The Extraterrestrial Cause for the Cretaceous-Tertiary Extinction
In the 570-million-year period for which abundant fossil remains are available,
there have been five great biological crises, during which many groups
of organisms died out. The great extinction is used to define the boundary
between the Cretaceous and Tertiary periods (Ct-boundary). There, 65 million
years ago the dinosaurs died out, but why?
Picture of the Kt- boundary (taken from the web, origin lost)
When the tiny layer of clay dating back to 65 million years ago became
a matter of interest for scientists, nobody had linked it to the extinction
of the dinosaurs. During the 70s, Isabella Premoli Silva, a paleontologist
at the University of Milan, was carrying out research on the microfossils
in the rocks of Bottaccione Gorge. She got
interested in the tiny layer of clay because it pointed out a discontinuity
in the evolution of the microscopic organisms she was studying. In fact,
by observing through the microscope sections taken from the lower part
of the red debris in Bottaccione Gorge, a variety of foraminiferas can
be found: globorotalias and globigerinas, but above all globotruncanas.
After reaching an advanced development, this variety of microrganisms suddenly
disappears, to be substituted by a single type of globigerinas, the so
called "eugubina" one, in the upper part of the red debris. The paleontologist
immediately associated the K - T border with the tiny layer of clay which
showed no microfossils inside, as well as to the drastic disappearance
of two thirds of plant and animal life on the Earth. She wanted to find
out how long it had taken the layer to form in order to determine how long
life had almost disappeared from the planet. She contacted Walter Alvarez,
a geologist at the University of Berkley, California, who was then in Gubbio
because he was carrying out research on paleomagnetism. Professor Alvarez
went back to California and carried out analyses together with his research
group, which included his father, the physicist Luis Alvarez, and the nuclear
chemists Helen Michel and Frank Asaro. While analyzing the Bottaccione
Gorge rocks, they used a standard method, which consisted in measuring
the time of sedimentation of a layer of rock by the quantity of iridium
it contains. The presence of iridium in the rocks on the surface of the
Earth is actually small, and it is mainly due to "cosmic dust", that is
to say, the continuous rain of micrometeorites which constantly falls down
from the sky and goes along with the formation of the rocky layers constituting
the surface of the planet. Unfortunately the standard method did not allow
the American scientists to answer the Italian paleontologist’'s question
of how long the tiny layer of clay from Bottaccione Gorge had taken to
form: surprisingly enough, it contained a quantity of iridium which was
far higher than normal conditions would suggest. Such concentrations could
not only be due to "cosmic dust": some other causes had to be found. Professor
Premoli Silva’s question about how long life had disappeared on the Earth
remained unsolved , but the problem of what had happened in that period
of time started to be a matter of interest for the American researcher.Walter
Alvarez and his research group concentrated on what could have caused such
a vast presence of Iridium in the tiny layer of clay in the rocks in Bottacione
Gorge. There were many possible explanations: the explosion of a supernova,
or a comet or meteorite falling from the sky. A Supernova is a star which
becomes extremely bright because of the explosion of its nucleous. A supernova
is a terminal phase in the life of a star, which can then become a cloud
of gas or a compact celestial body such as a pulsar or a black hole. The
hypothesis of a supernova was taken into consideration because the explosion
of a nearby star could explain the presence of iridium
on the surface of the Earth, but it was rejected because not only iridium
but also radioactive material would have been found. Comets are celestial
bodies belonging to the solar system. They have a small mass, and their
orbit is in general more elongated than that of the planets. They become
visible to the human eye when they get near to the sun, when they are far
away from the sun the only thing that can be seen of them is the nucleous,
an irregular shaped mass composed of ices mixed with soot-like matter -
perhaps carbon in the form of fine dust. As comets gravitate around the
sun, the icy components sublimates and frees the dust, and a coma of gas
and dust around the nucleous is formed. Because of solar wind the coma
around the nucleous is blown in the direction opposite the sun, developing
two tails, one of dust and one of gas. Most comets come from a spherical
region surrounding the solar system called the Oort cloud, and may come
out of this cluod and enter the solar system when disturbed by the gravitational
force of a nearby star.Comets may leave particles of dust along their orbit,
and when the Earth crosses these ordits full of dust there is the so-called
meteorid shower.This hypothesis was rejected because a comet is mostly
made of water in the form of ice.
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