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There are lots of hypotheses about what causes mass extinctions, one more fanciful than the other. Two ideas taken more seriously are volcanoes and cosmic impacts. Now a new hypothesis suggests algae may be the killer behind the world's great fauna annihilations.

The idea was introduced in "Hypothesis for the role of toxin-producing algae in Phanerozoic mass extinctions based on evidence from the geologic record and modern environments" , a paper published in the March 2009 issue of the peer-reviewed journal Environmental Geosciences, and also presented in a talk on 19 October 2009, at the annual meeting of the Geological Society of American in Portland, Oregon, USA. (The full paper by Castle and Rodgers from March can be downloaded from this Clemson University site).

Castle and Rodgers have spent two years analyzing data from ancient stromatolite structures finding evidence that blue-green algae, which produce poisons and deplete oxygen, were present in sufficient quantities to kill off untold numbers of plants and animals living on land or in the sea. The geologic record demonstrates a pronounced increase in abundance and environmental range of algae, including stromatolitic cyanobacterial mats, coincident with the first four of the five major Phanerozoic (the last 545 million years) mass extinctions. During these past events of algal expansion, population decline of animals could have been caused by effects of algal blooms, including algal-produced toxins, at a scale sufficient to generate a fossil record of mass extinction.


Mass extinctions have often been attributed to climate changes, sea level, volcanic activity, and asteroids. Castle and Rodgers claim that these causes are contributors, but algae were the mass killer.

Environmental changes such as climatic warming, sea level fluctuation, and increased nutrient supply may have promoted algal blooms over vast expanses of marine to freshwater environments. and indeed, in my humble opinion toxic algae may just be another contributor together with other factors leading to (too) harsh environmental conditions.


Schematic profiles illustrating the influence of climate-induced sea level change on algal growth. (A) Sea level is low, shelves are narrow, and water temperatures are less favorable for algal growth during periods of cool global climate. (B) During periods of warm global climate, sea level is high resulting in extensive areas of shallow marine and coastal environments favourable for algal growth. Warm water temperatures promote the growth of algal blooms, domal and columnal stromatolites, and stromatolitic mats, which increases the potential for toxin production and release.

• http://www.clemson.edu/media-relations/article.php?article_id=2336
  
• http://www.redorbit.com/news/science/1771782/clemson_researchers_say_algae_key_to_mass_extinctions/index.html?source=r_science
  
• http://www.terradaily.com/reports/Killer_Algae_A_Key_Player_In_Mass_Extinctions_999.html
  
• http://www.usnews.com/science/articles/2009/10/21/killer-algae-a-key-player-in-mass-extinctions.html
  

The classical "Big Five" mass extinctions are End Ordovician, Late Devonian, End Permian, End Triassic, and End Cretaceous (see note below). The Holocene extinction event is referred to as the Sixth Extinction, that is the extinction event that is taking place NOW. This means indeed that the earth's 6th great mass extinction is occurring while you read this post.

A study published in the international journal Conservation Biology is the first comprehensive review of more than 24,000 scientific publications related to conservation in the Oceanic region. Compiled by a team of 14 scientists, it reveals a sorry and worsening picture of habitat destruction and species loss. It also describes the deficiencies of and opportunities for governmental action to lessen this mounting regional and global problem. The review highlights destruction and degradation of ecosystems as the main threat. In Australia, agriculture has altered or destroyed half of all woodland and forests. Around 70% of the remaining forest has been damaged by logging. Loss of habitats is behind 80% of threatened species, the report claims.

According to the IUCN Red List of Threatened Species, 2008

• Nearly 17,000 of the world's 45,000 assessed species are threatened with extinction (38 percent). Of these, 3,246 are in the highest category of threat, Critically Endangered, 4,770 are Endangered and 8,912 are Vulnerable to extinction.
  
• Nearly 5,500 animal species are known to be threatened with extinction and at least 1,141 of the 5,487 known mammal species are threatened worldwide.
  
• In 2008, nearly 450 mammals were listed as Endangered, including the Tasmanian Devil (Sarcophilus harrisii), after the global population declined by more than 60 percent in the last 10 years.
  
• Scientists have catalogued relatively little about the rest of the world's fauna: only 5 percent of fish, 6 percent of reptiles, and 7 percent of amphibians have been evaluated. Of those studied, at least 750 fish species, 290 reptiles, and 150 amphibians are at risk.
  
• The average extinction rate is now some 1,000 to 10,000 times faster than the rate that prevailed over the past 60 million years.
  

Extinction certainly threatens amphibians [http://www.pnas.org/content/105/suppl.1/11466.abstract] — frogs, salamanders, and caecilians. A detailed worldwide assessment and subsequent updates show that one-third or more of the 6,300 species are threatened with extinction.

