View source for Earth:Cretaceous–Paleogene boundary

{{Short description|Geological formation between time periods}}
[[File:KT boundary 054.jpg|thumb|300px|Badlands near Drumheller, Alberta, Canada, where glacial and post-glacial erosion have exposed the K–Pg boundary along with much other sedimentation (the exact boundary is a thin line not obviously visible).]]
[[Image:Cretaceous Paleogene clay at Geulhemmergroeve.jpg|thumb|Complex Cretaceous-Paleogene clay layer (gray) in the Geulhemmergroeve tunnels near Geulhem, the Netherlands. Finger is on the actual K–Pg boundary.]]
The '''Cretaceous–Paleogene '''('''K–Pg''')''' boundary''', formerly known as the '''Cretaceous–Tertiary '''('''K–T''')''' boundary''',{{efn|This former designation has as a part of it a term, '[[Earth:Tertiary|Tertiary]]' (abbreviated as '''T'''), that is now discouraged as a formal geochronological unit by the [[Organization:International Commission on Stratigraphy|International Commission on Stratigraphy]].<ref>{{cite book |editor1-last=Gradstein |editor1-first=Felix M. |editor2-last=Ogg |editor2-first=James G. |editor3-last=Smith |editor3-first=Alan G. |year=2004 |title=A geologic time scale 2004 |location=Cambridge, UK |publisher=Cambridge University Press |isbn=978-0-521-78142-8 }}</ref>}} is a [[Earth:Geology|geological]] [[Earth:Stratigraphy|signature]], usually a thin [[Earth:Stratum|band]] of rock containing much more [[Chemistry:Iridium|iridium]] than other bands. The K–Pg boundary marks the end of the [[Earth:Cretaceous|Cretaceous]] Period, the last period of the [[Earth:Mesozoic|Mesozoic]] [[Physics:Era#Geological era|Era]], and marks the beginning of the [[Earth:Paleogene|Paleogene]] Period, the first period of the [[Earth:Cenozoic|Cenozoic]] Era. Its age is usually estimated at 66 million years,<ref>{{cite web|url=http://stratigraphy.org/ICSchart/ChronostratChart2013-01.pdf|title=International Chronostratigraphic Chart|publisher=International Commission on Stratigraphy|year=2012|access-date=2013-12-18|url-status=dead|archive-url=https://web.archive.org/web/20130717121504/http://stratigraphy.org/ICSchart/ChronostratChart2013-01.pdf|archive-date=2013-07-17}}</ref> with [[Physics:Radiometric dating|radiometric dating]] yielding a more precise age of 66.043 ± 0.011 Ma.<ref name=":0">{{Cite journal|title = Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary|author1=Renne |date = 2013|journal = Science|doi = 10.1126/science.1230492|pmid = 23393261|display-authors=0 |volume=339 |issue = 6120|pages=684–7|bibcode=2013Sci...339..684R|s2cid = 6112274}}</ref>

The K–Pg boundary is associated with the [[Earth:Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction event]], a [[Biology:Extinction event|mass extinction]] which destroyed a majority of the world's Mesozoic species, including all [[Biology:Dinosaur|dinosaur]]s except for [[Biology:Bird|bird]]s.<ref name="Fortey">{{cite book|author=Fortey, R|title=Life: A Natural History of the First Four Billion Years of Life on Earth|publisher=Vintage|year=1999|pages=238–260|isbn=978-0-375-70261-7}}</ref>

Strong evidence exists that the extinction coincided with a large meteorite impact at the [[Earth:Chicxulub crater|Chicxulub crater]] and the generally accepted scientific theory is that this impact triggered the extinction event.

The word "Cretaceous" is derived from the Latin "creta" (chalk). It is abbreviated K (as in "K–Pg boundary") for its German translation "Kreide" (chalk).<ref>{{Cite web|url=https://www.geologypage.com/2014/04/cretaceous-period.html|title = Cretaceous Period|date = 15 April 2014}}</ref>

