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Mass extinctions and their possible causes
During a mass-extinction a significant part of all life forms on Earth will become extinct over a relative short period of time. The mass-extinction at the end of the Cretaceous period which caused the dinosaurs to go extinct is probably the best known mass-extinction amongst the general public. There were however more periods were similar events took place. One example is the end of the Permian period mass-extinction, an estimated 95% off all marine life went extinct (Raup 1979).
There are several theories that can explain mass-extinction events. First off all the most ‘popular’ cause of mass-extinction has to be a large meteorite impacting the earth’s surface. Second a global increase in volcanic activity could also have led to mass extinction. A third possibility is rapid changing of sea levels. Below we will discuss the different possibilities what caused some of the best known mass extinction in Earth’s history.
- The late Ordovician mass extinction.
- The Permian – Triassic boundary.
- The Cretaceous - Paleocene boundary.
- The Eocene – Oligocene boundary.
The Late Ordovicium Extinction
Changes in sea levels have almost for certain played a big role in late Ordovician extinctions and also at the P/T boundary. Changes in sea levels could have caused changes in salt- and oxygen-levels in parts of our Earth’s oceans. Oceans contains a big number of habitats and ecological niches. When sea levels changed and habitats were disturbed, many species could go extinct when they didn’t adapt fast enough. Sea levels have a significant correlation with the rate of glaciation, how much water migrates to and from glaciers and the icecaps.
One way or the other climate change probably has played a significant role in all mass extinctions, even if that change is caused by another event (e.g. meteorite impact). The most important form of climate change leading to mass extinction is probably glaciation. Climate change triggers extinction when it leads to altering or disappearing of so called ‘ecological niches’. The organisms affected by this have to migrate to a suitable area or adapt, when they fail to do so, they will go extinct. Another example of climate change is increased amount of rain, this is thought of to be the cause of the mass extinction at the end of the Triassic period.
The Permian - Triassic Boundary
During the Carboniferous and Permian periods organisms like crinoids, ammonites, corals and fish were thriving. But at the end of the Permian period the largest mass extinction event in the history of the Earth took place, whipping out a big portion of life on Earth. Many causes are appointed to the P/T boundary mass extinction, meteorite impact, see level changes, anoxic conditions and increased volcanic activity. Teichert 1990 even implicates that there wasn’t any abrubt dying/extinction of species at the P/T boundary but it gradually took place over a period of 6-10 million years. More recent studies however show that actually was a abrupt mass extinction event. At the end of the Permian period there was a marine regression. This fact also makes the study of the P/T boundary more difficult, because there aren’t that many continuous sections that cross the P/T boundary.
Plankton and Nekton show an abrupt decrease in populations at the P/T boundary (Valentine, 1986; Li etal., 1991). Estimates are that at least 95% of all marine species and almost three quarters of all vertebrate families went extinct (Raup, 1979). On land, large groups of insects went extinct, 99% of alle reptile genera and more than 95% of the plant life. At the P/T boundary clearly there is evidence of a global environmental disturbance after which stress-tolerant species flourish (Rampino et al, 1996).
The organic calcium levels decrease in many sections at the boundary. Also large shifts in ratios of Carbon, Oxygen, Sulfur and Strontium isotopes are shown at the P/T boundary (Xu et al., 1989; Magaritz et al., 1988; Holser et al., 1991). The negative carbon-isotope shifts at first were described as gradual shifts (Erwin, 1993; Magaritz et al., 1988; Holser et al., 1991). But studies of the GK-I core from the Alps, show it to be a sudden/abrupt shift (Rampino et al., 1996). A negative shift of oxygen-18 isotope indicates a significant rise in temperature.
In the same GK-I core a 5-7 times increased level of Iridium was found at the boundary. At different globally spread locations one also discovered elevated levels of Iridium similar to the levels discovered in the GK-I core. In China certain sections show micro-tectites (Xu et al., 1989). All this information is coherent with a meteorite impact at the P/T boundary. On the other hand layers of clay at the boundary in China show a much more volcanic character (Zhou & Kyte., 1989). The Araguainha impact crater, found in Brazil (Engelhardt et al., 1992) has the correct timing but is too small to cause the P/T mass extinction by itself.
