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Chinese Science Bulletin

© 2009 SCIENCE IN CHINA PRESS Springer

The and the End- mass : a critical review

Andy SAUNDERS† & Marc REICHOW

Department of Geological Sciences, University of Leicester, Leicester, LE1 7RH, United Kingdom The association between the Siberian Traps, the largest continental flood province, and the largest-known mass at the end of the Permian period, has been strengthened by re- cently-published high-precision 40Ar/39Ar dates from widespread localities across the Siberian province[1]. We argue that the impact of the volcanism was amplified by the prevailing late Permian environmental conditions―in particular, the hothouse , with sluggish oceanic circulation, that was leading to widespread oceanic anoxia. Volcanism released large masses of sulphate aerosols and , the former triggering short-duration volcanic winters, the latter leading to long-term warming. Whilst the mass of CO2 released from individual eruptions was small compared with the total mass of carbon in the atmosphere- system, the long ‘mean lifetime’ of atmospheric CO2, com- pared with the eruption flux and duration, meant that significant accumulation could occur over periods of 105 . Compromise of the systems (by curtailment of photosynthesis, de- struction of biomass, and warming and acidification of the ) probably led to rapid atmospheric

CO2 build-up, warming, and shallow-water anoxia, leading ultimately to mass extinction.

continental flood , oceanic anoxia, radiometric dating, CO2, SO2

The last half billion years of history have been cycle. Carbon excursions also occur at several punctuated by several episodes of rapid . other intervals in the geological record, for example Some of these changes led to mass , when during the (~183 Ma), and at the Palaeo- substantial portions of the Earth’s fauna and flora, un- cene- Thermal Maximum (~55 Ma), although the able to cope with the rapidity of change, were destroyed scale of the accompanying mass extinctions, and the in geologically short periods of time. The causes of these extent of oceanic anoxia, were smaller. mass extinctions are debated and are of immediate con- It is simplistic to invoke one mechanism to explain all cern, not least because anthropogenically-driven climate extinction events. However, any mechanism needs to be change may be about to trigger the next mass extinction. sufficiently powerful, and perhaps operating in synergy The ‘Big Five’ mass extinction events originally rec- with other processes, to disrupt the global climate and ognised by Raup and Sepkoski[2] are the Ordovi- ultimately the global , and within a rela- cian- (at 444 Ma), -Carboniferous (360 tively short period time. Furthermore, any process has to Ma), end-Permian (251 Ma), end- (200 Ma), and cause ecosystems to collapse and exterminate large end- (65.6 Ma). Mass extinctions are now swaths of species. Certainly, a sufficiently large meteor- also recognised at the Cambro- boundary ite or comet impact could cause a mass extinction, and (488 Ma) and during the Permian (260 Ma: the Guada- Received June 27, 2008; accepted September 17, 2008 lupian extinction). Several of these extinctions are asso- doi: 10.1007/s11434-008-0543-7 † ciated with oceanic anoxic events and carbon isotope Corresponding author (email: [email protected]) Supported by the Natural Environment Research Council, UK (Grant No. NE/ excursions, indicating major disruption to the carbon C003276) www.scichina.com | csb.scichina.com | www.springerlink.com Chinese Science Bulletin | January 2009 | vol. 54 | no. 1 | 20-37 time salts and mass extinctions has been recognised foralong The association between flood ba- of basaltic . and are characterised by rapid and voluminous effusion basalt provinces are a type of , continental eruptions. Continental flood realisation that the causemay beintrinsic tothe planet: mechanism needstobeinvoked. tinctions is sparse or non-existent; therefore some other dence formajor impacts at the time ofothermass ex- not alone cause the extinction. Furthermore, the evi- extinctions (Figure 1) is much better than that between meteorite impacts and the correlation between eruption events and extinctions tinction thatmillion occurred251 yearsago. The Earth’s Andy being ‘entirely fortuitous’ issmall Deccan, respectively). The like largest flood basalt events (, Central Atlantic, and and Cretaceous are all contemporaneous withthe three large mass extinctions at the end of the Permian, Triassic tope excursions areincluded even strongerifoceanic growing evidence ceous mass extinction, 65.6 million years ago, there is Chicxulub impact crater in Mexi whilst there is aclose match between the timing ofthe Extinctionrateversustime(continuousline,bluefi Figure 1 anic plateaus,theCaribbean(CP)Kerguelen(KP),andOntong Traps, Siberian the eruptions of the with correspond theCentral continental thePermo-Triassic, massextinctions, floodbasalts ofthelargest Three (redbars). Triassic- andthe Cret Over theOver last two decades, therehasagrowing been In thisreview wefocus [4–9] , and for the period of the last 300 million years, [3] that even thismassive impact did [10,11] anoxic events and carbon iso- SAUNDERS on the end-Permian mass ex- . The correlation becomes in the dataset. The three three The dataset. the in lihood of this correlationlihood [11] co and the end-Creta- . etal.ChineseScience Bulletin eld represent multiple-intervalmarinegenera)compared with eruptionag aa(J)aeicue.Mdfe fe e.[1. Java (OJP)areincluded.Modifiedafter ref.[11]. Atlantic Magmatic Province, and the , Deccan Atlantic MagmaticProvince,andthe respectively

flood basalt province, the SiberianTraps cided with the eruption of the largest-known continental and whose echoes continue to the present day. It coin- continental flood basalt province, but less than a half of Not without good cause isthis recognised asthelargest province TheSiberiancontinental flood basalt 1 h rpincudhv rgee h lblcii. the eruptioncouldhavetriggeredglobalcrisis. ing environmental changes nature and timing ofthe extinction and the accompany- post-date the extinction event? Then we consider the instance, do the eruptions precede, accompany, or Permo-Triassic boundary section at Meishan, China. For using recently-obtained dates from Siberiaand the Traps, and the timing and duration oftheir eruption, by Permian. We consider initially the main features of the mary trigger for the massextinction at the end of the Traps Siberian for the asapri- evidence the assess we scale disruption of thecarbon and sulphur cycles.Here, , which wasitself associated with global- oceanic a to major hasbeenattributed extinction rine whose effects lasted for several millions ofyears ‘Great Dying’ began denly, theenvironmental cat end-Guadal at the tinction, ecosystemswere still recoveringfrom arecentmass ex-

| January 2009 | vol. 54 | no. 1 | || vol.54no.1 2009 January ― an eventthat started rapidly, upian (~260 Ma), when sud- to assess whether and how 20-37 astrophe that led to the to the led that astrophe

[14 ― aceous-Tertiary, 16] . Three . Three oce- . The ma- [11 es of es of ― 13] 21 ,

GEOLOGY REVIEW

it is now seen at the Earth’s surface; the bulk of it is drilling in the West Siberian Basin, derived from several buried beneath thick sedimentary cover. The most obvi- thousand commercial and scientific boreholes, reveals ous outcrop of the Siberian continental flood basalt extensive subcrops of Permo-Triassic basalts and rare province (or Siberian Traps) is located on the Siberian overlying the older, pre-Trap basement. Most , a region that has been tectonically stable since of these volcanic rocks are now buried beneath thick Precambrian times (Figure 2). Virtually the entire out- deposits of and Cenozoic sedimentary rocks, crop comprises rock of basaltic composition: basalt but occasionally Permo-Triassic basalts and rhyolites is the dominant rock type, but there are also abundant outcrop on the basin margins. For example, there are basaltic volcaniclastic deposits and shallow intrusions widespread outcrops of basalt in the coal-rich Kuznetsk (mostly sills), especially around the SE fringes of the Basin, and thick sequences of basaltic and rhyolitic lava province[17]. The area of the province preserved on the are found in the Semeitau region (eastern Kazakhstan), craton is of the order of 2.5 million km2, including the near Chelyabinsk in the south-eastern Urals, and at region underlain by upper crustal intrusives. The thick- Vorkuta in the Polar Urals (Figure 2). Permo-Triassic ness ranges from more than 3 km near Noril’sk and basaltic rocks also occur in the far north on the Taimyr Maymecha-Kotuy, thinning towards the south and east. Peninsula and, from seismic and magnetic data, are However, this represents only a fraction of the total known to extend beneath the sediment infill of the province. Information from scientific and commercial Yenesei-Khatanga Trough and the Kara [18].