==Proposed causes==
===Chicxulub crater===
{{Infobox terrestrial impact site
| name = Chicxulub crater
| other_name = Chicxulub impact structure
| photo = Yucatan chix crater.jpg
| photo_caption = Imaging from [[Organization:NASA|NASA]]'s Shuttle Radar Topography Mission [[Engineering:STS-99|STS-99]] reveals part of the {{convert|180|km|abbr=on}} diameter ring of the crater. The numerous [[Earth:Sinkhole|sinkhole]]s clustered around the trough of the crater suggest a prehistoric [[Earth:Oceanic basin|oceanic basin]] in the depression left by the impact.<ref name="NASA PIA03379">{{cite web|title=PIA03379: Shaded Relief with Height as Color, Yucatan Peninsula, Mexico|url=http://photojournal.jpl.nasa.gov/catalog/PIA03379|work=[[Earth:Shuttle Radar Topography Mission|Shuttle Radar Topography Mission]]|publisher=[[Organization:NASA|NASA]]|access-date=October 28, 2010}}</ref>
| map = North America
| map_caption = Location of Chicxulub crater
| coordinates ={{coord|21|24|0|N|89|31|0|W|type:landmark_scale:5000000_region:MX|display=inline}}
| confidence = Confirmed
| diameter = {{convert|150|km|abbr=on}}
| depth = {{convert|20|km|abbr=on}}
| age = 66.043 ± 0.011 Ma <br/>Cretaceous–Paleogene boundary<ref name="RenneDeino2013">{{cite journal|last1=Renne|first1=P. R.|last2=Deino|first2=A. L.|last3=Hilgen|first3=F. J.|last4=Kuiper|first4=K. F.|last5=Mark|first5=D. F.|last6=Mitchell|first6=W. S.|last7=Morgan|first7=L. E.|last8=Mundil|first8=R.|last9=Smit|first9=J.|title=Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary|journal=Science|volume=339|issue=6120|year=2013|pages=684–687|issn=0036-8075|doi=10.1126/science.1230492|pmid=23393261|bibcode=2013Sci...339..684R|s2cid=6112274|url=https://eps.harvard.edu/files/eps/files/renne.kt_.science.2013.pdf}}</ref>
| exposed = No
| drilled = Yes
| imp_size = {{convert|10–15|km|abbr=on}}
| bolide = Carbonaceous chondrite
| country = {{MEX}}
| state = Yucatán
}}

{{main|Earth:Chicxulub crater}}

[[File:LWA with Walt.JPG|thumb|right|[[Biography:Luis Walter Alvarez|Luis]] (left) and his son Walter Alvarez (right) at the K-T Boundary in Gubbio, Italy, 1981]]

In 1980, a team of researchers led by Nobel prize-winning physicist [[Biography:Luis Walter Alvarez|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 contain a [[Chemistry:Concentration|concentration]] of [[Chemistry:Iridium|iridium]] hundreds of times greater than normal. They suggested that this layer was evidence of an impact event that triggered worldwide [[Astronomy:Impact winter|climate disruption]] and caused the [[Earth:Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction event]], a mass extinction in which 75% of plant and animal species on Earth suddenly became extinct, including all non-[[Biology:Bird|avian]] [[Biology:Dinosaur|dinosaur]]s.<ref name="Alvarez">{{cite journal |last1=Alvarez |first1=L.W. |last2=Alvarez |first2=W. |last3=Asaro |first3=F. |last4=Michel |first4=H. V. |year=1980 |title=Extraterrestrial cause for the Cretaceous–Tertiary extinction |journal=Science |volume=208 |issue=4448 |pages=1095–1108 |bibcode=1980Sci...208.1095A |doi=10.1126/science.208.4448.1095 |pmid=17783054|citeseerx=10.1.1.126.8496 |s2cid=16017767 }}</ref>

When it was originally proposed, one issue with the "[[Biology:Alvarez hypothesis|Alvarez hypothesis]]" (as it came to be known) was that no documented crater matched the event. This was not a lethal blow to the theory; while the crater resulting from the impact would have been larger than {{convert|250|km|abbr=on}} in diameter, Earth's geological processes hide or destroy craters over time.<ref name="Keller">{{cite journal|vauthors=Keller G, Adatte T, Stinnesbeck W, Rebolledo-Vieyra, Fucugauchi JU, Kramar U, Stüben D|year=2004|title=Chicxulub impact predates the K-T boundary mass extinction|journal=PNAS|volume=101|pages=3753–3758|doi=10.1073/pnas.0400396101|pmid=15004276|issue=11|pmc=374316 |bibcode = 2004PNAS..101.3753K |doi-access=free}}</ref>

