Editing Permian–Triassic extinction event (section)

=== Methane clathrate gasification ===
{{Main|Clathrate gun hypothesis}}
{{Further|Arctic methane emissions}}[[Methane clathrate]]s, also known as methane hydrates, consist of molecules of methane trapped in the crystal lattice of ice. This methane, produced by [[methanogens|methanogen]] microbes, has a {{nobr| {{sup|13}}C ''⁄'' {{sup|12}}C [[isotope analysis|isotope ratio]]}} about 6% below normal ({{delta|13|C}} −6.0%). At the right combination of pressure and temperature, clathrates form near the surface of [[permafrost]] and in large quantities on [[continental shelf|continental shelves]] and nearby seabed at water depths of at least {{convert|300|m|ft|abbr=on}}, buried in sediments up to {{convert|2000|m|ft|abbr=on}} below the sea floor.<ref name="Dickens2001">{{cite journal |author=Dickens, G.R. |year=2001 |title=The potential volume of oceanic methane hydrates with variable external conditions |journal=[[Organic Geochemistry]] |volume=32 |issue=10 |pages=1179–1193 |doi=10.1016/S0146-6380(01)00086-9|bibcode=2001OrGeo..32.1179D }}</ref>

Massive release of methane from these clathrates may have contributed to the PTME, as scientists have found worldwide evidence of a swift decrease of about 1% in the{{nobr| {{sup|13}}C ''⁄'' {{sup|12}}C ratio }}in [[Carbonate minerals|carbonate]] rocks from the end-Permian.<ref name="Twitchett" /><ref>
{{cite journal |vauthors=Palfy J, Demeny A, Haas J, Htenyi M, Orchard MJ, Veto I |year=2001 |title=Carbon isotope anomaly at the Triassic–Jurassic boundary from a marine section in Hungary |journal=[[Geology (journal)|Geology]] |volume=29 |issue=11 |pages=1047–1050 |bibcode=2001Geo....29.1047P |doi=10.1130/0091-7613(2001)029<1047:CIAAOG>2.0.CO;2 |issn=0091-7613}}</ref> This is the first, largest, and fastest of a series of excursions (decreases and increases) in the ratio, until it abruptly stabilised in the middle Triassic, followed soon afterwards by the recovery of calcifying shelled sealife.<ref name="Payne2004Local" /> The seabed probably contained [[methane hydrate]] deposits, and the lava caused the deposits to dissociate, releasing vast quantities of methane.<ref>{{cite journal |vauthors=Reichow MK, Saunders AD, White RV, Pringle MS, Al'Muhkhamedov AI, Medvedev AI, Kirda NP |year=2002 |title={{nobr| {{sup|40}}Ar ''⁄'' {{sup|39}}Ar }} dates from the West Siberian Basin: Siberian flood basalt province doubled |url=http://eprints.gla.ac.uk/468/1/Reichow_.pdf |journal=[[Science (journal)|Science]] |volume=296 |issue=5574 |pages=1846–1849 |bibcode=2002Sci...296.1846R |doi=10.1126/science.1071671 |pmid=12052954 |s2cid=28964473}}</ref>
