Primeval 16 – K–Pg Extinction Event

 

The end of the Cretaceous was one of the largest extinction events in the existence of the Earth, the Cretaceous–Paleogene extinction event, also called the K-T Extinction or K–Pg Extinction. Based on marine fossils, it is estimated that 75% or more of all species were wiped out by the K–Pg extinction.(1)David Jablonski and W. G. Chaloner (1994) “Extinctions in the fossil record (and discussion)” Philosophical Transactions of the Royal Society of London, Series B, Volume 344, Number 1307, Pages 11–17 In terrestrial ecosystems all animals weighing more than a kilogram disappeared.(2)Sarah Fecht (May 31, 2012) “Baby Boom: Did Retained Juvenile Traits Help Birds Outlive Dinosaurs?” Scientific American It marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opening the Cenozoic Era that continues today.

Earth 65 Million Years Ago

Earth 65 Million Years Ago

As originally proposed by a team of scientists led by Luis Alvarez, it is now generally believed that the extinction was partially caused by a massive asteroid impact which resulted in catastrophic effects on the global environment, including a lingering impact winter that made it impossible for plants and plankton to carry out photosynthesis.(3)Luis W. Alvarez et al. (1980) “Extraterrestrial cause for the Cretaceous–Tertiary extinction” Science, Volume 208, Number 4448, Pages 1095–1108 The event appears to have impacted all continents at the same time. Dinosaurs are known from the Maastrichtian Age of North America, Europe, Asia, Africa, South America, and Antarctica,(4)D. B. Weishampel and P. M. Barrett (2004) “Dinosaur Distribution” In David B Weishampel et al. editors The Dinosauria, (Second Edition) Pages 517–606. University of California Press but are unknown from the Cenozoic Eon anywhere in the world. Similarly, fossil pollen show devastation of the plant communities in areas as far apart as New Mexico, Alaska, China, and New Zealand.(5)D. J. Nichols and K. R. Johnson (2008) Plants and the K–T Boundary.

In 1980, a team of researchers consisting of Nobel prize-winning physicist Luis Alvarez, his son geologist Walter Alvarez, and chemists Frank Asaro and Helen Michel discovered that sedimentary layers found all over the world at the K-Pg boundary contain a concentration of iridium many times greater than normal. Iridium is extremely rare in Earth’s crust because it is a heavy element that bonds easily to iron, and therefore most of it travelled with the iron as it sank into Earth’s core during planetary differentiation. As iridium remains abundant in most asteroids and comets, the Alvarez team suggested that an asteroid struck the Earth at the time of the K-Pg boundary.

Dinosaur at K-T Extinction

Dinosaur at K-T Extinction

This hypothesis was viewed as radical when first proposed, but additional evidence soon emerged. The boundary clay was found to be full of minute spherules of rock, crystallized from droplets of molten rock formed by the impact.(6)J. Smit and J. Klaver (1981) “Sanidine spherules at the Cretaceous-Tertiary boundary indicate a large impact event”. Nature, Volume 292, Number 5818, Pages 47–49 Shocked quartz and other minerals were also identified in the Cretaceous–Paleogene boundary.(7)B. F. Bohor et al. (1987) “Shocked Quartz in the Cretaceous-Tertiary Boundary Clays: Evidence for a Global Distribution” Science, Volume 236, Number 4802, Pages 705–709 Shocked minerals have their internal structure deformed, and are created by intense pressures such as those associated with nuclear blasts or meteorite impacts. The identification of giant tsunami beds along the Gulf Coast and the Caribbean also provided evidence for impact(8)J. Bourgeois et al. (1988) “A tsunami deposit at the Cretaceous-Tertiary boundary in Texas”. Science, Volume 241, Number 4865, Pages 567–570, and suggested that the impact may have occurred nearby as did the discovery that the K-Pg boundary became thicker in the southern United States, with meter-thick beds of debris occurring in northern New Mexico.

Further research identified the giant Chicxulub crater, buried under Chicxulub on the coast of Yucatán, Mexico as the source of the K-Pg boundary clay. Identified in 1990(9)A. R. Hildebrand et al. (1991) “Chicxulub crater: a possible Cretaceous/Tertiary boundary impact crater on the Yucatán peninsula, Mexico.” Geology, Volume 19, Pages 867-871 based on work by geophysicist Glen Penfield in 1978, the crater is oval, with an average diameter of roughly 180 kilometres (110 miles), about the size calculated by the Alvarez team.(10)K. O. Pope et al. (1996) “Surface expression of the Chicxulub crater” Geology, Volume 24, Number 6, Pages 527–530 The discovery of the crater provided conclusive evidence for a K-Pg impact, and strengthened the hypothesis that the extinction was caused by an impact. In a 2013 paper, Paul Renne of the Berkeley Geochronology Center reported that the date of the asteroid event is 66 million years ago based on argon–argon dating.(11)Paul Renne et al. (2013) “Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary” Science, Volume 339, Number 6120, Pages 684-687

