Astrophysical Drivers of 62 Myr Periodicity in Fossil Biodiversity

There have long been discussions of apparent periodicity in the fossil record, but in 2004 there appeared a study which showed high statistical significance for a 62 million year periodicity going back about 500 million years. We have re-examined and verified this result. It is clear that the 62 My cycle strongly dominates. Some discussion of our analysis of the cycle, and an alternate causal mechanism can be found here.

The discoverers of the cycle considered but could not construct a plausible mechanism to drive this peridocity. We have proposed a definite theory to explain this. Descriptive news stories from New Scientist, National Geographic.com, and Space.com are available. A somewhat more complete summary is available in Physics Today.

Our galaxy is a thin disk. The whole solar system moves around the galaxy in a complicated motion, an eccentric orbit, also bobbing up and down

within the disk as it goes. We noticed that times of major fossil biodiversity drop seemed to coincide with the times when the Sun has bobbed "up", or more specifically galactic north, perpendicular to the plane of the disk. It turns out that they agree to better than one chance in ten million. Now our galaxy's "north" happens to lie very close to the direction of the Virgo Cluster of galaxies, the biggest mass concentration in our galaxy's neighborhood. Our galaxy naturally is falling toward this cluster, it turns out at about 200 kilometers per second through the hot gas that

surrounds galaxies. Such motion should produce a shock wave, such as the one imaged here from a laboratory experiment . In fact, our Sun and solar system have a shock structure surrounding them as the Sun moves through the hot gas in our galaxy. We expect that just as the shock at the boundary of our solar system produces cosmic rays, the shock at the north side of our galaxy should too. The galaxy has one extra feature that makes it different: a turbulent magnetic field, strongest inside the disk of the galaxy. This magnetic field shields us from the cosmic rays except when the sun bobs up out of the disk on the north side, and the shielding distance is thin. Many more high-energy cosmic rays can reach us, many of them energetic enough to get past the Earths protective magnetic field and reach the ground. This can cause all kinds of problems with the climate as well as directly damage the molecules of life.

This model does an excellent job of explaining the diversity cycle; but of course it was constructed to do so. It can also predict something new. Because the galaxy is not perfectly smooth, the Sun bobs up to different heights each time, and this motion has been calculated back into the past. When the Sun bobs up further, the damage can be worse. We define the damage at each event as the biodiversity minimum subtracted from the preceeding peak, to get a drop. Let's plot this "extinction strength" along with the cosmic ray flux predicted by our model for each of the upward excursions in the last 500 million years:

as you can see, the fit predicted by the model is in amazing agreement: extinction strength goes in a one-to-one correspondence with the increase in cosmic rays.

This work appeared in Astrophysical Journal. A story on it had the highest-hit count on the website Science Now, a publication of Science magazine, in August 2007. In 2008, we found confirmation of this cycle in an independent database, providing strong evidence that it is likely to be real.

It has been widely discussed as an input into the question of Galactic habitable zones.

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