Cretaceous-Tertiary extinction event

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Image:KT boundary 054.jpg The Cretaceous-Tertiary extinction event was a period of massive extinction of species that occurred 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 many others, is unknown. Many forms of life perished, encompassing approximately 50% of all plant and animal families (genera), including the non-avian dinosaurs. Many possible causes of the mass extinctions have been proposed. The most widely accepted current hypothesis is that an object from space produced an impact event on Earth.

The extinction event is also known as the K-T extinction event and its geological signature as the KT boundary ("K" is the traditional abbreviation for the Cretaceous period, named from the Latin for chalk, creta, which in German is kreide and in Greek is kreta. "K" is used to avoid confusion with the Carboniferous period, abbreviated as "C").

1 Casualties of the extinction
2 Theories
3 Further skepticism
4 Other mass extinctions
5 See also
6 References
7 External links

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Casualties of the extinction

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A broad range of organisms became extinct at the end of the Cretaceous, the most conspicuous being the dinosaurs. While dinosaur diversity appears to have declined in the last ten million years of the Cretaceous, at least in North America, many species are known from the Hell Creek, Lance Formation and Scollard Formation, including six or seven families of theropods and a similar number of ornithischians dinosaurs. Birds were the sole survivors among Dinosauria, but they also suffered heavy losses. A number of diverse groups became extinct, including Enantiornithes and Hesperornithiformes. The last of the pterosaurs also vanished; mammals suffered as well, with marsupials and multituberculates experiencing heavy losses; placentals were less affected. The great sea reptiles of the Cretaceous, the mosasaurs and plesiosaurs, also fell victim to extinction. Among mollusks, 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. In North America, as many as 57% of the plants species may have become extinct as well. Much less is known about how the K-T event affected the rest of the world, due to the absence of good fossil records spanning the K-T boundary. It should be emphasized that the survival of a group does not mean that the group was unaffected: a species may be 99% annihilated by an asteroid strike, yet still manage to survive.

Darkness from an impact-generated dust cloud (Alvarez et al. 1980) may have been supplemented by associated gases. Darkness resulted in loss of photosynthesis both on land and in the oceans. On land preferential survival may be closely tied to animals that were not in food chains directly dependent on plants. Dinosaurs, both herbivores and carnivores, were in plant-eating food chains.

Mammals of the Late Cretaceous were not herbivores. Many mammals fed on insects, larvae, worms, snails etc., which in turn fed on dead plant matter. During the crisis when green plants disappeared, mammals may have survived, because they lived in "detritus-based" food chains. Soon after the K/T extinction the mammals radiated into plant-eating lifestyles, and were soon followed by other mammals that became carnivores.

In stream communities few groups of animals became extinct. Stream communities tend to be less reliant on food from living plants and are more dependent on detritus that washes in from land. The stream communities may also have been buffered from extinction by their reliance on detritus-based food chains. (See Sheehan and Fastovsky, Geology, v. 20, p. 556-560.) Similar, but more complex patterns have been found in the oceans. For example, animals living in the water column are almost entirely dependent on primary production from living phytoplankton. Many animals living on or in the ocean floor feed on detritus, or at least can switch to detritus feeding. Extinction was more severe among those animals living in the water column than among animals living on or in the sea floor.

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Theories

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Alvarez hypothesis

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 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 killed off by an impact event from a ten-kilometer-wide asteroid. Two facts supporting the theory are the relative abundance of iridium in many asteroids and the similarity between the isotopic composition of iridium in asteroids and K-T layers, which differs from the that of terrestrial iridium.

Iridium is very rare on the Earth's surface, but is found more commonly in the Earth's interior and in extraterrestrial objects such as asteroids and comets. Furthermore, chromium isotopic anomalies found in Cretaceous-Tertiary boundary sediments strongly supports the impact theory and suggests that the impact object must have been an asteroid or a comet composed of material similar to carbonaceous chondrites.

The blast resulting from such an impact 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 the late Cretaceous [1]. This high O₂ level would have supported intense combustion. The level of atmospheric O₂ plummeted in the early Tertiary period.

In addition, the worldwide cloud would have blocked sunlight for months, decreasing photosynthesis and thus depleting food resources. This period of reduced sunlight, a "long winter," may also have been a factor in the extinctions. Gradually skies cleared but greenhouse gases from the impact caused an increase in temperature for many years.

The impact target rocks also produced acidic rainfall that would have affected natural habitats. However, recent research suggests this effect was relatively minor. Chemical buffers would have reduced the impact, and the survival of animals vulnerable to acid rain effects (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 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.

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Chicxulub crater

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Image:Chicxulub radar topography.jpg 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 the Chicxulub 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. 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. The age of the Chicxulub crater has been revised to approximately 300ky before the K-T boundary. This dating is based on evidence collected in Northeast Mexico, detailing multiple stratigraphic layers containing impact spherules, the earliest of which occurs some 10 meters below the K-T boundary. This chronostratigraphic thickness is thought to represent 300ky. This finding supports the theory that one or many impacts were contributary, but not causal, to the K-T boundary mass extinction.

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Deccan traps

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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 that a group of physicists was 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 crater has been recently reported in the sea floor off the west coast of India 2. This, the Shiva crater, 450-600 kilometres in diameter, 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. [2].

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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.

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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.

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Further skepticism

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 linkage between an impact and any other incident of mass extinction, although research on other events also implicates 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. (A study by Fastovsky & Sheehan (1995) counters that there is no evidence for a slow, 10-million-year decline of dinosaurs.) Slower mechanisms are needed to explain slow 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, still a brief period in geological time.

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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 Cambrian-Ordovician extinction, End Ordovician, Triassic-Jurassic, Late Devonian, and the Permian-Triassic, which is the largest extinction event ever recorded.

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