Chicxulub Impact as Seen From Orbit

Chicxulub Impact as Seen From Orbit

Most paleontologists now agree that an asteroid did hit the Earth at approximately the end of the Cretaceous, but there is an ongoing dispute whether the impact was the sole cause of the extinctions.(12)G. Keller et al. (2004) “Chicxulub impact predates the K–T boundary mass extinction”. Proceedings of the National Academy of Sciences, Volume 101, Number 11, Pages 3753–3758 In 1996 paleontologist Sankar Chatterjee of Texas Tech University drew attention to the proposed and much larger 600 km (370 miles) Shiva crater and the possibility of a multiple-impact scenario. Several other craters also appear to have been formed about the time of the K-Pg boundary, suggesting the possibility of near simultaneous multiple impacts. In addition to the 180 km (110 miles) Chicxulub Crater, there is the 24 km (15 miles) Boltysh crater in Ukraine, the 20 km (12 miles) Silverpit crater, a suspected impact crater in the North Sea, and the controversial and much larger Shiva crater. Any other craters that might have formed in the Tethys Ocean would have been obscured by tectonic events like the relentless northward drift of Africa and India.(13)L. Mullen (October 13, 2004) “Debating the Dinosaur Extinction” Astrobiology Magazine Chatterjee is confident that Shiva was one of many impacts, stating:

…the K-T extinction was definitely a multiple-impact scenario.- Sankar Chatterjee(14)M. R. Rampino and B. M. Haggerty (1996) "The “Shiva Hypothesis”: Impacts, mass extinctions, and the galaxy." Earth, Moon, and Planets. Volume 72 Numbers 1-3 Pages 441-460

impactcraters

Multiple Impact Theory

Chatterjee further claim that the Shiva impact may have been the triggering event for the Deccan Traps as well as a contributing factor to the accelerated movement of the Indian plate in the early Paleogene.(15)Sankar Chatterjee (2003) “The Shiva Crater: Implications for Deccan Volcanism, India-Seychelles Rifting, Dinosaur Extinction, and Petroleum Entrapment at the KT Boundary” Seattle Annual Meeting, Paper Number 60-8 The Deccan Traps are a large igneous province located on the Deccan Plateau of west-central India and one of the largest volcanic features on Earth. They consist of multiple layers of solidified flood basalt that together are more than 2 km (1.2 miles) thick and cover an area of 500,000 km² (190,000 square miles), having a volume of 512,000 km³ (123,000 cubic miles). The Deccan Traps began forming around 66 million years ago, at the end of the Cretaceous period. The bulk of the volcanic eruption occurred at the Western Ghats near Mumbai around 66 million years ago. The most recent evidence shows that the traps erupted over a period of 800,000 years at the K–Pg boundary, and therefore may be responsible for the delayed recovery of the biosphere after the extinction.(16)G. Keller et al. (2008) “Main Deccan volcanism phase ends near the K–T boundary: Evidence from the Krishna-Godavari Basin, SE India” Earth and Planetary Science Letters, Volume 268, Number 3–4, Pages 293–311 The release of volcanic gases, particularly sulfur dioxide, during the formation of the traps would have contributed to climate change. Data points to an average drop in temperature of only 2°C in this period,(17)D. L. Royer (2004) “CO2 as a primary driver of Phanerozoic climate” Geological Society of America Today, Volume 14, Number 3, Pages 4–10 hardly enough to have caused an extinction.

Ultimately impact theories in any time period can only explain very rapid extinctions, since the dust clouds and possible sulfuric aerosols would wash out of the atmosphere in a fairly short time, possibly within a decade.(18)D. A. Kring (2003) “Environmental consequences of impact cratering events as a function of ambient conditions on Earth” Astrobiology, Volume 3, Number 1, Pages 133–152 In the case of the K-Pg Extinction Event though, the situation is further complicated by the fact that most of the species that died out were extinct prior to the impacts. At the peak of the Mesozoic, there were no polar ice caps, and sea levels are estimated to have been from 100 to 250 meters (300 to 800 ft) higher than they are today. There is clear evidence that sea levels fell in the final stage of the Cretaceous by more than at any other time in the Mesozoic era. In some Maastrichtian stage rock layers from various parts of the world, the later layers are terrestrial; earlier layers represent shorelines and the earliest layers represent seabeds. These layers do not show the tilting and distortion associated with mountain building, therefore the likeliest explanation is a drop in sea level. There is no direct evidence for the cause of the drop in sea level, but the explanation currently accepted as most likely is that the mid-ocean ridges became less active and therefore sank under their own weight.(19)N. MacLeod et al. (1997) “The Cretaceous–Tertiary biotic transition” Journal of the Geological Society, Volume 154, Number 2, Pages 265–292