The Chicxulub crater is an [[Astronomy:Impact crater|impact crater]] buried underneath the Yucatán Peninsula in Mexico.<ref name="eid">{{Cite Earth Impact DB | name = Chicxulub | access-date = December 30, 2008 }}</ref> Its center is located near the town of Chicxulub, after which the crater is named.<ref name=penfield>Penfield, Glen. Interview: ''The Dinosaurs: Death of the Dinosaur''. 1992, WHYY.</ref> It was formed by a large [[Astronomy:Asteroid|asteroid]] or [[Astronomy:Comet|comet]] about {{convert|10–15|km|abbr=on}} in diameter,<ref name="science" >{{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=1214–18 |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=9 December 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. |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 }}</ref><ref>{{cite news|url=https://www.bbc.com/news/science-environment-39922998|title=Dinosaur asteroid hit 'worst possible place'|first=Jonathan|last=Amos|date=May 15, 2017|work=BBC News}}</ref> the [[Astronomy:Chicxulub impactor|Chicxulub impactor]], striking the Earth. The date of the impact coincides precisely with the Cretaceous–Paleogene boundary (K–Pg boundary), slightly more than 66 million years ago.<ref name=RenneDeino2013/>

The crater is estimated to be over {{convert|150|km|abbr=on}} in diameter<ref name=eid/> and {{convert|20|km|abbr=on}} in depth, well into the [[Earth:Continental crust|continental crust]] of the region of about {{convert|10–30|km|abbr=on}} depth. It makes the feature the second of the [[Earth:List of impact craters on Earth#Confirmed impact craters listed by size and age|largest confirmed impact structures on Earth]], and the only one whose peak ring is intact and directly accessible for scientific research.<ref name="nyt1">{{cite news|url=https://www.nytimes.com/2016/11/18/science/chicxulub-crater-dinosaur-extinction.html|author=St. Fleur, Nicholas|work=The New York Times|title=Drilling Into the Chicxulub Crater, Ground Zero of the Dinosaur Extinction|date=17 November 2016|access-date=4 November 2017}}</ref>

The crater was discovered by Antonio Camargo and Glen Penfield, [[Physics:Geophysics|geophysicists]] who had been looking for [[Chemistry:Petroleum|petroleum]] in the Yucatán during the late 1970s. Penfield was initially unable to obtain evidence that the geological feature was a crater and gave up his search. Later, through contact with Alan Hildebrand in 1990, Penfield obtained samples that suggested it was an impact feature. Evidence for the impact origin of the crater includes [[Chemistry:Shocked quartz|shocked quartz]],<ref name="Becker2002">{{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 }}</ref> a [[Physics:Gravity anomaly|gravity anomaly]], and [[Chemistry:Tektite|tektite]]s in surrounding areas.

In 2016, a scientific drilling project drilled deep into the [[Astronomy:Peak ring|peak ring]] of the impact crater, hundreds of meters below the current sea floor, to obtain rock core samples from the impact itself. The discoveries were widely seen as confirming current theories related to both the crater impact and its effects.

The shape and location of the crater indicate further causes of devastation in addition to the dust cloud. The asteroid landed right on the coast and would have caused gigantic [[Tsunami|tsunami]]s, for which evidence has been found all around the coast of the Caribbean and eastern United States—marine sand in locations which were then inland, and vegetation debris and terrestrial rocks in marine sediments dated to the time of the impact.<ref>{{cite journal |last1=Smit |first1=J. |last2=Roep |first2=T.B. |last3=Alvarez |first3=W. |last4=Montanari |first4=A. |last5=Claeys |first5=P. |last6=Grajales-Nishimura |first6=J.M. |last7= Bermudez |first7=J. |year=1996 |title=Coarse-grained, clastic sandstone complex at the K/T boundary around the Gulf of Mexico: Deposition by tsunami waves induced by the Chicxulub impact? |journal=Geological Society of America Special Papers |volume=307 |pages=151–182 |url=https://we.vub.ac.be/~dglg/Web/Claeys/pdf/Smit-etal-96.pdf |access-date=19 August 2021}}</ref><ref>{{cite journal |last1=Schulte |first1=Peter |last2=Smit |first2=Jan |last3=Deutsch |first3=Alexander |last4=Salge |first4=Tobias |last5=Friese |first5=Andrea |last6=Beichel |first6=Kilian |title=Tsunami backwash deposits with Chicxulub impact ejecta and dinosaur remains from the Cretaceous-Palaeogene boundary in the La Popa Basin, Mexico: Cretaceous-Palaeogene event deposit, La Popa Basin, Mexico |journal=Sedimentology |date=April 2012 |volume=59 |issue=3 |pages=737–765 |doi=10.1111/j.1365-3091.2011.01274.x|s2cid=131038473 }}</ref>

