The last Pleistocene Ice Age started to end about 20 000 years ago; after this temperatures rose more-or-less steadily till the end of the Pleistocene. This warming trend was interrupted by three reversals, in which the temperatures fell sharply, known as the Oldest, Older and Younger Dryas (so named because they can be identified in drilled core samples by increases in the abundance of the pollen of the alpine plant Dryas octopetala). The Oldest Dryas occurred between about 18 000 and 15 000 years ago, the Older Dryas between about 14 000 and 13 500 years ago (though this is not well calibrated & other dates are often quoted), and the Younger Dryas between about 12 800 and 11 500 years ago (the end of the Younger Dryas is generally also considered to be the end of the Pleistocene and the beginning of the Holocene).
Ice Age fauna such as Wooly Mammoths either had their last flourish in the Younger Dryas, or were wiped out by it, depending on which people you talk to. BBC.
The causes of these temperature reversals are not entirely clear and are therefore the subject of considerable scientific debate, the most popular theories being that they are the result of melting ice-sheets dumping large amounts of cool fresh water into the oceans, both cooling the oceans and altering the flow of ocean currents, and therefore causing periods of cooling and flooding as part of a general warming trend (this is a predicted outcome of modern anthropogenic global warming, and was used to good effect by science fiction writer John Wyndham in his 1953 novel The Kraken Wakes, in which alien invaders deliberately melt the Earth's ice caps, cooling the climate and flooding our cities), or that these cooler periods are the result of Milankovitch Cycles, long term variations in the Earth's tilt which effect the climate.
How Milankovitch Cycles work. Lyndon State College of Vermont.
In 2007 a group of scientists at a meeting of the American Geophysical Union proposed that the Younger Dryas event was caused by an impact event, with one or more large objects either slamming into the Earth, or exploding in the upper atmosphere and causing widespread devastation and a cooling in the Earth's climate, based upon samples from sites across North America that they interpreted as containing impact related material. This was not well received, as such cooling events can clearly occur without impact events, and over the next few years much of the evidence was debunked; minerals were shown to have been misidentified, and in one case 'shock-related carbon spherules' were shown to be fungus spores.
Artist's impression of a possible impact event at the start of the Younger Dryas. Florida Frontiers/Neily Trappman Studios.
In a paper published in the Proceedings of the National Academy of Sciences, a team of scientists lead by Isabel Israde-Alcántara of the Departamento de Geología y Mineralogía at the Instituto de Investigaciones Metalúrgicas at Universidad Michoacana de San Nicólas de Hidalgo announce the results of a study of deposits in Lake Cuitzeo in central Mexico which they interpret as evidence supporting the Younger Dryas impact theory.
The location and topography of Lake Cuitzeo. From Israde-Alcántara et al. (2012) supplementary material.
Israde-Alcántara et al. report the discovery of a horizon in cores from Lake Cuitzeo that they interpret as the start of the Younger Dryas, at which they find extensive charcoal, magnetic grains, framboidal spherules and weekly magnetic volcanic glass, material they interpret as indicative of an impact event.
Scanning Electron Microscope images of possible impact grains from Lake Cuitzeo. (A, B) Magnetic spherules with dendritic (branching) surface patterns. (C) Framboidal (raspberry-patterned) pyrite spherules. (D) Collisional magnetic impact spherules (?). (E) Light micrograph of (D). (F) Teardrop-shaped particle with dentritic surface pattern. (G) Light micrograph of (F). Scale for (D-G) not given in original. From Israde-Alcántara et al. (2012).
However there are a number of problems with this diagnosis.
Firstly there is the dating of the horizon to the beginning of the Younger Dryas. Israde-Alcántara et al. attempted to gain carbon-dates for the sequence (itself not a perfect method), but were unable to do so, apparently because the sediments had been overturned by turbulence, which should have set alarm-bells ringing. Instead they dated the onset of the Younger Dryas as a peak in organic carbon content in the sediments, above which the amount of oak pollen was dramatically reduced, and the amount of grass pollen dramatically increased.
This is widely seen at the onset of the Younger Dryas, but is not indicative of it. It probably shows the death of an oak forrest and its replacement by grasslands, which might be caused by global climatic breakdown, but could also be caused by a local forrest fire; on its own it is not evidence of anything.
Oak pollen became much less abundant at the start of the Younger Dryas, as climatic breakdown killed off many oak forests. University of Arizona.
Then there are the particles themselves. Alcántara et al. observe that similar particles can be generated by volcanic eruptions, but that such eruptions do not typically spread them over a wide area, and therefore reject a volcanic origin for the particles. However the particles were not gathered over a wide area, but from a single lake, surrounded by volcanoes. This does not enable a volcanic origin to be ruled out (indeed taken with a local die-off of oak forests, this is quite a good explanation). Even if the particles were scattered over a wide area, Alcántara et al. would need to establish that an unusual volcanic event, in which such particles were scattered over a wider area, was a less likely explanation than a major impact event.
Alcántara et al. are clearly enthusiastic supporters of impact events to explain terrestrial phenomena. They are keen to stress the similarities between the putative Younger Dryas impact event, and the impact event at the end of the Cretaceous. Unfortunately this event has been repeatedly shown to have occurred before the end of the period, implying that it cannot have been responsible for the extinction event.
Most scientist have moved on from this theory, and those that have not are forced to deal with the survival of multiple groups of Cretaceous animals into the Palaeocene (for example a forthcoming paper by Landman et al. in the journal Acta Palaeonotologica Polonica, proposing the survival of Ammonites beyond the end of the Cretaceous, as defined by impact stratigraphy). This is nonsensical; the geological periods were defined by their fossils, as this has been the easiest way to date rocks for much of the history of geology. To start redefining geological periods to fit in with impact events that didn't cause mass extinction events would be close to insanity.
Scientists need to be enthusiastic about their work to succeed, but when this enthusiasm clouds judgement in favor of pet theories, then the science produced will inevitably be poor. The best science is often produced by scientists able to reject their own preferred theories (which also has the advantage of saving them the embarrassment of somebody else doing it).