Comets and extinctions; Cause and effect?

Micropaleontological study of the sedimentary record from the Montagnais impact structure, located on the shelf off Nova Scotia, has shown that the impact had neither regional nor global effects on biological diversity. This result provides new evidence for the lower threshold of extinctions due to the impacts, indicating that impacting bolides must be larger than 3 km in diameter to cause extinctions. We use this evidence to construct simplified curves to test relations between mass extinctions, bolide diameter, and impact periodicity. These data point to reoccurrence of mass extinctions of amplitude comparable to the Cretaceous/Paleogene event an average of once in every 100 to 500 m.y., whereas the probability of extinction of life on Earth triggered by an impact of a bolide larger than 60 km in diameter has an average frequency of once in 1 b.y. Data presented in this chapter do not support a direct parallel between the 26-m.y. extinction periodicity and the impact cratering record in Earth.