MASS EXTINCTION

Major extinction events

Although the Cretaceous-Tertiary (or K-T) extinction event is the most well-known because it wiped out the dinosaurs, a series of other mass extinction events has occurred throughout the history of the Earth, some even more devastating than K-T. Mass extinctions are periods in Earth's history when abnormally large numbers of species die out simultaneously or within a limited time frame. The most severe occurred at the end of the Permian period when 96% of all species perished. This along with K-T are two of the Big Five mass extinctions, each of which wiped out at least half of all species. Many smaller scale mass extinctions have occurred, indeed the disappearance of many animals and plants at the hands of man in prehistoric, historic and modern times will eventually show up in the fossil record as mass extinctions.

Ordovician-Silurian mass extinction

The third largest extinction in Earth's history, the Ordovician-Silurian mass extinction had two peak dying times separated by hundreds of thousands of years. During the Ordovician, most life was in the sea, so it was sea creatures such as trilobites, brachiopods and graptolites that were drastically reduced in number.

Late Devonian mass extinction

Three quarters of all species on Earth died out in the Late Devonian mass extinction, though it may have been a series of extinctions over several million years, rather than a single event. Life in the shallow seas were the worst affected, and reefs took a hammering, not returning to their former glory until new types of coral evolved over 100 million years later.

Permian mass extinction

The Permian mass extinction has been nicknamed The Great Dying, since a staggering 96% of species died out. All life on Earth today is descended from the 4% of species that survived.

Triassic-Jurassic mass extinction

During the final 18 million years of the Triassic period, there were two or three phases of extinction whose combined effects created the Triassic-Jurassic mass extinction event. Climate change, flood basalt eruptions and an asteroid impact have all been blamed for this loss of life.

Cretaceous-Tertiary mass extinction

The Cretaceous-Tertiary mass extinction - also known as the K/T extinction - is famed for the death of the dinosaurs. However, many other organisms perished at the end of the Cretaceous including the ammonites, many flowering plants and the last of the pterosaurs.

Importance of mass Extinction events in Evolution

Mass extinctions have sometimes accelerated the evolution of life on Earth. When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the new dominant group is "superior" to the old and usually because an extinction event eliminates the old dominant group and makes way for the new one.

For example mammaliformes ("almost mammals") and then mammals existed throughout the reign of the dinosaurs, but could not compete for the large terrestrial vertebrate niches which dinosaurs monopolized. The end-Cretaceous mass extinction removed the non-avian dinosaurs and made it possible for mammals to expand into the large terrestrial vertebrate niches. Ironically, the dinosaurs themselves had been beneficiaries of a previous mass extinction, the end-Triassic, which eliminated most of their chief rivals, the crurotarsans.

Causes of particular mass extinctions

Flood basalt events: Eleven occurrences, all associated with significant extinctions. Only five of the major extinctions coincided with flood basalt eruptions and that the main phase of extinctions started before the eruptions.

Basaltic eruptions can have series of interrelated effects. A basaltic eruption could have

1.     produced dust and particulate aerosols which inhibited photosynthesis and thus caused food chains to collapse both on land and at sea

2.     emitted sulfur oxides which were precipitated as acid rain and poisoned many organisms, contributing further to the collapse of food chains

3.     emitted carbon dioxide and thus possibly causing sustained global warming once the dust and particulate aerosols dissipated.

Flood basalt events occur as pulses of activity punctuated by dormant periods. As a result they are likely to cause the climate to oscillate between cooling and warming, but with an overall trend towards warming as the carbon dioxide they emit can stay in the atmosphere for hundreds of years.

It is speculated that massive volcanism caused or contributed to the End-Permian, End-Triassic and End-Cretaceous extinctions.

2. Sea-level falls

Sea-level falls could reduce the continental shelf area (the most productive part of the oceans) sufficiently to cause a marine mass extinction, and could disrupt weather patterns enough to cause extinctions on land. But sea-level falls are very probably the result of other events, such as sustained global cooling or the sinking of the mid-ocean ridges.Sea-level falls are associated with most of the mass extinctions, including all of the "Big Five"—End-Ordovician, Late Devonian, End-Permian, End-Triassic, and End-Cretaceous.

3. Impact events

The impact of a sufficiently large asteroid or comet could have caused food chains to collapse both on land and at sea by producing dust and particulate aerosols and thus inhibiting photosynthesis. Impacts on sulfur-rich rocks could have emitted sulfur oxides precipitating as poisonous acid rain, contributing further to the collapse of food chains. Such impacts could also have caused megatsunamis and/or global forest fires.

The Shiva hypothesis proposes that periodic gravitational disturbances cause comets from the Oort cloud to bombard earth every 26 to 30 million years.

4. Ocean asteroid impact

Carbon Dioxide (CO2) is soluble in sea water and is present in very large quantities. It mostly reports as the bicarbonate radical (−HCO3) which is only stable at temperatures below 50°C.Sea surface temperatures are normally below 50°C, but can easily exceed that temperature when an asteroid strikes the ocean thereby inducing a large thermal shock. Under those circumstances very large quantities of CO2 erupt from the ocean. As a heavy gas, the CO2 can quickly spread around the world in concentrations sufficient to suffocate air breathing fauna, selectively at low altitudes.Asteroid impacts with the ocean may not leave obvious signs, but these impacts have the potential to be far more devastating to life on earth than impacts with land.