A vast release of methane might cause significant global warming since methane is a very powerful [[Methane#Methane as a greenhouse gas|greenhouse gas]]. Strong evidence suggests the global temperatures increased by about 6&nbsp;°C (10.8&nbsp;°F) near the equator and therefore by more at higher latitudes: a sharp decrease in oxygen isotope ratios ({{nobr|{{sup|18}}O ''⁄'' {{sup|16}}O}});<ref>{{cite journal |vauthors=Holser WT, Schoenlaub HP, Attrep Jr M, Boeckelmann K, Klein P, Magaritz M, Orth CJ, Fenninger A, Jenny C, Kralik M, Mauritsch H, Pak E, Schramm JF, Stattegger K, Schmoeller R |year=1989 |title=A unique geochemical record at the Permian/Triassic boundary |journal=[[Nature (journal)|Nature]] |volume=337 |issue=6202 |pages=39–44 |bibcode=1989Natur.337...39H |doi=10.1038/337039a0 |s2cid=8035040}}</ref> the extinction of ''[[Glossopteris]]'' flora (''Glossopteris'' and plants that grew in the same areas), which needed a cold [[climate]], with its replacement by floras typical of lower paleolatitudes.<ref>{{cite journal |author=Dobruskina, I.A. |year=1987 |title=Phytogeography of Eurasia during the early Triassic |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=58 |issue=1–2 |pages=75–86 |bibcode=1987PPP....58...75D |doi=10.1016/0031-0182(87)90007-1}}</ref> It was also suggested that a large-scale release of methane and other [[greenhouse gas]]es from the ocean into the atmosphere was connected to the [[anoxic event]]s and euxinic (sulfidic) events at the time, with the exact mechanism compared to the 1986 [[Lake Nyos disaster]].<ref name="Ryskin 2003">{{cite journal |last=Ryskin |first=Gregory |date=September 2003 |title=Methane-driven oceanic eruptions and mass extinctions |journal=[[Geology (journal)|Geology]] |volume=31 |issue=9 |pages=741–744 |bibcode=2003Geo....31..741R |doi=10.1130/G19518.1}}</ref>

The clathrate hypothesis seemed the only proposed mechanism sufficient to cause a global 1% reduction in the {{nobr| {{sup|13}}C ''⁄'' {{sup|12}}C ratio }}.<ref>{{cite journal |last1=Krull |first1=Evelyn S. |last2=Retallack |first2=Gregory J. |date=1 September 2000 |title=<sup>13</sup>C depth profiles from paleosols across the Permian–Triassic boundary: Evidence for methane release |url=https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/112/9/1459/183682/13C-depth-profiles-from-paleosols-across-the?redirectedFrom=fulltext |journal=[[Geological Society of America Bulletin]] |volume=112 |issue=9 |pages=1459–1472 |bibcode=2000GSAB..112.1459K |doi=10.1130/0016-7606(2000)112<1459:CDPFPA>2.0.CO;2 |issn=0016-7606 |access-date=3 July 2023|url-access=subscription }}</ref><ref name="Erwin1993" /> While a variety of factors may have contributed to the ratio drop, a 2002 review found most of them insufficient to account for the observed amount:<ref name="Berner2002" /> 
* Gases from volcanic eruptions have a{{nobr| {{sup|13}}C ''⁄'' {{sup|12}}C ratio }}about 0.5 to 0.8% below standard ({{delta|13|C}} −0.5 to −0.8%), but a 1995 assessment concluded that the observed 1.0% worldwide reduction would have required eruptions massively larger than any found.<ref name="Dickens1995">{{cite journal |vauthors=Dickens GR, O'Neil JR, Rea DK, Owen RM |year=1995|title=Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene |journal=[[Paleoceanography and Paleoclimatology]] |volume=10 |issue=6 |pages=965–971 |doi=10.1029/95PA02087 |bibcode=1995PalOc..10..965D}}</ref> (However, this analysis addressed only CO<sub>2</sub> produced by the magma itself, not from interactions with carbon bearing sediments, as described below.)
