Difference between now and past mass extinctions

Look back to the "Big Five", they were all physically caused events so that they might seem to have little or even nothing to tell us about the Sixth mass extinction event -- which is related to human activities [1]. Obviously, human activities do a strong impact to the ecosystem which may directly lead to species destruction in the modern world. For instance, transformation of the landscape, pollution, overexploitation of species, etc. 
In order to compare the sixth mass extinction(refer to human impact) with the previous events, scientists have identified a "background rate" of species extinctions from the fossil record[2]. A widely-used metric is E/MSY (extinctions per million species years)

Previous extinctions
It is estimated from the fossil extinctions that the background rates took place in million-year-or-more timeframes. Therefore, it is difficult to compare the current rate of extinctions-- just occurring over periods of decades to centuries. According the research，the average E/MSY over the fossil record is about 1.8 which means there were fewer than 2 species became extinct every million species-years on average. To break the time into less than 1,000 years the most common E/MSY is zero.

Current Extinction Rate
Over the past 1,000 years, it has been found the average extinction rate is 24 E/MSY that is about 13 times larger than the background rate. If the data is broken into 1-year bins, the maximum extinction rate over that period is bout 693 E/MSY which is 385 times larger than the background rates.

To sum up, human activities are the most different thing between the 6th mass extinction and the "big five". Their influences are causing faster rate extinctions that may lead to an acceleration for the coming 6th mass extinction. However, there is another hypothesis that the extinction event has been already arrived.  

Reference:
1. http://www.actionbioscience.org/newfrontiers/eldredge2.html.
2. http://www.skepticalscience.com/sixth-mass-extinction.html

A time trip: back to the ancient time

After the blogs written before, we know that the "big five" events have destroyed more than 50% to 95% of all organisms that have ever lived on Earth. Here is a video that gives a conclusion of all "big five" events and leads us back to the ancient time.

This trip made me to think about our current situation carefully. As most of the speculations about what causes a mass extinction are related to the climate change, it is a dangerous signal for us that the concentration of CO2 is rising rapidly and the global temperature is getting higher and higher in recent years. 

Are we causing the sixth mass extinction?

Cretaceous-Paleogene extinction event

Cretaceous-Paleogene extinction event, also called the K-T extinction where K stands for Cretaceous and T for Tertiary, is the most well known extinction event in the world and is the last one of the "Big five". It happened about 65 million years ago. During the this boundary of time, about 40-65% of marine species died out from the Earth ecosystem[1]. Almost all the large vertebrates on Earth, on land, in sea, and in the air suddenly became extinct including coccoliths, planktonic foraminifera, ammonites, gastropods, sponges, marine reptiles, dinosaurs and mammals. 
For this mass extinction event, there are still arguments existed about just how short it was[2]. It was certainly sudden in geological terms and may have been catastrophic by anyone's standards.

Why it happened? There have been many bad theories to explain this extinction event and the causes are still being debated by paleontologists. Based on researches, it is believed that a major factor was an asteroid about 10 kilometers in diameter that struck what is now the Yucatán peninsula in Mexico[3]. The impact led to many serious effects, for instance, global forest fires, possibly a period of cold weather because of sunlight-blocking dust and smoke, etc. Another hypothesis of the cause is a massive bout of volcanism.  Here is a video that gives an explanation to the K-T mass extinction in details.

Reference

1. MacLeod, N. in press. End-Cretaceous extinctions. In R. C. Selley, L. R. M. Cocks and I. R. Plimer (eds.), Encyclopedia of Geology. Academic Press, London.

2. http://www.ucmp.berkeley.edu/education/events/cowen1b.html
3. http://paleobiology.si.edu/geotime/main/htmlversion/cretaceous4.html

Causes for Triassic-Jurassic mass extinction

The Triassic-Jurassic mass extinction is the event owns most of the debates for the causes to it. Several theories for explaining this mass extinction have been suggested, but none of them have answered why it happened.

Generally, evidences have shown there were gradual climate changes, a pulse of oceanic acidification or sea-level fluctuations at that time. They can be causes of mass extinction but according to the TJME situation, those causes cannot explain why event happened so suddenly [1].

Another possible cause-asteroid impact also seems not enough for causing such a sudden big mass extinction as the carbon record presents that there were no big asteroid impacts at that period of time [2].