5. Sustained and significant global cooling

Sustained global cooling could

·      kill many polar and temperate species and force others to migrate towards the equator;

·      reduce the area available for tropical species;

·      often make the Earth's climate more arid on average, mainly by locking up more of the planet's water in ice and snow.

The glaciation cycles of the current ice age are believed to have had only a very mild impact on biodiversity, so the mere existence of a significant cooling is not sufficient on its own to explain a mass extinction.

It has been suggested that global cooling caused or contributed to the End-Ordovician, Permian-Triassic, Late Devonian extinctions, and possibly others. Sustained global cooling is distinguished from the temporary climatic effects of flood basalt events or impacts.

6.Sustained and significant global warming

This would have the opposite effects:

·      expand the area available for tropical species;

·      kill temperate species or force them to migrate towards the poles;

·      possibly cause severe extinctions of polar species;

·      often make the Earth's climate wetter on average, mainly by melting ice and snow and thus increasing the volume of the water cycle.

It might also cause anoxic events in the oceans.

Global warming as a cause of mass extinction is supported by several recent studies.The most dramatic example of sustained warming is the Paleocene-Eocene Thermal Maximum, which was associated with one of the smaller mass extinctions. It has also been suggested to cause the Triassic-Jurassic extinction event, during which 20% of all marine families went extinct. Furthermore, the Permian–Triassic extinction event has been suggested to have been caused by warming. Human-caused global warming is contributing to extinctions today.

7.Clathrate gun methane eruptions

Clathrates are composites in which a lattice of one substance forms a cage around another. Methane clathrates (in which water molecules are the cage) form on continental shelves. These clathrates are likely to break up rapidly and release the methane if the temperature rises quickly or the pressure on them drops quickly—for example in response to sudden global warming or a sudden drop in sea level or even earthquakes. Methane is a much more powerful greenhouse gas than carbon dioxide, so a methane eruption ("clathrate gun") could cause rapid global warming or make it much more severe if the eruption was itself caused by global warming.

It has been suggested that "clathrate gun" methane eruptions were involved in the end-Permian extinction and in the Paleocene–Eocene Thermal Maximum, which was associated with one of the smaller mass extinctions.

8. Anoxic events

Anoxic events are situations in which the middle and even the upper layers of the ocean become deficient or totally lacking in oxygen. Their causes are complex and controversial, but all known instances are associated with severe and sustained global warming, mostly caused by sustained massive volcanism.

It has been suggested that anoxic events caused or contributed to the Ordovician–Silurian, late Devonian, Permian–Triassic and Triassic–Jurassic extinctions, as well as a number of lesser extinctions. On the other hand, there are widespread black shale beds from the mid-Cretaceous, which indicate anoxic events but are not associated with mass extinctions.

9. Hydrogen sulfide emissions from the seas

During the Permian–Triassic extinction event the warming also upset the oceanic balance between photosynthesising plankton and deep-water sulphate-reducing bacteria, causing massive emissions of hydrogen sulphide which poisoned life on both land and sea and severely weakened the ozone layer, exposing much of the life that still remained to fatal levels of UV radiation.

10. Oceanic overturn

Oceanic overturn is a disruption of thermohaline circulation which lets surface water (which is more saline than deep water because of evaporation) sink straight down, bringing anoxic deep water to the surface and therefore killing most of the oxygen-breathing organisms which inhabit the surface and middle depths. It may occur either at the beginning or the end of a glaciation, although an overturn at the start of a glaciation is more dangerous because the preceding warm period will have created a larger volume of anoxic water

It has been suggested that oceanic overturn caused or contributed to the late Devonian and Permian–Triassic extinctions.

11. A nearby nova, supernova or gamma ray burst

A nearby gamma ray burst at the End-Ordovician extinction would be powerful enough to destroy the Earth's ozone layer, leaving organisms vulnerable to ultraviolet radiation from the sun. Gamma ray bursts are fairly rare, occurring only a few times in a given galaxy per million years.

12. Geomagnetic reversal

Increased geomagnetic reversals will weaken Earth's magnetic field destroy magnetosphere, long enough to expose the atmosphere to the solar winds, causing oxygen ions to escape the atmosphere, resulting in a disastrous drop on oxygen. Additionally, Magnetosphere destruction will cause the earth to be bombarded with Alpha, beta, gamma and X rays, wiping out lives.

13. Plate tectonics

Movement of the continents into some configurations can cause or contribute to extinctions in several ways:

·      by initiating or ending ice ages;

·      by changing ocean and wind currents and thus altering climate;

·      by opening seaways or land bridges which expose previously isolated species to competition for which they are poorly adapted (for example, the extinction of most of South America's native ungulates and all of its large metatherians after the creation of a land bridge between North and South America).

Occasionally plate tectonics creates a super-continent which includes the vast majority of Earth's land area, which is likely to reduce the total area of continental shelf (the most species-rich part of the ocean) and produce a vast, arid continental interior which may have extreme seasonal variations.