Editing Extinction event (section)

==Patterns in frequency==
Various authors have suggested that extinction events occurred periodically, every 26 to 30&nbsp;million years,<ref>{{cite magazine | vauthors = Beardsley T  |year = 1988 |title = Star-struck? |magazine =[[Scientific American]] |volume = 258 |issue = 4 |pages = 37–40 |doi = 10.1038/scientificamerican0488-37b |bibcode = 1988SciAm.258d..37B}}</ref><ref name=Raup1984>{{cite journal | vauthors = Raup DM, Sepkoski JJ | title = Periodicity of extinctions in the geologic past | journal =  Proceedings of the National Academy of Sciences| volume = 81 | issue = 3 | pages = 801–805 | date = February 1984 | pmid = 6583680 | pmc = 344925 | doi = 10.1073/pnas.81.3.801 | bibcode = 1984PNAS...81..801R | doi-access = free }}</ref> or that diversity fluctuates episodically about every 62 million years.<ref name=Rohde2005>
Different cycle lengths have been proposed; e.g. by {{cite journal | vauthors = Rohde RA, Muller RA | title = Cycles in fossil diversity | journal = Nature | volume = 434 | issue = 7030 | pages = 208–210 | date = March 2005 | pmid = 15758998 | doi = 10.1038/nature03339 | s2cid = 32520208 | bibcode = 2005Natur.434..208R }}</ref> Various ideas, mostly regarding [[Astronomy|astronomical]] influences, attempt to explain the supposed pattern, including the presence of a [[Nemesis (hypothetical star)|hypothetical companion star]] to the Sun,<ref>{{cite web | vauthors = Muller RA |title = Nemesis |website = Muller.lbl.gov |publisher = [[Lawrence Berkeley Laboratory]] |url = http://muller.lbl.gov/pages/lbl-nem.htm |access-date = 2007-05-19}}</ref><ref>
{{cite journal | vauthors = Melott AL, Bambach RK | date = July 2010 | title = Nemesis reconsidered | journal = [[Monthly Notices of the Royal Astronomical Society]] | volume = 407 | issue = 1 | pages = L99–L102 | doi = 10.1111/j.1745-3933.2010.00913.x | doi-access = free | arxiv = 1007.0437 | bibcode = 2010MNRAS.407L..99M | s2cid = 7911150 | url = http://www.centauri-dreams.org/?p=13357 | access-date = 2010-07-02 }}</ref> oscillations in the galactic plane, or passage through the Milky Way's spiral arms.<ref name="Gillmana2008">
{{cite journal | vauthors = Gillman M, Erenler H | year = 2008 | title = The galactic cycle of extinction | journal = International Journal of Astrobiology | volume = 7 | issue = 1 | pages = 17–26 | doi = 10.1017/S1473550408004047 | bibcode = 2008IJAsB...7...17G | issn = 1475-3006 | citeseerx = 10.1.1.384.9224 | s2cid = 31391193 | url = http://oro.open.ac.uk/11603/1/S1473550408004047a.pdf | access-date = 2018-04-02 }}
</ref> However, other authors have concluded that the data on marine mass extinctions do not fit with the idea that mass extinctions are periodic, or that ecosystems gradually build up to a point at which a mass extinction is inevitable.<ref name=Alroy_2008>{{cite journal | vauthors = Alroy J | title = Colloquium paper: dynamics of origination and extinction in the marine fossil record | journal =  Proceedings of the National Academy of Sciences of the United States of America| volume = 105 | issue = Supplement 1 | pages = 11536–11542 | date = August 2008 | pmid = 18695240 | pmc = 2556405 | doi = 10.1073/pnas.0802597105 | doi-access = free | bibcode = 2008PNAS..10511536A }}</ref> Many of the proposed correlations have been argued to be spurious or lacking statistical significance.<ref>{{cite journal | vauthors = Bailer-Jones CA | date = July 2009 | title = The evidence for and against astronomical impacts on climate change and mass extinctions: A review | journal = International Journal of Astrobiology | volume = 8 | issue = 3 | pages = 213–219 | doi = 10.1017/S147355040999005X |bibcode = 2009IJAsB...8..213B |arxiv = 0905.3919 | s2cid = 2028999|issn=1475-3006}}</ref><ref>{{cite journal | vauthors = Overholt AC, Melott AL, Pohl M | year = 2009 | title = Testing the link between terrestrial climate change and galactic spiral arm transit | journal = The Astrophysical Journal | volume = 705 | issue = 2 | pages = L101–03 | doi = 10.1088/0004-637X/705/2/L101 | bibcode=2009ApJ...705L.101O |arxiv = 0906.2777 | s2cid = 734824}}</ref><ref>{{Cite journal | vauthors = Erlykin AD, Harper DA, Sloan T, Wolfendale AW |date=2017 | veditors = Smith A |title=Mass extinctions over the last 500 myr: an astronomical cause? |journal=Palaeontology |language=en |volume=60 |issue=2 |pages=159–167 |doi=10.1111/pala.12283|bibcode=2017Palgy..60..159E |s2cid=133407217 |doi-access=free }}</ref> Others have argued that there is strong evidence supporting periodicity in a variety of records,<ref name="Melott2011">{{cite journal | vauthors = Melott