Then, massive volcanic eruptions, which would emit carbon dioxide or sulfur dioxide and aerosols, could cause global warming or cooling also can be the reason for the event [3]. The exist evidence proofs that there was an increase in  carbon dioxide in the atmosphere at that boundary.

Except the debating of the causes, there are also some one argued for that TJME was not the product of a single major event [4]. They preferred to not treat the event as a mass extinction event but a simply prolonged turnover of species across a considerable amount of time.

Reference:

1. T.M.Quan, et al. (2008) Nitrogen isotope and trace metal analyses from the Mingolsheim core: Evidence for redox variations across the Triassic-Jurassic boundary", Global Biogeochemical Cycles

2. S.Roff. (2011) Dark days of the Triassic: Lost world

3. J.F.Hubert et al. (2001) Stability of atmospheric CO₂ levels across the Triassic/Jurassic boundary

4. http://www.britannica.com/EBchecked/topic/1523109/end-Triassic-extinction

An introduction to Triassic-Jurassic Mass Extinction event

The Triassic-Jurassic Mass Extinction took place about 200 million years ago, just 50 million years after the "Great dying" event. There were 2 or 3 phases of extinction in this event and it happened just less than 10,000 years -- can be treated a blink of a geological eye.

In this event, more than half of all species died out on Earth. It killed about 20% of marine families and 30% of marine genera. Many of the "mammal-like raptiles", which are known as therapsids,  became extinct in the mass extinction. Besides, all large cruotarsans (non-dinosaur archosaurs, the ancestors of modern-day crocodiles, alligators and gavials) and most large  amphibians were also died out after that period of time[1].

At this boundary of time, dinosaurs were appeared on Earth[2], but only about 1-2% of fauna. As the mass extinction wiped out those life mentioned above, the living environment became better for dinosaurs as there were less food competition. After the event, the percentage of dinosaurs increased to 50-90% of fauna. Because of these reasons, the Triassic-Jurassic Mass Extinction is often treated as the event leading the stage for dinosaurs to dominant the world at that time.

Reference:

1. http://www.wisegeek.com/what-is-the-triassic-jurassic-extinction-event.htm

2. http://io9.com/Triassic-jurassic-extinction-event/

Permian-Triassic Mass Extinction event

The Permian-Triassic (P-Tr) Mass Extinction event occurred 252 million years ago.

Because of its severe level, it was nicknamed "The Great Dying" and well known as "the mother of all mass extinction events" . In this event, no class of life was escaped from the extinction: trees, lizards, plants, insects, fish, etc. all were nearly devastated. About 96% of all marine species and 70% of terrestrial vertebrate species on Earth died out (Benton, 2005).  The figure on the left hand side presents the world at the Permian time. 

For a clearer state of extinction losses, here are one table and one figure that both show the comparison between the diversity of marine life before and after the P-Tr mass extinction from Erwin (1989).

         

Figure on the left shows the sea-bed that has richness of reef life and the burrowing infauna before the P-Tr, and the right side's figure with an absence of such species gives a strong impact compare to the left one. 

Why did it happen?! Why this event was so severe?!

Here is a video from youtube which gives an explanation of the "Great Dying".

Consequently, this mass extinction event was caused by a combination of causes:

severe climate and environmental changes, contemporary volcanic activities, methane hydrate gasification and maybe the asteroid impact and some other reasons. They formed these sequence of catastrophes and each one worse than the previous that resulted in this greatest mass extinction over the time.  

And as the "Great Dying" event did so great the damage to Earth, it demanded about up to 10 million years for recovery. According to Chen,et al. (2009), this also change fundamentally the ecosystem of Earth and after this period,   the modern-type biota and ecosystem formed in the Triassic time when the dinosaurs appeared.

Reference:

Benton M J (2005). When life nearly died: the greatest mass extinction of all time. London: Thames & Hudson.

Erwin, D.H. (1989) The end-Permian mass extinction – what really happened and did it matter? Trends Ecol.

Z.Q.Chen et.al (2009). Permian-Triassic mass extinction and subsequent recovery: an update. Journal of the Geological Society of Australia: Taylor & Francis

Late Devonian Mass Extinction event

What is it?

