Paleontologists have shown that the rate of extinction of species varies dramatically with time (see the Figure).
The major extinction events, shown as peaks marked by letters (G, R, etc.) occur at the boundaries between previously established geologic epochs. For example, perhaps the best known mass extinction occurred at the boundary between the Cretaceous and Tertiary periods, around 65,000,000 years ago (marked with an M in the figure). The figure shows that roughly half of all marine animal species were wiped out at that time. More famously, all dinosaurs were wiped out as well. The largest (in the sense of highest percentage of extinctions) of all mass extinction events occurred at the end of the Permian period, around 240,000,000 years ago (marked with a G in the figure). Roughly 90 percent of all marine animals went extinct at the end of the Permian.
Various reasons have been proposed for the extinction of the dinosaurs, including climate change, competition from mammals, massive volcanic eruptions, and asteroid or comet impacts. Deciding which, if any, of these is correct is a subtle task.
In the case of the end Cretaceous we now have a very compelling story; An asteroid, with an approximate diameter of 10km, struck what is now the Yucatan peninsula. The first hint that such an impact occurred was provided by the discovery of an excess of irridium at the Cretaceous-Tertiary boundary at Gubbio, Italy. Subsequently, this irridium anomoly was found, at a level of a few parts per billion, at a large number of sights around the globe where the boundary could be accessed. An example of the data can be seen by clicking here; the top left figure shows the geologic structure of the Cretaceous-Tertiary boundry (marked T/C) at a particular location, the top right figure shows the irridium abundance near the boundary, and the lower figure shows the gamma ray spectrum produced by neutron activation of a sample of material taken from the boundary region.
Irridium is very rare in the Earth's crust, although it is more abundant in the mantle. It is also relatively abundant in meteorites, and hence in asteroids. The discovery paper, by Alvarez, Alvarez, Assaro suggested that the irridium anamoly was the result of an asteroid (or comet) striking the Earth. The impact would vaporize the impactor, launching irridium into the atmosphere, later to fall back and cover most of the Earth.
It took about ten years to actually locate the impact.
The remnants of the crater can be seen in maps showing slight changes in the local vertical (gravity). This formation is now called the Chicxulub (Chik-shu-lub) crater.Geological evidence that the structure is an impact crater includes the recovery of impact melts (obtained by drilling) showing evidence of shocking, large volumes of tektites (small glassy spherules believed to be produced by the high temperatues associated with an impact) found in Cretaceous-Tertiary boundary material found in Wyoming and Mexico as well as other locations, and evidence for tsunamis or tidal waves at the Cretaceous-Tertiary boundary throughout the Gulf of Mexico. Radioisotope dating places the crater at the Cretaceous-Tertiary boundary, as do studies of the foraminifera above and below the level associated with the impact.
Alvarez and company suggested a link between the impact and the extinctions; dust launched into the atmosphere would block out the sun, killing plants and hence disrupting food chains. Other proposed killing mechanisms include a global fire triggered by the reentry of impact ejecta into the atmosphere (the ejecta heat up as they fall back, an radiate infrared radiation strong enough to burn any organic material exposed on the surface), acid rain produced by sulfur found in rocks in the impact region or by nitrogen in the atmosphere, and global warming produced by green house gases.
It has been suggested that increased volcanic activity may also have
caused mass extinctions. The Deccan traps in India, a large volcanic structure,
have been dated to the same time as the C/T boundary. It is also
very suggestive that the end of the Permian, when the largest mass extinction
occurred, is rougly coincident with the formation age of the Siberian traps,
an even larger volcanic structure. However, recent work by Mukhopadhyay,
Farley, and Montanari strongly suggests that the marine extinctions occurred
in less than 10,000 years, followed by a very rapid recovery, also on a
10,000 to 100,000 year time scale.
The following figure and table illustrate their results.
Since the Deccan event lasted more than 500,000 years, an impact would appear to be the more likely cause of the marine extinctions.
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