* A reduction in organic activity would extract {{sup|12}}C more slowly from the environment and leave more of it to be incorporated into sediments, thus reducing the{{nobr| {{sup|13}}C ''⁄'' {{sup|12}}C ratio. }} [[Biochemistry|Biochemical]] processes preferentially use the lighter isotopes since chemical reactions are ultimately driven by electromagnetic forces between atoms and lighter isotopes respond more quickly to these forces, but a study of a smaller drop of 0.3 to 0.4% in {{nobr| {{sup|13}}C ''⁄'' {{sup|12}}C }} ({{delta|13|C}} −3 to −4&nbsp;‰) at the [[Paleocene-Eocene Thermal Maximum]] (PETM) concluded that even transferring all the organic [[carbon]] (in organisms, soils, and dissolved in the ocean) into sediments would be insufficient: Even such a large burial of material rich in {{sup|12}}C would not have produced the 'smaller' drop in the {{nobr| {{sup|13}}C ''⁄'' {{sup|12}}C ratio }} of the rocks around the PETM.<ref name="Dickens1995" />
* Buried sedimentary organic matter has a {{nobr| {{sup|13}}C ''⁄'' {{sup|12}}C ratio }} 2.0 to 2.5% below normal ({{delta|13|C}} −2.0 to −2.5%). Theoretically, if the sea level fell sharply, shallow [[marine sediment]]s would be exposed to oxidation. But 6,500–8,400 gigatonnes (1 gigatonne = {{10^|12}} kg) of organic carbon would have to be oxidized and returned to the ocean-atmosphere system within less than a few hundred thousand years to reduce the {{nobr| {{sup|13}}C ''⁄'' {{sup|12}}C ratio }} by 1.0%, which is not thought to be a realistic possibility.<ref name="Erwin1993" /> Moreover, sea levels were rising rather than falling at the time of the extinction.<ref name="White" />
* Rather than a sudden decline in sea level, intermittent periods of ocean-bottom [[hyperoxia]] and [[Anoxic sea water|anoxia]] (high-oxygen and low- or zero-oxygen conditions) may have caused the {{nobr| {{sup|13}}C ''⁄'' {{sup|12}}C ratio }} fluctuations in the Early Triassic;<ref name="Payne2004Local" /> and global anoxia may have been responsible for the end-Permian blip. The continents of the end-Permian and early Triassic were more clustered in the tropics than they are now, and large tropical rivers would have dumped sediment into smaller, partially enclosed ocean basins at low latitudes. Such conditions favor oxic and anoxic episodes; oxic/anoxic conditions would result in a rapid release/burial, respectively, of large amounts of organic carbon, which has a low {{nobr| {{sup|13}}C ''⁄'' {{sup|12}}C ratio }} because biochemical processes use the lighter isotopes more.<ref name="SchragBernerEtAl2002">{{cite journal |vauthors=Schrag DP, Berner RA, Hoffman PF, Halverson GP |year=2002 |title=On the initiation of a snowball Earth |journal=[[Geochemistry, Geophysics, Geosystems]] |volume=3 |issue=6 |pages=1–21 |doi=10.1029/2001GC000219 |bibcode=2002GGG.....3.1036S |doi-access=free }}
Preliminary abstract at {{cite web | author=Schrag, D.P. | date=June 2001 | title=On the initiation of a snowball Earth | publisher=Geological Society of America | url=http://gsa.confex.com/gsa/2001ESP/finalprogram/abstract_8038.htm | access-date=2008-04-20 | url-status=dead | archive-url=https://web.archive.org/web/20180425115243/https://gsa.confex.com/gsa/2001ESP/finalprogram/abstract_8038.htm | archive-date=2018-04-25 }}</ref> That or another organic-based reason may have been responsible for both that and a late Proterozoic/Cambrian pattern of fluctuating {{nobr| {{sup|13}}C ''⁄'' {{sup|12}}C ratios.}}<ref name="Payne2004Local" />

However, the clathrate hypothesis has also been criticized. Carbon-cycle models that include consideration of roasting carbonate sediments by volcanism confirm that it would have had enough effect to produce the observed reduction.<ref name="Berner2002">
