Another new paper on the Younger Dryas boundary event – by Andrew Overholt and Adrian Melott. They are thinking in terms of increased levels of C14 and Bryllium 10 at the boundary are actually evidence of an extraterrestrial object entering the Earth's atmosphere – and breaking up. In other words, the meme has definitely shifted from a comet impact to an airburst – see http://cosmictusk.com/melot/
Melott et all (2010) have previously explored atmospheric air bursts and this new paper was inspired, it would seem, by the recent Russian meteor (earlier in the year). They thin comet fragments exploding in the atmosphere can spray around and deposit nitrates and ammonium that is potentially detectable in ice core isotopes – and all this is consistent with Tunguska (1908). However, they also recognise that the presence of increased cosmic rays does not necessarily imply a comet or bolide event as other things can cause showers of cosmic rays to rain on the atmosphere.
Apparently, some comets have a high C14 content – but the amount of C14 deposited by small comets is insignificant compared with the average C14 mass residing in the atmosphere – or coming from the Sun, and elsewhere. To make a noticeable change to the C14 record very large deposits are necessary and therefore such events are rare. Results from their research show that measureable amounts of C14 will be deposited in the atmosphere by large long period comets. Short period comets, they say, do not deposite a significant measureable impact on C14 levels in the atmosphere. This is an interesting paper as some people have used fluctuating C14 levels in the atmosphere to say, or to deny, planetary influences on the Sun – but how measureable are these fluctuations (usually considered to be extremely small – see http://tallbloke.wordpress.com/2013/07/28/cameron-and-schussler-no-evide…