What Really Killed The Dinosaurs?
The Last Day of the Dinosaurs - The Mystery Continues to Deepen
|What Really Killed The Dinosaurs?
The Last Day of the Dinosaurs - The Mystery Continues to Deepen
Until recently most scientists thought they knew what killed off the
dinosaurs. A 10km-wide meteorite had smashed into the Yucatan peninsula
in Mexico, causing worldwide forest fires, tsunamis several kilometres
high, and an 'impact winter' - in which dust blocked out the sun for
months or years.
The discovery that birds are a type of dinosaur showed that dinosaurs in general are not, in fact, extinct as is commonly stated.
However, all non-avian dinosaurs as well as many groups of birds did suddenly become extinct approximately 65 million years ago. Many other groups of animals also became extinct at this time.
It was thought that the dinosaurs were blasted, roasted
and frozen to death, in that order.
But now a small but
vociferous group of scientists believes there is increasing evidence
that this 'impact' theory could be wrong. That suggestion has generated
one of the bitterest scientific rows of recent times.
theory: The impact theory was beautifully simple and appealing. Much of
its evidence was drawn from a thin layer of rock known as the 'KT
This layer is 65 million years old (which is around the time
when the dinosaurs disappeared) and is found around the world exposed in
cliffs and mines. For supporters of the impact theory, the KT
boundary layers contained two crucial clues.
In 1979 scientists
discovered that there were high concentrations of a rare element called
iridium, which they thought could only have come from an asteroid.
underneath the iridium was a layer of 'spherules', tiny balls of rock
which seemed to have been condensed from rock which had been vaporized
by a massive impact.
On the basis of the spherules and a range of
other evidence, Dr Alan Hildebrand of the University of Calgary deduced
that the impact must have happened in the Yucatan peninsula, at the
site of a crater known as Chicxulub.
Chemical analysis later confirmed
that the spherules had indeed come from rocks within the crater. Challenging
the theory: A group of scientists led by Prof Gerta Keller of Princeton
and Prof Wolfgang Stinnesbeck of the University of Karlsruhe begged to
They uncovered a series of geological clues which suggests the
truth may be far more complicated. In short, that the crater in the
Yucatan is too old to have killed off the dinosaurs. They
concentrated on a series of rock formations in Mexico where the iridium
layer was separated from the spherule layer by many metres of sandstone.
That opinion sparked a massive row, as the supporters of the impact
theory such as Prof Jan Smit of Vrije Universiteit, Amsterdam, rubbished
Keller's ideas. Smit argued that the sandstone had been deposited by
massive tsunami waves caused by the asteroid, and so did not undermine
the idea of a single impact.
But Keller's team found evidence -
such as ancient worm burrows - that suggested that the deposition of the
sandstone had been interrupted many times. They concluded that there
was a gap of some 300,000 years between the deposition of the spherules
(from the Chicxulub crater) and the iridium (from an asteroid).
Therefore there must have been two impacts.
In March 2010 an international panel of scientists endorsed the asteroid hypothesis, specifically the Chicxulub impact, as being the cause of the extinction.
The K–T boundary is a geological signature, usually a thin band, dated 65.5 million years ago. K is the traditional abbreviation for the Cretaceous period, and T is the abbreviation for the Tertiary period.
The boundary marks the end of the Mesozoic era and the beginning of the Cenozoic era, and is associated with the Cretaceous–Tertiary extinction event, a mass 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 miles) space rock hurtled into earth at Chicxulub.
The rock's size could be approximately the entire size of Martian moon Deimos (mean radius 6.2 km); the collision would have released the same energy as 100 teratonnes of TNT, over a billion times the energy of the bombs dropped on Nagasaki and Hiroshima).
Using estimates of the total amount of iridium in the K–T 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 miles) in diameter, about the size of Manhattan. Such a large impact would have had approximately the energy of 1 x 108 megatons, i.e. about 2 million times as great as the most powerful thermonuclear bomb ever tested.
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. 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.
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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.
The extinctions at the end of the Cretaceous were caused by one or more catastrophic events, such as massive asteroid or meteorite impacts, or increased volcanic activity.
These geological events may have reduced sunlight and hindered photosynthesis, leading to a massive disruption in Earth's ecology.
This high O2 level would have supported intense combustion. The level of atmospheric O2 plummeted in the early Tertiary 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–T 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.