A theory of terrestrial catastrophism

It is argued that the passage of the solar system through galactic spiral arms containing planetesimals may be responsible for the solar system cratering flux record, the current distribution of comets and the existence of many short-lived bodies in the solar system, as well as many biostratigraphically recognizable events in earth history. The steep decline in the cratering rate of the moon and inner planets deduced for the solar system's first two billion years, brief periods of enhanced bombardment and the constant cratering rate for the last three billion years are discussed in terms of the decay of primordial bodies and the capture of planetesimals into the Oort cloud upon the sun's passage through Gould's belt of the Orion arm. The existence of interstellar planetesimals may have implications for the origins of the solar system and the spiral arms, and it is proposed that, in addition to the comets, many dynamically evolving asteroids and satellites within the solar system may be the survivors of capture episodes. Temporal coincidences between the ages of large terrestrial impact craters and charges in stratigraphy patterns indicating transitions between geological periods, catastrophic extinctions and ice ages are presented.