Chicxulub Crater a few Millennia after the Impact

Chicxulub Crater a few Millennia after the Impact

A severe drop on sea level would have greatly reduced the aquatic 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. Researchers conclude that this change would have been insufficient to cause the observed level of ammonite extinction. The sea level regression would also have caused climate changes, partly by disrupting winds and ocean currents and partly by reducing the Earth’s albedo and therefore increasing global temperatures.(20)C. R. Marshall and P. D. Ward (1996) “Sudden and Gradual Molluscan Extinctions in the Latest Cretaceous of Western European Tethys” Science, Volume 274, Number 5291, Pages 1360–1363

The drop in sea level also resulted in the loss of inland seas, such as the Western Interior Seaway of North America. The loss of these seas greatly altered habitats, removing coastal plains that ten million years before had been host to diverse communities such as are found in the rocks of the 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 marine environments suffered.(21)Archibald David (2004) “Dinosaur Extinction” In David B. Weishampel et al. editors The Dinosauria, (Second Edition) Pages 672–684

References   [ + ]

1. David Jablonski and W. G. Chaloner (1994) “Extinctions in the fossil record (and discussion)” Philosophical Transactions of the Royal Society of London, Series B, Volume 344, Number 1307, Pages 11–17
2. Sarah Fecht (May 31, 2012) “Baby Boom: Did Retained Juvenile Traits Help Birds Outlive Dinosaurs?” Scientific American
3. Luis W. Alvarez et al. (1980) “Extraterrestrial cause for the Cretaceous–Tertiary extinction” Science, Volume 208, Number 4448, Pages 1095–1108
4. D. B. Weishampel and P. M. Barrett (2004) “Dinosaur Distribution” In David B Weishampel et al. editors The Dinosauria, (Second Edition) Pages 517–606. University of California Press
5. D. J. Nichols and K. R. Johnson (2008) Plants and the K–T Boundary.
6. J. Smit and J. Klaver (1981) “Sanidine spherules at the Cretaceous-Tertiary boundary indicate a large impact event”. Nature, Volume 292, Number 5818, Pages 47–49
7. B. F. Bohor et al. (1987) “Shocked Quartz in the Cretaceous-Tertiary Boundary Clays: Evidence for a Global Distribution” Science, Volume 236, Number 4802, Pages 705–709
8. J. Bourgeois et al. (1988) “A tsunami deposit at the Cretaceous-Tertiary boundary in Texas”. Science, Volume 241, Number 4865, Pages 567–570
9. A. R. Hildebrand et al. (1991) “Chicxulub crater: a possible Cretaceous/Tertiary boundary impact crater on the Yucatán peninsula, Mexico.” Geology, Volume 19, Pages 867-871
10. K. O. Pope et al. (1996) “Surface expression of the Chicxulub crater” Geology, Volume 24, Number 6, Pages 527–530
11. Paul Renne et al. (2013) “Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary” Science, Volume 339, Number 6120, Pages 684-687
12. G. Keller et al. (2004) “Chicxulub impact predates the K–T boundary mass extinction”. Proceedings of the National Academy of Sciences, Volume 101, Number 11, Pages 3753–3758
13. L. Mullen (October 13, 2004) “Debating the Dinosaur Extinction” Astrobiology Magazine
14. M. R. Rampino and B. M. Haggerty (1996) "The “Shiva Hypothesis”: Impacts, mass extinctions, and the galaxy." Earth, Moon, and Planets. Volume 72 Numbers 1-3 Pages 441-460
15. Sankar Chatterjee (2003) “The Shiva Crater: Implications for Deccan Volcanism, India-Seychelles Rifting, Dinosaur Extinction, and Petroleum Entrapment at the KT Boundary” Seattle Annual Meeting, Paper Number 60-8
16. G. Keller et al. (2008) “Main Deccan volcanism phase ends near the K–T boundary: Evidence from the Krishna-Godavari Basin, SE India” Earth and Planetary Science Letters, Volume 268, Number 3–4, Pages 293–311
17. D. L. Royer (2004) “CO2 as a primary driver of Phanerozoic climate” Geological Society of America Today, Volume 14, Number 3, Pages 4–10
18. D. A. Kring (2003) “Environmental consequences of impact cratering events as a function of ambient conditions on Earth” Astrobiology, Volume 3, Number 1, Pages 133–152
19. N. MacLeod et al. (1997) “The Cretaceous–Tertiary biotic transition” Journal of the Geological Society, Volume 154, Number 2, Pages 265–292
20. C. R. Marshall and P. D. Ward (1996) “Sudden and Gradual Molluscan Extinctions in the Latest Cretaceous of Western European Tethys” Science, Volume 274, Number 5291, Pages 1360–1363
21. Archibald David (2004) “Dinosaur Extinction” In David B. Weishampel et al. editors The Dinosauria, (Second Edition) Pages 672–684