Theories
Impact Theory (Alvarez hypothesis). In 1980, a team of researchers, led by Nobel Prize-winning physicist Luis Alvarez, 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. They suggested that the dinosaurs had been killed off by an impact event from a ten-kilometer-wide asteroid. The theory is supported by 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 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 percent) during the late Cretaceous. This high O2 level would have supported intense combustion. The level of atmospheric O2 plummeted in the early Tertiary (Paleogene) 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 would have cleared, but greenhouse gases from the impact would be assumed to cause an increase in temperature for many years.
Radar topography reveals the 180 kilometer (112 mile) wide ring of the Chicxulub crater (image courtesy NASA/JPL-Caltech)
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. The discovery of the Chicxulub Crater in the Yucatan, as well as various types of debris in North America and Haiti, has 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 300,000 years 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 finding supports the theory that one or many impacts were contributory, but not causal, to the K-T boundary mass extinction.
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. 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.
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.
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 million years) that is not found in earth rocks. However, analysis of the boundary layer sediments revealed the absence of Pu-244, thus essentially countering this hypothesis.
Overview of explanation. 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 [[[dinosaur]]s 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.
Holocene extinction event or the "Sixth Extinction”
The Holocene extinction event is a name customarily given to the widespread, ongoing extinction of species during the modern Holocene epoch. The extinctions vary from mammoths to dodos, to species in the rainforest dying every year. Because some believe the rate of this extinction event is comparable to the "Big Five" mass extinctions, it is also known as the Sixth Extinction, although the actual numbers of extinct species are not yet similar to the major mass extinctions of the geologic past.
The Holocene epoch extends from the present day to back about 11,500 years ago. An interglacial period, the Holocene starts late in the retreat of the Pleistocene glaciers. Human civilization dates entirely to the Holocene.
In broad usage, the Holocene extinction event includes the remarkable disappearance of large mammals, known as megafauna, by the end of the last ice age 9,000 to 13,000 years ago. Such disappearances have been considered as either a response to climate change, a result of the proliferation of modern humans, or both. These extinctions, occurring near the Pleistocene/Holocene boundary, are sometimes referred to as the Pleistocene extinction event or Ice Age extinction event.
The observed rate of extinction has risen dramatically in the last 50 years. There is no general agreement on whether to consider more recent extinctions as a distinct event or merely part of a single escalating process. Only during these most recent parts of the extinction have plants also suffered large losses.
The Pleistocene or Ice Age extinction
The Ice Age extinction event is characterized by the extinction of many large mammals weighing more than 40 kg (88 lb). In North America, around 33 of 45 genera of large mammals went extinct, in South America 46 of 58, in Australia 15 of 16, in Europe 7 of 23, and in sub-Saharan Africa only 2 of 44. Only in South America and Australia did the extinction occur at family levels or higher. The two main hypotheses concerning this extinction are: (1) the animals died off due to climate change (the retreat of the polar ice cap), and (2) the animals were exterminated as a result of human activity: The "prehistoric overkill hypothesis" (Martin 1967).
The prehistoric overkill hypothesis is not universally applicable and is imperfectly confirmed. For instance, there are ambiguities around the timing of sudden extinctions of marsupial Australian megafauna. Biologists note that comparable extinctions have not occurred in Africa, where the fauna evolved with hominids. Post-glacial megafaunal extinctions in Africa have been spaced over a longer interval. In North America, the culture that has been connected with the wave of extinctions is the paleo-Indian culture associated with the Clovis people, who were thought to throw spears to kill large animals. The chief opposition to the prehistoric overkill hypothesis has been that populations of humans, such as the Clovis culture, were too small to be ecologically significant.
An alternative to the theory of human responsibility is Tollmann's bolide theory, a more controversial hypothesis, which claims that the Holocene was initiated by an extinction event caused by bolide (asteroid or meteorite) impacts.
Among the major megafauna exterminated about 9,000 to 15,000 years ago were the woolly mammoth, the woolly rhinoceros, the Irish elk, the cave lion, the cave bear, and saber-toothed cats.
Recent extinctions
In more recent years, within the past 2,000 years, a large number of species have become extinct in ways more clearly linked to human dispersal or activity. Around 1500 C.E., several species became extinct in New Zealand after Polynesian settlers arrived, including ten species of Moa (giant flightless ratite birds). It is currently estimated that among the bird species of the Pacific, some 2,000 species have gone extinct since the arrival of humans (Steadman 1995). In Madagascar, starting with the arrival of humans about 2,000 years ago, nearly all of the island's megafauna became extinct, including the Aepyornism, or elephant bird (a giant flightless ratite bird); 17 of 50 species of lemur; and a giant tortoise. Starting about 500 years ago, a number of species became extinct upon human settlement of the Indian Ocean islands, including several species of giant tortoise on the Seychelles and the Macscarene islands. Notable examples of modern extinctions of mammal fauna include the Thylacine or Tasmanian tiger (Thylacinus cynocephalus); the Quagga (a zebra relative); the Dodo, the giant flightless pigeon of Mauritius; the Great Auk of islands in the north Atlantic; and the Passenger Pigeon of North America, which became extinct in 1914.