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Wednesday, May 02, 2007

Extinctions due to Galactic interference?

Adrian Mellot and collegues at the University of Kansas have come out with an explanation for the 62 Ma periodicity in marine extinctions/radiations on earth discovered by Rhode and Muller a few years back. The movement of the Sun north of the galactic plane is argued to increase cosmic ray influx to the earth causing a higher rate of mutation and subsequent extinctions. The periodicity is derived from an analysis of marine fossils in the record (see Figure at the top of the page). Mellot and colleagues re-analyzed the same dataset. Basically, they looked for periodicity in the extinction/radiation cycle by looking at residuals following a cubic spline fit to the data. The statistical analysis employed by Mellot and colleagues was slightly different, but the same 62 Ma periodocity was observed.
The argument that there is a celestial driver to radiations and extinctions on earth is nothing new. "Nemesis" stars have been proposed for the extinction periodicity, asteroids and comets have been blamed for mass extinctions as have gamma-ray bursts (GRB's). GRB's, asteroid and comets are aperiodic so only the Nemesis and this new proposal fit the periodicity models. Nemesis has largely been rejected because the passage of a companion star would cause far more havoc than mere extinction of life, but the movement of the sun above the galactic plane may be more reasonable.
Here's what troubles me about this paper. I had the opportunity to sit with Adrian Mellot for a few hours a couple weeks back and also to listen to his presentation on the subject. What struck me was that the periodicity ends at about 140-160 Ma. That is, the 62 Ma cycle is only clearly evident in the 360 Ma interval prior to about 140-160 Ma. While sitting listening to Mellot talk, I realized that 140-160 million years ago is close to the age of the oldest oceanic crust. Why is that important? Remember that the periodicity is based on the marine fossil record. Since there is no significant amount of oceanic crust older than ~160 Ma, the pre-160 Ma fossil record is only that which is preserved on continental margins or inland continental seas. Continents make up ~1/3 of the earth so, at first glance 2/3 of the fossil record might be missing from the analysis. One could argue that most marine diversity is to be found in shallow seas and therefore the available fossil record is a sufficient sample. If so, then why doesn't the periodicity appear in the most robustly sampled fossil record (the most recent 160 Ma)?
This is an interesting story and I am sure that Mellot, Lieberman etc will be looking at this more carefully and attempting to explain the lack of periodicity during the last 160 Ma. Stay tuned.....


Joe Meert

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