Mass Extinctions

I.G. Kenyon

Mass Extinction - DefinitionA significant proportion of

species become extinct (between 30% and 95%)

The extinctions operate across a wide range of

environments and lifestyles

The extinctions occurred rapidly (over a few million

years) and were due to one or more physical factors

5 Major Mass Extinctions identified since the Cambrian Extinction Event Date of Extinction

End of Ordovician 443 Million years ago

Late Devonian 375 Million years ago

End of Permian 251 Million years ago

End of Triassic 200 Million years ago

End of Cretaceous 65 Million years ago

The 5 Major Mass Extinctions

Gaps between extinctions vary from 51 Ma to 135 Ma with an average gap of 94.5

Ma

Extinctions During the Phanerozoic

The Phanerozoic is from 542 Ma (Cambrian) to the present

Diversity Curves through the Phanerozoic

End Triassic Mass Extinction possibly caused by submarine volcanism associated with the

break up of Pangea

End of Ordovician Mass Extinction – 443 Ma

70% of marine species became extinctTropical faunas badly affected especially coral reefs

Main groups affected Trilobites, Graptolites, Echinoids, Brachiopods

Late Devonian Mass Extinction – 375 Ma A series of events that lasted around 10 Ma

Cephalopods, Fish and Corals most affected

End Permian Mass Extinction – 251 Ma

The largest mass extinction event removing 95% of marine species and 50% of marine families

Trilobites, Cephalopods, Bryozoans, Corals, Crinoids badly affected

Major faunal and floral overturn on land Marks the boundary between dominance

by the Palaeozoic and Modern Fauna

End Triassic Mass Extinction – 200 Ma Multiple event mostly affected the land

where over 95% floral species eliminated

Around 30% marine species became extinct – mainly reef dwellers, Ceratites, Brachiopods and Bivalves

End Cretaceous Mass Extinction – 65 Ma

Around 70% of all species wiped outDinosaurs, Reptiles, Ammonites, Belemnites,

Brachiopods, Bivalves, Foraminifera

Causes of Mass Extinctions Supercontinent FormationExtra-Terrestrial ImpactsFlood Basalt Eruptions

Methane Hydrates and Global WarmingRapid and Major Glaciation Events

Supercontinent Formation Pangea formed at the

end of the Permian

Resulted in fewer continental shelves and lack of habitat for

shallow marine organisms

Coincided with huge decline in the numbers of shallow

marine species

Caused rapid fluctuations in climate, unstable weather

patterns and extreme aridity in the interior of the land mass

Supercontinent Formation A single continent reduces the

input of nutrients to oceans from rivers and estuaries

This decreases the amount of nutrients available for shallow

water marine life and may have also altered the salinity

Supercontinent Formation A supercontinent positioned close to one of the poles can

initiate major glaciations

Glaciation causes sea levels to fall and there is a significant

reduction in shallow water marine environments

A supercontinent located over a pole can also lead to the coverage

of the Earth in ice, a condition known as ‘Snowball Earth’

Extra-Terrestrial Impacts Asteroids 1 km diameter strike the Earth every 500,000 years

Large collisions with 5 km diameter asteroids occur approximately about

once every 10 million years The last known impact of an object of 10km diameter or larger was 65Ma

Extra-Terrestrial Impacts

Local destruction of habitats – impact blast, shockwave, ignition of wildfires

Billions of tonnes of debris injected into the atmosphere resulting in rapid global

cooling

Extra-Terrestrial Impacts

Impact in the sea – billions of tonnes of water vapour

injected into the atmosphere resulting in a greenhouse effect and rapid

global warming

Flood Basalt Eruptions

Eruptions last between 0.5 and 2.0 million years and can erupt enough basalt to cover the whole of the USA to a

depth of a kilometreLocal destruction of habitats

and initiation of wildfires

Flood Basalt Eruptions

Billions of tonnes of carbon dioxide and

sulphur dioxide released into the atmosphere

during eruptions

Both are powerful greenhouse gases and will contribute to very rapid global warming

Flood Basalt Eruptions

When it rains the sulphur dioxide will come back down to Earth dissolved in rainwater and

the acid rain will kill vegetation on a large scale

With vegetation dying, all food chains will be affected and also the oceans may become acidified with disastrous effects on marine life

Methane Hydrates in Ocean Floor Sediments Large volumes currently locked into ocean floor sediments

