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Rampino, M.R., 1998: The galactic theory of mass extinctions: An update. Celest. Mech. Dynam. Astron., 69, 49-58.
Astronomical and geological evidence is consistent with the hypothesis that mass extinctions of life on Earth are related to impacts with comets whose flux is partly modulated by the dynamics of the Milky Way Galazy. Geologic evidence for impact (ejecta and large impact craters) has been found at times of mass extinction events, and the record of large dated craters shows a significant correlation with extinctions. Statistical analyses suggest that mass extinction events exhibit a periodic component of about 30 Myr, and periodicities of 30±0.5 Myr and 35±2 Myr have been extracted from sets of well-dated large imact craters. These results suggest periodic or quasi-periodic showers of impactors, probably Oort Cloud comets, with an approcimately 30 or 36 Myr cycle. The best explanation for these proposed quasi-periodic comet showers involves the Sun's vertical oscillation through the galactic disk, which may have a similar time cycle between crossings of the galactic plane. Further refinement of the model will depend on the identification and quantification of the dark matter component in the galactic disk, and discovery and accurate dating of additional impact creater.
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