Criticism of Comet Impact Hypotheses as Cause for the Younger Dryas
Since a group of twenty-six scientists presented their findings at the American Geophysical Union in New Mexico in May 2007, the idea of a comet encounter as taking place approximately 13,000 BP has at least been discussed. However, not unexpectedly, much criticism has been directed towards this new hypothesis. One article produced by the BBC News department highlights the critique to date. The article titled “Mammoth – Killing Comet Questioned, was written by Jason Palmer and dated January 29, 2009, was not generally supportive of the Younger Dryas comet hypothesis. Dr Andrew Scott of the University of London has done some work on fire frequencies (also one of the important cornerstones of the comet scenario). It is Dr. Scott’s conclusion that greater fires frequencies extended for five thousand year period both during and before and after the Younger Dryas, and were not therefore just confined to the Younger Dryas.
A second criticism also directed at the Younger Dryas comet theory is suggested by Dr. Wallace Broecker, a well-known geochemist from Columbia University. Dr. Broecker indicates that a second important signature for the comet encounter at the beginning of the Younger Dryas, nanodiamonds, rather than only increasing at times of comet encounters, they are also prevalent in the present day without any significant comet presence. As it turns out, this may be an unfair criticism of the impact hypothesis, as some recent sampling by Dr. James Kennett of the University of Oregon registered increases in a specific type of nanodiamonds that are hexagonal in shape. These nanodiamonds, called lonsdaleites have been found on Santa Rosa Island, one of the present day Channel Islands that lie off the coast of California. Dated at approximately 13,000 BP, or the beginning of the Younger Dryas, what is so interesting about these nanodiamonds are they are only associated with meteorites and comet impacts. (Live Science, Jeanna Brynner, July 20, 2009). The first criticism too (that of wildfires frequency not being particularly great at the Younger Dryas), may not be accurate either as Dr. Richard Firestone, of the Lawrence Berkley National Laboratory in California suggests that an important limitation to Dr. Scott’s work is that it is of low resolution,thereby not permitting a definitive date to be assigned to increased fire frequencies (Mammoth – Killers Comet Questioned, by Jason Palmer BBC News). Apart from this, there are a number of other important signs indicating a comet encounter at the beginning of the Younger Dryas. Not only are there increased nanodiamonds, but there are also carbon spherules, another distinctive proxy allegedly associated with meteor impact events. Also, rare on Earth elements such as helium 3, potassium 40 and iridium (detected in a sample taken from the Greenland Ice Cap) all become prevalent during the Younger Dryas. Finally, another important indicator also found within the high resolution Greenland Ice Cores, are significant ammonium spikes. Ammonia, produced as a by-product of wildfires must be regarded as being anomalous during cold phases such as the Younger Dryas. Normally, this particular gas increases during warmer intervals, so the increase during the Younger Dryas is all the more surprising. The fact that most ammonia gas arriving at Greenland comes from North America is also very interesting, and is cause to consider the comet impact thesis more strongly.
