Decades passed between the first suggestion that the mass extinction at the Cretaceous-Palaeogene boundary was caused by an extra-terrestrial impact, and this hypothesis becoming accepted by most geologists as evidence — the global iridium anomaly, shocked quartz grains and tektite glass spherules, and finally an impact crater — accumulated.
An extra terrestrial impact has also been suggested as the cause of the Younger Dryas, the last major cold event of the last ice age (Firestone et al 2007). Will this hypothesis be resolved any faster?
Firestone et al (2007), writing the prestigious journal Proceedings of the National Academy of Sciences (PNAS), suggest that thin layers overlying Clovis sites containing (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 extra-terrestrial helium are evidence of an impact causing forest fires, ending the Clovis tool-making era, and initiating the Younger Dryas. Pinter et al (2011) held a “requiem” for the impact hypothesis, showing that these supposed signatures of an impact could all be explained by ordinary processes or misinterpretations. For example, the charcoal could be the result of ordinary forest fires, the carbon spherules could be misinterpreted fungal spores and the nano-diamonds could be other forms of carbon. In short, Pinter et al found no evidence of an impact and questioned the rigour of scientists who ‘readily jump aboard the “impact bandwagon”’ at the first sign of unusual evidence.
But the hypothesis refused to be buried, Bunch et al (2012) and Wittke et al (2013), both in PNAS, report widespread deposition of microspherules they interpret as being formed in the intense heat of an impact. A major problem with these studies for inferring that an impact initiated the Younger Dryas is the chronology. The abrupt start of the Younger Dryas is well dated and defined in the Greenland ice cores; radiocarbon-based chronologies from archaeological sites or elsewhere are never going to be sufficient to prove synchronicity.
The obvious solution to this is to search in the layer of the ice core at the beginning of the transition into the Younger Dryas for evidence of an impact. This is what Petaev et al (2013) have done, yet again published in PNAS, finding a platinum spike that is synchronous with the start of the Younger Dryas, and indicates that the Younger Dryas started with a “cataclysm”. There is no iridium spike, the indicator of the dinosaur-exterminating impact, which Firestone et al (2007) had reported and Pinter et al (2012) had rubbished. Curiously the possibly impact-related platinum spike is not synchronous with the ammonium and nitrate spikes that indicate large amount of biomass burning at the start of the Younger Dryas. A large impact should be accompanied by a large nitrate spike because the impact will ionise the atmosphere (Melott et al 2010). Rather than being evidence of a cataclysm, I think this Pt spike spikes the impact hypothesis. An impact cannot both cause a Pt spike in Greenland at the initiation of the Younger Dryas and forest fires some decades later.
Perhaps the Pt spike represents the local fallout of a small impact unrelated to a later cataclysmic impact that left microspherules all over North America, but no platinum group element evidence in Greenland. It certainly should be possible to test if the Pt spike represents a local impact or one with the potential to cause global climate change by testing if it is found in ice of the same age in cores from other regions, for example the Himalaya.
Even if there is an extra-terrestrial impact at the time of the initiation of the Younger Dryas, it is a major step to claim a causal relationship. One problem is the cold events within the late-glacial interstadial. The isotopic excursions for these events are almost as large as that for the Younger Dryas, but the temperature recovers quickly, rather than having a long cold phase. There is no suggestion that each of these cold event is preceded by an impact, so clearly the climate system 13000 years ago is unstable even without impacts. Is there any reason to invoke an impact for the Younger Dryas?