Крета-Палеоген жойылуына астероид соқтығысының себептігі

Asteroid impact hypothesis as cause of the Cretaceous–Paleogene extinction

The Alvarez hypothesis posits that the mass extinction of the non avian dinosaurs and many other living things during the Cretaceous–Paleogene extinction event was caused by the impact of a large asteroid on the Earth. Prior to 2013, it was commonly cited as having happened about 65 million years ago, but Renne and colleagues (2013) gave an updated value of 66 million years. Evidence indicates that the asteroid fell in the Yucatán Peninsula, at Chicxulub, Mexico. The hypothesis is named after the father and son team of scientists Luis and Walter Alvarez, who first suggested it in 1980. Shortly afterwards, and independently, the same was suggested by Dutch paleontologist Jan Smit. In March 2010, an international panel of scientists endorsed the asteroid hypothesis, specifically the Chicxulub impact, as being the cause of the extinction. A team of 41 scientists reviewed 20 years of scientific literature and in so doing also ruled out other theories such as massive volcanism. They had determined that a space rock in diameter hurtled into earth at Chicxulub. For comparison, the Martian moon Phobos has a diameter of , and Mount Everest is just under The collision would have released the same energy as , over a billion times the energy of the atomic bombs dropped on Hiroshima and Nagasaki. A 2016 drilling project into the peak ring of the crater strongly supported the hypothesis, and confirmed various matters that had been unclear until that point. These included the fact that the peak ring comprised granite (a rock found deep within the Earth) rather than typical sea floor rock, which had been shocked, melted, and ejected to the surface in minutes, and evidence of colossal seawater movement directly afterwards from sand deposits. Crucially, the cores also showed a near complete absence of gypsum, a sulfate containing rock, which would have been vaporized and dispersed as an aerosol into the atmosphere, confirming the presence of a probable link between the impact and global longer term effects on the climate and food chain.

In 1980, a team of researchers led by Nobel prize winning physicist Luis Alvarez, his son, geologist Walter Alvarez, and chemists Frank Asaro and Helen Vaughn Michel, discovered that sedimentary layers found all over the world at the Cretaceous–Paleogene boundary (K–Pg boundary, formerly called Cretaceous–Tertiary or K–T boundary) contain a concentration of iridium hundreds of times greater than normal. Previously, in a 1953 publication, geologists Allan O. Kelly and Frank Dachille analyzed global geological evidence suggesting that one or more giant asteroids impacted the Earth, causing an angular shift in its axis, global floods, firestorms, atmospheric occlusion, and the extinction of the dinosaurs. There were other earlier speculations on the possibility of an impact event, but without strong confirming evidence.

The location of the impact was unknown when the Alvarez team developed their hypothesis, but later scientists discovered the Chicxulub Crater in the Yucatán Peninsula, now considered the likely impact site. In April 2019, a paper was published in PNAS which describes evidence from a fossil site in North Dakota that the authors say provides a "postimpact snapshot" of events after the asteroid collision "including ejecta accretion and faunal mass death". The team found that the tektites that had peppered the area were present in amber found on the site and were also embedded in the gills of about 50 percent of the fossil fish. They were also able to find traces of iridium. The authors – who include Walter Alvarez – postulate that shock of the impact, equivalent to an earthquake of magnitude 10 or 11, may have led to seiches, oscillating movements of water in lakes, bays, or gulfs, that would have reached the site in North Dakota within minutes or hours of the impact. This would have led to the rapid burial of organisms under a thick layer of sediment. Coauthor David Burnham of the University of Kansas was quoted as saying "They’re not crushed, it’s like an avalanche that collapses almost like a liquid, then sets like concrete. They were killed pretty suddenly because of the violence of that water. We have one fish that hit a tree and was broken in half." According to a high resolution study of fossilized fish bones published in 2022, the Cretaceous Paleogene asteroid which caused mass extinction impacted during the Northern Hemisphere spring.

Although a 2010 paper published in Science that declared that the extinction of the dinosaurs was caused by Chicxulub was co authored by 41 scientists, dozens of other scientists challenged both the paper's methods and its conclusions. A leading critic of the Alvarez hypothesis is Gerta Keller, who has focused on Deccan Traps volcanism as a likely cause of a more gradual extinction. Despite the fact that the Alvarez hypothesis has overwhelming support from the scientific community, Keller has continued to advocate for research into alternate theories. The Deccan Traps are an area of volcanic flood basalts in Western India spanning ~1.3 million square kilometers that were created by massive volcanic activity during the same time period in which the Chicxulub impact occurred. Prior to Keller's research, the timeframe of the Deccan Traps' eruptions had a significantly large range of error, making it difficult to draw strong conclusions regarding their connection to the K Pg extinction. Along with ocean acidification, ozone reduction, acid rain, and a release of harmful gases, she asserts that these conditions were sufficient to have initiated the mass extinction. as well the more recent assertion that a tsunami generated by the impact created the unusual sediment layer. Keller additionally claims that the impact did not cause as much ecological damage as is widely believed,