• http://www.science.unsw.edu.au/news/extinction-crisis-oceania/
• http://www.guardian.co.uk/environment/2009/jul/28/species-extinction-hotspots-australia
• http://wwf.org.au/news/queensland-land-clearing/

In Danish:
• http://politiken.dk/videnskab/article759526.ece

Note on 5 previous mass extinctions:

• Cretaceous-Tertiary. 65 million years ago, the dinosaurs were wiped out in a mass extinction that killed nearly a fifth of land vertebrate families, 16% of marine families and nearly half of all marine animals.
  
  
• End of Triassic. About 200 million years ago, lava floods erupting from the central Atlantic are thought to have created lethal global warming, killing off more than a fifth of all marine families and half of marine genera.
  
  
• Permian-Triassic. The worst mass extinction took place 250 million years ago, killing 95% of all species.
  
  
• Late Devonian. About 360 million years ago, a fifth of marine families were wiped out, alongside more than half of all marine genera.
  
  
• Ordovician-Silurian. About 440 million years ago, a quarter of all marine families were wiped out.
  

A major global extinction in the Middle Permian 260 to 270 million years ago preceded the huge end-Permian extinction. This extinction is also known as the Guadalupian Mass Extinction. New research suggests it may have been caused by volcanic eruptions in what is now China.

Wignall et al. have studied the Emeishan flood basalt province in south-west China. The eruption in the Emeishan province unleashed around half a million km² of lava, covering an area 5 times the size of Wales. The eruptions occurred in a shallow sea so that the lava appears today as a distinctive layer of igneous rock sandwiched between layers of sedimentary rock containing easily datable fossilised marine life. A close temporal link between the onset of eruptions and extinction suggests a cause-and-effect scenario. Cooling and acid rain (caused by sulphur dioxide (SO₂) effusion and sulfate aerosol formation) and consequent environmental deterioration are candidates for this link.

The study is published in the 29 May 2009 edition of the journal Science:
Volcanism, Mass Extinction, and Carbon Isotope Fluctuations in the Middle Permian of China
Vol. 324. no. 5931, pp. 1179 - 1182
DOI: 10.1126/science.1171956
• http://www.sciencemag.org/cgi/content/abstract/324/5931/1179

There is a relevant press release from the University of Leeds:
• http://www.leeds.ac.uk/media/press_releases/current09/volcanic.htm

And a plenitude of media coverage - a.o.:
• http://www.eurekalert.org/pub_releases/2009-05/uol-ave052709.php
• http://www.physorg.com/news162738601.html
• http://www.scientificblogging.com/news_articles/global_extinction_event_volcanoes_did_it_says_hypothesis
• http://news.yahoo.com/s/ap/20090528/ap_on_sc/us_sci_volcano_extinction_2
• http://www.scientificamerican.com/blog/60-second-science/post.cfm?id=new-evidence-for-volcanoes-as-sourc-2009-05-28
• http://www.abc.net.au/science/articles/2009/05/29/2584220.htm
• http://www.terradaily.com/reports/Ancient_eruption_killed_off_worlds_sea_life_scientists_999.html
• http://www.nytimes.com/aponline/2009/05/30/science/30volcano-wire.html?_r=1&partner=rss&emc=rss
• http://blogs.discovermagazine.com/80beats/2009/05/29/in-the-permian-period-erupting-super-volcanoes-may-have-killed-half-the-planet/

Oceanic anoxic events or Anoxic events occur when the Earth's oceans become completely depleted of oxygen (O₂) below the surface levels.

Detailed stratigraphic studies of Cretaceous black shales from many parts of the world have indicated that two Oceanic Anoxic Events were particularly significant in terms of their impact, one in the early Aptian (~120 Ma), sometimes called the Selli Event (or OAE 1a) after the Italian geologist, Raimondo Selli (1916–1983), and another at the Cenomanian–Turonian boundary (~93 million years ago), sometimes called the Bonarelli Event (or OAE 2) after the Italian geologist, Guido Bonarelli (1871–1951).

OAE2 was the big one. It was the most global, the most dramatic of a half-dozen OAEs during the exceptional warmth of the mid-Cretaceous period 120 million to 80 million years ago. The young Atlantic Ocean was as narrow as a few hundred kilometers, the sea ran free between Europe and Africa and into the western Pacific, and high sea levels drove the ocean up onto the continents.

Palm trees grew in what would be Alaska, large reptiles roamed in northern Canada and the ice-free Arctic Ocean warmed to the equivalent of a tepid swimming pool. But the depths of the ocean suddenly became starved of oxygen, wiping out swathes of marine life - a mass extinction. The sea floor around the globe became a lethal black ooze, that much later turned into black shale and partly transformed into oil.

After the extinction, levels of greenhouse gases in the atmosphere dropped and Earth lurched into a sudden, but short-lived, period of cooling. What was the cause of this extraordinary event? Palaeoceanographers looking for triggers of OAEs, especially OAE2, have long turned their attention to large volcanic eruptions. A shift in lead isotopes recorded at the very onset of OAE2 in Italy supported that idea (Science, 27 April 2007, p. 527 - Humongous Eruptions Linked to Dramatic Environmental Changes), but the evidence remained regional in scale.

