Mass Extinction

The classical explanation for the lesser and greater mass extinctions of life included climatic change (global greenhouse to icehouse shifts), sea-level shift, extensive volcanic activity (with resultant damage to atmosphere and ocean systems), disease, and plate-tectonic continental motion (convergence or divergence of land masses). With the 1980 publication of a widely cited paper in the journal Science (by American physicist L.W. Alvarez and others) that showed strong evidence of comet or asteroid impact at the 65-million year old mass extinction boundary (the Cretaceous-Tertiary boundary), many investigations shifted toward possible cosmic impact as the cause of other mass extinctions. In a 1994 paper, geologists M.R. Rampino and B.M. Haggerty marshaled evidence that at least 16 of the 23 extinction peaks identified by J.J. Sepkoski were either strongly connected to impact events (with known craters on Earth) or indirectly connected to impacts on Earth (e.g., chemical traces of impact dust or shocked materials). Subsequent work on this issue has increased the number of mass extinction peaks associated with known impacts on Earth. In particular, there is now strong to good evidence of major impact events (perhaps more than one impact per extinction) at three of the great mass extinction events (i.e., the Permian-Triassic boundary 245 million years ago; impact dust and shocked materials in some places); the Triassic-Jurassic boundary (208 million years ago; same age as Manicouagan crater, Canada); and the Cretaceous-Tertiary boundary (65 million years ago; same age as Chicxulub crater in Mexico and Boltysh crater in Ukraine and global clay layer with impact dust and shocked materials).

Whether the main cause of mass extinctions, especially great mass extinctions over geologic time is more likely to be cosmic impacts or some more Earth-bound factor, the fossil record clearly shows that they have occurred in the past. There is no reason to assume that they will not happen on Earth in the future. An understanding of these events, by careful study of the stratigraphic and paleontologic record, may help scientists better understand how to care for the world as it is known today. It has been suggested that the modern rate of species loss on Earth is comparable to the great mass extinctions of the past. While it is difficult to relate observations of modern faunal and floral species loss to observations taken from the fossil record, there is a warning for all in the record of the past. Extinction is forever, and mass extinctions profoundly change the faunal and floral characteristics of Earth's ecosystems after they occur.