A geochemical perspective on the causes and periodicity of mass extinctions

Environmental change, including changes in biogeochemical cycles, climate, and sea level, is the primary cause of extinctions that result from mechanisms external to evolutionary dynamics. Evidence that extraordinary tectonism, including volcanism, sea-floor spreading, and eustatic sea level changes, took place prior to and at the Cretaceous-Tertiary boundary (K-TB) is sufficient to account for the environmental changes that led to mass extinctions. A coincident impact of an extraterrestrial object cannot be conclusively ruled out. Some mineralogic evidence suggests a scenario that includes impacts, but this does not rule out tectonism. The K-TB is certainly the best studied and most often discussed extinction boundary, but study of other extinction episodes and other potential extinction causes will now shed more light on mechanisms than continued study of the K-TB.