The asteroid landed in a bed of [[Chemistry:Anhydrite|anhydrite]] ({{chem|Ca|S|O|4}}) or [[Chemistry:Gypsum|gypsum]] (CaSO<sub>4</sub>·2(H<sub>2</sub>O)), which would have ejected large quantities of [[Chemistry:Sulfur trioxide|sulfur trioxide]] {{chem|S|O|3}} that combined with water to produce a [[Chemistry:Sulfuric acid|sulfuric acid]] [[Chemistry:Aerosol|aerosol]]. This would have further reduced the sunlight reaching the Earth's surface and then over several days, precipitated planet-wide as [[Earth:Acid rain|acid rain]], killing vegetation, plankton and organisms which build shells from [[Chemistry:Calcium carbonate|calcium carbonate]] (coccolithophorids and molluscs).<ref>[http://www.livescience.com/43960-asteroid-extinction-sulfur-ocean-acidification.html ''Dinosaur-Killing Asteroid Triggered Lethal Acid Rain,'' Livescience, March 09, 2014]</ref><ref>{{cite journal |last1=Ohno |first1=Sohsuke |last2=Kadono |first2=Toshihiko |last3=Kurosawa |first3=Kosuke |last4=Hamura |first4=Taiga |last5=Sakaiya |first5=Tatsuhiro |last6=Shigemori |first6=Keisuke |last7=Hironaka |first7=Yoichiro |last8=Sano |first8=Takayoshi |last9=Watari |first9=Takeshi |last10=Otani |first10=Kazuto |author11-link=Takafumi Matsui |last11=Matsui, Takafumi |last12=Sugita |first12=Seiji |title=Production of sulphate-rich vapour during the Chicxulub impact and implications for ocean acidification |journal=Nature Geoscience |date=April 2014 |volume=7 |issue=4 |pages=279–282 |doi=10.1038/ngeo2095|bibcode=2014NatGe...7..279O }}</ref>

===Deccan Traps===<!-- This section is linked from Cretaceous-Tertiary extinction event -->
{{main|Earth:Deccan Traps}}

Before 2000, arguments that the [[Earth:Deccan Traps|Deccan Traps]] [[Earth:Flood basalt|flood basalt]]s caused the extinction were usually linked to the view that the extinction was gradual, as the flood basalt events were thought to have started around 68&nbsp;Ma and lasted for over 2 million years. However, there is evidence that two thirds of the Deccan Traps were created within 1 million years about 65.5&nbsp;Ma, so these eruptions would have caused a fairly rapid extinction, possibly a period of thousands of years, but still a longer period than what would be expected from a single impact event.<ref>{{cite journal|vauthors=Hofman C, Féraud G, Courtillot V|year=2000|title=40Ar/39Ar dating of mineral separates and whole rocks from the Western Ghats lava pile: further constraints on duration and age of the Deccan traps|journal=Earth and Planetary Science Letters|volume=180|issue=1–2|pages=13–27|doi=10.1016/S0012-821X(00)00159-X|bibcode=2000E&PSL.180...13H}}</ref><ref name="Duncan">{{cite journal|title=Rapid eruption of the Deccan flood basalts at the Cretaceous/Tertiary boundary|author1=Duncan, RA  |author2=Pyle, DG|year=1988|journal=Nature|volume=333|issue=6176|pages=841–843|doi=10.1038/333841a0|bibcode = 1988Natur.333..841D |s2cid=4351454 }}</ref>

The Deccan Traps could have caused extinction through several mechanisms, including the release of dust and sulfuric aerosols into the air which might have blocked sunlight and thereby reduced photosynthesis in plants. In addition, Deccan Trap volcanism might have resulted in carbon dioxide emissions which would have increased the [[Earth:Greenhouse effect|greenhouse effect]] when the dust and aerosols cleared from the atmosphere.<ref name="Duncan"/>

