Editing Chicxulub crater (section)

==Discovery==
In the late 1970s, geologist [[Walter Alvarez]] and his father, [[Nobel Prize]]–winning scientist [[Luis Walter Alvarez]], put forth their theory that the [[Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction]] was caused by an impact event.<ref name="Bates_1992"/><ref name="newyorker_2019-03-29"/> The main evidence of such an impact was contained in a thin layer of clay present in the [[Cretaceous–Paleogene boundary]] (K–Pg boundary) in [[Gubbio|Gubbio, Italy]]. The Alvarezes and colleagues reported that it contained an [[iridium anomaly|abnormally high concentration of iridium]], a chemical element rare on Earth but common in asteroids.<ref name="Bates_1992"/><ref>{{Cite conference |last1=Alvarez, W. |author-link=Walter Alvarez |last2=Alvarez, L.W. |author-link2=Luis Walter Alvarez |last3=Asaro, F. |last4=Michel, H.V. |year=1979 |editor2-last=Birkelund, T. |title=Anomalous iridium levels at the Cretaceous/Tertiary boundary at Gubbio, Italy: Negative results of tests for a supernova origin |location=[[University of Copenhagen]], Copenhagen, Denmark| volume=2 |pages=69 |book-title=Cretaceous/Tertiary Boundary Events Symposium |editor=Christensen, W.K.}}; {{cite journal | first1=Luann | last1=Becker | url=http://www.miracosta.edu/home/kmeldahl/articles/blows.pdf | title=Repeated Blows | access-date=January 28, 2016 | journal=Scientific American | year=2002 | volume=286 | issue=3 | pages=76–83 | bibcode=2002SciAm.286c..76B | doi=10.1038/scientificamerican0302-76 | pmid=11857903 | archive-date=December 8, 2003 | archive-url=https://web.archive.org/web/20031208144031/http://www.miracosta.edu/home/kmeldahl/articles/blows.pdf | url-status=live }}</ref> [[Iridium]] levels in this layer were as much as 160 times above the background level.<ref name="Alvarez et al-1980" /> It was hypothesized that the iridium was spread into the atmosphere when the impactor was [[vaporized]] and settled across Earth's surface among other material thrown up by the impact, producing the layer of iridium-enriched clay.<ref>{{Cite web |first=Hillary|last=Mayell |date=May 15, 2005 |title=Asteroid Rained Glass Over Entire Earth, Scientists Say |url=http://news.nationalgeographic.com/news/2005/04/0415_050418_chicxulub.html |url-status=dead |archive-url=https://web.archive.org/web/20160918074556/http://news.nationalgeographic.com/news/2005/04/0415_050418_chicxulub.html |archive-date=September 18, 2016 |access-date=October 1, 2007 |website=[[National Geographic Society|National Geographic]] News}}</ref> At the time, there was no consensus on what caused the Cretaceous–Paleogene extinction and the boundary layer, with theories including a nearby [[supernova]], [[climate change]], or a [[geomagnetic reversal]].<ref name="Alvarez et al-1980">{{Cite journal |last1=Alvarez |first1=Luis |last2=Alvarez |first2=Walter |last3=Asaro |first3=Frank |last4=Michel |first4=Helen |date=June 6, 1980 |title=Extraterrestrial Cause for the Cretaceous-Tertiary Extinction |journal=[[Science (journal)|Science]] |volume=208 |issue=4408 |pages=1095–1108 |bibcode=1980Sci...208.1095A |doi=10.1126/science.208.4448.1095 |issn=0036-8075 |pmid=17783054 |s2cid=16017767}}</ref>{{rp|1095}} The Alvarezes' impact hypothesis was rejected by many paleontologists, who believed that the lack of fossils found close to the K–Pg boundary—the "three-meter problem"—suggested a more gradual die-off of fossil species.<ref name="newyorker_2019-03-29" /><ref name="Alvarez_2008"/>

