Chalk-Paleogene border

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The moment of impact (artist's impression)

The Cretaceous-Paleogene boundary , also known as the KP boundary , formerly the Cretaceous-Tertiary boundary , also called the K / T boundary , is the point in time of a geological event 66 million years ago that marked the transition from the Cretaceous to the Paleogene (until 2000 " Tertiary "). The Cretaceous-Paleogene boundary marks the beginning of one of the five largest mass extinctions in Earth's history, which in particular ended the era of the dinosaurs . This geological cardinal point also forms the transition between the Mesozoic Era and the Cenozoic Era .

However, this is not a fixed point in time in the conventional sense. The geological and biological upheaval is made up of a wide variety of environmental changes that occurred in staggered chronological order and spanned periods from a few days to several hundred thousand years. The impact of one or more asteroids , coupled with greatly increased volcanic activity, caused a serious change in fauna and flora . Geological features of the Cretaceous-Paleogene boundary include an iridium anomaly , indicating a large asteroid impact, as well as extensive quantities of ash and pebbles that must have formed under extreme heat. It is currently very likely that these events are directly related to global extinction.

The International Commission on Stratigraphy (ICS), the previous name "tertiary" replaced in 2000 by "Paleogene" the term chalk-Paleogene boundary (English: Cretaceous-Paleogene boundary or K-Pg boundary ) is in the German and international Technical literature is now in common use.

Evidence of asteroid impacts

Computer generated map of Chicxulub crater based on the gravity anomalies

An essential indicator for the hypothesis of one or more impacts is the unusually high iridium content of many rocks near the Cretaceous-Paleogene boundary . Since the earth's mantle is poor in iridium compared to stone meteorites , it is assumed that the dust raised by the impact can be found in these layers. Receives additional support the hypothesis of an asteroid strike by an abnormality of chromium - isotope distribution in the same layer in which the Iridium anomaly occurs. The chromium isotope distribution is normally homogeneous on earth. While in the case of the iridium anomaly it was stated that volcanic activities could also cause iridium enrichment, the isotope anomaly in the case of chromium can only be explained by the admixture of extraterrestrial material. Further mineralogical traces of the impact consist of results of the pressure wave and the high temperatures, such as changed (“shocked”) quartz structures ( planar deformation elements, PDFs ), stishovites , zircons , diamond crystals and glass spheres ( microtectites ). These structures occur worldwide and decrease quantitatively proportionally with the distance from the crater. A possible candidate for the impact body is an asteroid with a composition similar to carbonaceous chondrites ; the latter have the same chromium isotope distribution as the KP boundary layer. Since a comet presumably consists of ice and dust particles, the composition of which is similar to the carbonaceous chondrites , a comet cannot be ruled out as a possible impact body.

History and scenario of the impact hypothesis

An outcrop in Trinidad Lake State Park , Colorado . The dashed line runs along the KP boundary.
Preparation of a sequence of layers containing the KP border in the Maastricht Natural History Museum

In June 1980, the research team led by the physicist and Nobel Prize winner Luis Walter Alvarez and his son, the geologist Walter Alvarez , published the discovery of an iridium anomaly at the Cretaceous-Paleogene boundary . The resulting assumption of a large asteroid impact that led to the extinction of dinosaurs , among others , was the beginning of a long discussion about the pros and cons of the hypothesis presented by father and son Alvarez.

In search of the possible impact point of the impactor in 1991 on the Mexican Yucatán peninsula , a 180 km large crater was found below the village of Chicxulub Puerto and was covered by younger sediments . This did not end the scientific controversy about the so-called Chicxulub impact . Even if the age and size of the crater seemed to fit into the scheme of the Cretaceous-Paleogene Crisis, several counter-hypotheses were put forward, including the one that it was not the impact, but the magmatic eruption of the Indian Deccan Trapps , that caused the mass extinction at the Cretaceous-Paleogene -Border had forced. In addition, sediment investigations as part of a deep drilling program with the research vessel Saipem 10000 seemed to confirm that the Chicxulub crater had formed 300,000 years before the actual KP boundary layer.