The Devonian period can be divided into Early, Middle and Late epochs and they are then subdivided into several stages. The Late Devonian Mass Extinction (LDME) happened 359 million years ago around the Frasnian and Famennian (F-F) stages of the Late Devonian.  As one of the “Big Five” event, the Late Devonian time devastated most marine and terrestrial life on Earth. It was considered to be a prolongation for the marine biotic crisis extending for 20-25 million years from the late Middle Devonian to the Devonian/Carboniferous boundary when taken place of 8-10 extinction events. Two events among these are considered to be the most serious. The first occurred at the Frasnian-Famennian boundary is known as the Kellwasser event which is considered as the severest event during that period.The other, severe as 70% as the Kellwasser one, happened at the boundary with the Carboniferous, is known as the Hangenbery event. 

What happened in the event?

This event made a severe impact on floral and fauna on Earth. At the end of the Frasnian, 60% of existing taxa suffered the extinction. 13% to 38% families are estimated to disappear with further lowering the taxonomic hierarchy with losses of 55% to 60% genus and 70% to 82% species (Goehring, 2001). Besides, 43% to 50% plant species of terrestrial life were also lost during the LDME.

(Figure of the Late Devonian period)

Why did it happen?

The suggested possible causes of the LDME are mostly summarized into those four parts:

climate changes, tectonics, sea level fluctuations and asteroid impacts. None of them can solely cause the extinction, many believed that the LDME was caused by the combination of those factors. 

Climate changes:

One possible factor causing the Devonian extinction is global cooling as it can lead to the disruption of marine environments. The cooling was suggested to be caused by the paleogeography. This was theorized that the ocean between Laurussia and Gondwana closed at the F-F boundary which may disrupt the low-latitute warm water circumequatorial flow. Then restricted circulation and anoxic conditions in warmer water basins on the easter margin would be created as the high-latitute colder water would flow into equatorial areas on the west margins of the joined continents. This caused the subtropical reef and perireef life suffering higher extinction rate. Besides, global cooling can cause a lower water temperature which would accompany with the glaciation event.

Another speculation for the LDME proposed by Thompson and Newton is a lethal increase in temperature. Isotopic oxygen and carbon studies have proved this point as they presented an increase of temperature to over 60EC at the F-F boundary, while the general range was 36EC to 54EC over the earlier Devonian period. As many marine life live closer to their physiologic tolerance upper temperature limits, a small change could take them into intolerable temperatures. Besides, warmer water could hold less oxygen which leads to an anoxic condition. 

Tectonics:

Many theories for the LDME have been derived from tectonic mechanisms. Fisher and Arthur made a climate model according to tectonic megacycles, changes related to phases of accelerated plate tectonic activity. By the increase of magma upwelling and spreading at mid-ocean ridges, ocean would change to continents. Then, with increased global volcanism, carbon dioxide would be added into the atmosphere. On the other side, less carbon dioxide in the atmosphere may be caused by the slowing of plate tectonics which can lead to an icehouse cycle. There rapid condition changes could result in a global ecosystem collapse.

Sea level fluctuations:

Although less evidences can show the LDME was related to the sea level fluctuations as these periods suffered transgressions and regressions without an associated extinction event, some reasons can still prove this point. For instance, regression would be caused by glaciation. This would result in loss of habitat for sessile shallow marine organisms or deadly hyper-saline conditions in shallow water. On the other hand, the transgression, can lead to the "drowning" of perched reef ecosystem together with an influx of anoxic deeper water into shallow areas. This can kill sessile sea floor marine organisms. 

Asteroid impacts:

This aspect has become a popular theory that caused extinction events. A theory suggested the LDME was the first bolide-induced event. The asteroid would need a diameter greater than 10 kilometers. If the asteroid that impacted earth satisfying this condition, it would destroy life in the target area, result in earthquakes, tsunamis, wildfires and ballistic molten debris. The consequences for those events can listed as following:

1. Affect shallow marine ecosystems (Tsunamis)

2. The atmosphere would be heated sufficiently leading a combination of nitrogen and oxygen to generate nitric oxide and nitric acid. Rain then falling in high concentrations could pollute upper surface waters and kill phytoplanktonic life (Blast).  

3. The environment could be poisoned by dioxins and aromatic hydrocarbons produced by wildfires.

4. The sunlight could be blocked disrupting the photosynthesis (Global dust clouds).

5. Icehouse effect that decrease the global temperature.

Reference:

S. Goehring (2001) The Late Devonian Mass Extinction Event. Elizabeth City, NC.

http://www.bbc.co.uk/nature/extinction_events/Late_Devonian_extinction