The Alvarezes, joined by [[Frank Asaro]] and [[Helen Vaughn Michel|Helen Michel]] from [[University of California, Berkeley]], published their paper on the iridium anomaly in ''[[Science (journal)|Science]]'' in June 1980.<ref name="Alvarez et al-1980" /> Almost simultaneously [[Jan Smit (paleontologist)|Jan Smit]] and Jan Hertogen published their iridium findings from Caravaca, Spain, in ''[[Nature (journal)|Nature]]'' in May 1980.<ref name="Smit and Hertogen-1980" /> These papers were followed by other reports of similar iridium spikes at the K–Pg boundary across the globe, and sparked wide interest in the cause of the K–Pg extinction; over 2,000 papers were published in the 1980s on the topic.<ref name="Alvarez_2008"/>{{rp|82}}<ref name="Weinreb_2002"/> There were no known impact craters that were the right age and size, spurring a search for a suitable candidate.<ref name="newyorker_2019-03-29"/> Recognizing the scope of the work, Lee Hunt and Lee Silver organized a cross-discipline meeting in [[Snowbird, Utah]], in 1981. Unknown to them, evidence of the crater they were looking for was being presented the same week, and would be largely missed by the scientific community.<ref name="Alvarez_2008"/>{{rp|83–84}}<ref name="Weinreb_2002"/>

[[File:Chicxulub impact - artist impression.jpg|thumb|alt=A painting depicting the asteroid impacting Earth, creating the Chicxulub crater| Artist's impression of the asteroid slamming into tropical, shallow seas of the sulfur-rich [[Yucatán Peninsula]] in what is today [[Geography of Mexico|Southeast Mexico]].<ref name="nhm.ac.uk">{{Cite web |last=Osterloff |first=Emily |year=2018 |title=How an asteroid ended the age of the dinosaurs |url=https://www.nhm.ac.uk/discover/how-an-asteroid-caused-extinction-of-dinosaurs.html |url-status=live |archive-url=https://web.archive.org/web/20220426174952/https://www.nhm.ac.uk/discover/how-an-asteroid-caused-extinction-of-dinosaurs.html |archive-date=April 26, 2022 |access-date=May 18, 2022 |publisher=[[Natural History Museum, London|Natural History Museum]] |location=London, England}}</ref> The aftermath of the asteroid collision, which occurred approximately 66 million years ago, is believed to have caused the [[Extinction event|mass extinction]] of non-avian dinosaurs and many other species on Earth.<ref name="nhm.ac.uk" /> The impact spewed hundreds of billions of tons of sulfur into the atmosphere, producing a worldwide blackout and freezing temperatures which persisted for at least a decade.<ref name="nhm.ac.uk" />]]
In 1978, geophysicists Glen Penfield and Antonio Camargo were working for the Mexican state-owned oil company Petróleos Mexicanos ([[Pemex]]) as part of an airborne magnetic survey of the [[Gulf of Mexico]] north of the [[Yucatán Peninsula]].<ref name="Verschuur_1996"/>{{rp|20–21}} Penfield's job was to use geophysical data to scout possible locations for oil drilling.<ref name="Bates_1992"/> In the offshore magnetic data, Penfield noted anomalies whose depth he estimated and mapped. He then obtained onshore [[gravity anomaly|gravity data]] from the 1940s. When the gravity maps and [[Magnetic anomaly|magnetic anomalies]] were compared, Penfield described a shallow "bullseye", {{convert|180|km|abbr=on|sp=us}} in diameter, appearing on the otherwise non-magnetic and uniform surroundings—clear evidence to him of an impact feature.<ref name="Bates_1992"/><ref name="Penfield">{{Cite journal |last=Penfield |first=Glen |year=2019 |title=Unlikely Impact |url=https://explorer.aapg.org/story?articleid=55293 |url-status=live |journal=AAPG Explorer |volume=40 |issue=12 |pages=20–23 |archive-url=https://web.archive.org/web/20210116205711/https://explorer.aapg.org/story?articleid=55293 |archive-date=January 16, 2021 |access-date=December 12, 2019}}</ref> A decade earlier, the same map had suggested a crater to contractor Robert Baltosser, but Pemex corporate policy prevented him from publicizing his conclusion.<ref name="Verschuur_1996"/>{{rp|20}}

