Luis, left, and his son Walter Alvarez, right, at the K-T Boundary in Gubbio, Italy, 1981
The Alvarez hypothesis posits that the mass extinction of the dinosaurs and many other living things during the Cretaceous–Paleogene extinction event was caused by the impact of a large asteroid on the Earth.
Prior to 2013, it was commonly cited as having happened about 65
million years ago, but Renne and colleagues (2013) gave an updated value
of 66 million years. Evidence indicates that the asteroid fell in the Yucatán Peninsula, at Chicxulub, Mexico. The hypothesis is named after the father-and-son team of scientists Luis and Walter Alvarez, who first suggested it in 1980. Shortly afterwards, and independently, the same was suggested by Dutch paleontologist Jan Smit.
In March 2010, an international panel of scientists endorsed the asteroid hypothesis, specifically the Chicxulub impact,
as being the cause of the extinction. A team of 41 scientists reviewed
20 years of scientific literature and in so doing also ruled out other
theories such as massive volcanism. They had determined that a 10–15 km
(6–9 mi) space rock hurtled into earth at Chicxulub. For comparison, the
Martian moon Phobos
is 11 km (7 mi) and Mount Everest is just under 9 km (5.6 mi). The
collision would have released the same energy as 100,000,000 megatonnes
of TNT (4.2×1023 J), over a billion times the energy of the atomic bombs dropped on Hiroshima and Nagasaki.
A 2016 drilling project into the peak ring of the crater strongly
supported the hypothesis, and confirmed various matters that had been
unclear until that point. These included the fact that the peak ring
comprised granite (a rock found deep within the earth) rather than
typical sea floor rock, which had been shocked, melted, and ejected to
the surface in minutes, and evidence of colossal seawater movement
directly afterwards from sand deposits. Crucially the cores also showed a
near complete absence of gypsum,
a sulfate-containing rock, which would have been vaporized and
dispersed as an aerosol into the atmosphere, confirming the presence of a
probable link between the impact and global longer-term effects on the
climate and food chain.
History
In 1980, a team of researchers led by Nobel prize-winning physicist 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 (K–Pg boundary, formerly called Cretaceous–Tertiary or K–T boundary) contain a concentration of iridium
hundreds of times greater than normal. Iridium is extremely rare in the
Earth's crust because it is very dense and has the affinity for iron
that characterizes the siderophile elements (see Goldschmidt classification),
and therefore most of it sank into the Earth's core while the earth was
still molten. The Alvarez team suggested that an asteroid struck the
earth at the time of the Cretaceous–Paleogene boundary.
Previously, in a 1953 publication, geologists Allan O. Kelly and Frank Dachille
analyzed global geological evidence suggesting that one or more giant
asteroids impacted the Earth, causing an angular shift in its axis,
global floods, fire, atmospheric occlusion, and the extinction of the dinosaurs. There were other earlier speculations on the possibility of an impact event, but without strong confirming evidence.
Evidence
The evidence for the Alvarez impact hypothesis is supported by chondritic meteorites and asteroids which contain a much higher iridium concentration than the Earth's crust. The isotopic
ratio of iridium in meteorites is similar to that of the
Cretaceous–Paleogene boundary layer but significantly different from the
ratio in the Earth's crust. Chromium isotopic anomalies found in Cretaceous–Paleogene boundary sediments are similar to that of an asteroid or a comet composed of carbonaceous chondrites. Shocked quartz granules, glass spherules and tektites,
indicative of an impact event, are common in the Cretaceous–Paleogene
boundary, especially in deposits from around the Caribbean. All of these
constituents are embedded in a layer of clay, which the Alvarez team
interpreted as the debris spread all over the world by the impact. The location of the impact was unknown when the Alvarez team developed their hypothesis, but later scientists discovered the Chicxulub Crater in the Yucatán Peninsula, now considered the likely impact site.
Badlands near Drumheller, Alberta where erosion has exposed the K–Pg boundary.
Using estimates of the total amount of iridium in the K–Pg layer, and
assuming that the asteroid contained the normal percentage of iridium
found in chondrites,
the Alvarez team went on to calculate the size of the asteroid. The
answer was about 10 kilometers (6 mi) in diameter, about the size of
Manhattan. Such a large impact would have had approximately the energy of 100 million megatons, i.e. about 2 million times as great as the most powerful thermonuclear bomb ever tested.
Paul Renne of the Berkeley Geochronology Center has reported that the date of the asteroid event is 66,038,000 years ago, plus or minus 11,000 years, based on the radioactive decay of argon. He further posits that the mass extinction of dinosaurs occurred within 33,000 years of this date.