Figure 2 Map of present-day western Siberia, showing the distribution of Permo-Triassic igneous rocks. The main outcrops of basaltic and volcaniclastic rocks, and the extent of intrusive rocks (sills and dykes) (on the Siberian craton, to the east) are indicated in green. Local- ised outcrops of basalt occur in the Kuznetsk Basin, Taimyr Peninsula, and Urals. Basalts subcropping beneath the West Siberian Basin (left centre, between the Siberian craton and the Urals) and Yenesei-Khatanga Trough are shown in red (the latter are speculative). Also shown are the major that occur beneath the West Siberian Basin (Ur: Urengoy; Kh: Khudosey), and the locations of some of several thousand boreholes that have been drilled, mostly for hydrocarbon exploration. The putative limit of the Siberian large igneous province is indicated by the dashed line. Modified after ref. [1].

22 Andy SAUNDERS et al. Chinese Science Bulletin | January 2009 | vol. 54 | no. 1 | 20-37 been partly covered by basalt intrusivesmillionis 3 km mate’ forthe total volume of eruptives and shallow-level the squares of the errors. Sources ofdata:the squaresoferrors.Sources Reichow greensymbols: etal. calculated toFCS=28.02Ma).Error propagation was establishedby includingtheuncertainty age asthesquare ofeach rootof beneath a layer ofbasalt some 12km thick, or an area lion km south Rifts, run the length ofthe basin before fading out to the of rifts, the largest of which, the Urengoy and Khudosey is riven by major N-S trending faults, producing aseries Much of the basement beneath the West Siberian Basin West Basinand Siberian Part ofthe problem relatesto the role of rifting in the accurate figureof area (and reference age of Bed 25. Bed 25 is represented by the blue bar, blue the by isrepresented 25 Bed Bed25. hasan of and age reference Andy timingofSiberianRelative Traps volcanismandthePerm Figure 3 area ofapproximately5×10 crops asmaximuma extent, then weestimate that an moved by . Taking the limits ofthe existing out- quences and because an unknown amount has been re- substantial portion is covered by thick sedimentary se- flood basalt province is difficult to estimate, because a The original total area (and volume) oftheSiberian ExtentandvolumeoftheSiberian Traps 1.1 ther 1 km ofbasalt below this level abandoned, with seismic datasuggesting at least a fur- penetratedmore than 1 kmof basaltic lava before being example, superdeep borehole plotted relative Bed 25 at Meishan, which istaplotted relativeBed25atMeishan, West SiberianBasin lion km [19] 3 3 . Manyoftherifts are flooredwithbasalt.For would bury an area equivalent to the entire UK either way. To put this into perspective, 3 mil- [25] . Data for MeishanBed28arefromReichow etal. 3 , with an error of about, withmil- an errorof 1 SAUNDERS volume) willever be known. Khatanga-Yenesei Trough. 6 km SG-6 in the Urengoy Rift Urengoy in the SG-6 [1] . Itisunlikely that an 2 of Siberia may have may have ofSiberia [20] etal.ChineseScience Bulletin ken astheculmination . A ‘working esti- of the extinction event of the

o-Triassic mass extinction, modified after Reichow etal.

[1] [1] New other, and indicate an eruption age of about 250 Ma. to a common standard, these all lie within error of each slightly but significantly older than the Maymecha-Kotuy, U/Pbgive concordiaagesthatare together with perovskite, and baddeleyite from (see below). Published TheageoftheSiberian Traps 1.2 the West Siberian Basin (~1.3×10 larger than recent estimat However, even the minimum estimate issignificantly the size of China beneath a layer about 300m thick. Noril’sk separates) /bae niaearnefo 5. . o211± U/Pb ages indicate arange from 251.7± 0.4 to 251.1 ± tend to be very precise; themost recently determined Nonetheless, whilst there are fewerU/Pb ages, they do Permo-Triassic extinction horizon within error of the these regions was alsoarou in the Polar Urals confirm that the age of eruption in Tunguska, Taimyr, theKuznetsk Basin and fromVorkuta ; red symbols: Noril’sk; redsymbols: .

| January 2009 | vol. 54 | no. 1 | || vol.54no.1 2009 January andbaddeleyites fro 40 Ar/ 40 Ar/ 39 [14,22,23] Ar age of 249.83 ± 0.15 Ma (Renne et al. et (Renne Ma ±0.15 249.83 of Ar age 6 km 39 40 Ar data Ar Ar/ 3 ) , Putorana [32] [21] 39 ; negative values represent ages older than the thanthe agesolder valuesrepresent ; negative Ar determinations (whole-rock and . [14,22,23] [1] forbasaltsfrom Noril’sk, Lower 40 [25] Ar/ , Putorana [14] (Figure3).When normalised 39 exist for basalts from nd 250 Ma.These ages are es forthe Deccan Traps , Maymecha-Kotuy Ar ages obtained for the 20-37 m intrusions atNoril’sk, [14] [16]

, Maymecha-Kotuy atMeishan, China

40 Ar/ [16] ; all ages re- all ages ; 39 [1] the sum of sum of the . Data are Ar ages. [24] [24] and , and 23

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0.3 Ma for magmatism in the Maymecha-Kotuy re- 2 The Permo-Triassic extinction horizon [26] gion . and the Siberian Traps: Are they contem- With the presently available age data, it is not possi- poraneous events? ble to determine precisely how long it took to emplace the whole of the Siberian Traps. However, using both The internationally-recognised Global Stratotype Sec- U/Pb and Ar-Ar dates (corrected for the inter-technique tion and Point of the Permo-Triassic (P-Tr) boundary is differences), it is entirely possible that the bulk of the located within Section D, at Meishan, Changxing province was emplaced within less than 2 million years, County, South China[30] (Figure 4). The section com- but it may have been much shorter than this, possibly prises dolomite, limestone, marl and mudstone, with significantly less than 1 million years. Recent palaeo- occasional clay layers derived from volcanic ash fallout, magnetic and radiometric studies of the Deccan Traps, all deposited in a marine environment. The biostrati- , suggest that the bulk of those lavas were erupted in graphical boundary between the Permian and Triassic is less than 600000 years[27,28]. Timescales of 300000 years defined as the first occurrence of the conodont have been estimated[29] for the 4 to 6 km of lava erupted Hindeodus parvus, located at the base of Bed 27c at in East Greenland at 55 Ma. If these estimates are correct, Meishan[31]. The main extinctions occurred slightly ear- a timescale of less than 1 Ma for the emplacement of the lier, and are recorded within Beds 24 through 26, with [32] bulk of the Siberian Traps seems realistic. the peak extinction rate at the base of Bed 25 .

