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Zheng, H. Ordovician–Silurian Extinction Events. Encyclopedia. Available online: https://encyclopedia.pub/entry/30736 (accessed on 07 December 2023).
Zheng H. Ordovician–Silurian Extinction Events. Encyclopedia. Available at: https://encyclopedia.pub/entry/30736. Accessed December 07, 2023.
Zheng, Handwiki. "Ordovician–Silurian Extinction Events" Encyclopedia, https://encyclopedia.pub/entry/30736 (accessed December 07, 2023).
Zheng, H.(2022, October 23). Ordovician–Silurian Extinction Events. In Encyclopedia. https://encyclopedia.pub/entry/30736
Zheng, Handwiki. "Ordovician–Silurian Extinction Events." Encyclopedia. Web. 23 October, 2022.
Ordovician–Silurian Extinction Events
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The Ordovician–Silurian extinction events, when combined, are the second-largest of the five major extinction events in Earth's history in terms of percentage of genera that became extinct. This event greatly affected marine communities, which caused the disappearance of one third of all brachiopod and bryozoan families, and further numerous groups of conodonts, trilobites, and graptolites. The Ordovician–Silurian extinction occurred during the Hirnantian stage of the Ordovician Period and the subsequent Rhuddanian stage of the Silurian Period. The last event, the Late Ordovician mass extinction (LOME), is dated in the interval of 455 to 430 million years ago, lasting from the Middle Ordovician to Early Silurian, thus including the extinction period. This event was the first of the big five Phanerozoic events and was the first to significantly affect animal-based communities. In May 2020, geologists reported that this mass extinction, the Late Ordovician mass extinction, 445 million years ago, may have been the result of global warming. Almost all major taxonomic groups were affected during this extinction event. Extinction was global during this period, eliminating 49–60% of marine genera and nearly 85% of marine species. Brachiopods, bivalves, echinoderms, bryozoans and corals were particularly affected. Before the late Ordovician cooling, temperatures were relatively warm and it is the suddenness of the climate changes and the elimination of habitats due to sea-level fall that are believed to have precipitated the extinctions. The falling sea level disrupted or eliminated habitats along the continental shelves. Evidence for the glaciation was found through deposits in the Sahara Desert. A combination of lowering of sea level and glacially driven cooling were likely driving agents for the Ordovician mass extinction.

ordovician–silurian silurian climate changes

1. Impact

The extinction occurred 443.8 million years ago, during the Great Ordovician Biodiversification Event.[1] It marks the boundary between the Ordovician and following Silurian period. During this extinction event there were several marked changes in biologically responsive carbon and oxygen isotopes. The spread of anoxia (the absence of oxygen) greatly affected the organisms that lived in this time period.[2] This complexity may indicate several distinct closely spaced events, or particular phases within one event.

At the time, most complex multicellular organisms lived in the sea, and around 100 marine families became extinct, covering about 49%[3] of faunal genera (a more reliable estimate than species). The brachiopods and bryozoans were decimated, along with many of the trilobite, conodont and graptolite families.

Statistical analysis of marine losses at this time suggests that the decrease in diversity was mainly caused by a sharp increase in extinctions, rather than a decrease in speciation.[4] Several groups of marine organisms with a planktonic lifestyle, more exposed to UV radiation than groups that lived in the benthos, suffered severely during the late Ordovician. Organisms that dwelled in the plankton were affected before benthic organisms during the mass extinction, and species dwelling in shallow water were more likely to become extinct than species dwelling in deep water.[5]

2. Possible Causes

A reconstruction showing Cameroceras shells sticking out of the mud, the result of the Ordovician-Silurian Extinction event. https://handwiki.org/wiki/index.php?curid=1577400