In April 2019 a paper was published in PNAS which describes evidence from a fossil site in North Dakota
that the authors say provides a "postimpact snapshot" of events after
the asteroid collision "including ejecta accretion and faunal mass
death". The team found that the tektites that had peppered the area were present in amber
found on the site and were also embedded in the gills of about 50
percent of the fossil fish. They were also able to find traces of
iridium. The authors – who include Walter Alvarez – postulate that shock
of the impact, equivalent to an earthquake of magnitude 10 or 11, may
have led to seiches,
oscillating movements of water in lakes, bays, or gulfs, that would
have reached the site in North Dakota within minutes or hours of the
impact. This would have led to the rapid burial of organisms under a
thick layer of sediment. Coauthor David Burnham of the University of
Kansas was quoted as saying “They’re not crushed, it’s like an avalanche
that collapses almost like a liquid, then sets like concrete. They were
killed pretty suddenly because of the violence of that water. We have
one fish that hit a tree and was broken in half.”
Impact
The most
easily observable consequence of such an impact would be a vast dust
cloud which would block sunlight and prevent photosynthesis for a few
years, an event called an impact winter. This would account for the extinction of plants and phytoplankton
and of all organisms dependent on them (including predatory animals as
well as herbivores). But small creatures whose food chains were based on
detritus
would have a reasonable chance of survival. It is estimated that
sulfuric acid aerosols were injected into the stratosphere, leading to a
10–20% reduction of solar transmission normal for that period. It
would have taken at least ten years for those aerosols to dissipate.
Global firestorms may have resulted as incendiary fragments from the blast fell back to Earth. Analyses of fluid inclusions in ancient amber suggest that the oxygen content of the atmosphere was very high (30–35%) during the late Cretaceous. This high O2 level would have supported intense combustion. The level of atmospheric O2 plummeted in the early Paleogene Period. If widespread fires occurred, they would have increased the CO2 content of the atmosphere and caused a temporary greenhouse effect once the dust cloud settled, and this would have exterminated the most vulnerable survivors of the "long winter".
The impact may also have produced acid rain,
depending on what type of rock the asteroid struck. However, recent
research suggests this effect was relatively minor. Chemical buffers
would have limited the changes, and the survival of animals vulnerable
to acid rain effects (such as frogs) indicate this was not a major contributor to extinction.
Impact hypotheses can only explain very rapid extinctions, since
the dust clouds and possible sulphuric aerosols would wash out of the
atmosphere in a fairly short time – possibly under ten years.
Although further studies of the K–Pg layer consistently show the
excess of iridium, the idea that the dinosaurs were exterminated by an
asteroid remained a matter of controversy among geologists and paleontologists for more than a decade.
Criticism
Amongst others, Charles B. Officer and Gerta Keller have been critical of the theory.
Officer and Jake Page describe in their The Great Dinosaur Extinction Controversy how
- A dust cloud from an asteroid impact could not have existed because many marine plants that require uninterrupted sunlight were unaffected
- The iridium deposits appear to be the work of volcanoes
- The extinctions occurred gradually, not instantaneously "Even if a meteoric impact occurred at K-T time, it simply could not explain the extinction record."
- The Chicxulub structure is a volcanic sequence of late Cretaceous age; it is not an impact meltsheet of Cretaceous-Tertiary age
- Papers disputing the Alvarez hypothesis were summarily rejected by journals without review
Keller has focussed on Deccan Traps volcanism as a likely cause of a more gradual extinction.
2016 Chicxulub crater drilling project
In 2016, a scientific drilling project drilled deep into the peak ring of the Chicxulub impact crater, 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. They confirmed that the rock composing the peak ring had been
subjected to immense pressures and forces and had been melted by immense
heat and shocked by immense pressure from its usual state into its
present form in just minutes; the fact that the peak ring was made of
granite was also significant, since granite is not a rock found in
sea-floor deposits, it originates much deeper in the earth and had been
ejected to the surface by the immense pressures of impact; that gypsum, a sulfate-containing rock that is
usually present in the shallow seabed of the region, had been almost
entirely removed and must therefore have been almost entirely vaporized
and entered the atmosphere, and that the event was immediately followed
by a huge megatsunami
(a massive movement of sea waters) sufficient to lay down the largest
known layer of sand separated by grain size directly above the peak
ring.
These strongly support the hypothesis that the impactor was large
enough to create a 120-mile peak ring, to melt, shock and eject basement granite from the midcrust
deep within the earth, to create colossal water movements, and to eject
an immense quantity of vaporized rock and sulfates into the atmosphere,
where they would have persisted for a long time. This global dispersal
of dust and sulfates would have led to a sudden and catastrophic effect
on the climate worldwide, large temperature drops, and devastated the food chain.