In Siberia at the time of the P/T boundary large amounts of ‘flood basalts’ were formed and also in South China spread over a large region there was increased volcanic activity. Repeatedly the connection between global volcanic activity and the P/T mass extinction is suggested. This could be caused by rapid cooling as a consequence of sulfuric-aerosols or a greenhouse effect due to increased Carbon dioxide in the atmosphere.
Research on samples from Spitsbergen, Slovenia and Italia prove that the oceans showed anoxic conditions at the end of the Permian Period, as well on high as on low (paleo) latitudes (Wignall & Twitchett., 1996). These conditions applied to the shelf all the way up to undeep water above the ‘storm wave base’. These anoxic conditions perhaps played a role in the P/T mass extinction.
According Erwin, 1996, it is likely that more than one factor together have led to the P/T extinction. The mechanism behind the big extinction has to meet a number of conditions. First of all it has to be consistent with available geological, geo-chemical and climate data. Second, it had to take place at the correct moment. Third, the event had to influence the marine life as well as terrestrial life. Last but not least, the event had to be of a certain scale, making it plausible to have caused the mass extinction.
Meeting those conditions make it plausible that multiple factors played a significant role in the P/T mass extinction. There is proof for a meteorite impact, but not enough to justify that it was ‘the cause’. Possibly it was the combination of oceanic regression at the end of the Permian period and increased volcanic activity leading to a greenhouse effect, add up a big meteorite impact. This should have caused such a big impact on the environment that the oceans could become largely anoxic and caused the extinction of many organisms. To come to an ultimate conclusion on this matter, much more research is needed.
Since they have found Iridium in clay-layers, proving that there was a large meteorite impact at the K-Pg boundary (Alvarez et al., 1980; Smit & Hertogen, 1980) , more and more evidence for a large meteorite impact has been discovered. One discovery is the global existence of the ‘boundary clay’. Also the presence of ‘shocked’ minerals and micro-tektites at the boundary can only be explained by a meteorite impact. The most convincing evidence would be a large impact crater, indeed just that was found at Yucatan, Mexico (Chicxulub crater) after years of research. The impact causing such a crater could have caused the K-Pg mass extinction.
Calculations show that the Chicxulub meteorite had to have a diameter of 10km to cause the crater with a diameter of 150km. This impact is comparable with a huge nuclear explosion. Effects must have been catastrophic. First a shockwave travelled across the Earth’s surface. The intense heat and storms radiating from the impact must have caused large fires on a huge scale (Wolbach et al., 1986). A cloud of water vapour and dust coming from those fires led to month’s dark conditions and acid rain falling to earth (Vellekoop et al., 2014). These dark conditions made photosynthesis close to impossible. From isotope research it appears the oceans showed a 8°C temperature increase directly after the ‘boundary’. This indicates a global warming / greenhouse effect due to increased carbon dioxide levels derived from the fires. Studies of foraminifera (Carbon isotope studies) show that photosynthesis in the photic zone was close to zero just above the K-Pg boundary. The effect on life on Earth was devastating, mostly impacting larger organisms which depended on vegetation as their main food source.
Another possible cause for mass extinctions is increased vulcanic activity. E.g. the Deccan traps in India are also suggested to be the cause of the K-Pg mass extinction, however other researchers say that those were formed 1Ma before the K-Pg mass extinction. Increased volcanic activity can catapult a large amount of gas and dust into the Earth’s atmosphere, which in term has a devastating effect on animal and plant life. One option is rapid cooling of the temperature on earth as a consequence of the dust blocking the sunlight, or the opposite might happen as well, global warming through elevated levels of carbon dioxide in the atmosphere.
Studies were performed to look for evidence of meteorite impacts as cause for other large mass extinctions like the P/T and Eocene / Oligocene mass extinctions. For the E/O mass extinction evidence is found indicating another large meteorite impact. Still there is a lot of discussion as to what caused the K-Pg mass extinction, but most scientists agree on a large meteorite impact as the most plausible cause for this event. The role of meteorite impacts in a mass extinction will probably always be subject of discussion as there is also proof of large impacts which not have triggered a mass extinction.
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Author: Herman Zevenberg
Translated by: Ivo Kesselaer
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