The "predating" of the impact met with criticism and is again unlikely in view of the latest research results. The application of modern dating methods and analysis techniques with very low tolerance ranges led to the result that the impact event and the KP boundary layer coincide precisely in time. The impact winter following the impact is now considered factually secured. Until recently, scientists were of the opinion that biodiversity and the stability of ecosystems were on the decline at the end of the Cretaceous . New studies are increasingly providing evidence that the ecological situation in the late Maastrichtian was more stable than assumed for a long time, despite possible environmental pollution from volcanic emissions. Thus it was left to the Chicxulub impact to set the end point for the Mesozoic fauna.

The currently most likely scenario is based on the assumption that 66.040 million years ago (± 0.032 million years) an approximately 14 km large asteroid hit a tropical shallow sea at a speed of 20 km / s in the area of ​​what is now the Gulf of Mexico , with the presumed impact angle of around 45 to 60 degrees corresponded to a worst-case scenario with regard to the environmental impact. The impactor evaporated almost completely within a second, but due to the force of the explosion, which was likely to be heard all over the globe, a few thousand cubic kilometers of carbonate and evaporite rock were hurled over long distances as glowing ejecta into the stratosphere , to a smaller extent far beyond that. In addition to the direct effects of the impact such as megatsunamis , a supersonic pressure wave and earthquakes with a magnitude of 11 or 12, wildfires occurred around the world, the extent and duration of which has not yet been finally clarified. Within a few days, a large amount of soot and dust clouds spread throughout the atmosphere, absorbing sunlight for months and causing a global drop in temperature. In addition, according to a recent and based on climate models study, a layer could sulfuric acid - aerosols have contributed significantly to global permafrost period over several years, with a decrease of the surface temperature by at least 26 ° C throughout much of the world.

A hypothesis presented by several well-known geoscientists in April 2015 assumes that the long “smoldering” Dekkan-Trapp in what is now West India recorded a considerable increase in its activity due to the impact energy of 3 × 10 23 joules and the tectonic shock waves triggered by it . According to this hypothesis, the short-term discharge of 70 percent of all Dekkan-Trapp flood basalts is due to the Chicxulub impact. This scenario, which has received little attention so far, is being discussed more and more intensively in the scientific literature.

The oceanic and mainland ecosystems were equally affected by the subsequent biological crisis. 75 percent of the species fell victim to mass extinction, including not only dinosaurs, but also ammonites , almost all calciferous foraminifera and, to a large extent, birds. After a cold phase presumably lasting several decades, rapid warming began, leading to heat stress , due to billions of tons of carbon dioxide released by the impact as a result of the evaporation of oceanic floors. The duration of the extreme greenhouse effect is estimated at around 50,000 years before the climate probably only finally stabilized after several hundred thousand years.

Impact hypothesis Permian / Triassic

Also, another big mass extinction - at the transition between Palaeozoic and Erdmittelalter ( Paläozoikum and Mesozoikum ) - is hypothetically attributed to a meteorite ( " Perm - Trias -Impakt"). However, neither a significantly increased iridium concentration nor isotope anomalies are documented for this limit in the corresponding rock strata. In 2006, a gravity anomaly was detected using satellite data in the southern polar Wilkesland region . Radar images provided evidence of the existence of a 480 km large impact crater deep under the Antarctic ice sheet with a presumed age of 250 million years. This means that the Wilkesland crater could be traced back to the largest known impact in the history of the earth, whose destructive potential would have considerably exceeded that of the Chicxulub meteorite . It has not yet been possible to provide direct evidence, for example through geological analyzes on site based on corresponding deep boreholes .

