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