Chapter 4: Catastrophe: Impact of comets and asteroids

The size distribution of comets and asteroids that move through the Solar System on Earth-crossing orbits, and the record of impact craters on the terrestrial planets can be used to determine the average intervals among impacts of various sizes. The apparent coincidence of the impact (or impacts) with the abrupt extinctions, including that of the dinosaurs, and the possibility that climatic and environmental changes caused by the impact led to the extinctions, revived interest in the physical effects of large impact events. The carbon cycle involves the transfer of carbon between the solid Earth and the ocean/atmosphere system. Carbon dioxide releases associated with metamorphic decarbonation of sediments at subduction zones, mantle out gassing at mid-ocean ridges and the chemical weathering of carbonate rocks, represent the primary sources of carbon dioxide to the oceans and atmosphere. The mix of species can be quite variable from one site to another, and thus differences in the depth zone in which calcification takes place among calcareous plankton and species-specific vital effects in isotope fractionation, can affect the isotopic composition of bulk material.