The relevance of the background impact flux to cyclic impact/mass extinction hypotheses

Abstract

The belief that mass extinctions of species on Earth have occurred on a ∼26- to 33-my cycle is supported by some rather equivocal geological evidence. This has prompted a search for cosmic phenomena that could subject the Earth at regular intervals to bombardment by showers of comets, with resulting damage to the biosphere. A crucial assumption that an impact-driven mass extinction cycle would automatically show up in the geological record is questioned. Might the background flux of random impacts distort the cycle and render it unrecognizable? Computer simulation of the impact bombardment of the Earth over a 250-my period, in which the background impact flux is overlaid by a 26-my comet shower cycle, showed a periodicity in the mass extinction data between 24 and 33 my in ∼40–60% of runs, dependent on the magnitude of the background flux chosen, and only ∼20–40% indicated the “true” 26-my periodicity. Thus, background impact “noise” can beidentified as an additional constraint on cyclic impact/mass extinction hypotheses.