In the years when the Deccan Traps theory was linked to a slower extinction, Luis Alvarez (who died in 1988) replied that [[Biology:Paleontology|paleontologists]] were being misled by sparse data. While his assertion was not initially well-received, later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely or at least partly due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in [[Sea level|sea level]] and massive volcanic eruptions that produced the Indian Deccan Traps, and these may have contributed to the extinctions.<ref>{{cite book|author=Alvarez, W|title=T. rex and the Crater of Doom|year=1997|publisher=Princeton University Press|isbn=978-0-691-01630-6|pages=[https://archive.org/details/trexcraterofdoo000alva/page/130 130–146]|url-access=registration|url=https://archive.org/details/trexcraterofdoo000alva/page/130}}</ref>

===Multiple impact event===
Several other craters also appear to have been formed about the time of the K–Pg boundary. This suggests the possibility of nearly simultaneous multiple impacts, perhaps from a fragmented asteroidal object, similar to the [[Astronomy:Comet Shoemaker–Levy 9|Shoemaker–Levy 9]] cometary impact with [[Astronomy:Jupiter|Jupiter]]. Among these are the [[Earth:Boltysh crater|Boltysh crater]], a {{Convert|24|km||abbr=on}} diameter impact crater in {{wipe|Ukraine}} {{nowrap|(65.17 ± 0.64&nbsp;Ma);}} and the [[Earth:Silverpit crater|Silverpit crater]], a {{Convert|20|km||abbr=on}} diameter impact crater in the North Sea {{nowrap|(60–65&nbsp;Ma).}} Any other craters that might have formed in the [[Earth:Tethys Ocean|Tethys Ocean]] would have been obscured by erosion and tectonic events such as the relentless northward drift of Africa and India.<ref>{{cite journal|author=Mullen, L|date= October 13, 2004|title= Debating the Dinosaur Extinction| url= http://www.astrobio.net/news/article1243.html|journal=[[Astronomy:Astrobiology Magazine|Astrobiology Magazine]]|access-date=2007-07-11 |archive-url=https://web.archive.org/web/20110603232108/http://www.astrobio.net/exclusive/1243/debating-the-dinosaur-extinction |archive-date=2011-06-03 |url-status=usurped}}</ref><ref>{{cite journal|author=Mullen, L| date= October 20, 2004 | title=Multiple impacts|url=http://www.astrobio.net/news/article1253.html|journal=[[Astronomy:Astrobiology Magazine|Astrobiology Magazine]]|access-date=2007-07-11 |archive-url=https://web.archive.org/web/20080709072051/http://astrobio.net/news/article1253.html |archive-date=2008-07-09 |url-status=usurped}}</ref><ref name="Mullen Shiva">{{cite journal|author=Mullen, L|date=November 3, 2004|title=Shiva: Another K–T impact?|url=http://www.astrobio.net/news/article1281.html|journal=Astrobiology Magazine|access-date=2007-07-11 |archive-url=https://web.archive.org/web/20110804221725/http://www.astrobio.net/exclusive/1281/shiva-another-k-t-impact |archive-date=2011-08-04 |url-status=usurped}}</ref>

A very large structure in the sea floor off the west coast of India was interpreted in 2006 as a crater by three researchers.<ref>{{cite journal|author1=Chatterjee, S |author2=Guven, N |author3=Yoshinobu, A |author4=Donofrio, R  |name-list-style=amp |year=2006|title=Shiva structure: a possible K–Pg boundary impact crater on the western shelf of India|journal=Special Publications of the Museum of Texas Tech University|issue=50|url=http://www.nsrl.ttu.edu/publications/specpubs/sps/SP50.pdf|access-date=2007-06-15}}</ref> The potential [[Earth:Shiva crater|Shiva crater]], {{convert|450|-|600|km|abbr=on}} in diameter, would substantially exceed Chicxulub in size and has been estimated to be about 66&nbsp;mya, an age consistent with the K–Pg boundary. An impact at this site could have been the triggering event for the nearby Deccan Traps.<ref>{{cite journal|title=The Shiva Crater: Implications for Deccan Volcanism, India-Seychelles rifting, dinosaur extinction, and petroleum entrapment at the KT Boundary|url=http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_58126.htm|author1=Chatterjee, S|author2=Guven, N|author3=Yoshinobu, A|author4=Donofrio, R|name-list-style=amp|year=2003|journal=Geological Society of America Abstracts with Programs|volume=35|issue=6|pages=168|access-date=2007-08-02|archive-date=2008-10-12|archive-url=https://web.archive.org/web/20081012085555/http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_58126.htm|url-status=dead}}</ref> However, this feature has not yet been accepted by the geologic community as an impact crater and may just be a sinkhole depression caused by salt withdrawal.<ref name="Mullen Shiva"/>