Stable under low temperatures of deep ocean

Methane Hydrates in Ocean Floor Sediments

Global warming may result in deep ocean

temperatures rising and the release of large volumes of

methane from ocean sediments

The rapid release of large amounts of methane into the atmosphere will result

in highly accelerated global warming

Methane Hydrates in Permafrost

Glaciations-Possible Causes

Supercontinents positioned in high latitudes/close to the poles

Milankovitch cycles: Precession, Obliquity and Eccentricity

Glaciations-Effects on Ecosystems

Loss of habitat as ice masses growCould eventually lead to ‘Snowball Earth’ scenarioContraction of climatic belts towards the equator

Global cooling, decreased productivity of primary producers

End Cretaceous Mass Extinction – 65 Ma

A large asteroid or meteorite (10km in diameter) collided with the Earth 65 million years ago

Evidence - Asteroid Impact Location

Location of possible impact site discovered on the Yucatan Peninsula in Mexico at Chixulub

The impact structure is a circular depression about 180 km in diameter

Evidence for Asteroid Impact Location

A. Gravity survey onshore and offshore of the Yucatan PeninsulaB. 3D map of gravity and magnetic field variations reveals the

Chicxulub crater, now buried beneath tons of sediment. This view is looking down at the surface, from an angle of about 60°.

A

B

Evidence – The K-T Boundary LayerThe presence of a thin 2cm

layer of iridium-rich clay found all over the world

within sedimentary rocks

The K-T Boundary Layer dates the same everywhere

at 65.5 Ma +/- 0.3 Ma

Iridium is a transition element, rare on Earth but

found in meteorites.

First proposed by Luis Alvarez in 1980

Evidence – Shocked Quartz

Shocked quartz is found worldwide, in a thin layer

at the boundary between Cretaceous and Tertiary rocks.

It was first discovered at nuclear testing sites and later in craters caused by meteorite impacts as

at the Barringer Crater near Flagstaff, Arizona in the USA

Evidence – Shocked QuartzShocked quartz has a microscopic structure

different from normal quartz.

Under intense pressure, but relatively low temperature,

the crystalline structure of quartz is deformed along planes inside the crystal.

These planes, which show up as lines under a

microscope, are called planar deformation features

(PDFs), or shock lamellae.

Evidence – Tektites (Glass Spherules)Tektites (from Greek

tektos, molten) are natural glass rocks up to a few

centimetres in size.

Most scientists agree they are formed by the impact

of large meteorites on Earth’s surface.

Tektites are black or olive-green in colour and their

shape varies from rounded to quite irregular.

Tektites (Glass Spherules) from 1 to 8 mm in diameter are found within a radius of 600 to 1,000km of the Chixulub Crater in Mexico

Evidence – Soot from Wildfires

The K-T Layer has high concentrations of carbon in many locations,suggesting that the asteroid impact may have generated wildfires.

Large areas of vegetation would have been destroyed in a short time,soot fell to Earth and was incorporated into sedimentary rocks

Evidence – Tsunami DepositsIt is thought that the asteroid

impact occurred in the sea and initially produced a crater 100

km wide and 30 km deep

This would have displaced vast volumes of seawater and

generated a series of very large tsunamis possibly over

100 metres in height

The tsunamis would have travelled great distances inland, and in Texas at Waco there are

large-scale sedimentary deposits thought to be of tsunami origin.

Evidence – Tsunami DepositsThe tsunami deposits in Waco, Texas are thought to be from

the waves generated by the K-T asteroid impact.

The sediments are estimated to have been deposited at least

300km inland by the tsunamis!

Animation to show Impact of a large asteroid with the Earth

The impactor's estimated size was about 10 km in diameter and is estimated to have released 4×1023 joules of energy,

equivalent to 100,000,000 megatons of TNT on impact.

Results published in 2007 suggest that the impactor that wiped out the dinosaurs and other life forms on Earth 65 million years ago can been traced back to a break-up event

in the main asteroid belt more than 100 million years earlier.

Origin of the K-T Asteroid

Flood Basalt Eruptions 66 Ma – The Deccan TrapsErupted mainly over a period of 30,000 years

Over 2000 metres thick and cover 500,000 km²May have originally covered 1,500,000 km²Caused a global drop in temperature of 2°C

The term Trap is derived from the Dutch for stairs and refers to the step-like landscape of the area

That’s All Folks!