Cretaceous-Tertiary extinction event - Alvarez hypothesis
In 1980, a team of researchers led by Nobel-prize-winning physicist Luis Alvarez, his ***, geologist Walter Alvarez, and a group of colleagues discovered that fossilized sedimentary layers found all over the world at the Cretaceous-Tertiary boundary, 65.5 million years ago contain a concentration of iridium hundreds of times greater than normal. The end of the Cretaceous coincided with the end of the dinosaurs. It was in general a period of extraordinary mass extinction, leading to the Tertiary Period of the Cenozoic Era, in which mammals came to dominate on Earth. The paper suggested that the dinosaurs had been ****** off by the impact of a ten-kilometer-wide asteroid on Earth (see impact event). Two facts supporting this conclusion are that
- iridium is relatively abundant in many asteroids, and
- the isotopic composition of iridium in K-T layers resembles that of asteroids more closely than that of terrestrial iridium.
Iridium is very rare on Earth's surface, but much more common in the Earth's interior as well as in extraterrestrial objects, such as asteroids and comets. Furthermore, chromium isotopic anomalies are found in Cretaceous-Tertiary boundary sediments which strongly supports the impact theory and suggests that the impactor must have been an asteroid or a comet composed of material similar to carbonaceous chondrites.
The resulting blast would have been hundreds of millions of times more devastating than the most powerful nuclear weapon ever detonated, may have created a hurricane of unimaginable fury, and certainly would have thrown massive amounts of dust and vapor into the upper atmosphere and even into space.
A global firestorm may have resulted as the incendiary fragments from the blast fell back to Earth. Analyses of fluid inclusions in ancient amber suggest that the oxygen (O
2) content of the atmosphere was very high (30 - 35%) during the late Cretaceous. This high O
2 atmospheric content would have supported massive combustion. The level of atmospheric O
2 plummeted in the early Tertiary.
In addition, the worldwide cloud would have ****** off sunlight for months, resulting in a darkness that prevented photosynthesis and depleting food resources. During this interval of reduced sunlight a "long winter" may have also been involved in the extinction. Gradually skies cleared but greenhouse gases from the impact caused an increase in temperature for many years.
The impact target rocks also produced acid rains that would have inflicted further hardship on the environment, but recent work suggests this was relatively *****. Chemical buffers would have reduced the effect, and survival of ******* prone to acid rain damage (such as frogs) indicate this was not a major contributor to extinction (see Kring, D.A. GSA Today v. 10, no.8).
Although further studies of the K-T layer consistently showed the excess of iridium, the idea that the dinosaurs were exterminated by an asteroid remained a matter of controversy among geologists and paleontologists for over a decade.
Cretaceous-Tertiary extinction event - Chicxulub crater
One problem with the "Alvarez hypothesis" (as it came to be known) was that no documented crater matched the event. This was not a lethal blow to the theory; although the crater resulting from the impact would have been 150 to 200 kilometers in diameter, Earth's geological processes tend to hide or destroy craters over time. The discovery by Alan R. Hildebrand and Glen Penfield of a crater buried under Chicxulub in the Yucatan as well as various types of debris in North America and Haiti have lent credibility to this theory (see Chicxulub Crater). Most paleontologists now agree that an asteroid did hit the Earth 65 million years ago, but many dispute whether the impact was the sole cause of the extinctions.
Cretaceous-Tertiary extinction event - Deccan traps
Several paleontologists remained skeptical about the impact theory, as their reading of the fossil record suggested that the mass extinctions did not take place over a period as short as a few years, but instead occurred gradually over about ten million years, a time frame more consistent with longer term events such as massive volcanism. Several scientists think the extensive volcanic activity in India known as the Deccan Traps may have been responsible for, or contributed to, the extinction. A partial reason for the rejection of the impact theory may have been a certain general distrust of a group of physicists intruding into the paleontologists' domain of expertise.
Luis Alvarez, who died in 1988, replied that paleontologists were being misled by sparse data. His assertion did not go over well at first, but later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely or at least partly due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level and massive volcanic eruptions in India (Deccan Traps sequence), and these may have contributed to the extinctions.
A very large impact crater has been recently reported in the sea floor off the west coast of India 2. This, the Shiva crater (450/600 km diam.), has also been dated at about 65 million years at the K-T boundary. The researchers suggest that the impact may have been the triggering event for the Deccan Traps. However, this feature has not yet been accepted by the geologic community as an impact crater and may just be a sinkhole depression caused by salt withdrawal.
Cretaceous-Tertiary extinction event - Multiple impact event
Several other craters also appear to have been formed at the K-T boundary. This suggests the possibility of near simultaneous multiple impacts from perhaps a fragmented asteroidal object, similar to the Shoemaker-Levy 9 cometary impact with Jupiter.
- Boltysh crater (24 km diam., 65.17 ± 0.64 Ma old) in Ukraine
- Silverpit crater (20 km diam., 60-65 Ma old) in the North Sea
- Eagle Butte crater (10 km diam., < 65 Ma old) in Alberta, Canada
- Vista Alegre crater (9.5 km diam., < 65 Ma old) in Paraná State, Brazil
Note: Ma means million years.
Cretaceous-Tertiary extinction event - Supernova hypothesis
Another proposed cause for the K-T extinction event was cosmic radiation from a relatively nearby supernova explosion. The iridium anomaly at the boundary could support this hypothesis. The fallout from a supernova explosion should contain the plutonium isotope Pu-244, the longest-lived plutonium isotope (half-life 81 Myr), that is not found in earth rocks. However, analysis of the boundary layer sediments revealed the absence of Pu-244, thus essentially disproving this hypothesis.
