraterrestrial impact hypothesis. Proceedings of the National Academy of Sciences - PNAS, 106(43), 18155-18158. https://doi.org/10.1073/pnas.0907857106
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There is a cave in the mind."
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The best prophet of the future is the past."
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What would be ugly in a garden constitutes beauty in a mountain."
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Always my soul hungered for less than it had"
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