Massive extinction occurred at the Cretaceous-Paleogene boundary about 65.5 million years ago. Researchers led by Dr. Luis Alvarez who was a winner of Nobel Prize in Physics have presented a hypothesis that the extinction was caused by an asteroid impact of 10 kilometer diameter. The theory has sparked an international controversy. A scientific circle has supported the hypothesis since the Chicxulub crater, an impact crater of 180 kilometer diameter at the latest Cretaceous, was discovered in Yucatan Peninsula, Mexico in1991. However, some researchers presented other theories such as Deccan Trap Theory　of powerful volcanic eruption and multiple impact theories. A group insisted that the Chicxulub impact had no relation to the massive extinction because the impact occurred 300.000 years before the extinction.

 An international research group led by Dr. Peter Schulte at Friedrich-Alexander Universität Erlangen-Nürnberg, FAU has reviewed the reported research results including geological evidences, physical properties of impact craters and numerical models. The group consists of 41 researchers from 12 countries in different fields such as geology, paleontology, geophysics and planetary science. The research group includes Assistant Professor Kazuhisa Goto at Disaster Control Research Center, Tohoku University and Director Takafumi Matsui at Planetary Exploration Research Center, Chiba Institute of Technology. They participated in the research as an expert in environment disturbances caused by impacts and a crater formation process, and have studied on the relation between the massive extinction and the Chicxulub impact.

 The results shows that multiple environmental changes caused by the Chicxulub impact can uniformly explain the mass extinction at the Cretaceous-Paleogene boundary. There are three evidences:

1) Geological layers at the latest Cretaceous reported at 350 places in the world include substances dating back to the Chicxulub impact. It is considered that the impact influenced on the whole earth because layers have systematically been thinner and impact origin substances have decreased in size around the Chicxulub crater.

2) The Chicxulub impact and the massive extinction occurred at the same period. The layers in the world show that the both occurrence times precisely coincided. It is clarified that the hypothesis that the Chicxulub impact has no relation to the extinction is groundless.

3) Numerical calculation shows an amount and atmospheric residence time of gases including dust, sulfate and soot emitted by the Chicxulub impact and forest fire. These may have caused photosynthesis inhibition of phytoplankton for a long time. Large creatures such as dinosaurs became extinct because they were unable to gain food due to extinction of photosynthetic organisms that were at the bottom of the food chain.

 Volcanic eruption has been continued for about a million years through the Cretaceous-Paleogene boundary. However, it had a miner impact on environments, and a temperature change at this period was less than 2 degrees C. When volcanic activities were the most intensified, massive extinction did not occur. The theory can not prove that volcanic activities were weak at the latest Cretaceous. There are other hypotheses that several giant impacts except the Chicxulub impact caused the massive extinction. However, it is assumed that the Chicxulub impact was the only giant impact occurred at this period based on a research to precisely measure iridium concentration for 10 million years through the latest Cretaceous.

 It has been considered that there have been no researchers to properly prove the Chicxulub impact theory because there were too many controversies over asteroid impacts at the Cretaceous-Paleogene boundary. It is noteworthy in this study that researchers in multidisciplinary fields such as geology, paleontology, geophysics and planetary science have got together to uniformly review theories and to show evidences of the massive extinction caused by the Chicxulub impact. The research is important for the science history in adopting a new method to settle a longtime scientific dispute. The research has ended a dispute over the mass extinction at the Cretaceous-Paleogene boundary in the commemorative year of the 30th anniversary of the Dr. Luis Alvarez’s discovery.

 It is clarified that a scale of supracrustal environment disturbances caused by the asteroid impact at the latest Cretaceous was large enough to trigger the mass extinction. However, its estimate accuracy has not been adequate. Integrated Ocean Drilling Program (IODP) is currently ongoing. The project is expected to help precisely explain details including the scale of the impact crater and the formation process.

 The research results have been published online in Science, U.S. academic journal, on March 5, 2010. The paper’s title is _The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary._ The research have been partly conducted with the aid of a Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Science.

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