In the journal Nature of 16 July 2008, palaeoceanographers report geographically broad-based isotopic evidence for a volcano-OAE2 link. They measured the element osmium (a rare metal) in sediments across OAE2 from Italy—which was in the Tethys seaway between Europe and Africa at the time—and just off northeast South America, which was then in the opening Atlantic. At both sites, they found that the ratio of osmium-187 to osmium-188 dropped dramatically just before the extinction event. The drop indicates that a huge amount of molten magma, carrying a higher proportion of the heavier isotope, was discharged into the oceans.

During that time period, the only volcanic site large enough to spew forth the necessary amount of magma in a few hundred of thousand years was a known volcanic plateau underneath the modern-day Caribbean. However, the researchers say that the huge lava flows thought to have been involved (and forming the bed of the present-day Caribbean) would have preceded the extinction by up to 23,000 years.

Next big question: what caused the huge eruption in the first place?

References
Turgeon, S. C. & Creaser, R. A. Nature 454, 323–326 (2008)
Kerr, Caribbean Megaeruptions Drove a Global Ocean Crisis, Science of 18 july 2008, p. 327.

• http://www.physorg.com/news135432196.html
• http://www.terradaily.com/reports/Death_in_the_deep_Volcanoes_blamed_for_mass_extinction_999.html
• http://news.bbc.co.uk/2/hi/science/nature/7510541.stm
• http://www.nature.com/news/2008/080716/full/news.2008.957.html

The debate is still going on about what lead to the mass extinction 251 million years ago - the end of Permian. According to the media “British researchers, reporting on Sunday in the journal Nature Geoscience, ruled out a leading theory that the oceans became starved of oxygen and rich with sulphide, causing marine life to die out.” I have not read the report in Nature Geoscience, and the short notes in TerraDaily and PhysOrg seem a bit vague, and even confusing, but the team of scientists apparently used a two-dimensional computer model of atmospheric chemistry to test the theory. I take it, however, that the theory in question was more or less the following.

A severe anoxic event at the end of the Permian could have made sulphate-reducing bacteria the dominant force in oceanic ecosystems, causing massive emissions of hydrogen sulphide (a stinking gas best known for its rotten egg smell) which poisoned plant and animal life on both land and sea, as well as severely weakening the ozone layer, exposing much of the life that remained to fatal levels of ultraviolet radiation. See Kump et al. 2005. This theory has the advantage of explaining the mass extinction of plants, which ought otherwise to have thrived in an atmosphere with a high level of carbon dioxide. Fossil spores from the end of Permian further support the theory: many show deformities that could have been caused by ultraviolet radiation, which would have been more intense after hydrogen sulphide emissions weakened the ozone layer.

A range of killing scenarios have indeed been discussed over the past 20 years or more. Apart from oceanic anoxia, we have heard about major continental plate movements, sea level changes, salinity changes, pH changes, global warming (as of late), and the two prime suspects: asteroid impact (in Antarctica ?) or mass volcanism (the Siberian traps), or maybe a combination of some of these events.

Whatever the culprit, it is still an intriguing question. A good review on large igneous provinces and mass extinctions, by Wignall, was brought by Earth Science Reviews in March 2001, unfortunately not with free access.

Benton & Twitchett wrote a good review “How to kill (almost) all life: the end-Permian extinction event” in Trends in Ecology & Evolution of 18 July 2003. A reprint of this paper can be downloaded as pdf file by clicking here. This paper contains the essence of the book “When Life Nearly Died. The greatest mass extinction of all time”. Michael J Benton, Thames & Hudson, 2003.

There may be some confusion over whether certain geochemical signatures at the Permo-Triassic boundary represent causes or effects of the extinction.

One suggestion for a scenario combining different processes is: the impact of an extraterrestrial body (like an asteroide) could have caused sudden mass mortality of organisms in sea and on land. The accompanying pressure wave could have triggered both the release of methane from hydrates stored on the bottom of the oceans and the
initiation of Siberian volcanism. After the initial events of the first 10,000–30,000 years, the reduction of land plant productivity lasted for millions of years, leading to a drop in global organic carbon burial and a shift of the major site of burial from the terrestrial to the marine environments. This would help to explain the very low level of Triassic coal deposition as compared with that during the Permian. Initial increased organic burial in the marine environment, caused by the sudden input of dead organisms, led to a higher frequency of basins with anaerobic conditions and a greater burial of sedimentary pyrite in the early Triassic. A suggestion out of many.

Maybe the flood basalt is temporarily on the winning hand, but undoubtedly the debate will go on for a long time to come.

Recent media story:
• http://www.terradaily.com/reports/Stinking_seas_not_to_blame_for_mother_of_all_mass_extinctions_999.html
• http://www.physorg.com/news125509388.html

Other (older) references:
• http://www.pnas.org/cgi/content/full/99/7/4172?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=Berner&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT
• http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V62-42HFR73-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=400a45eff6acca36ec2f140df0fd425d
• http://www.gsajournals.org/perlserv/?request=get-abstract&doi=10.1130%2FG21295.1