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The structure of the earth's history is based on changes in fauna and flora. The transition between the Mesozoic Era and the Cenozoic Era marks the end of the dinosaurs and is of great interest. Strictly speaking, it is the Cretaceous-Tertiary boundary (K/T) - named after the Cretaceous (youngest part of the Mesozoic) and the Tertiary (now Paleogene, oldest part of the Cenozoic) 66 million years ago. At that time, around three quarters of all animal and plant species died.
"It was in 1980 when the physicist Luis Walter Alvarez and his son, the geologist Walter Alvarez, found a few centimeters thick, yellowish clay layer ('boundary clay') with strikingly high iridium values near Gubbio in Umbria," begins Christian Köberl, geochemist and impact researcher at the University of Vienna, in his comments for the book "Science Adventures - Explorers between the Alps, the Orient and the Arctic Ocean".
On June 6, 1980, the paper that began the search for the cause of the iridium anomaly appeared in "Science" under the title "Extraterrestrial causes for the Cretaceous–Tertiary extinction." The authors postulated that the iridium could come from enrichment in seawater or - more likely - from an extraterrestrial event. In the original text they concluded: "In brief, our hypothesis suggests that an asteroid struck the earth, formed an impact crater, and some of the dust-sized material ejected from the crater reached the stratosphere and was spread around the globe." The thesis put forward at the time seemed entirely plausible, but that wasn't enough: "We would like to find the crater produced by the impacting object."
While a gigantic volcanic eruption was initially discussed as a possible cause of the iridium anomaly, everything became clear in 1984 with the discovery of shocked quartz grains in the "boundary clay" by Bruce F. Bohor (USA): it must have been an impact. Shocked quartz has a deformed crystal lattice and is formed under extremely high pressure, which only occurs on Earth during meteorite impacts. The "boundary clay" is also found in Austria: in Gams near Hieflau (Styria) and in Elendgraben near Gosau (Upper Austria). If you want something more convenient, you can also find a large piece of rock with "boundary clay" in the Natural History Museum in Vienna (room 8).
When Bevan M. French (NASA) discovered shocked quartz in eastern Montana (USA), he also thought he had found the possible impact crater: the Manson crater in Iowa (USA). Again it was the magazine "Science" where his thesis - "Impact Event at the Cretaceous-Tertiary Boundary: A Possible Site" - could be read on October 19, 1984. He recommended exploring the Manson Crater in more detail: “Should be studied in more detail.” And that's how it happened.
In addition to the Manson crater, the research results of which were ultimately published by Köberl together with Raymond R. Anderson in 1996, the Kara crater in northern Russia, hard on the coast of the Arctic Ocean, was also a topic for research. At 70 million years old, it was too old, but still of interest. Köberl published the results in 1990 together with colleagues.
In parallel with this research, there was increasing evidence that it could be the Chicxulub crater on the Yucatán Peninsula where the asteroid had hit.
As early as the 1970s, Glen T. Penfield and Antonio Camargo-Zanoguera from the Mexican oil company Pemex had used geophysical methods to find a huge circular structure underground in the Gulf of Mexico and on the Yucatán mainland. They even thought of a meteorite impact at the time. Only after reading the above-mentioned work by Alvarez father and son and co-authors in "Science" (1980) did they go to the professional public at a conference in Tulsa (Oklahoma, USA) in 1981.
Here the science journalist Carlos Byars heard her talk, took up her results and wrote "Mexican Site May be Link to Dinosaur's Disappearance" in 1981. But no one read it. “The message went past the right experts, the impact researchers,” said Köberl.
The decisive step only took place in 1991. Alan Hildebrand, a doctoral student from Arizona, had studied the specialist literature and, with Penfield and Camargo-Zanoguera, published the thesis in the journal "Geology" that the structure in question, with a diameter of 180 kilometers, must have been the result of an impact. They relied on a few samples from previous oil wells where they found shocked quartz. They concluded: "This impact may have caused the K-T extinctions."
“Scientific drilling was now required,” said Köberl. In the spring of 1995, the University of Mexico (UNAM) undertook two 700-meter-deep drillings that uncovered breccias (rubble rocks) typical of an impact. Köberl says again: "These samples were hardly accessible to the international professional public; the Mexicans had their hands on them." This was followed in 2001/02 by deeper drilling on the southern edge of the crater, which was on the mainland, a cooperation between the Mexican university and international experts (International Continental Scientific Drilling Program, ICDP). “But we wanted to know what it looks like in the center of the crater? Its ring structures are unique in the world,” said Köberl, who was one of the applicants for the third drilling program. This drilling (Chicx-03b), a collaboration between ICDP and ECORD (European Consortium for Ocean Research Drilling), as part of IODP (Integrated Ocean Discovery Program), ran from April 5 to May 31, 2016 under the name "Expedition 364, Chicxulub K-Pg Impact Crater". Drilling took place in shallow water (20 meters) to a final depth of 1,334.69 meters.
The results: Scientific and understandable
The drilling samples from 2016 are located in Bremen and are available to the scientific community. Ludovic Ferrière, an expert from the Natural History Museum, was there to describe the rock samples in detail. "I myself wouldn't have been able to go to Bremen for a month as general director back then," Köberl formally apologizes. Only one of the results of the drilling is highlighted. As part of his dissertation at the University of Vienna, Jean-Guillaume Feignon calculated the pressure of the asteroid impact in the Chicxulub crater to be twelve to 15 GPa (gigapascals). Not only quartz, but also feldspars, apatites and titanites were shocked, i.e. deformed in the crystal lattice. New studies look at the angle of impact, as recently shown in STANDARD under the title "Dino-Killer: Asteroid hit the Earth at the 'deadliest angle'".
Köberl and Alwin Schönberger have summarized further information about the Chicxulub crater and impacts in general in their book "Attention Rockfall. Asteroids & Meteorites. Deadly Danger and Cradle of Life". (Thomas Hofmann, June 6, 2020)