===Maastrichtian marine regression===
Clear evidence exists that sea levels fell in the final stage of the Cretaceous by more than at any other time in the [[Earth:Mesozoic|Mesozoic]] era. In some [[Earth:Maastrichtian|Maastrichtian]] [[Physics:Stage (stratigraphy)|stage]] rock layers from various parts of the world, the later ones are terrestrial; earlier ones represent shorelines and the earliest represent seabeds. These layers do not show the tilting and distortion associated with [[Physics:Orogeny|mountain building]]; therefore, the likeliest explanation is a [[Earth:Marine regression|regression]], that is, a buildout of sediment, but not necessarily a drop in sea level. No direct evidence exists for the cause of the regression, but the explanation which is currently accepted as the most likely is that the [[Earth:Plate tectonics|mid-ocean ridges]] became less active and therefore sank under their own weight as sediment from uplifted orogenic belts filled in structural basins.<ref name="MacLeod">{{cite journal |author1=MacLeod, N. |author2=Rawson, P.F. |author3=Forey, P.L. |author4=Banner, F.T. |display-authors=2 |year=1997 |title=The Cretaceous–Tertiary biotic transition |journal=Journal of the Geological Society |volume=154 |issue=2 |pages=265–292 |issn=0016-7649 |doi=10.1144/gsjgs.154.2.0265 |url=http://jgs.geoscienceworld.org/content/154/2/265 |url-access=subscription |bibcode=1997JGSoc.154..265M |s2cid=129654916 }}</ref><ref>{{cite journal |last1=Liangquan |first1=Li |last2=Keller |first2=Gerta |year=1998 |title=Abrupt deep-sea warming at the end of the Cretaceous |journal=Geology |volume=26 |issue=11 |pages=995–8 |doi=10.1130/0091-7613(1998)026<0995:ADSWAT>2.3.CO;2 |bibcode=1998Geo....26..995L }}</ref>

A severe regression would have greatly reduced the [[Earth:Continental shelf|continental shelf]] area, which is the most species-rich part of the sea, and therefore could have been enough to cause a ''marine'' mass extinction. However, research concludes that this change would have been insufficient to cause the observed level of [[Biology:Ammonite|ammonite]] extinction. The regression would also have caused climate changes, partly by disrupting winds and ocean currents and partly by reducing the Earth's [[Earth:Albedo|albedo]] and therefore increasing global temperatures.<ref name="Marshall">{{cite journal|author1=Marshall, C. R.  |author2=Ward, P.D.|year=1996|title=Sudden and Gradual Molluscan Extinctions in the Latest Cretaceous of Western European Tethys|journal=Science|volume=274|issue=5291|pages=1360–1363|doi=10.1126/science.274.5291.1360|pmid=8910273|bibcode = 1996Sci...274.1360M |s2cid=1837900}}</ref>

Marine regression also resulted in the reduction in area of epeiric seas, such as the [[Earth:Western Interior Seaway|Western Interior Seaway]] of North America. The reduction of these seas greatly altered habitats, removing [[Earth:Coastal plain|coastal plain]]s that ten million years before had been host to diverse communities such as are found in rocks of the [[Earth:Dinosaur Park Formation|Dinosaur Park Formation]]. Another consequence was an expansion of freshwater environments, since continental runoff now had longer distances to travel before reaching oceans. While this change was favorable to freshwater vertebrates, those that prefer [[Earth:Ocean|marine]] environments, such as [[Biology:Shark|shark]]s, suffered.<ref name=AF04>{{cite book |last=Archibald |first=J. David |author2=Fastovsky, David E. |editor=Weishampel, David B. |editor2=Dodson, Peter |editor3=Osmólska, Halszka |title=The Dinosauria |edition=2nd |year=2004|publisher=University of California Press |location=Berkeley |isbn=978-0-520-24209-8 |pages=672–684 |chapter=Dinosaur Extinction}}</ref>

