Mass Extinctions The struggle for life on earth
- Slides: 50
Mass Extinctions The struggle for life on earth
Mass Extinction - Definition • A 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
The “Big Five” 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
Percentage of Species Wiped Out • • • Ordovician-Silurian - 85% Late Devonian - 82% Permian-Triassic - 96% End Triassic - 76% Cretaceous-Tertiary - 76%
Extinctions During the Phanerozoic The Phanerozoic is from 542 Ma (Cambrian) to the present
End of Ordovician Mass Extinction – 443 Ma 70% of marine species became extinct Tropical 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
Permian Mass Extinction 100% trilobites 98% crinoids 97% foraminifera 99% radiolaria 100% blastoids 100% eurypterids 97% ammonites 100% acanthodians 96% brachiopods
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 out Dinosaurs, Reptiles, Ammonites, Belemnites, Brachiopods, Bivalves, Foraminifera
End-Cretaceous Mass Extinction 100% belemnites 100% ammonites few radiolaria 100% dinosaurs 100% pterosaurs few plants 100% mosasaurs 100% ichthyosaurs 90% foraminifera
Dinosaur extinction theories
Causes of Mass Extinctions Supercontinent Formation Extra-Terrestrial Impacts Flood Basalt Eruptions Methane Hydrates and Global Warming Rapid 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 10 km diameter or larger was 65 Ma
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 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 kilometre Local 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
Siberian volcanism in the Permian period
Methane Hydrates in Ocean Floor Sediments Large volumes currently locked into ocean floor sediments Stable under low temperatures of deep ocean
What is methane hydrate? Methane molecule (CH 4) in a 'cage' of H 2 O molecules Structure held together by hydrogen bonds "The ice that burns"
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 grow Could eventually lead to ‘Snowball Earth’ scenario Contraction of climatic belts towards the equator Global cooling, decreased productivity of primary producers
End Cretaceous Mass Extinction – 65 Ma K-T extinction A large bolide (asteroid or meteorite 10 km in diameter) collided with the Earth 65 million years
And then what? • • • Fireball Tsunami Wildfires Dust and darkness Acid rain Increased CO 2 and global warming
Evidence - Asteroid Impact Location of possible impact site discovered on the Yucatan Peninsula in Mexico at Chicxulub The impact structure is a circular depression about 180 km in diameter
Evidence for Asteroid Impact Location A B A. Gravity survey onshore and offshore of the Yucatan Peninsula B. 3 D 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°.
Evidence – The K-T Boundary Layer The presence of a thin 2 cm 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 Quartz Shocked 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. Tektites (Glass Spherules) from 1 to 8 mm in diameter are found within a radius of 600 to 1, 000 km of the Chixulub Crater in Mexico Most scientists agree they are formed by the impact of large meteorites on Earth’s surface. Tektites are black or olivegreen in colour and their shape varies from rounded to quite irregular.
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 Deposits It 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 Deposits The 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 300 km 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 megatons of TNT on impact.
Never mind the bolides…… Flood Basalt Eruptions 66 Ma – The Deccan Traps Erupted mainly over a period of 30, 000 years Over 2000 metres thick and cover 500, 000 km² May have originally covered 1, 500, 000 km² Caused 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
The Sixth Mass Extinction? NASA • Are we now in the midst of a sixth mass extinction? http: //en. wikipedia. org/wiki/Geologic_time_scale • Following their evolution humans have caused huge species loss, and this has accelerated in since the 19 th century • Species are becoming extinct at a rate of about 4000/year, 100/day, 1 species every 15 minutes. Today
Meanwhile…. the K-T debate continues… • 31 August 2002 “Cold was killing dinosaurs long before the asteroid commonly thought to have been their downfall hit, according to scientists. ” • 11 June 2002 “Dino heatwave recorded in leaves” … “analysis of fossil leaves from 65 million years ago shows there was a sudden and dramatic rise in carbon dioxide in the Earth's atmosphere. “
… and the P-Tr debate rumbles on… • 6 June 2002 “A huge outpouring of molten rock 250 million years ago may have been the decisive factor in the deaths of nearly all lifeforms on the Earth at that time. ” • 23 February 2001 “Earth's biggest mass extinction 251 million years ago was triggered by a collision with a comet or asteroid, US scientists say. ”
http: //www. firstscience. com/SITE/articles/mac_f 2. asp
Conclusions • The largest extinctions of the last 300 m. y. correlate with massive volcanism. • At least one also correlates with a significant meteorite impact. • There are far more impact and volcanic events than there are mass extinctions. • This rules out simple causal relationships between volcanism and extinctions, or impact and extinctions. • The “Murder on the Orient Express” hypothesis shows that combinations of driving factors are probably more important. • These complications lead to conflicting reports in the media
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