Figure 4 Lithological and carbon isotope (δ 13C) variation through the Permo-Triassic section at Meishan, China. The left-hand panel shows an expanded part of the section. Lithology based on Yin et al.[30] and ADS (field observations, 2005). Carbon are from refs. [58, 61] (raw data supplied by authors). The 40Ar/39Ar age of Bed 25 is taken from Renne et al.[16], and Bed 28 from Reichow et al.[1] (both dates relative to FCS = 28.02 Ma). (Note that Mundil et al.[34] obtained a U/Pb age of 252.4 ± 0.3 Ma on zircons from Bed 25; see text for discussion). The mass extinction event occurs in Bed 24, culminating at Bed 25[32].

24 Andy SAUNDERS et al. Chinese Science Bulletin | January 2009 | vol. 54 | no. 1 | 20-37 give aprecisedurationforthemass extinctionevent means that it is not possible, with the current data, to resolution lackof This younger’. unresolvably but 252.6 ± 0.3 Ma, and the P-Tr boundary tobe‘slightly Meishan niques used in the preparation of the dated zircons from included U/Pbagesin this diagram because the tech- from variouspartsofthe Siberian province.We have not Mundil et al. from Meishan yielded an age of 252.4 ± 0.3 Ma, and ages for zirconsfrom Meishan. U/Pb ages. This process resulted in systematically older remove theeffects oflead loss and to give concordant bedsby annealing and partial dissolutionHF in acid,to ~300 kayounger thanthecrystals inashBed25. an age of249.25±0.14Ma, whichmakes them atleast slightly younger ashBed 28 at Meishan, and determined Andy 40 40 0.15 Ma tracted fromBed 25yielded a ex- feldspar sanidine of techniques. dating are two sets of conflicting data from rich sourceof isotopic agedates, butunfortunately there at 252.4 ± 0.3 Ma by U/Pb Meishan, the culminationofthe extinction eventisdated From the greater age); this is a discrepancy that needs addressing. ages of Siberian zircons (i.e., give them a slightly be sure that annealing and leaching will not affect the al. et Mundil taken by Bowring et al. Ma)was ± 0.3 (251.4 of which ages, younger the U/Pb yielded an age of250.7 ± 0.3 Ma. Bed 25 yielded two sections. Bed 28,above theP-Tr boundary Meishan,at layers, fromMeishan and other Chinese Permo-Triassic multi- U/Pb agesin zirconsa seriesofclay from reported by severalworkers ages of the extinction event and of the Siberian Traps, as 3 that there is strong evidence of synchrony between the tween the error inthe the decay constant for Ar/ Ar/ Bowring The volcanic ash layers at Meishan have provided a Clearly, there isasystema 39 39 Ar Ar andU/Pb dating techniques,due either to an [34] [16,33] [16] 40 40 andfrom Siberia

Ar/ 40 . Figure 3 shows therelative difference be- Ar/ [34] [34] Ar/ et al. . We 39 39 estimate the age of the biotic crisis to be pre-treated zircon crystals from theash Ar data alone, it is apparent from Figure 39 Arage ofMeishan Bed 25,and basalts Ar referencestandard, [1] [33] have analysed feldspars from the

determined sing [33] SAUNDERS [34] [1,14 to bethe age oftherock. [26] , and 249.83 ±0.15 Ma by tic difference between the 40 ― The ash layer in Bed 25 layerin Theash are different; we cannot Ar/ 16,25,26] [35] 39 40 Ar249.83 ageof ± . Thus, Bed 25 at Thus, Bed25 . etal.ChineseScience Bulletin . Ar/ or toanerrorin le-crystal and 39 Ar and U/Pb Ar and [34] .

ednl fteepaeeto h ieinTas n pendently of the emplacement of the Siberian Traps, and stagnation. Such processes could have occurred inde- increasing oceanic stratification, and associated oceanic mixing, and latitudinal oceaniccirculation decreasing ocean warming and loss of polar icesheets, would be CO shan Traps) were erupted at atime ofhigh atmospheric imply that the Siberian Traps (and the preceding Emei- rect, however, theymay be very important, because they niques have inherently large error bars. If they are cor- should be used with caution, because the applied tech- and mass extinction. been a factor in the subseque during the late Permian volume) intheEarlyPermian, to as much as 10×PIL similarto pre-industrial (PIL; 280 levels oscillations on at least Ma time scales warmed, albeit withevidence forstrong global climate Permian Period, the Earth’s climate had progressively South China Plates. During the 50 million years ofthe of , and waspartially enclosed by theNorth and 5). The Palaeo-Tethys Ocean lay along the eastern edge Pangea, surroundedby the were configured in the north-south During the Permian Period, the main continental masses the Permian Changingenvironmental conditionsin 3 would also have contributed substantial CO the Emeishan (~260 Ma) and SiberianTraps (~251 Ma) gaea at about 280 Ma. Towards the end of thePermian Pan- subduction-related volcanismspread innorthern gest thatatmospheric laeosols and leaf stomatal densities) and modelling sug- determinations (forexample from compositions of pa- progressive CO the resultof One significant system. mosphere/ocean tainly asaresult of build-up ofatmospheric CO acterised by increasing global temperatures, almost cer- Overall, thePermian periodappearsto have been char- interrupted by periods of abrupt cooling (Figure 6). glaciations, followed by general warming began with ice-house conditions, similar in extent to the belts,and increasing aridity) masses (perhaps as aresult ofanabsence ofextensive caused by diminished weathering of continental land- | January 2009 | vol. 54 | no. 1 | || vol.54no.1 2009 January The increasing CO 2 . This unusual atmospheric composition may have 2 bidu,adcneun topei- build-up, and consequent atmospheric- 2 p inthePermianmay have been CO [36 2 ― varied from concentrations concentrations from varied Panthalassic(Figure Ocean 41] nt dramatic climate change 20-37 (Figure 6). These data

[37,39] [36] . The Period . The Period , and wide-, and μ 2 to theat- L· 2 . Proxy − 1 by 25

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Figure 5 End-Permian (~251 Ma) global palaeogeographic reconstruction showing the locations of the main landmasses and oceans, the major studied Permo-Triassic boundary sections, and the Siberian Traps. Boundary sections: 1, South China, including Meishan; 2, Iran; 3, Slovenia; 4, Austria; 5, Greenland; 6, Spitsbergen; 7, Deep-water sections in Panthalassa (location indeterminate); 8, Karoo Basin; 9, Antarc- tica. Map adapted from C. R. Scotese (PALEOMAP website), http://www.scotese.com.