Possible factors for the mass extinction at the KP border

Comprehensive analyzes of known mass extinctions have recently led to a deeper understanding of the mechanisms and relationships between these events. The specialist literature on this topic had increased almost tenfold between 1984 and 2004, with increasing consideration of interdisciplinary research. This led to the realization that mass extinctions do not necessarily have to be linked to long-term geological processes, but have often taken a catastrophic and time-limited course. In addition, there is a growing body of evidence to support the assumption that many mass extinctions in the history of the earth were linked to serious climate change events and their consequences.

In addition to a single impact, a number of alternative models of the Cretaceous-Paleogene Extinction are being considered, such as a whole series of meteorite or comet impacts at short intervals. However, the absence of correspondingly large impact craters in the period in question speaks against this assumption. In addition, the possibility of a Verneshot is rejected by experts due to the lack of a reliable database, as is the existence of a hypothetically postulated giant crater on the Indian subcontinent ( Shiva impact structure ).

On the other hand, in addition to the rapid decline in vegetation cover, the considerable acidification of large areas of the sea may have played an important role during the transition phase from the Cretaceous to the Paleogene. The amounts of sulfur dioxide and various nitrogen oxides released by the impact and the Dekkan-Trapp volcanism were significant and estimated to have been in the range of 1 to 2 trillion tons. This not only led to extremely acid rain , but also to the destabilization of the oceanic biotopes , mainly due to a temporary lack of oxygen and the rapid drop in the pH value . With the extensive reduction in nannoplankton , many dependent organisms lost their food sources, which had an impact on the entire marine ecosystem in the form of a chain reaction. Thus, the mass extinction in the seas reached similar dimensions as that on the mainland. More recent studies consistently assume that the biological crisis on the Cretaceous-Paleogene border was caused exclusively by the Chicxulub impact.

literature

  • Steven M. Stanley: Historical geology , translated from the American by Volker Schweizer, Erika Kraatz u. Reinhart Kraatz, Heidelberg a. a. 2001, ISBN 3-8274-0569-6 , pp. 528-540.
  • Markus Harting: On the Cretaceous / Tertiary transition in NE Mexico: Geochemical characterization of the Chicxulub impact projecta . Dissertation at the University of Karlsruhe, Fac. F. Civil engineering, geo- and environmental sciences, 2004. Abstract online (PDF)
  • J. David Archibald : Dinosauria extinction and the end of an Era - what the fossils say , Columbia University Press 1996
  • Peter Skelton: The Cretaceous World , Cambridge University Press, 2002, ISBN 0-521-53843-2
  • József Pálfy: Disasters in the history of the earth. Global extinction? Schweizerbart, Stuttgart 2005, ISBN 3-510-65211-8 .

Web links

Commons : Cretaceous-Paleogene Boundary  - Collection of Images, Videos and Audio Files