Penfield presented his findings to Pemex, who rejected the crater theory, instead deferring to findings that ascribed the feature to volcanic activity.<ref name="Penfield" /> Pemex disallowed release of specific data, but let Penfield and Camargo present the results at the 1981 [[Society of Exploration Geophysicists]] conference.<ref name="Weinreb_2002"/> That year's conference was under-attended and their report attracted little attention, with many experts on impact craters and the K–Pg boundary attending the Snowbird conference instead. Carlos Byars, a ''[[Houston Chronicle]]'' journalist who was familiar with Penfield and had seen the gravitational and magnetic data himself, wrote a front-page story<ref>{{Cite news |last=Byars |first=Carlos |date=1981-12-13 |title=Mexican site may be a link to dinosaurs' disappearance |url=https://infoweb.newsbank.com/apps/news/openurl?ctx_ver=z39.88-2004&rft_id=info%3Asid/infoweb.newsbank.com&svc_dat=AMNEWS&req_dat=0D1C2A34C3EF45E0&rft_val_format=info%3Aofi/fmt%3Akev%3Amtx%3Actx&rft_dat=document_id%3Aimage%252Fv2%253A14DB39C1C40322B4%2540EANX-NB-1642F1C7C1E6C3CA%25402444952-1642EDDD95D04DD1%25400-1642EDDD95D04DD1%2540 |access-date=2024-09-01 |work=[[Houston Chronicle]] |pages=1, 18 |type=Newspaper [digital microfilm] |edition=Final |via=[[NewsBank]] |volume=81 |issue=61}}</ref> on Penfield and Camargo's claim, but the news did not propagate widely.<ref name="Verschuur_1996"/>{{rp|23}}

Although Penfield had plenty of geophysical data sets, he had no rock cores or other physical evidence of an impact.<ref name="Bates_1992"/> He knew Pemex had drilled exploratory wells in the region. In 1951, one well bored into what was described as a thick layer of [[andesite]] about {{convert|1.3|km|ft|sp=us}} down. This layer could have resulted from the intense heat and pressure of an Earth impact, but at the time of the borings it was dismissed as a [[lava dome]]—a feature uncharacteristic of the region's geology.<ref name="Bates_1992"/> Penfield was encouraged by [[William C. Phinney]], curator of [[lunar rock]]s at the [[Johnson Space Center]], to find these samples to support his hypothesis.<ref name="Penfield" /> Penfield tried to secure site samples, but was told they had been lost or destroyed. When attempts to return to the drill sites to look for corroborating rocks proved fruitless, Penfield abandoned his search, published his findings and returned to his Pemex work.<ref name="Bates_1992"/> Seeing the 1980 ''Science'' paper, Penfield wrote to Walter Alvarez about the Yucatán structure, but received no response.<ref name="Weinreb_2002"/>

Alvarez and other scientists continued their search for the crater, although they were searching in oceans based on incorrect analysis of glassy [[spherule]]s from the K–Pg boundary that suggested the impactor had landed in open water.<ref name="Alvarez_2008"/> Unaware of Penfield's discovery, [[University of Arizona]] graduate student [[Alan R. Hildebrand]] and faculty adviser William V. Boynton looked for a crater near the [[Brazos River]] in [[Texas]].<ref name="Alvarez_2008"/> Their evidence included greenish-brown clay with surplus iridium, containing [[shocked quartz]] grains and small weathered glass beads that looked to be [[tektite]]s.<ref name="Hildebrand et al_1991-09"/> Thick, jumbled deposits of coarse rock fragments were also present, thought to have been scoured from one place and deposited elsewhere by an impact event. Such deposits occur in many locations but seemed concentrated in the [[Caribbean Basin]] at the K–Pg boundary. When Haitian professor Florentine Morás discovered what he thought to be evidence of an ancient volcano on [[Haiti]], Hildebrand suggested it could be a telltale feature of a nearby impact. Tests on samples retrieved from the K–Pg boundary revealed more tektite glass, formed only in the heat of asteroid impacts and high-yield [[nuclear weapon|nuclear detonations]].<ref name="Bates_1992"/>