Figure 6 Estimated atmospheric carbon dioxide levels during the Permian, shown as parts per million by volume (μL·L−1) and relative to pre-industrial levels (PIL: 280 μL·L−1), against a background of low (blue) and warm (red) global temperatures. Adapted from Montañez et al.[36] for the Cisuralian, and from Royer[41] for the later Permian. Note that there are considerable uncertainties in these estimates, which are based on modelling and proxy data, but they indicate a general warming, caused by increasing atmospheric CO2 levels, throughout much of the Permian. been well underway before the end-Permian crisis. In subducted, but a deep water section from the Pantha- addition, the Permian atmosphere may also have been lassic Ocean has been preserved in accreted terranes in characterised by low oxygen concentrations[39]. Japan[42]. There are several recent overviews of the end-Per- 4 Changing environmental conditions mian mass extinction event, and the accompanying eco- across the Permo-Triassic boundary logical and environmental changes[43―50]. The Permian The majority of the preserved marine P-Tr boundary Period was characterised by a rich faunal and floral di- sections are located on the continental shelves within the versity which was strongly reduced during the end- Palaeo-Tethys realm, but some are from the Panthalassic and end-Permian extinction events. There Ocean, and several sections are terrestrial (Figure 5). is some debate about the number of taxa that became Deep-water sections are rare, because most have been extinct at the end of the Permian, but it is thought that at

26 Andy SAUNDERS et al. Chinese Science Bulletin | January 2009 | vol. 54 | no. 1 | 20-37 events were diachronous also strong evidence that of the extinction of benthic communities, but there is continental shelveswasprobablythemain direct cause anoxic event werelong-livedoceansthe in gripofa super- OAE:a world’s the that Triassic, the indicate into well extended oxic during the P-Tr transition, and that this anoxia often servation that somany shallow-watersections werean- organisms that utilise oxygen for respiration. The ob- large volumes of the oceans become uninhabitable to maysurface waters oxygenated. become Asaresult, in oxygen and, in extreme cases, only the wind-churned ing an OAE, a large part of the water column isdepleted (OAE) in the past is, in co sediments. However, evidence the , ortothe water trapped within bottom restrictedeuxinia are tois oceans are oxygenated to alldepths, and anoxia and reduced sulphur) and, in the most extreme examples, (presence of oxygen) dissolved (absence of anoxia oxygen), solved evidence of oceanic dysoxia (reduced content of dis- the be aproxyforseawater)(Figure 4) light ( fall in showasignificant localities) other (and several Shangsi Meishan and sequences at The and sulphurisotopes. changes at the P-Tr extinction horizon, including carbon Andy ests at high latitudes during the latest Permian palaeosol data indicate the existence of deciduous for- which are small, tree-like mosses peared to be replaced by lycopsids (e.g., Europe was covered by conife ished considerably. During the late Permian,much of land the varietyofinsects, and forestsdimin- benthic communities were particularly affected, and on 83% of all genera went extinct. In the marine realm, the least 90% of marine species, 57% of all families, and especially atthecommunity level faunal and floral diversity persistedwellinto Triassic, the Meishan, signature, and hence the carbon cycle, of ocean water. At −6‰, indicating major changes in the carbon isotope A common feature ofmany P-Trmarine sections is Several important geochemical signatures show major δ δ 13 12 δ δ C C) carbon measuredC) carbon incarbonate, considered to 13 carb δ δ C 13 carb excursion varies, butistypically around [42] C (the ratio of isotopically heavy ( carb . ofanoxic water onto the [53] values fall before the culmination of values fall before the culmination of . Atthepresent time, theEarth’s [54] ntrast, frequently found. Dur- found. frequently ntrast, SAUNDERS olated watermasses such as . the shallow-water anoxic r forests,but these disap- for oceanic anoxic events [13] [51] [33,55 . . Palynological and etal.ChineseScience Bulletin ― 62] . The size of . Pleuromeia) [52] 13 . Low C) to

from Siusi,northern Italy and at Bálvány, Hungary. However, the P-Tr section ing, the extinction horizon preserved at Meishan, China falling again further up-section (Figure 4) before recovers slightly It then of Bed24. verytop the sudden proximately 3‰,before ap- Bed25), by Bed24 to (top of mass extinction the duced sulphur, held in H phate reduction bacteria, lead to the formation of re- development of oceanic anoxia, andthe activity of sul- tion andpoisoningofoxygen breathers into theatmosphere, andleadingtowidespreadsuffoca- Musashi et al. δ δ and notrestrictedtothePalaeo-Tethys Ocean. the perturbation of the carbon cycle was global in extent, seamounts from thePanthalassic Ocean, suggesting that sions in shallowmarine sediments preserved in accreted Flood basaltsarecharacterised Theimpact ofvolcanism 5 shallow anoxicwaters types ofbacteria(greensul rising intothephotic zone,thusallowingwater specific such models invoke deeply sourced,sulphide-rich anoxic been usedtoarguetopes, thathas photic-zone euxinia; for stpclyhayadlgtslht,rsetvl. isotopically heavy andlightsulphate,respectively. water into the upper part of the ocean column) toform then be re-oxidised (e.g., by upwelling of the reduced cally heavy sulphur in the anoxic seawater. Thesemay topically light Sin the sulphide component, and isotopi- version leads to strong isotopic fractionation, with iso- the predominant sulphur species is sulphate, SO anic anoxia and euxinia. In normal, oxygenated seawater sions must be due to the processes associated with oce- canic eruptions isunlikely. by the addition of isotopically light sulphur from vol- 10 of sulphur in the oceans (currently approximately 1.3 × breach the ocean surface, thus releasing H releasing breach theoceansurface,thus may, into theshallowocean) extreme cases, most inthe excursion’, (i.e.,bringingthe anoxicwater cline-upward patterns. Kaiho et al. et Kaiho patterns. positive excursions in 34 | January 2009 | vol. 54 | no. 1 | || vol.54no.1 2009 January 6 Sulfur isotopes ( It is evidence of this It isevidence Gt of S), generating such large isotopic excursions S sulphate of about 15‰ immediately prior to, and and dur- to, prior 15‰ immediately of about [57] reportnegative carbon isotope excur- δ δ 34 δ δ S) show moreS) show complex excursion [63 [61,67,68] nature, supportedbycarboniso- 34 2 ― S S [66] (solution) 65] sulphate Rather, these isotopic excur- phur bacteria)toflourishin report an abrupt fallin shows both negative and 20-37 . The so-called ‘chemo- . Theso-called by prolificeffusive erup- ly fallingafurther 3‰ at . Due to the large mass or in pyrite. Thisor inpyrite. con-