The rapid onset of the continental glaciation on Gondwana determined by its position in the South Pole area; the cooling; the hydrodynamic changes through the entire water column in the World Ocean; and the corresponding sea level fall, which was responsible for the reduction of shelf areas and shallow-water basins, i.e., the main ecological niche of the Ordovician marine biota, were main prerequisites of the stress conditions. Similar to other mass extinction events, these processes were accompanied by volcanism, impact events, a corresponding reduction of the photosynthesis and bioproductivity, the destruction of food chains, and anoxia. The appearance and development of terrestrial plants and microphytoplankton, which consumed atmospheric carbon dioxide, thus, diminishing the greenhouse effect and promoting the transition of the climatic system to the glacial mode, played a unique role in that period.[6]

2.1. Volcanism and Weathering

The late Ordovician glaciation was preceded by a fall in atmospheric carbon dioxide (from 7,000 ppm to 4,400 ppm).[7][8] The dip is correlated with a burst of volcanic activity that deposited new silicate rocks, which draw CO2 out of the air as they erode. A major role of CO2 is implied by a 2009 paper.Cite error: Closing </ref> missing for <ref> tag

This incurred a shift in the location of bottom-water formation, shifting from low latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which was accompanied by increased deep-ocean currents and oxygenation of the bottom-water. An opportunistic fauna briefly thrived there, before anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters. Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.

2.2. Metal Poisoning

Toxic metals on the ocean floor may have dissolved into the water when the oceans' oxygen was depleted. An increase in available nutrients in the oceans may have been a factor, and decreased ocean circulation caused by global cooling may also have been a factor. [9]

The toxic metals may have killed life forms in lower trophic levels of the food chain, causing a decline in population, and subsequently resulting in starvation for the dependent higher feeding life forms in the chain.[10][11]

2.3. Gamma-Ray Burst

Some scientists have suggested that the initial extinctions could have been caused by a gamma-ray burst originating from a hypernova within 6,000 light-years of Earth (in a nearby arm of the Milky Way galaxy). A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, exposing surface-dwelling organisms, including those responsible for planetary photosynthesis, to high levels of extreme ultraviolet radiation.[5][12][13][14] Although the hypothesis is consistent with patterns at the onset of extinction, there is no unambiguous evidence that such a nearby gamma-ray burst ever happened.

3. End of the Event

The end of the second event occurred when melting glaciers caused the sea level to rise and stabilize once more. The rebound of life's diversity with the sustained re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving orders.

Following such a major loss of diversity, Silurian communities were initially less complex and broader niched. Highly endemic faunas, which characterized the Late Ordovician, were replaced by faunas that were amongst the most cosmopolitan in the Phanerozoic, biogeographic patterns that persisted throughout most of the Silurian.[15]

These end Ordovician–Silurian events had nothing like the long-term impact of the Permian–Triassic and Cretaceous–Paleogene extinction events. Nevertheless, a large number of taxa disappeared from the Earth over a short time interval,[15] eliminating and changing diversity.

