At http://scienceblogs.com/highlyallochthonous/2007/10/out-of-the-ice-age-into-the-as.php April 23rd … ‘Out of the Ice Age, into the Asteroid shower’ is a post by Chris Rowan which begins, ‘21,000 years ago, at the peak of the last glacial, ice sheets covered most of western Europe and North America (a bit of an exaggeration but never mind), and by 10,000 years ago a warming climate had melted them away …’.
Research from ice and sediment cores all agree that the ice retreat was not smooth but jerky. Episodes of more rapid warming and fairly rapid cooling were superimposed on the longer term warming trend – which includes the 1500 year Younger Dryas event when mean annual temperatures dropped sharply and glaciers reappeared in western Scotland, North Wales and the Lake District (but note the assumption of an underlying ‘warming trend’ as in the Milankovitch model rather than a simple description of the facts, a retreat of the glaciers from NW Europe and NE North America followed by warmer temperatures that were punctuated by the Older and Younger Dryas events).
He then goes on to describe the Firestone et al theory of an impact event/airburst and compares the evidence with the old theory, quoting Piotrowski et al = changes to the thermahaline circulation model. He suggests the two mechanisms might be complimentary – which is a nice kind of breakthrough. Basically, the impact or airburst event would have left behind a lot of dust and aerosols in the atmosphere which would have chilled the temperature of the earth as sunlight was filtered. The opaque sunlight then resulted in changes to the thermahaline circulation model ushering in a lengthy cooling period (aided and abetted by background cycles).
The comments that follow the post involve dating methodology (as one thread) and highlight the problems involved in actually getting a handle on a precise time lapse – not least, it seems to me, is the 1500 years assigned to the YD (which is exactly the period of one of those cycles so beloved of early computer orientated science). It is also a fact that the Oldest Dryas event was deemed to last some 3000 years = two such cycles. Anyway, leaving aside suspicious thoughts a commenter says, ‘one problem is C14 dating which is not precise or accurate during the deglaciation event’. He appears to be referring to what is known as a C14 plateau where labs produce dates that fluctuate over a period of time – but at this time the plateau is fairly long in comparison with historical C14 plateaus. He ends by saying what is required is a well-dated sequence.
Another commenter, Thomas Palm, notes an impact would cause a short cooling lasting several years – not the 1500 years of the YD. This is the problem in a nutshell. Hence, if an impact or airburst event melted lots of ice and this found it’s way into the North Atlantic that might have very quickly shut down the thermahaline system. A nice marraige between both ideas.
Other comments include – there were some pretty weird things going on in the radiocarbon world during the late glacial period. Others also do not trust C14 methodology – but some say dendrochronological support of C14 dates are reliable. Suspicious minds might say it’s odd that dendro supports so neatly the status quo as far as historical chronology is concerned. Was C14 used to support dendro dates in order to produce C14 dates supported by dendro? However, since then a problem has emerged as C14/dendro dates appear to date the Thera eruption consistently to around the late 17th century BC whereas in conventional chronology it is located in the late reign of Hatshepsut (or thereabouts) some two hundred or so years later. It seems an element of wishful thinking is assumed by anybody who thinks dendro assisted C14 is reliable – but why rock the boat?
It seems that RS Telford has written a couple of papers, in 2004, criticising ‘all age-depth models’ which might be useful to get hold of.
Finally, a geologist then admits that when it comes to age-dating methods he leaves it to the experts – the labs. There comes a time when you have to accept the ages knowing they will be modified down the road – and just publish what you have found. He fully expects the methods for determining accurate dating will change. The same goes for archaeologists I assume.