===Supernova hypothesis===
Another discredited cause for the K–Pg extinction event is cosmic radiation from a nearby [[Astronomy:Supernova|supernova]] explosion. An [[Chemistry:Iridium anomaly|iridium anomaly]] at the boundary is consistent with this hypothesis. However, analysis of the boundary layer sediments failed to find {{chem|244|Pu}},<ref>{{cite journal|title=Could a Nearby Supernova Explosion have Caused a Mass Extinction?|author1=Ellis, J  |author2=Schramm, DN|journal=Proceedings of the National Academy of Sciences|year=1995|volume=92|pages=235–238|pmid=11607506|doi=10.1073/pnas.92.1.235|issue=1|pmc=42852|bibcode = 1995PNAS...92..235E |arxiv=hep-ph/9303206|doi-access=free }}</ref> a supernova byproduct{{clarify|date=January 2022}} which is the longest-lived [[Chemistry:Plutonium|plutonium]] isotope, with a [[Physics:Half-life|half-life]] of 81 million years.

===Verneshot===
An attempt to link volcanism - like the Deccan Traps - and impact events causally in the ''other'' direction compared to the proposed Shiva crater is the so-called [[Earth:Verneshot|Verneshot]] hypothesis (named for Jules Verne), which proposes that volcanism might have gotten so intense as to "shoot up" material into a ballistic trajectory into space before it fell down as an impactor. Due to the spectacular nature of this proposed mechanism, the scientific community has largely reacted with skepticism to this hypothesis.

===Multiple causes===
It is possible that more than one of these hypotheses may be a partial solution to the mystery, and that more than one of these events may have occurred. Both the Deccan Traps and the Chicxulub impact may have been important contributors. For example, the most recent dating of the Deccan Traps supports the idea that rapid eruption rates in the Deccan Traps may have been triggered by large seismic waves radiated by the impact.<ref>{{cite journal |last1=Richards |first1=Mark A. |last2=Alvarez |first2=Walter |last3=Self |first3=Stephen |last4=Karlstrom |first4=Leif |last5=Renne |first5=Paul R. |last6=Manga |first6=Michael |last7=Sprain |first7=Courtney J. |last8=Smit |first8=Jan |last9=Vanderkluysen |first9=Loÿc |last10=Gibson |first10=Sally A. |title=Triggering of the largest Deccan eruptions by the Chicxulub impact |journal=Geological Society of America Bulletin |date=November 2015 |volume=127 |issue=11–12 |pages=1507–1520 |doi=10.1130/B31167.1|bibcode=2015GSAB..127.1507R |s2cid=3463018 |url=https://escholarship.org/uc/item/86f3521g }}</ref><ref>{{cite journal |last1=Renne |first1=Paul R. |last2=Sprain |first2=Courtney J. |last3=Richards |first3=Mark A. |last4=Self |first4=Stephen |last5=Vanderkluysen |first5=Loÿc |last6=Pande |first6=Kanchan |title=State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact |journal=Science |date=2 October 2015 |volume=350 |issue=6256 |pages=76–78 |doi=10.1126/science.aac7549|pmid=26430116 |bibcode=2015Sci...350...76R |s2cid=30612906 |doi-access=free }}</ref>

==See also==
*[[Earth:Climate across Cretaceous–Paleogene boundary|Climate across Cretaceous–Paleogene boundary]]
*[[Earth:Sub-Paleogene surface|Sub-Paleogene surface]]
*[[Earth:Tanis (fossil site)|Tanis (fossil site)]]

==References and notes==
=== Explanatory notes ===
{{notelist}}

=== References ===
{{Reflist|2}}

==External links==
* {{cite web |first=Robert |last=Sanders |url=https://news.berkeley.edu/2019/03/29/66-million-year-old-deathbed-linked-to-dinosaur-killing-meteor/ |title=66 million-year-old deathbed linked to dinosaur-killing meteor |date=29 March 2019 |publisher=Berkeley News}}
* {{cite news |url=https://www.newyorker.com/magazine/2019/04/08/the-day-the-dinosaurs-died |title=The day the dinosaurs died |newspaper=New Yorker |first=Douglas |last=Preston |date=29 March 2019}}
{{KT boundary}}
{{Impact cratering on Earth}}

{{DEFAULTSORT:Cretaceous-Paleogene boundary}}
[[Category:Cretaceous geology]]
[[Category:Extinction events]]
[[Category:Paleogene geology]]

{{Sourceattribution|Cretaceous–Paleogene boundary}}