[69] . 2 [62] S gas directly S gasdirectly . Similarly, 4 2 − . The . The 27

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tion of magma. The 1783/1784 eruption of Laki, anthropogenic CO2 is slowly extracted from the atmos- , for example created a basaltic flow with a vol- phere by the biosphere (e.g., photosynthesis) and disso- ume of about 15 km3 over a period of a few months[70]. lution in the oceans. The ocean-atmosphere-biosphere In the case of larger eruptions, a single eruptive event system will achieve a new equilibrium, but the total may last years or decades, producing a flow field com- carbon in the system will be raised until the excess car- prising many individual lava flows with a combined bon is removed as carbonates, or buried as organic car- [71,72] volume of several thousand cubic kilometres . bon. Such geological sequestration occurs on timescales Well-documented flow fields in the of 105 years or longer, and consequently a residue of volcanic province, in the Northwest USA, are at least CO2 will remain in the atmosphere for millennia after [73] two orders of magnitude greater volume than Laki . the original injection. Between 1850 and 2000, of the Unlike explosive volcanic eruptions, where dust and gas total of 471 Gt of C that were released by burning of is rapidly injected into the atmosphere, and is then re- fuel, roughly two thirds were sequestered by the moved by fallout and precipitation over a period of little oceans and biomass, leaving about 175 Gt in the atmos- more than a or so, a flood basalt eruption leads to phere. The bulk of this carbon will continue to be re- prolonged release of gas and aerosols; the duration of moved by the oceans and biomass, but it is estimated the eruption far exceeds the residence time of some of that between 17% and 33% will still remain in the at- the atmospheric pollutants, especially SO2 and halogens. mosphere after 1 ka, and a residue of about 5% to 7% of There are three important volcanic products to con- ‘semi-permanent’ CO2 will remain in the atmosphere for sider. The first is CO (leading to global warming), the 2 at least 100 ka, until it is removed by long-term silicate second is SO , and its product sulphate aerosol (global 2 weathering processes[76,77]. This long ‘tail’ in the draw- cooling), whilst the third is the halogens and associated down profile for anthropogenic carbon gives it a ‘mean compounds (depletion of ozone and increasing transmis- lifetime’ in the atmosphere of around 30 to 35 ka, much sion of light). longer than is generally supposed. 5.1 Carbon dioxide Let us now return to the eruption of the Traps. A A 2000 km3 basalt flow could release as much as 12 Gt ‘mean lifetime’ of 30 to 35 ka for CO2 in the atmosphere of C (44 Gt CO )1). If we assume that the flow is erupted is much greater than the average eruptive frequency of 2 [27,28] over a period of 10 years, then this represents an annual flow fields (100 to 1000 years ), so it is reasonable flux of 1.2 Gt of carbon added to the atmosphere. The to consider average eruption fluxes. Assume that the pre-industrial atmosphere contained a little over 600 Gt bulk of the Traps were erupted in 600000 years, and that 6 3 of carbon as CO (currently ~800 Gt C)[75], so this would they have a total eruptive volume of 3×10 km . The 2 3 3 represent an addition of about 0.2 per year %. Given that average output is 5 km basalt per year, or 5000 km of the Late Permian atmosphere may have contained up to basalt per 1000 y; this equates to 30 Gt of C per ka. As- sume then that 5% to 10% of this carbon remains in the 5 to 10 times more CO2 than the pre-industrial level (see above), this initially appears to be an insignificant addi- atmosphere for at least 100000 years. This means that tion, and unlikely to trigger a global crisis. However, if every 1000 years, between 1.5 and 3 Gt of C is added to we consider the province as a whole, the eruption of the atmosphere and effectively remains there. Over a between 2×106 and 3×106 km3 of basalt could release period of 100 ka, between 150 and 300 Gt of ‘semi- 12000 to 18000 Gt of C, enough to significantly change permanent’ C will have been added to the atmosphere, the carbon content of even a Permian atmosphere. Note plus a substantial amount to the oceans and biomass that the eruption of 18000 Gt of C over 1 million years (between 2700 to 2850 Gt). For the lifetime of the prov- equates to only 0.018 Gt per year, a fraction of the cur- ince (600 ka), the gross addition to the atmosphere, rent output from burning of fossil fuels (~ 7 Gt C/a). ocean and biomass could be as much as 18000 Gt of C. At the present day, the ratio of CO in the oceans and The fate of the injected CO2 is crucial here. Let us ini- 2 tially consider the present-day situation. The bulk of the atmosphere is about 50:1. However, in the end-Permian

1) This assumes that the magma contained 9000 μg·g−1 C, and that 90% of the C was degassed on eruption and cooling. McCartney et al.[74] estimate that 1 km3 of Hawaiian magma emits 5 Mt of C.

28 Andy SAUNDERS et al. Chinese Science Bulletin | January 2009 | vol. 54 | no. 1 | 20-37 with injectionofJurassicsills tive hydrothermalthe KarooBasin,associated ventsin byan importantthediscoveryprocess, asnoted ofputa- Convection of heatby groundwater islikely tohavebeen host rockthatundergoes may berestricted. pyrolysis the chilledmargins oftheintrusion,thenvolume of sill ordyke isimpeded bylowthermal the conductivity of example,stood. If,for theenergy releasedfrom acooling andhost rocksis,however,tween intrusion poorly under- isotopes(Sec carbon seawater inducinglargeit iscapableof negativeexcursioninthe isotopically verylight,hence carbon isalso cally-derived triggering acatastrophicgreenhousecrisis.Theorgani- (as CH amounts ofcarbon to releaserapidlyvast mian.model isattractivebecauseithasthepotential The end ofthePer- sill-induced coal-burningoccurredatthe Andy stratosphere causes cooling ofthe lower atmosphere by Injection of sulphur (assulphate/sulphuric acid) into the Sulphur DioxideandHalogens 5.2 hanced duetosilicateweatheringastheclimate warms. counter these effects, carbonsequestration will be en- , or by reduction of biomass. To atmospheric concentration ofCO higher atmospheric loading. both ocean warming and acidification, resulting in a muchlower, been may have ratio this to due oceans, al. hasbeenlease by proposedby sillinjection Svensenet rock types. Methanere- in awiderangeofsedimentary methanecoal, more (includinggashydrates),dispersed or may sedimentary Thecarbon theform of basins. bein injection ofmagma anddykes into assills carbon-bearing the CO figures do not take into account that for shorter periods, atmosphere will be slightly less than this. Note that these carbon, so the total retained by the oceans, biomass and mation of carbonates, or permanent burial oforganic have been extracted by reaction with silicates and for- over a period of100000 years. A fractionofthis will laeocene, and Retallack and Jahren laeocene, andRetallack tion mechanisms, eitherby CO Nor do they consider any compromise tothe sequestra- there will still be between 10% and 15% remaining). 33% C remaining in the atmosphere, and after 10 ka, years after the injection, th [78] There mayThere beanadditional sourceofCO An addition of 300Gt to the atmosphere will raise the to explain the global warming at the end of the Pa- theglobalwarmingend ofthe toexplain atthe 2 loading will be much greater (e.g., even 1000 SAUNDERS erewill bebetween 17% and [80] tion 7).Theinteractionbe- 2 . saturation of the oceans, oceans, the of saturation 2 by about 160 etal.ChineseScience Bulletin

