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Rampino, M.R., and R.B. Stothers, 1984: Terrestrial mass extinctions, cometary impacts and the sun's motion perpendicular to the galactic plane. Nature, 308, 709-712, doi:10.1038/308709a0.
An analysis of the data of Raup and Sepkoski (1984) is reported which suggests that the dominant cyclicity in major marine extinctions during at least the past 250 Myr is 30±1 Myr, with the standard deviation of an individual episode being ±9 Myr. This terrestrial cycle is found to be strongly correlated with the time needed for the solar system to oscillate vertically about the galactic plane. It is argued that galactic triggering or forcing of terrestrial biological crises may arise as a result of collisions or close encounters of the solar system with clouds of gas and dust, which would gravitationally perturb the solar system's family of comets and thereby increase the flux of comets and meteorites near the earth, leading to large-body impacts. A dominant cyclicity of 31±1 Myr is found for the observed age distribution of impact craters on earth, the phase of this cycle agreeing with the shown by the major biological crises.
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