{{cite journal| author = Berner, R.A.| year = 2002| title = Examination of hypotheses for the Permo-Triassic boundary extinction by carbon cycle modeling |journal = [[Proceedings of the National Academy of Sciences of the United States of America]] |volume=99 |issue=7 |pages=4172–4177 |doi=10.1073/pnas.032095199 |doi-access=free |pmid=11917102 |pmc=123621  |bibcode = 2002PNAS...99.4172B }}</ref><ref name="Benton2003">{{cite journal|author1 = Benton, Michael James  | author1-link = Michael Benton | author2=Twitchett, R.J.| year = 2003 |title=How to kill (almost) all life: The end-Permian extinction event | journal = [[Trends in Ecology & Evolution]] |volume = 18 | issue = 7 | pages = 358–365 |doi = 10.1016/S0169-5347(03)00093-4 }}</ref> Also, the pattern of isotope shifts expected to result from a massive release of methane does not match the patterns seen throughout the Early Triassic. Not only would such a cause require the release of five times as much methane as postulated for the PETM, but would it also have to be reburied at an unrealistically high rate to account for the rapid increases in the {{nobr| {{sup|13}}C ''⁄'' {{sup|12}}C ratio }} (episodes of high positive {{delta|13|C}}) throughout the early Triassic before it was released several times again.<ref name="Payne2004Local" /> The latest research suggests that greenhouse gas release during the extinction event was dominated by volcanic carbon dioxide,<ref>{{cite journal |last1=Cui |first1=Ying |last2=Li |first2=Mingsong |last3=van Soelen |first3=Elsbeth E. |last4=Peterse |first4=Francien |last5=M. Kürschner |first5=Wolfram |date=7 September 2021 |title=Massive and rapid predominantly volcanic {{CO2}} emission during the end-Permian mass extinction |journal= [[Proceedings of the National Academy of Sciences of the United States of America]] |volume=118 |issue=37 |pages=e2014701118 |doi=10.1073/pnas.2014701118 |pmid=34493684 |pmc=8449420 |bibcode=2021PNAS..11814701C |doi-access=free }}</ref> and while methane release had to have contributed, isotopic signatures show that thermogenic methane released from the Siberian Traps had consistently played a larger role than methane from clathrates and any other biogenic sources such as wetlands during the event.<ref name="WuEtAl2021NatureCommunications">{{cite journal |last1=Wu |first1=Yuyang |last2=Chu |first2=Daoliang |last3=Tong |first3=Jinnan |last4=Song |first4=Haijun |last5=Dal Corso |first5=Jacopo |last6=Wignall |first6=Paul Barry |last7=Song |first7=Huyue |last8=Du |first8=Yong |last9=Cui |first9=Ying |date=9 April 2021 |title=Six-fold increase of atmospheric ''p''{{CO2}} during the Permian–Triassic mass extinction |url=https://www.researchgate.net/publication/350759904 |journal=[[Nature Communications]] |volume=12 |issue=1 |page=2137 |doi=10.1038/s41467-021-22298-7 |pmid=33837195 |pmc=8035180 |bibcode=2021NatCo..12.2137W |s2cid=233200774 |access-date=2024-03-26}}</ref>

Adding to the evidence against methane clathrate release as the central driver of warming, the main rapid warming event is also associated with marine transgression rather than regression; the former would not normally have initiated methane release, which would have instead required a decrease in pressure, something that would be generated by a retreat of shallow seas.<ref>{{cite journal |last1=Shen |first1=Shu-Zhong |last2=Cao |first2=Chang-Qun |last3=Henderson |first3=Charles M. |last4=Wang |first4=Xiang-Dong |last5=Shi |first5=Guang R. |last6=Wang |first6=Yue |last7=Wang |first7=Wei |date=January 2006 |title=End-Permian mass extinction pattern in the northern peri-Gondwanan region |url=https://www.sciencedirect.com/science/article/abs/pii/S1871174X06000072 |journal=[[Palaeoworld]] |volume=15 |issue=1 |pages=3–30 |doi=10.1016/j.palwor.2006.03.005 |access-date=26 May 2023|url-access=subscription }}</ref> The configuration of the world's landmasses into one supercontinent would also mean that the global gas hydrate reservoir was lower than today, further damaging the case for methane clathrate dissolution as a major cause of the carbon cycle disruption.<ref>{{Cite journal |last1=Majorowicz |first1=J. |last2=Grasby |first2=S. E. |last3=Safanda |first3=J. |last4=Beauchamp |first4=B. |date=1 May 2014 |title=Gas hydrate contribution to Late Permian global warming |url=https://www.sciencedirect.com/science/article/pii/S0012821X14001460 |journal=[[Earth and Planetary Science Letters]] |volume=393 |pages=243–253 |doi=10.1016/j.epsl.2014.03.003 |bibcode=2014E&PSL.393..243M |issn=0012-821X |access-date=12 January 2024 |via=Elsevier Science Direct|url-access=subscription }}</ref>