[79] suggestthat 2 , involving 4 orCO μ L· − 2 ), 1

phere lower stratosphere have sufficient vigour for aerosol injection into the fountains and their attendant eruption columns could fire where the eruption, basalt flood of sites at occur acted withevaporatedeposits inter- ascending the of some because rich, phur sul- particularly maybeen have eruptions Siberian the stratosphere islower than atthe equator.Furthermore, eruptions, where the boundary between troposphere and Siberian the of latitude high by the aided may be jection structure similar totoday’s, the Laki observations to a full-blown flood basalt erup- acted to warm thenear-surface layers. Extrapolation of bilitythethat sulphur-rich atmospherein thelower gases Thismay have been coincidental, but there is the possi- 0.02 Gt of SO ters of 1783ters 1784 and maticcooling ofthe lower atmosphere duringwin- the phate injection into the lower stratosphere led to dra- properties tothoseofvolcanicsulphateaerosols. particulates, which have different reflectance/absorption as thenuclear scenarios also involve injection ofcarbon effects ofvolcanicaerosols, Whilst these models are indicative of the proved its quality forphotosynthesis because the aerosols diffused the incident light and im- tion ofPinatubo led to increased biomass production, may bebeneficial for plant development; the 1991 erup- Small amounts ofsulphate injection into the stratosphere atmosphere. the through light of transmission the in several years ordecades, and leading to a large reduction be sustained for theduration ofthe eruption, lasting could release between 3 and 10 Gt mospheric loading ofsulphates successive eruptive events could lead to massive at- the Philippines Chichón in Mexico and the 1991 eruption ofPinatubo in cooling was observed after the 1982 eruption of El reflection and absorption ofincident sunlight. Such European summer of 1783 was also particularly hot particularly wasalso 1783 summerof European heating depending onthelocation andtime ofyear mate on a global scale, with both extreme cooling and nuclear exchangeindicate cat of the atmospheric effects of dustproduced by a limited Modelling are largely unknown. loadings larger aerosol | January 2009 | vol. 54 | no. 1 | || vol.54no.1 2009 January Unlike the Pinatubo eruptio The Laki eruption of 1783/1784 suggests that sul- [84] , a major flood basalt eruption of 1000 km 2 [81] equivalentinjected were into theatmos- . High-level injection of sulphur could [70,82] . IfthePermian atmospherehad a [70,89,90] they aredifferentinasmuch then such stratospheric in- stratospheric such then [83] 20-37 . Curiously, however, the astrophicchanges tocli- [28,82] . n, wherean estimated

[71,72,82] . The release would would Therelease . [85] . The effects of , and rapidly rapidly , and [86 ― [91] 88] 29 3 . .

GEOLOGY REVIEW

tion is fraught with uncertainty. The likelihood is that the Due to the lack of constraining data, the effect on sulphate loading in the stratosphere would cause surface ecosystems of the volcanic sulphur release event is dif- cooling and, if the loading was sufficient, curtailment of ficult to predict. If the optical depth of the atmosphere photosynthesis (the ‘’ scenario[92]), whilst is reduced to the point where photosynthesis is affected, low altitude injection could cause surface heating. The then we would predict progressive die-off of primary possibility exists therefore for extremely cold winters producers both in the oceans and on land. This is test- and extremely hot summers, combined with a reduction able, because we would anticipate (a) greater effects in in photosynthesis and plant viability, and lasting for pe- Northern Hemisphere ecosystems (where the volcanism riods of several years or decades. occurred); (b) that if the control is primarily light occlu- Halogens released during the Laki 1783 eruption sion, then the effects may be greater in the polar and were responsible for local devastation of vegetation and near-polar regions, and upon marine plant species; (c) subsequent famine in Iceland[70]. Beerling et al.[93] mod- that if the control is primarily temperature reduction, this elled the release and stratospheric injection of HCl from may affect tropical and subtropical terrestrial plant spe- large-scale flood basalt eruption columns, and organo- cies to a greater extent. Either way, we would expect a halogens from pyrolysis of dispersed sedimentary or- progressive reduction in the range of plant types; the ganic compounds, to show that these could result in sub- worst case scenario may be the wholesale destruction of stantial stratospheric ozone loss. It is argued that this led plant systems, and their temporary replacement by to increased ultraviolet-B radiation at the Earth’s surface, non-photosynthetic ecosystems and the production of [95] leading to mutation in plant species[94]. the characteristic ‘fungal spike’ . The recovery of Flood basalt volcanism has the potential, therefore, to these systems after individual eruptions would depend create a massive pollution event, at least hemispheric on the survival of plants in refugios, and the viability of and possibly global in scale, with serious consequences seeds, spores and root systems. Injection of HCl and for global climate. The processes operate on different organohalogens into the stratosphere, compromise of the timescales, and in different directions. On the short time , and increased atmospheric transparency to [93] scale (effectively for the duration of the flood basalt UV-B could only add to the deteriorating surface eruption event; i.e., years to decades), injection of SO2 conditions. and the formation of sulphate aerosols in the strato- sphere may cause cooling of the lower atmosphere, and 6 The carbon isotope excursion (CIE) attenuation of light to the point where photosynthesis is As described earlier (Figure 4), a pronounced negative impaired. Lower level injection of sulphates may lead to CIE (of about 6‰) characterises most Permo-Triassic summer warming. Thus the climate may oscillate un- boundary sections, and CIEs also occur at several other comfortably, possibly catastrophically, between seasonal times in the geological record, including the Toarcian extremes, with consequent damage to ecosystems. Dur- and Palaeocene-Eocene Thermal Maximum (PETM) ing the late Permian, such cooling could be particularly events[96]. Due to the size of the oceanic reservoir, a catastrophic because the environment had been precon- negative shift in δ 13C of 6‰ requires massive input of ditioned by the long-term climatic warm conditions that 12 preceded the Siberian Traps. light carbon ( C) into the oceans. If we assume that the As soon as an individual eruption ceased, however, Permian ocean contained the same mass of carbon as the sulphate particulates would be removed from the at- present day oceans, approximately 40000 Gt, then a shift mosphere within a year or so, depending on the overturn in the isotopic value by −6‰ requires the addition of at 13 time of the stratospheric circulation (currently up 2 to 3 least 60000 Gt of carbon with δ C of −10‰, or 5000 13 years); sulphates in the lower atmosphere would have a Gt of C with δ C of −55‰, to the oceanic reservoir. residence time of a few weeks. The short-term cooling Could volcanically-derived carbon alone explain the pulses would be unlikely to affect ocean temperatures, observed oceanic CIE? This depends on the isotopic because of the high thermal capacity of the oceans. Thus, composition of the volcanic gases. To generate the re- development of substantial polar ice caps would not be quired −6‰ excursion in present-day ocean water, and expected; moreover, there may be no geological record assuming a total mass of carbon from the Siberian Traps of these ephemeral cooling events. of around 18000 Gt, the contaminant would need to