Individual evidence

  1. ^ RD Norris, BT Huber, J. Self-Trail: Synchroneity of the KT oceanic mass extinction and meteorite impact: Blake Nose, western North Atlantic . (PDF) In: Geology . 27, No. 5, May 1999, pp. 419-422. doi : 10.1130 / 0091-7613 .
  2. ^ LW Alvarez, W. Alvarez, F. Asaro, HV Michel: Extraterrestrial Cause for the Cretaceous-Tertiary Extinction . (PDF) In: Science . 208, June 1980, pp. 1095-1108.
  3. ↑ G. Keller, T. Adatte, W. Stinnesbeck, M. Rebolledo-Vieyra, JU Fucugauchi, U. Kramar, Doris Stüben: Chicxulub impact predates the KT boundary mass extinction . In: pnas . 101, No. 11, 2004, pp. 3753-3758. doi : 10.1073 / pnas.0400396101 .
  4. ↑ Peter Schulte: Comment on the paper "Chicxulub impact predates KT boundary: New evidence from Brazos, Texas" by Keller et al. (2007) . (PDF) In: Earth and Planetary Science Letters . No. 269, 2008, pp. 614-620. doi : 10.1016 / j.epsl.2007.11.066 .
  5. ^ Paul R. Renne, Alan L. Deino, Frederik J. Hilgen, Klaudia F. Kuiper, Darren F. Mark, William S. Mitchell III, Leah E. Morgan, Roland Mundil, Jan Smit: Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary . (PDF) In: Science . 339, No. 6120, February 2013, pp. 684-687. doi : 10.1126 / science.1230492 .
  6. ^ Robert Sanders: New evidence comet or asteroid impact was last straw for dinosaurs . In: UC Berkeley News Center, Feb. 7, 2013.
  7. ↑ Johan Vellekoop, Appy Sluijs, Jan Smit, Stefan Schouten, Johan WH Weijers, Jaap S. Sinninghe Damsté, Henk Brinkhuis: Rapid short-term cooling Following the Chicxulub impact at the Cretaceous-Paleogene boundary . In: pnas . 111, No. 21, May 2014. doi : 10.1073 / pnas.1319253111 .
  8. ^ Tyler R. Lyson, Antoine Bercovici, Stephen GB Chester, Eric J. Sargis, Dean Pearson, Walter G. Joyce: Dinosaur extinction: closing the '3 m gap' . In: The Royal Society, Biology Letters . July 2011. doi : 10.1098 / rsbl.2011.0470 .
  9. ↑ Zoltan Siki-Sava, Eric Buffetaut, Attila Ősi, Xabier Pereda-Suberbiola, Stephen L. Brusatte: Island life in the Cretaceous - faunal composition, biogeography, evolution, and extinction of land-living vertebrates on the Late Cretaceous European archipelago . In: ZooKeys . 469, January 2015, pp. 1–161. doi : 10.3897 / zookeys.469.8439 .
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  11. ↑ a b Michael J. Henehan, Andy Ridgwell, Ellen Thomas, Shuang Zhang, Laia Alegret, Daniela N. Schmidt, James WB Rae, James D. Witts, Neil H. Landman, Sarah E. Greene, Brian T. Huber, James R. Super, Noah J. Planavsky, Pincelli M. Hull: Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact . In: PNAS . 116, No. 43, October 2019. doi : 10.1073 / pnas.1905989116 .
  12. ↑ GS Collin, N. Patel, TM Davison, ASP Rae, JV Morgan, SPS Gulick, IODP-ICDP Expedition 364 Science Party: GL Christeson, E. Chenot, P. Claeys, CS Cockell, MJL Coolen, L. Ferrière, C. Gebhardt, K. Goto, H. Jones, DA Kring, J. Lofi, CM Lowery, R. Ocampo-Torres, L. Perez-Cruz, AE Pickersgill, MH Poelchau, C. Rasmussen, M. Rebolledo-Vieyra, U . Riller, H. Sato, J. Smit, SM Tikoo, N. Tomioka, J. Urrutia-Fucugauchi, MT Whalen, A. Wittmann, L. Xiao, KE Yamaguchi, Third-Party Scientists: N. Artemieva, TJ Bralower: A steeply-inclined trajectory for the Chicxulub impact . In: Nature Communications . May 11, 2020. doi : 10.1038 / s41467-020-15269-x .
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  14. ↑ Claire M. Belcher: reigniting the Cretaceous-Palaeogene debate firestorm . In: geology . 37, No. 12, 2009, pp. 1147-1148. doi : 10.1130 / focus122009.1 .
