Cretaceous–Paleogene boundary

The Cretaceous–Paleogene boundary, often called the K–Pg boundary, is a critical geological marker. It represents the transition between the Cretaceous Period and the Paleogene Period. This boundary occurred approximately 66 million years ago.

It marks the end of the Mesozoic Era and the start of the Cenozoic Era. Most importantly, this boundary is associated with a mass extinction event. This event destroyed roughly 75% of all plant and animal species on Earth. This included all non-avian dinosaurs, which are the dinosaurs that did not evolve into birds.

Scientists identify this boundary by looking for a specific signature in rock layers. This signature is usually a thin band of clay containing high levels of iridium. Iridium is a metal that is very rare in the Earth's crust.

The presence of so much iridium suggests that material from space arrived at this time. In 1980, a research team led by physicist Luis Alvarez and geologist Walter Alvarez discovered this phenomenon. They worked with chemists Frank Asaro and Helen Vaughn Michel. Their findings led to the Alvarez hypothesis. This theory suggests that a massive impact event triggered global climate disruption.

The most widely accepted cause for this extinction is the Chicxulub impact. A large asteroid or comet, known as the Chicxulub impactor, struck the Earth. The impact site is a massive crater buried under the Yucatán Peninsula in Mexico.

Geophysicists Antonio Camargo and Glen Penfield discovered the crater while searching for petroleum in the late 1970s. The crater is enormous in both diameter and depth. It is the second-largest confirmed impact structure on Earth. Scientists have even drilled into its peak ring to collect rock samples. These samples help confirm the devastating effects of the collision.

The impact caused several immediate and deadly environmental changes. Because the asteroid hit a coastal area, it likely triggered gigantic tsunamis. Evidence of these waves exists in marine sediments across the Caribbean and the eastern United States. Furthermore, the impactor landed in a bed of anhydrite or gypsum. These minerals contain sulfur. The impact ejected large amounts of sulfur trioxide into the atmosphere. This combined with water to create a sulfuric acid aerosol. This resulted in planet-wide acid rain. This acid rain killed many organisms, including plants and plankton. It also harmed creatures that build shells from calcium carbonate, such as molluscs.

Other geological factors may have contributed to the extinction. One major factor involves the Deccan Traps in India. These are massive flood basalt eruptions. Some evidence suggests these eruptions were very active around the time of the impact. They may have released dust and sulfur into the air, blocking sunlight. This would have reduced photosynthesis in plants. They also released carbon dioxide, which increases the greenhouse effect. Some scientists believe the seismic waves from the Chicxulub impact might have even triggered more rapid eruptions in the Deccan Traps. This suggests that multiple catastrophes may have worked together.

Sea level changes also played a role during the late Cretaceous. This process is known as a marine regression, where sea levels fall or sediment builds out. During the Maastrichtian stage, sea levels fell more than at any other time in the Mesozoic Era. This reduced the area of the continental shelf. The continental shelf is the most species-rich part of the ocean. Losing this habitat likely caused a marine mass extinction. This change also altered ocean currents and winds. It also expanded freshwater environments, which changed the balance of life between land and sea.

Scientists continue to investigate other possibilities and mysteries. Some researchers have looked at the possibility of multiple impacts occurring at once. They point to other craters, like the Boltysh crater in Ukraine, formed around the same time. There is also the potential Shiva crater near India, though it is not yet universally accepted as an impact site. Other theories, such as a nearby supernova or the "Verneshot" volcanic hypothesis, have been proposed. However, many of these lack the strong evidence found for the Chicxulub impact. The K–Pg boundary remains a central focus for understanding how massive changes shape life on Earth.