References

  1. Munnecke, A.; Calner, M.; Harper, D. A. T.; Servais, T. (2010). "Ordovician and Silurian sea-water chemistry, sea level, and climate: A synopsis". Palaeogeography, Palaeoclimatology, Palaeoecology 296 (3–4): 389–413. doi:10.1016/j.palaeo.2010.08.001. Bibcode: 2010PPP...296..389M.  https://dx.doi.org/10.1016%2Fj.palaeo.2010.08.001
  2. error
  3. Rohde & Muller; Muller, RA (2005). "Cycles in Fossil Diversity". Nature 434 (7030): 208–210. doi:10.1038/nature03339. PMID 15758998. Bibcode: 2005Natur.434..208R.  https://dx.doi.org/10.1038%2Fnature03339
  4. 2.0.CO;2. http://www.bioone.org/perlserv/?request=get-document&issn=0094-8373&volume=30&page=522. " id="ref_4">Bambach, R.K.; Knoll, A.H.; Wang, S.C. (December 2004). "Origination, extinction, and mass depletions of marine diversity". Paleobiology 30 (4): 522–542. doi:10.1666/0094-8373(2004)030<0522:OEAMDO>2.0.CO;2. http://www.bioone.org/perlserv/?request=get-document&issn=0094-8373&volume=30&page=522. 
  5. Melott, A.L. (2004). "Did a gamma-ray burst initiate the late Ordovician mass extinction?". International Journal of Astrobiology 3 (2): 55–61. doi:10.1017/S1473550404001910. Bibcode: 2004IJAsB...3...55M.  https://dx.doi.org/10.1017%2FS1473550404001910
  6. Barash, M. (November 2014). "Mass Extinction of the Marine Biota at the Ordovician–Silurian Transition Due to Environmental Changes". Oceanology 54 (6): 780–787. doi:10.1134/S0001437014050014. Bibcode: 2014Ocgy...54..780B.  https://dx.doi.org/10.1134%2FS0001437014050014
  7. Seth A. Young, Matthew R. Saltzman, William I. Ausich, André Desrochers, and Dimitri Kaljo, "Did changes in atmospheric CO2 coincide with latest Ordovician glacial–interglacial cycles?", Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 296, No. 3–4, 15 October 2010, Pages 376–388.
  8. Jeff Hecht, High-carbon ice age mystery solved, New Scientist, 8 March 2010 (retrieved 30 June 2014) https://www.newscientist.com/article/dn18618-highcarbon-ice-age-mystery-solved.html
  9. Bartlett, Rick; Elrick, Maya; Wheeley, James R.; Polyak, Victor; Desrochers, André; Asmerom, Yemane (2018). "Abrupt global-ocean anoxia during the Late Ordovician–early Silurian detected using uranium isotopes of marine carbonates". Proceedings of the National Academy of Sciences 115 (23): 5896–5901. doi:10.1073/pnas.1802438115. PMID 29784792. Bibcode: 2018PNAS..115.5896B.  http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=6003337
  10. Katz, Cheryl (2015-09-11). "New Theory for What Caused Earth's Second-Largest Mass Extinction". http://news.nationalgeographic.com/2015/09/15911-metals-extinction-ocean-oxygen-ordovician-silurian/. Retrieved 2015-09-12. 
  11. Vandenbroucke, Thijs R. A.; Emsbo, Poul; Munnecke, Axel; Nuns, Nicolas; Duponchel, Ludovic; Lepot, Kevin; Quijada, Melesio; Paris, Florentin et al. (2015-08-25). "Metal-induced malformations in early Palaeozoic plankton are harbingers of mass extinction" (in en). Nature Communications 6: Article 7966. doi:10.1038/ncomms8966. PMID 26305681. Bibcode: 2015NatCo...6.7966V.  http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=4560756
  12. Wanjek, Christopher (April 6, 2005). "Explosions in Space May Have Initiated Ancient Extinction on Earth". NASA. http://www.nasa.gov/vision/universe/starsgalaxies/gammaray_extinction.html. Retrieved 2008-04-30. 
  13. "Ray burst is extinction suspect". BBC. April 6, 2005. http://news.bbc.co.uk/1/hi/sci/tech/4433963.stm. Retrieved 2008-04-30. 
  14. Melott, A.L.; Thomas, B.C. (2009). "Late Ordovician geographic patterns of extinction compared with simulations of astrophysical ionizing radiation damage". Paleobiology 35 (3): 311–320. doi:10.1666/0094-8373-35.3.311.  https://dx.doi.org/10.1666%2F0094-8373-35.3.311
  15. Harper, D. A. T.; Hammarlund, E. U.; Rasmussen, C. M. Ø. (May 2014). "End Ordovician extinctions: A coincidence of causes". Gondwana Research 25 (4): 1294–1307. doi:10.1016/j.gr.2012.12.021. Bibcode: 2014GondR..25.1294H.  https://dx.doi.org/10.1016%2Fj.gr.2012.12.021
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