Cretaceous-Tertiary Boundary

The 66-million-year-old stratigraphic boundary between the end of the Cretaceous Period and the beginning of the Tertiary Period. This boundary also marks the end of the Mesozoic Era and the beginning of the  Cenozoic Era. The K-T boundary is now also called the K-Pg (Cretaceous–Paleogene) boundary, because of a recent stratigraphic revision. In many places the boundary is marked by a distinctive clay layer, often enriched in Iridium (Ir) relative to the layers above and below. The K-T boundary marks a global extinction event at 66 Ma (or more precisely 66.043 ± 0.011 Ma) famous for killing off most of the dinosaurs (except birds, of course!) and two-thirds of all species on Earth. However, small mammals, turtles, crocodiles, birds, redwood trees and many others species survived. There is compelling evidence that a massive asteroid hit Earth 66 Ma leaving behind the large (150-180 km wide and 20 km deep) Chixculub Crater off the coast of Mexico, along with disturbed geologic deposits (iridium and shocked quartz) consistent with an asteroid impact.

The impact would have immediately initiated tidal waves, earthquakes, forest fires, and other natural disasters around the world. Then, the emission of dust and particles would have blotted out the sun for months and may have covered the entire surface of the Earth for years or even an entire decade. Most damaging to life on Earth would have been the shock production of carbon dioxide, a greenhouse gas, released by the destruction of carbonate rocks. Over a decade or longer, sunlight would have been blocked from reaching the surface of the Earth, not only cooling the surface dramatically but also disrupting the growth of plants, algae and certain bacteria that rely on photosynthesis. In effect, the impact caused an extinction event not just from the immediate impact itself, but from the long lasting effects that caused the destruction of the lowest levels of the foods chain.

Some or all content above used with permission from J. H. Wittke.