30 Andy SAUNDERS et al. Chinese Science Bulletin | January 2009 | vol. 54 | no. 1 | 20-37 eventual thermal destabilisation of the hydrates warming of ocean bottom watersand permafrost, and mechanism: global atmospheric warming, leading to 24 isotope excursion preserved at Meishan in the top ofBed pears to have been already underway before the carbon oceans in the dysoxia by triggering or , pheric atmos- warming, global catastrophic by either tinction, sive. The twogroups of mechanisms arenot mutually exclu- sins coal- and other hydrocarbon-bearing sedimentary ba- includes models that propose injection ofbasalt into like hydrothermally venting chimneys.This group now pyrolised,or releasing themethane throughdiatreme- from 500to 24000 Gt of C volume ispoorly constrained, with estimates ranging sive deposits ontheoceanfloor, and inpermafrost. Their cian very low from hydrates, because organicmethane has Andy average an have bearing sediments Injection ofsills and dykes into orbeneath hydrate- the release is a direct consequence ofmagmatic activity: groups of ideas about this. The first group suggests that hydrates inthe first pla What process could have triggered the release of the gas nothing is known about mass ofhydrates inthe past. much as400 mayGt C bestored in permafrost. Virtually calculates that theremay benomore than 2500Gt. As els ofCO may havecontained asmuch as 10 × pre-industrial lev- Furthermore, given that the late Permian atmosphere mantle of most carbon. estimates than significantly lower al. tions in British ColumbiaTibet. and Similarly, Sluijs et oxia predated thecarbon isotope excursion in P-Tr sec- also noted that theonset of extinction and oceanic an- also reachedbyothers magma alone could create th Therefore it seems unlikely thatCO 40000 Cthan dissolved more CIEs at the end-Permian [106] [32,58,62] Did thereleasemethane of lead directlymass to ex- A plausible explanation for theCIE is release of [79,80,103] [96,100] report that environmental change, as recorded by [104] . Twitchett etal. δ δ . At the present day, hydrates constitute exten- 2 ? ? Importantly, the P-Tr mass extinction ap- , then the oceans must have contained a lot alot contained must have the oceans , then . The second. The group invokesa lessdirect 13 C ( 60‰). This−60‰). has been proposed forthe δ δ [78] 13 . Thesedimentary rocksare heated C value of about [5,9,97] [105] ce? There arece?two currently There SAUNDERS . [43,98] , andWignall and Newton Gt (see earlier comments). Gt(seeearliercomments). [101] e total CIE, a conclusion , PETM , although Milkov , although etal.ChineseScience Bulletin 2 from degassing 20‰, which is is which −20‰, [99] and Toar- and [43,98 ― 100] [102] [54] .

the endofPermian. entire extinction process, or all of the anoxic events, at the carbon isotopic excursion was not responsible forthe tions strongly suggest that the mechanism thatcreated throughout the LowerTriassic [42,54]. These observa- beginning at the start ofthe Late Permian and continuing oceanic anoxia associated with P-Tr event, the anoxia could account fortheobserved extent and duration of therefore arguable whether releaseof hydrates alone PETM, also predate the carbon isotope excursion. It is biological and biochemical indictors atthe time ofthe p The combination ofincreasingly high atmospheric CO bean tendto unfort Disasters 7 Discussion ades, there would have been massive release ofCO flow unit was emplaced, over durations of yearsto dec- the current data (seesections 2and 3, above). Aseach (Ar-Ar time) or~252 Ma (U/Pbtime) is consistent with set of Siberian Trap volcanism, but afigure of ~251 Ma penetration bywarm, densesalinesurfacewaters. strong equatorial evaporationmay have led to deep meant sluggish latitudinal ocean circulation, although ened polar-driventhermohaline circulationwould have weak- or Absent forests. deciduous support to enough polar icecaps; indeed, the climate atthepoleswaswarm been deserts,there and do appear not to have been any high atmospheric high CO mass extinctionevent(Figure7). this coincidence that led, ultimately, to the world’s worst as seriouslyunfort flood basaltsthen beganto erupt can onlybedescribed vention. That the world’s largest known continental have led toa mass extinction without any furtherinter- Traps260 Ma at both possibly enhanced by the eruption of theEmeishan levels and global temperatures in the Late Permian global temperatures on upaemyhv e osaoa amn fte bound sulphatemay haveled toseasonal warming ofthe troposphere- although atmosphere, lower of the cooling phur, convertedsulphate to aerosols,would thenleadto On short timescales (years), sphere,depending the on height the eruption of columns. SO O | January 2009 | vol. 54 | no. 1 | || vol.54no.1 2009 January As yet, we do not know the precise timing of the on- Pre-Trap conditions in the Late Permian included 2 2 . Substantial areas of equatorial Pangaea may have andhalogens into the troposphere and lower strato- ―led to conditions that could, arguably, unate for the Earth system, but it was was it but system, Earth the for unate [37,38] 2 and associated high average and, possibly, low atmospheric the stratosphere-bound sul- stratosphere-bound the unate confluence of events. 20-37

― 31 2 2 ,

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Figure 7 Schematic diagram showing possible relationships leading to oceanic anoxia and global species extinctions (bottom right). The left hand side of the diagram shows long timescale processes (>Ma) operating during the Permian. A steady build-up of CO2 in the atmosphere led to global warming, loss of polar ice, and reduction in ocean circulation. Flood basalt volcanism (right-hand side of the diagram) at the end of the Permian injected both SO2 and CO2 into the atmosphere. The former led to short-term (decadal) cooling and impairment of photosyn- thesis and, possibly, widespread destruction of plants (marine and terrestrial); the CO2 injection would lead to longer-term (ka) warming and ocean acidification. CO2 content of the atmosphere would be expected to rise more rapidly as ocean temperatures and acidity increased, and carbon sequestration mechanism were impaired. Carbon with a light isotopic signature (from methane hydrate or sediment-bound organic material) may have been released by direct injection of magma into carbon-bearing strata[78―80], or by warming of deep ocean waters. However, the late occurrence of the main negative carbon-isotope excursions suggests that methane hydrate release (Figure 4) was likely not the direct cause of the mass extinction event. Modified after Wignall[9] and White[46]. lower atmosphere. Extreme oscillation in surface tem- questration. Firstly, sulphate aerosols led to light occlu- peratures would have caused die back of plants, espe- sion and short-term climatic oscillations, causing cially those acclimatised to the late Permian hothouse. In die-back of plants. Rapid carbon sequestration by bio- the more extreme cases of aerosol injection, light may mass growing after the eruption episode could, however, have been attenuated to the point that photosynthesis have partly offset this. Second, and perhaps more serious, was seriously compromised or curtailed, especially at was the reduced capacity of the oceans to retain the vast higher latitudes. Halogens and any organohalogens that amounts of CO2 that were produced by the volcanic entered the stratosphere could have led to degrading of province as a whole. If storage capacity was exceeded at the ozone layer resulting, in enhanced surface UV-B, as anytime (for example, by increased ocean water tem- [93] described by Beerling et al. . peratures and acidity), then atmospheric CO2 could have

CO2 from the degassing lavas and magma conduits, risen much more rapidly, leading to hothouse conditions. and from interaction of sills and dykes with carbon- The likely shallow calcium carbonate compensation bearing sedimentary formations, then accumulated in the depth in the Permo-Triassic oceans appears to be con- atmosphere and oceans. After the eruption of a flow unit, sistent with such ocean acidification. Sluggish ocean most of the CO2 would be sequestered by the oceans and circulation and stagnation may have also slowed CO2 rejuvenating biomass, but at least 5% of the injected transfer to the deeper oceans waters. 13 CO2 would remain in the atmosphere for at least 100 ka. The slight decline in δ Ccarbonate recorded in latest Thus the atmospheric CO2 increased after each succes- Permian Bed 24 at Meishan (Figure 4) is consistent with sive eruption, even though repose times between indi- the release of successive batches of volcanogenic CO2, vidual eruptions may have been between 100 and 1000 perhaps accompanied by isotopically light carbon from years. Two processes may have compromised CO2 se- pyrolysis of sedimentary carbon (along the lines of

