KT Boundary Extinction

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K/T Boundary Extinction

• Cretaceous-Tertiary boundary (end of Mesozoic)
• Second largest mass extinction
• Half of life on Earth died out

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K/T Boundary Extinction

• Tropical groups suffered most
• Seawater cooling, global regression
• Many animals in decline before K/T boundary
• Mammals, birds, turtles, crocodiles, lizards,
  snakes and amphibians unaffected

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K/T Boundary

• Cretaceous-Tertiary (K/T) boundary (65 Ma)
• Second largest mass extinction in Earths history
• Half of life on Earth died out (3/4 species)

http//www.mhhe.com/earthsci/geology/mcconnell/gti
/metaphor.htm
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What did Earth look like 65 Ma?
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Mass Extinctions Crises in the History of Life

• Greatest mass extinction took place at the end of
  the Paleozoic Era
• K/T extinction has attracted more attention
  because it affected dinosaurs

http//www.lpl.arizona.edu/SIC/impact_cratering/Ch
icxulub/totaldiversity.jpg
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What caused the K/T extinction?

• Volcanic eruptions?
• Regression?
• Climate change?
• Impact?
• All of the above?

http//www.psi.edu/projects/ktimpact/ktimpact.html
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What caused the K/T extinction?

• Numerous hypotheses proposed to explain Mesozoic
  extinctions
• One proposal has become popular since 1980
• based on a discovery in Italy
• 2.5-cm-thick clay layer at the Cretaceous-Tertiary
  boundary with a remarkably high concentration of
  iridium
• High iridium concentrations have now been
  identified at many other Cretaceous-Tertiary
  boundary sites

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Cretaceous-Tertiary Boundary

• K/T boundary site in Italy
• 2.5-cm-thick clay layer shows high concentration
  of iridium

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Boundary Clay

• Closeup view of the boundary clay in the Raton
  Basin, New Mexico

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Boundary Clay

• Deep sea core
• Recovered by Joides Resolution in 1997

http//www.usssp-iodp.org/Education/poster.html
http//www.nmnh.si.edu/paleo/blast/k_t_boundary.ht
m
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Iridium Anomaly

• Significance of the iridium anomaly
• iridium is rare in crustal rocks
• found in much higher concentrations in some
  meteorites
• Accordingly, some investigators propose
• meteorite impact explains the anomaly
• meteorite perhaps 10 km in diameter
• impact set in motion a chain of events leading to
  extinctions

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Iridium Anomaly Origin

• The iridium anomaly is real
• but its origin and significance are debated
• We know very little about the distribution of
  iridium in crustal rocks or how it may be
  distributed and concentrated
• Some geologists suggest that the iridium was
  derived from within Earth by volcanism, but this
  idea is not very well supported by evidence

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Boundary Sites

• Some Cretaceous-Tertiary boundary sites also
  contain
• soot
• shock-metamorphosed quartz grains

http//wwwdsa.uqac.ca/mhiggins/MIAC/chicxulub.htm
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Meteorite Impact Crater

• Proposed meteorite impact crater
• Centered on Chicxulub on the Yucatán Peninsula of
  Mexico
• Discovered in 1950s, interpreted to be volcanic

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Chicxulub Crater
www.ig.utexas.edu GAIL CHRISTESON
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Chicxulub Crater
Palaeo.gly.bris.ac.uk/Communication/Hanks/Fig6.htm
l
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Chicxulub Crater
http//wwwdsa.uqac.ca/mhiggins/MIAC/chicxulub.htm
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Chicxulub Crater
http//www.lpl.arizona.edu/SIC/impact_cratering/Ch
icxulubprpage/Chicxulub_drilling_hires.jpg
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Impact Site

• Centered on the town of Chicxulub, Yucatan
  Peninsula of Mexico
• The 180-km diameter structure lies beneath layers
  of sedimentary rock and appears to be the right
  age

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Evidence at Chicxulub

• Shocked quartz
• Tsunami deposits and tektites, small pieces of
  rock that were melted during the proposed impact
  and hurled into the atmosphere

http//wwwdsa.uqac.ca/mhiggins/MIAC/chicxulub.htm
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What happened?
The moment of impact 65 million years ago near
what is now the Yucatan Peninsula ...
... and the Chicxulub crater, a few days later.
Note the inner ring.
http//www.space.com/scienceastronomy/planetearth/
asteroid_jello_001122.html
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Impact Consequences

• According to the impact hypothesis
• 60 times the mass of the meteorite was blasted
  from the crust high into the atmosphere
• heat generated at impact started raging forest
  fires that added more particulate matter to the
  atmosphere
• Sunlight was blocked for several months
• caused a temporary cessation of photosynthesis
• food chains collapsed and extinctions followed

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Acid Rain

• With sunlight greatly diminished, Earth's surface
  temperatures were drastically reduced, adding to
  the biologic stress
• Another proposed consequence of an impact is that
  sulfuric acid (H2SO4) and nitric acid (HNO3)
  resulted from vaporized rock and atmospheric
  gases
• Both would have contributed to strongly acid rain
  that might have had devastating effects on
  vegetation and marine organisms

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Terrestrial and Marine Extinctions

• Even if a meteorite did hit Earth, did it lead to
  these extinctions?
• If so, both terrestrial and marine extinctions
  must have occurred at the same time
• To date, strict time equivalence between
  terrestrial and marine extinctions has not been
  demonstrated
• The selective nature of the extinctions is also a
  problem

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Impact Hastened Extinction?

• In the terrestrial realm
• large animals were the most drastically affected
• but not all dinosaurs were large
• crocodiles, close relatives of dinosaurs,
  survived although some species died out
• Some paleontologists think that dinosaurs, some
  marine invertebrates, and many plants were
  already on the decline and headed for extinction
  before the end of the Cretaceous
• A meteorite impact may have simply hastened the
  process

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Satisfaction?

• In the final analysis, Mesozoic extinctions have
  not been explained to everyones satisfaction
• Most geologists now concede that a large
  meteorite impact occurred, but we also know that
  vast outpourings of lava were taking place in
  what is now India
• Perhaps these brought about detrimental
  atmospheric changes

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Shallow Seas Withdrew

• Furthermore, the vast shallow seas that covered
  large parts of the continents had mostly
  withdrawn by the end of the Cretaceous
• Mild equable Mesozoic climates became harsher and
  more seasonal
• But the fact remains that these extinctions were
  very selective, and no single explanation
  accounts for all aspects of this crisis in life
  history