There is some good stuff to read on the Younger Dryas boundary event at http://wattsupwiththat.com/2012/06/16/younger-dryas-the-rest-of-the-story/ … a piece written by Rodney Chilton, author and catastrophist, followed by some excellent comments and terrific links – including a link to a pdf article by Anthony Perratt et al. The comments do come to a kind of conclusion – the jury is out. The impact theory has a major problem – why did temperatures switch suddenly at the end of the YD period, rising very quickly by 10 degrees. Also, how can an impact/airburst event cause the climate to enter a cooling period lasting some 1300 years (exact time is variable in different papers). In addition, Dryas events resemble very closely the Heinrich events, seven of which are scattered across the last Ice Age, and the more problematic Dansgaard-Oeschger events that seem to have occurred on a more regular basis (see comments for those unaware of this feature of Ice Age climate). Chilton does his best to present evidence against the ocean circulation model – and says an increase in C14, ammonia, nitrates, and beryllium 10 are also a feature of the YD period (and could have a cosmic origin). However, what is clear when reading the various papers on abrupt climate changes over the last 70,000 years or so, one can only agree with the commenter that says it is as if a switch clicks on and off at regular intervals – and the beginning of the YD is the switching off and the ending of the YD is the click back on. The process also seems to explain interstadials and stadials within the Ice Age, at regular and indisputable intervals – but what it is that sparks the swtich is not currently known. It is a mystery. The ocean circulation system is just one hypothesis – but even the switch that might set that in motion has been disputed. It gets more and more interesting.
However, all that is lost on science writer Richard Kerr (a sort of Tom Bridgman character with a holier than thou attitude to innovative science) who has once again got up the proverbial of George Howard – see http://cosmictusk.com/kerr-on-limb-science-writer-misleads-his-peeps/ … It came about as Kerr very quickly rubbished the recent PNAS paper (see two days ago) as reported by KSJT. Kerr is adamant – there is no evidence of impact and claims the YDB hypothesis is reliant on the extinction of the mammoths – which is a secondary affair as it is well known that mammoths were already in serious decline by the time of the Younger Dryas (their numbers were severely depleted by whatever it was that caused the end of the Late Glacial Maximum some 4000 years or so previously). The response is typical – but if you use earplugs and wear dark spectacles you see and hear no evil (or contrarian evidence). The babble even popped up on Wiki 'Talk' between the defenders of the Faith and the catastrophists.
Meanwhile, a paper read at the INQUA-Congress (conference) in 2011, by Marshall, Head, Clough and Fisher, had the title, Exceptional iridium concentrations found at the Alleroed-YD transition in sediments from Bodmin Moor in SW England – which brings the YDB event into home ground. So far, very little interest by geologists in this country but this paper might change things. Han Kloosterman is still out there looking for ways to bring the hypothesis to people's attention.