32 Andy SAUNDERS et al. Chinese Science Bulletin | January 2009 | vol. 54 | no. 1 | 20-37 of a majorof a source of CO leasing H tion. Breaching of the surface by the chemocline, re- tality andmay havebeen a primary causeoftheextinc- shallow waters could then have led to marine massmor- cases, euxinia. Accordingly, ascent of anoxic layers into ledhave toincreased oceanic anoxiaand,extremein stagnation and low atmospheric oxygen, would then Consequent global warming, accompanied by oceanic ditions were initiated byavolcanicprocess ditions wereinitiated hothouse con- That theextreme geological sequestration. Permian, due to lack of sufficient silicate weathering and CO questionable whether the process could have reversed. ism wasincidental to warming and extinction, then it is and extinction wereeventually corrected. If thevolcan- canism. This is because the processes leading to anoxia to the idea that events were, ultimately, caused by vol- events. That the system did finally recover lends support severity ofconditions experienced during the P-Tr tion, and the long duration of themain CIE, testify to the sills intoorganic-rich sedimentary basins?). from othersources (protracted volcanism, orinjection of of hydrates or incorporation ofisotopically light carbon utedmortality to amongair-breathers Andy development ofpermanent ice-sheets. to cause damage to ecosystems but not enough to trigger led to severe but short-lived volcanic winters, sufficient background, episodic flood basalt eruptions would have an essential part of the mass extinction. Against this extreme climate perturbations, this processmay not be al. et Svensen recover slowly by geological sequestration ofthe excess source could be turned off, thusallowing the system to Lower Triassic lowed by asuccession of isot Triassic,the appears toget more pronounced, and isfol- that the CIEexcursion continues up-sequence well into warmed sufficiently to trigger the hydrate release. Note reached a tipping point when the deep ocean waters had Perhap events. extinction mass late in the process, and seems tobeincidental to the ane hydrates, but this appears to have occurred relatively 24 at Meishan is consistent with rapid release of meth- The prolonged recovery af The sudden decrease in 2 appears tohave beenincreasing throughthe late 2 S gas from euxinic water,may have contrib- [62,107] [80] ), and oxidation of dying biomass. . Thissuggests eithrelease er further 2 SAUNDERS δ δ ― 13 also meantalso that the same ter the main mass extinc- C s thesystem hadsimply opic fluctuations in the carbonate etal.ChineseScience Bulletin [69] at the top of Bed Bed of top the at , but given the the given , but ― turning on

Traps (~2 Ma) geologically rapid accumulation of CO estimatesthe eruptionfor durationthethe bulk of of ing the eruption of the Traps. Even using the longest assic boundary. Secondly, the SO catastrophic forhumanity). Whilst we may not experi- but they would be a relatively short-lived events (albeit volcanic eruption. Either could cause an artificial winter, ogy wouldbe a nuclear exch phur remains in the lower atmosphere. The closest anal- eruption. Furthermore,most ofthis anthropogenic sul- magnitude less than that predicted froma flood basalt Mt SO release into the atmosphere peaked at ~68 MtS(or~136 climate change debate. Global anthropogenic sulphur canism fortunately has noco the CO atmosphere, and this is replenished from the oceans until the rate at which CO some important lessons to be derived. The key issue is If thescenario outlined above is correct, then there are Currentandfuturetrends 8 chemistry. The atmospheric CO that before ourcurrent experimentin atmospheric fore theeruption of the Traps, was very different from Firstly, the situation atthe end of the Permian, and be- heading for another Permo-Triassic extinction event. interglacialand pre-industrial period, CO culation was sluggish. Now we live in acomfortable were no substantial polar ice caps, and the oceanic cir- greenhouse event. Forest thri higher, and theEarth was already in the grips majorof a province. magnitude faster than the eruption of a flood basalt fossil fuels every year, which is at least twoordersof leasing about 7Gt of C to atmosphere by burning of 100’s ofka. Anthropogenic activities are currently re- system hasbeenreducedtopre-event levels. This takes mosphere; a persistent residue of CO cally sequester carbon dioxid appears to be the much longertime ittakes to geologi- in the atmosphere wouldoccur. The controlling factor CO low background, and it is unlikely that the atmospheric low. Thus,we are currently CO | January 2009 | vol. 54 | no. 1 | || vol.54no.1 2009 January There are,however,reasons 2 2 . wl iet h eesetmtdfrteProTi will rise to the levels estimated forthe Permo-Tri- 2 ) per annum in the 1980’s the in annum ) per 2 in the combined ocean/biomass/atmosphere 2 was added to the atmosphere dur- adding carbondioxide to a ved at highlatitudes, there ange, or a major explosive 20-37 e from the oceans and at- to believe that we are not unterpart inthe present 2 [108] concentration was far 2

released bythe vol- . This isan order of 2 remains in the 2 levels were 33 2

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ence a Permo-Triassic event, however, it does not mean different latitudes? that the Earth will escape lightly. Even if we cease all Specific to the end-Permian events, what were the hydrocarbon CO2 emissions now, higher levels of at- atmospheric and oceanic conditions prior to the eruption mospheric CO2 will persist for millennia, with conse- of the Traps? Can we better constrain the CO2 concen- quences for global temperatures, , and tration of the late Permian atmosphere? What made the melting of permafrost and methane hydrates. end-Permian extinction so severe and prolonged, and the Understanding the causes of the end-Permian and oceanic anoxia so prevalent? Was it the confluence of other mass extinctions is important to evaluating the events outlined in this review (including the prevailing present-day . Ancient events offer the global climate, volcanism, sill-injection into organic-rich opportunity to study the full sequence of events associ- sediments, and hydrate release), or are some of these ated with rapid climate change and ecosystem collapse, factors secondary? For example, we have little idea of from onset to recovery. However, much more needs to the mass of methane hydrates in the Late Permian ocean be accomplished. These include more accurate assess- floor. ment of flux rates of volcanic gases, which in turn re- These and many other questions will need to be ad- quire more accurate resolution of eruption volumes and dressed before we fully resolve the cause of the durations. Whilst radiometric dating has a role to play, it end-Permian mass extinction. However, there is a grow- is currently unable to provide the resolution (100’s of ing realisation that flood basalt volcanism has the poten- years) that are needed to evaluate the duration of erup- tial to be the primary trigger for mass extinctions, rather tion of flood basalt flow units and the intervening peri- than meteorite impacts or other external processes. The ods of quiescence. Fine-scale palaeomagnetic variations, fluxes of carbon dioxide from flood basalts and anthro- or studies of palaeosols between lavas may help. Model- pogenic burning of hydrocarbons are of sufficiently ling of the climatic effects of atmospheric injection of similar magnitude and timescale to fully justify more volcanic sulphate aerosols is underway (e.g. Highwood [89] [90] detailed studies of the relationships between flood ba- and Stevenson ; Chenet et al. ) but the consequences salts, oceanic anoxia, and mass extinctions. to the biosphere of massive sulphate injection into the stratosphere are poorly constrained: to what extent is We thank Chagqun Cao and Tony Riccardi for making available their raw transmission of solar radiation reduced, or its quality δ13C data used in Figure 4, and Mike Benton, Mike Widdowson and Paul changed, and what are the effects on photosynthesis at Wignall for their reviews.

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