Just to prove that it’s not a good week for high-impact asteroid theories, John Hawks points to a piece in Science which discusses a growing scepticism about Firestone et al.‘s proposal in PNAS last year that the Younger Dryas – the cold snap around 12-13,000 years ago which interrupted the last glacial-interglacial transition – was the result of an impact event on or above the North American ice sheet. An interesting discussion in the comments to my post on the PNAS paper implied that the radiocarbon dates being used to correlate the various impact indicators to the demise of the Clovis people and Pleistocene megafauna such as mammoths were probably not quite accurate enough to prove the link beyond doubt; now it seems that many of the impact indicators themselves are open to interpretation:
- Organic carbon was mainly in forms produced in intense fires – charcoal, soot and polycyclic aromatic hydrocarbons. But there are many more down-to-earth causes for any such fires, including human activity, which could also have produced these molecules.
- Other forms of carbon sometimes found in the layer included nanodiamonds and fullerenes, which are often associated with impacts. The helium trapped in the buckyball cages had much higher proportions of Helium-3 than you would expect for a terrestrial source. The reported nanondiamonds were based on NRM spectra rather than direct observation using an electron microscope, and experts claim that the analysis is flawed. The isolation of helium-bearing fullerenes has proven extremely difficult to replicate.
- A large number of magnetic microspherules – interpreted to be formed from the molten ejecta of an impact. Similar microspherules have been found accumulating on the roof of a researchers house, indicating both the possibility of a non-impact source and modern contamination of the outcrops being studied.
- A noticeable peak in the abundance of iridium, which is much more enriched in extraterrestrial bodies than the Earth’s crust (concentrations were generally below the limit of detection above and below the marker horizon). As reported in the original paper, duplicate analyses of YD boundary layer samples failed to pick up iridium enrichment at four out of the nine sites; although the authors claimed this was due the anomalous iridium being concentrated in a particular grain fraction which is sometimes only present in only small quantities (‘the nugget effect), others dispute that the sample sizes were small enough for this to be a problem. Furthermore, claims of a similar iridium spike in ice-core records is disputed by the scientist who analysed them.
It seems that although several indicators have been tied together to support the impact theory, the most definitive evidence, such as a crater, or the presence of shocked quartz, is lacking; and to fit the geochemical data, you have to assume that the impactor had a rather unusual composition. This is not an inference that should be made lightly, but some scientists clearly feel it has been, and in the fascinated glare of the world’s media no less. Perhaps the most forthright expression of this feeling is in January’s GSA today, where Nicholas Pinter and Scott Ishman argue that both the Younger Dryas impact hypothesis, and the megatsunami/chevron story, “appear to be spectacular explanations on long fishing expeditions for shreds of support.” Furthermore, “the term ‘chevron’ should be purged from the impact-related literature.” By normal scientific writing standards, this is DEFCON 1 language. Is is justified, do you think? Are people being a little too hasty to blame asteroid impacts for every vaguely sudden-looking jerk in the geological record? Or do you think that these groups might be on to something? Personally, although it’s clear that the burden of proof remains with the proponents in both cases, I wonder if Walter Alverez is suddenly getting a feeling of deja vu.