AL, Bambach RK | year = 2011 | title = A{{grey|[n]}} ubiquitous ~62&nbsp;Myr periodic fluctuation superimposed on general trends in fossil biodiversity. I. Documentation | journal = Paleobiology | volume = 37 | pages = 92–112 | doi = 10.1666/09054.1 | arxiv = 1005.4393 | s2cid = 1905891 }}</ref> and additional evidence in the form of coincident periodic variation in nonbiological geochemical variables such as Strontium isotopes,<ref name="Melott et al. 2012">{{cite journal | vauthors = Melott AL, Bambach RK, Petersen KD, McArthur JM | display-authors = etal | year = 2012 | title = A ~60 Myr periodicity is common to marine-87Sr/86Sr, fossil biodiversity, and large-scale sedimentation: what does the periodicity reflect? | journal = Journal of Geology | volume = 120 | issue = 2 | pages = 217–226 | arxiv = 1206.1804 | bibcode = 2012JG....120..217M | doi = 10.1086/663877 | s2cid = 18027758 }}</ref> flood basalts, anoxic events, orogenies, and evaporite deposition. One explanation for this proposed cycle is carbon storage and release by oceanic crust, which exchanges carbon between the atmosphere and mantle.<ref>{{cite journal | vauthors = Müller RD, Dutkiewicz A | title = Oceanic crustal carbon cycle drives 26-million-year atmospheric carbon dioxide periodicities | journal = Science Advances | volume = 4 | issue = 2 | pages = eaaq0500 | date = February 2018 | pmid = 29457135 | pmc = 5812735 | doi = 10.1126/sciadv.aaq0500 | bibcode = 2018SciA....4..500M }}</ref>
{{Phanerozoic biodiversity}}
Mass extinctions are thought to result when a long-term stress is compounded by a short-term shock.<ref name=Arens2008>{{cite journal | vauthors = Arens NC, West ID | year = 2008 | title = Press-pulse: a general theory of mass extinction? | journal = Paleobiology| volume = 34 | issue = 4 | pages = 456–471 | doi = 10.1666/07034.1 | bibcode = 2008Pbio...34..456A | s2cid = 56118514| url = http://doc.rero.ch/record/16048/files/PAL_E3838.pdf }}</ref> Over the course of the [[Phanerozoic]], individual taxa appear to have become less likely to suffer extinction,<ref name=Wang2008>{{cite journal | vauthors = Wang SC, Bush AM | year = 2008 | title = Adjusting global extinction rates to account for taxonomic susceptibility | journal = Paleobiology | volume = 34 | issue = 4 | pages = 434–55 | doi = 10.1666/07060.1 | s2cid = 16260671 | url = http://www.swarthmore.edu/NatSci/swang1/Publications/}}</ref> which may reflect more robust food webs, as well as fewer extinction-prone species, and other factors such as continental distribution.<ref name=Wang2008/> However, even after accounting for sampling bias, there does appear to be a gradual decrease in extinction and origination rates during the Phanerozoic.<ref name=Alroy_2008/> This may represent the fact that groups with higher turnover rates are more likely to become extinct by chance; or it may be an artefact of taxonomy: families tend to become more speciose, therefore less prone to extinction, over time;<ref name=Alroy_2008/> and larger taxonomic groups (by definition) appear earlier in geological time.<ref name="Budd2003">{{cite journal | vauthors = Budd GE | title = The Cambrian fossil record and the origin of the phyla | journal = Integrative and Comparative Biology | volume = 43 | issue = 1 | pages = 157–165 | date = February 2003 | pmid = 21680420 | doi = 10.1093/icb/43.1.157 | doi-access = free }}</ref>

It has also been suggested that the oceans have gradually become more hospitable to life over the last 500 million years, and thus less vulnerable to mass extinctions,{{efn|
[[Dissolved oxygen]] became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of [[eutrophication]] and [[anoxic event]]s; and marine ecosystems became more diversified so that [[food chain]]s were less likely to be disrupted.
}}<ref>{{cite journal | vauthors = Martin RE | year = 1995 | title = Cyclic and secular variation in microfossil biomineralization: Clues to the biogeochemical evolution of Phanerozoic oceans | journal = Global and Planetary Change | volume = 11 | issue = 1 | pages = 1–23 | doi = 10.1016/0921-8181(94)00011-2 | bibcode = 1995GPC....11....1M }}</ref><ref>{{cite journal | vauthors = Martin RE | year = 1996 | title = Secular increase in nutrient levels through the Phanerozoic: Implications for productivity, biomass, and diversity of the marine biosphere | journal = PALAIOS | volume = 11 | issue = 3 | pages = 209–219 | doi = 10.2307/3515230 | jstor = 3515230 | bibcode = 1996Palai..11..209M }}
</ref>
but susceptibility to extinction at a taxonomic level does not appear to make mass extinctions more or less probable.<ref name=Wang2008/>