Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary

Impact Dating The large mass extinction of terrestrial and marine life—most notably, non-avian dinosaurs—occurred around 66 million years ago, at the boundary between the Cretaceous and Paleogene periods. But attributing the cause to a large asteroid impact depends on precisely dating material from the impact with indicators of ecological stress and environmental change in the rock record. Renne et al. (p. 684 ; see the Perspective by Pälike ) acquired high-precision radiometric dates of stratigraphic layers surrounding the boundary, demonstrating that the impact occurred within 33,000 years of the mass extinction. The data also constrain the length of time in which the atmospheric carbon cycle was severely disrupted to less than 5000 years. Because the climate in the late Cretaceous was becoming unstable, the large-impact event appears to have triggered a state-shift in an already stressed global ecosystem.