Impact crises, mass extinctions, and galactic dynamics: The case for a unified theory

A general model in which mass extinctions of life on Earth are related to impacts of asteroids and comets whose flux is partly modulated by the dynamics of the Milky Way Galaxy is supported by several lines of astronomical and geologic evidence. The flux of Earth-crossing objects, and calculations of the environmental effects of impacts, predict that collisions with large bodies (mostly comets) ≥5 km in diameter (≥10⁷ Mt TNT equivalent) could be sufficient to explain the record of ∼25 extinction pulses in the last 545 m.y. The same evidence implies that the five major mass extinctions are related to impacts of the largest bodies (≥10 km in diameter, ≥10 ⁸ Mt events). Tests of "kill curve" relationships for impact-induced extinctions suggest that a possible threshold for catastrophic global extinction pulses occurs with impacts that produce craters with diameters between ∼100 and 150 km. Seven of the recognized extinction peaks have thus far been correlated with concurrent (in some cases, multiple) stratigraphic impact markers and/or large impact craters. Recent statistical analyses support previous findings that the record of extinction events exhibits a periodic component of ∼30 m.y. Additionally, spectral peaks of 30 ± 0.5 m.y. and 35 ± 2 m.y. have been extracted from recently revised sets of welldated large impact craters. These results could be explained by periodic or quasi-periodic showers of Oort cloud comets. The pacemaker for such comet showers may involve the Sun's vertical oscillation through the galactic disk, with a similar cycle time between crossings of the galactic plane. Further tests of the model will involve the identification and quantification of the dark matter component in the galactic disk and improved resolution of the putative periodic signature in the record of large craters and extinction events.