PNAS is a high impact journal

Another month, another paper enthusiastically discussing the thin evidence for a extra-terrestrial impact at the start of the Younger Dryas is published in PNAS.

Wu et al (2013) report osmium isotope ratios from putative Younger Dryas boundary layers at various sites and conclude that at most sites there is no evidence of an extra-terrestrial osmium component. At one site in Pennsylvania they find some high-temperature spherules with an odd osmium isotope ratios. All told, not very persuasive evidence of a climate-altering impact.

Wu et al (2013) do discuss a possible impact crater at Sept Iles, Gulf of Saint Lawrence, reported in conference proceedings (ie minimal peer review) by Higgins et al (2011). Whereas Wu et al (2013) describe the crater as being provisionally dated to 12.9 ka, the start of the Younger Dryas, Higgins et al (2011) report that the crater is “clearly younger than Ordovician [485–443 million years ago]”. Basal sediments within the supposed impact crater date to 12.9 ka, but it is quite possible that these sediments represent the first sediments deposited after the crater was scoured during the LGM, rather than the first sediments after the crater was formed. If the 4 km crater was formed at the start of the Younger Dryas, ejecta should be obvious in lake and ocean cores near the site, provided there was some Younger Dryas sedimentation.

A Younger Dryas impact cannot explain the inter-Allerød cold period, the Older Dryas, or any of the Dansgaard–Oeschger events, unless there was a queue of impactors, all of which have failed to leave any convincing evidence. With ample evidence that the climate in the last glaciation was unstable, it seems entirely unnecessary to hypothesize that the Younger Dryas was uniquely driven by extra-terrestrial impacts.


Higgins et al (2011) Bathymetric and petrological evidence for a young (Pleistocene?) 4-km diameter impact crater in the Gulf of Saint Lawrence, Canada. Lunar and Planetary Science Conference XXXXII, March 7–11, 2011, abstr 1608

Wu et al (2013) Origin and provenance of spherules and magneticgrains at the Younger Dryas boundary. PNAS


About richard telford

Ecologist with interests in quantitative methods and palaeoenvironments
This entry was posted in climate, Peer reviewed literature and tagged , , , , . Bookmark the permalink.

3 Responses to PNAS is a high impact journal

  1. climategordon says:

    “Another month, another paper enthusiastically discussing the thin evidence for a extra-terrestrial impact at the start of the Younger Dryas is published in PNAS.”

    I couldn’t agree more! I thought I was the only person noticing this!

  2. downwithtime says:

    It’s not just the physical evidence of the impact that is lacking. As we point out in our critique of the 2012 Israde-Alcántara et al. paper (Gill et al., 2012), the ecological signal of a purported YD impact is entirely reliant on misinterpreting the paleoecological record. The press is tying the event to a purported sudden mass extinction of megafauna, widespread fire and massive cultural change in human populations, but all of these are non-existent.

    Marlon et al. (2009) did an excellent job of showing that widespread fire did not occur at the YD, Gill et al (2009) showed that megafaunal declines began much earlier than the YD, and the evidence of rapid turnover in Clovis culture is also weak (Holliday, 2010). Boslough et al (2012) have a pretty thorough refutation of the whole thing.

    Boslough M, Nicoll K, Holliday V, et al. 2012. Arguments and evidence against a Younger Dryas impact event. Geophysical Monograph Series 198: 13-26.

    Gill JL, Blois JL, Goring S, et al. 2012. Paleoecological changes at Lake Cuitzeo were not consistent with an extraterrestrial impact. Proceedings of the National Academy of Sciences, 109, E2243-E2243.

    Gill JL., Williams JW, Jackson ST, et al., 2009. Pleistocene megafaunal collapse, novel plant communities, and enhanced fire regimes in North America. Science 326: 1100-1103.

    Holliday VT, Meltzer DJ. 2010. The 12.9-ka ET impact hypothesis and North American Paleoindians. Current Anthropology 51: 575-607.

    Israde-Alcántara I, et al. (2012) Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis. Proc Natl Acad Sci USA 109:E738–E747.

    Marlon JR, Bartlein PJ, Walsh MK, et al. 2009. Wildfire responses to abrupt climate change in North America. Proceedings of the National Academy of Sciences 106: 2519-2524.

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