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Cretaceous-Tertiary extinction event
The Cretaceous-Tertiary (KT) extinction event, also known as the KT boundary (the K is from kreta, the greek root of the word cretaceous, meaning chalk), was a period of massive extinction of species, about 65.5 million years ago. It corresponds to the end of the Cretaceous Period and the beginning of the Tertiary Period.
The duration of this extinction event (like others) is unknown. Many forms of life perished (embracing approximately 50% of all genera), the most often mentioned among them being those of all the families of the three orders of dinosaurs. Many explanations for this event have been proposed, the most widely-accepted being the results of an impact on the Earth of an object from space.
Casualties of the Extinction
A wide range of organisms became extinct at the end of the Cretaceous period. The most conspicuous, of course, were the dinosaurs. While there is evidence that dinosaur diversity declined in the Late Cretaceous of North America, many species are known from the Hell Creek and Lance Formations of the Late Cretaceous. These include six or seven families of theropods and a similar number of ornithischians. Among the Dinosauria, the only survivors were the birds, but birds suffered heavy losses. A number of diverse groups became extinct, including the Enantiornithes and Hesperornithiformes; the last of the pterosaurs also went extinct. A number of mammal groups also became extinct. In the sea, many species of phytoplankton were wiped out. The great sea reptiles of the Cretaceous, the mosasaurs and plesiosaurs, also fell victim to extinction. Among molluscs, the ammonites, a diverse group of coiled cephalopods, were exterminated, as were the specialized rudist and inoceramid clams. Freshwater mussels and snails also suffered heavy losses in North America. Much less is known about how the K-T event affected the rest of the world. It should be emphasized that the survival of a group does not mean that the group was unaffected: a species might have been 99% annihilated by the asteroid strike, yet still manage to survive.
Impact event and iridium
In 1980, a team of researchers led by Nobel-prize-winning physicist Luis Alvarez, his son, 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 relatively high concentration of iridium, hundreds of times greater than normal. The end of the Cretaceous coincided with the end of the dinosaurs and 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 killed 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 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 content of the atmosphere was very high (30 - 35%) during late Cretaceous . This high O2 atmospheric content would have supported massive combustion. The level of atmospheric O2 plummeted in the early Tertiary. In addition the worldwide cloud would have choked off sunlight for years, resulting in a "long winter" that wiped out many existing species, as well as creating "acid rains" that would have inflicted further hardship on the environment.
Although further studies of the "Cretaceous-Tertiary" or "K-T" layer consistently showed the excess of iridium, the idea that the dinosaurs had been exterminated by an asteroid remained a matter of controversy among geologists and paleontologists for over a decade.
One problem was that no known 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. Still, finding a crater would have buttressed the "Alvarez hypothesis", as it came to be known. The discovery by Alan K. 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 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.
Other scientists think the extensive volcanic activity in India known as the Deccan Traps may have been responsible for, or contributed to, the extinction. However, paleontologists remained skeptical, 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 massive volcanism. There was also a certain general distrust of a group of physicists intruding into their 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 . Two other craters 3, the Boltysh crater (24 km diam., 65.17 ± 0.64 My old) in Ukraine and the Silverpit crater (20 km diam., 60-65 Myr old) in the North Sea, 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 such as the Shoemaker-Levy 9 cometary impact with Jupiter.
Skeptics remain. Although there is now general agreement that there was at least one huge impact at the end of the Cretaceous that led to the iridium enrichment of the K-T boundary layer, it is difficult to directly connect this to mass extinction, and in fact there is no clear linkages between an impact and any other incident of mass extinction although research on other events also implicate impacts.
One interesting note about the K-T Event is that most of the larger animals that survived were to some degree aquatic, implying that aquatic habitats may have remained more hospitable than land habitats.
The impact and volcanic theories can be labeled "fast extinction" theories. There are also a number of slow extinction theories. Studies of the diversity and population of species have shown that the dinosaurs were in decline for a period of about 10 million years before the asteroid hit. Slower mechanisms are needed to explain such extinctions. Climatic change, a change in Earth's magnetic field, and disease have all been suggested as possible slow extinction theories. As mentioned above, extensive volcanism such as the Deccan Traps could have been a long term event lasting millions of years although short in geologic terms.
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 radiation from a supernova explosion should contain the plutonium isotope Pu-244, a relatively short lived 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.
Other mass extinctions
It is worth noting that the Cretaceous extinction is neither the only mass extinction in Earth's history, nor even the worst. Previous extinction events have included the Triassic-Jurassic extinction event and the Permian-Triassic extinction event, which is the largest extinction event ever recorded.
References and external links
- Understanding the K-T Boundary - NASA-related website
- Shiva crater: Chatterjee et al. 2002 Volcanism, India-Seychelles Rifting, Dinosaur Extinction, and Petroleum Entrapment at the KT Boundary (GSA abstract)
- list of 172+ impact craters in Earth Impact Database with Crater name, Diameter, Age, Country, Latitude, Longitude, etc.
- Air bubbles, amber, and dinosaurs
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