Introduction
The Cretaceous-Paleogene (K-Pg) extinction event, also known as the Cretaceous-Tertiary extinction (K-T), in the late Cretaceous is one among the five disastrous mass extinctions in the history of earth that wiped out 75 percent of species, approximately 66 million years ago. This event is marked by a thin layer of sediment called the K-Pg Boundary which separates the age of reptiles and the age of mammals. It also marked the beginning of the Cenozoic era. The most widely known “impact theory”, hypothesized by Luiz Alvarez and others, proposed that the extinction was caused by the impact of an asteroid approximately 10 km in diameter at Chicxulub on Mexico’s Yucatan peninsula. The impact concerted with volcanism, climate change and sea regressions, might have ultimately led to the mass extinction.
Understanding the K-T extinction would help us understand mass extinctions and provide a complete picture of how the biosphere responds to an impact.
Chicxulub Impact
The rocks deposited during the cretaceous period and the tertiary period are separated by a thin clay layer containing high concentration of iridium, prompting Alvarez and his team to blame an extra-terrestrial object for the mass extinction as iridium is a constituent of asteroidal bodies and all the iridium on the earth had sequestered into the core long ago. This iridium anomaly at or near the KT boundary, hence, suggests that an asteroid was responsible for the extinction. Further, the discovery of the chicxulub crater strengthened the impact hypothesis and the results of the Chicxulub drilling project (2016) provided that an impact occurred around 66.043±0.011 million years ago based on argon-argon dating.
Effects of the Chicxulub Impact
The impact created a 145-180 km wide crater releasing around 100 tera tonnes of TNT and blasting millions of tons of sulphate into the atmosphere. The immediate effects of the impact included mega-tsunamis and earthquakes of magnitude greater than magnitude 11. Fires were ignited almost simultaneously due to the infrared radiation from the re-entrant ejecta of the impact but these fires were a local phenomenon as there is no sedimentological record of a continental scale fire [1]. A thorough analysis of the impact suggests that the asteroid hit a gypsum deposit, generating sulphate aerosols, causing over a decade of global cooling and intense pulses of acid rain, which according to impact models were estimated to last up to 12 years [2]. Consequently, photosynthetic activities were shut down for 6 to 8months due to particles blocking sunlight transmission [3]. The upwelling of deep ocean water as the consequence of the sulphate cooling drastically altered ocean stratification and circulation [4]
More than 90 percent of the plankton was extinguished, leading to the inevitable collapse of the oceanic food chain. As a result, the extinction of primary producers produced starvation high up in the food chain. The impact destabilized the ecosystem leading to the destruction of an entire species. Moreover, by the late Cretaceous, many groups of dinosaurs were experiencing negative net diversification, making them even more vulnerable [5].
The Deccan Traps
The Deccan Traps in India was formed by an extended period of volcanic activity. The impact may have increased the permeability that allowed magma to reach the surface and produce voluminous flows. It also was associated with a simultaneous increase in volcanism to which recovery may have been possible until the volcanism slowed down 500,000 years later [6] [7].
In addition to the stress imposed by the impact, it is likely that volcanism created a background of environmental stress - release of dust, blocking of sunlight and thereby reducing photosynthesis resulting in increased greenhouse effect - that made it difficult for the ecosystems to survive. The combination of the impact and the resulting increase in eruptive volume may have been responsible for the mass extinctions that occurred at the same time.
Conclusion
The temporal match between ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record led us to conclude that the Chicxulub impact triggered the mass extinction [2]. Although some effects of the theory are under scrutiny, alternative theories fail to explain the distribution of ejecta and the timing of mass extinction. Hence, the contributed effects of the impact, volcanism and also sea regressions during the late Cretaceous can be concluded as the cause of the mass extinction.
References
[1] M. E. C. A. R. S. A. R. H. A. C. S. Claire M. Belcher, "Fireball passes and nothing burns—The role of thermal radiation in the Cretaceous-Tertiary event: Evidence from the charcoal record of North America," Geology , 2003.
[2] L. A. I. A. J. A. A. P. J. B. P. R. B. T. J. B. G. L. C. Peter Schulte, "The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary," New Series, vol. 327, pp. 1214-1218, 2010.
[3] S. L. D. a. C. R. M. Kevin O. Pope, "Meteorite impact and the mass extinction of species at the Cretaceous/Tertiary boundary".
[4] A. B. Pope Kevin, "Energy, volatile production, and climatic effects of the Chicxulub Cretaceous/Tertiary impact".
[5] M. J. B. C. V. Manabu Sakamoto, "Dinosaurs in decline tens of millions of years before their final extinction".
[6] P. R. R. Matthew E. Clapham, "Flood basalts and mass extinctions".
[7] C. J. S. Paul R Renne, "State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact".
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