  15. ^ Douglas S. Robertson, William M. Lewis, Peter M. Sheehan, Owen B. Toon: Reevaluation of the heat-fire hypothesis . In: Journal of Geophysical Research: Biogeoscience . 110, No. 1, March 2013, pp. 329–336. doi : 10.1002 / jgrg.20018 .
  16. ↑ Julia Brugger, Georg Feulner, Stefan Petri: Baby, it's cold outside: Climate model simulations of the effects of the asteroid impact at the end of the Cretaceous . In: Geophysical Research Letters . 44, No. 1, January 2017, pp. 419-427. doi : 10.1002 / 2016GL072241 .
  17. ↑ Mark A. Richards, Walter Alvarez, Stephen Self, Leif Karlstrom, Paul R. Renne, Michael Manga, Courtney J. Sprain, Jan Smit, Loÿc Vanderkluysen, Sally A. Gibson: Triggering of the largest Deccan eruptions by the Chicxulub impact . (PDF) In: Geological Society of America Bulletin . April 2015. doi : 10.1130 / B31167.1 .
  18. ^ Paul R. Renne, Courtney J. Sprain, Mark A. Richards, Stephen Self, Loÿc Vanderkluysen, Kanchan Pande: State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact . (PDF) In: Science . 350, No. 6256, October 2015, pp. 76-78. doi : 10.1126 / science.aac7549 .
  19. ↑ Nicholas R. Longrich, Tim Tokaryk, Daniel J. Field: Mass extinction of birds at the Cretaceous-Paleogene (K-Pg) boundary . In: pnas . 108, No. 37, September 2011, pp. 15253-15257. doi : 10.1073 / pnas.1110395108 .
  20. ↑ Adrian P. Jones; David G. Price; Paul S. DeCarli; Richard Clegg: Impact Decompression Melting: A Possible Trigger for Impact Induced Volcanism and Mantle Hotspots? In: C. Koeberl and F. Martinez-Ruiz (eds.): Impact markers in the Stratigraphic Record . Springer Verlag, Berlin 2003, ISBN 3-540-00630-3 , pp. 91–120 ( Online [PDF; 459 kB ; accessed on January 2, 2019]).
  21. ^ Richard J. Twitchett: The palaeoclimatology, palaeoecology and palaeoenvironmental analysis of mass extinction events . (PDF) In: Palaeogeography, Palaeoclimatology, Palaeoecology . 232, No. 2-4, March 2006, pp. 190-213. doi : 10.1016 / j.palaeo.2005.05.019 .
  22. ^ Peter Ward: Under a Green Sky: Global Warming, the Mass Extinctions of the Past, and What They Can Tell Us About Our Future (2007). ISBN 978-0-06-113791-4
  23. ↑ Bärbel Hönisch , Andy Ridgwell, Daniela N. Schmidt, Ellen Thomas, Samantha J. Gibbs, Appy Sluijs, Richard Zeebe, Lee Kump, Rowan C. Martindale, Sarah E. Greene, Wolfgang Kiessling, Justin Ries, James C. Zachos , Dana L. Royer, Stephen Barker, Thomas M. Marchitto Jr., Ryan Moyer, Carles Pelejero, Patrizia Ziveri, Gavin L. Foster, Branwen Williams: The Geological Record of Ocean Acidification . (PDF) In: Science . 335, No. 6072, March 2012, pp. 1058-1063. doi : 10.1126 / science.1208277 .
  24. ↑ Vincent Courtillot and Paul Renne : On the ages of flood basalt events , Comptes Rendus Geosciences, Volume 335, 2003, pp. 113-140
  25. ↑ Pincelli M. Hull, André Bornemann, Donald E. Penman, Michael J. Henehan, Richard D. Norris, Paul A. Wilson, Peter Blum, Laia Alegret, Sietske J. Batenburg, Paul R. Bown, Timothy J. Bralower, Cecile Cournede, Alexander Deutsch, Barbara Donner, Oliver Friedrich, Sofie Jehle, Hojung Kim, Dick Kroon, Peter C. Lippert, Dominik Loroch, Iris Moebius, Kazuyoshi Moriya, Daniel J. Peppe, Gregory E. Ravizza, Ursula Röhl, Jonathan D Schueth, Julio Sepúlveda, Philip F. Sexton, Elizabeth C. Sibert, Kasia K. Śliwińska, Roger E. Summons, Ellen Thomas, Thomas Westerhold, Jessica H. Whiteside, Tatsuhiko Yamaguchi, James C. Zachos: On impact and volcanism across the Cretaceous-Paleogene boundary . (PDF) In: Science . 367, No. 6475, January 2020, pp. 266-272. doi : 10.1126 / science.aay5055 .