In 1990, Carlos Byars told Hildebrand of Penfield's earlier discovery of a possible impact crater.<ref>{{Cite book |last=Frankel, Charles |title=The End of the Dinosaurs: Chicxulub Crater and Mass Extinctions |date=1999 |publisher=[[Cambridge University Press]] |location=Cambridge, England|isbn=978-0-521-47447-4 |page=[https://archive.org/details/endofdinosaursch00fran/page/236 236]}}</ref>{{rp|50}} Hildebrand contacted Penfield and the pair soon secured two drill samples from the Pemex wells, which had been stored in [[New Orleans]] for decades.<ref name="Penfield" /> Hildebrand's team tested the samples, which clearly showed [[shock metamorphism|shock-metamorphic]] materials.<ref name="Bates_1992"/> A team of California researchers surveying satellite images found a [[cenote]] ([[sinkhole]]) ring centered on the town of [[Chicxulub Pueblo]] that matched the one Penfield saw earlier; the cenotes were thought to be caused by [[subsidence]] of [[bolide]]-weakened [[lithostratigraphy]] around the impact crater wall.<ref>{{Cite journal |last1=Pope KO |last2=Baines, K.H. |last3=Ocampo, A.C. |last4=Ivanov, B.A. |year=1997 |title=Energy, volatile production, and climatic effects of the Chicxulub Cretaceous/Tertiary impact |journal=[[Journal of Geophysical Research]] |publisher=[[American Geophysical Union]] |location=Washington, D.C.| volume=102 |issue=E9 |pages=245–264 |bibcode=1997JGR...10221645P |doi=10.1029/97JE01743 |pmid=11541145 |doi-access=free}}</ref> More recent evidence suggests the crater is {{convert|300|km|abbr=on}} wide, and the {{convert|180|km|adj=on|abbr=on|sp=us}} ring observed is an inner wall of the larger crater.<ref>{{Cite journal |first1=Vernon L.|last1=Sharpton  |first2=Luis E.|last2=Marin |date=May 1997 |title=The Cretaceous–Tertiary impact crater and the cosmic projectile that produced it |journal=[[Annals of the New York Academy of Sciences]] | publisher=[[Wiley-Blackwell]]| location=New York City| volume=822 |issue=1 |pages=353–380 |bibcode=1997NYASA.822..353S |doi=10.1111/j.1749-6632.1997.tb48351.x |pmid=11543120 |s2cid=11962090}}</ref> Hildebrand, Penfield, Boynton, Camargo, and others published their paper identifying the crater in 1991.<ref name="Alvarez_2008"/><ref name="Hildebrand et al_1991-09"/> The crater was named for the nearby town of [[Chicxulub Pueblo]]. Penfield also recalled that part of the motivation for the name was "to give the academics and NASA naysayers a challenging time pronouncing it" after years of dismissing its existence.<ref name="Penfield" />

In March 2010, forty-one experts from many countries reviewed the available evidence: twenty years' worth of data spanning a variety of fields. They concluded that the impact at Chicxulub triggered the mass extinctions at the K–Pg boundary.<ref name="newyorker_2019-03-29"/><ref name="SchulteOthers2010">{{Cite journal |last1=Schulte |first1=P. |last2=Alegret |first2=L. |last3=Arenillas |first3=I. |last4=Arz |first4=J. A. |last5=Barton |first5=P.J. |last6=Bown |first6=P.R. |last7=Bralower |first7=T.J. |last8=Christeson |first8=G.L. |last9=Claeys |first9=P. |display-authors=3 |year=2010 |title=The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary |url=http://www3.nd.edu/~cneal/CRN_Papers/Schulte10_Sci_Chicxulub.pdf |journal=Science |volume=327 |issue=5970 |pages=1214–1218 |doi=10.1126/science.1177265 |issn=0036-8075 |archive-url=https://web.archive.org/web/20111209132114/http://www.nd.edu/~cneal/CRN_Papers/Schulte10_Sci_Chicxulub.pdf |archive-date=December 9, 2011 |access-date=December 9, 2016 |last10=Cockell |first10=C.S. |last11=Collins |first11=G.S. |last12=Deutsch |first12=A. |last13=Goldin |first13=T.J. |last14=Goto |first14=K. |last15=Grajales-Nishimura |first15=J. M. |last16=Grieve |first16=R.A.F. |last17=Gulick |first17=S.P.S. |last18=Johnson |first18=K.R. |last19=Kiessling |first19=W. |last20=Koeberl |first20=C. |last21=Kring |first21=D. A. |last22=MacLeod |first22=K.G. |last23=Matsui |first23=T. |last24=Melosh |first24=J. |last25=Montanari |first25=A. |last26=Morgan |first26=J.V.|author26-link= Joanna Morgan |last27=Neal |first27=C.R. |last28=Nichols |first28=D.J. |last29=Norris |first29=R. D. |last30=Pierazzo |first30=E. |last31=Ravizza |first31=G. |last32=Rebolledo-Vieyra |first32=M. |last33=Reimold |first33=W.U. |last34=Robin |first34=E. |last35=Salge |first35=T. |last36=Speijer |first36=R.P. |last37=Sweet |first37=A.R. |last38=Urrutia-Fucugauchi |first38=J. |last39=Vajda |first39=V. |last40=Whalen |first40=M.T. |last41=Willumsen |first41=P.S. |pmid=20203042|bibcode=2010Sci...327.1214S |s2cid=2659741 }}; {{Cite news |last=Rincon, Paul |date=March 4, 2010 |title=Dinosaur extinction link to crater confirmed |publisher=[[BBC]] |url=http://news.bbc.co.uk/2/hi/science/nature/8550504.stm |url-status=live |access-date=March 5, 2010 |archive-url=https://web.archive.org/web/20191031034913/http://news.bbc.co.uk/2/hi/science/nature/8550504.stm |archive-date=October 31, 2019}}</ref> Dissenters, notably [[Gerta Keller]] of [[Princeton University]], have proposed an alternate culprit: the eruption of the [[Deccan Traps]] in what is now the [[Indian subcontinent]]. This period of intense [[volcanism]] occurred before and after the Chicxulub impact;<ref name="newyorker_2019-03-29"/><ref name="Keller et al-2020">{{Cite journal |last1=Keller |first1=Gerta |last2=Mateo |first2=Paula |last3=Monkenbusch |first3=Johannes |last4=Thibault |first4=Nicolas |last5=Punekar |first5=Jahnavi |last6=Spangenberg |first6=Jorge E. |last7=Abramovich |first7=Sigal |last8=Ashckenazi-Polivoda |first8=Sarit |last9=Schoene |first9=Blair |last10=Eddy |first10=Michael P. |last11=Samperton |first11=Kyle M. |last12=Khadri |first12=Syed F.R. |last13=Adatte |first13=Thierry |display-authors=3 |date=November 2020 |title=Mercury linked to Deccan Traps volcanism, climate change and the end-Cretaceous mass extinction |journal=Global and Planetary Change |volume=194 |pages=103312 |bibcode=2020GPC...19403312K |doi=10.1016/j.gloplacha.2020.103312 |s2cid=225275560|doi-access=free }}</ref> dissenting studies argue that the worst of the volcanic activity occurred ''before'' the impact, and the role of the Deccan Traps was instead shaping the evolution of surviving species post-impact.<ref>{{Cite journal |last1=Hull |first1=Pincelli M. |last2=Bornemann |first2=André |last3=Penman |first3=Donald E. |last4=Henehan |first4=Michael J. |last5=Norris |first5=Richard D. |last6=Wilson |first6=Paul A. |last7=Blum |first7=Peter |last8=Alegret |first8=Laia |last9=Batenburg |first9=Sietske J. |last10=Bown |first10=Paul R. |last11=Bralower |first11=Timothy J. |display-authors=3 |date=January 17, 2020 |title=On impact and volcanism across the Cretaceous-Paleogene boundary |journal=Science |language=en |volume=367 |issue=6475 |pages=266–272 |bibcode=2020Sci...367..266H |doi=10.1126/science.aay5055 |issn=0036-8075 |pmid=31949074 |s2cid=210698721|doi-access=free |hdl=20.500.11820/483a2e77-318f-476a-8fec-33a45fbdc90b |hdl-access=free }}</ref> A 2013 study compared [[isotope]]s in [[impact glass]] from the Chicxulub impact with isotopes in ash from the K–Pg boundary, concluding that they were dated almost exactly the same, and within experimental error.<ref name="RenneDeino2013"/>