Triassic–Jurassic extinction event
Template:Short description Template:Annotated image/Extinction The Triassic–Jurassic (Tr-J) extinction event (TJME), often called the end-Triassic extinction, marks the boundary between the Triassic and Jurassic periods, Template:Ma. It represents one of five major extinction events during the Phanerozoic, profoundly affecting life on land and in the oceans.
In the seas, about 23–34% of marine genera disappeared; corals, bivalves, brachiopods, bryozoans, and radiolarians suffered severe losses of diversity and conodonts were completely wiped out, while marine vertebrates, gastropods, and benthic foraminifera were relatively unaffected. On land, all archosauromorph reptiles other than crocodylomorphs, dinosaurs, and pterosaurs became extinct. Crocodylomorphs, dinosaurs, pterosaurs, and mammals were left largely untouched, allowing them to become the dominant land animals for the next 135 million years. Plants were likewise significantly affected by the crisis, with floral communities undergoing radical ecological restructuring across the extinction event.
The cause of the TJME is generally considered to have been extensive volcanic eruptions in the Central Atlantic Magmatic Province (CAMP), a large igneous province whose emplacement released large amounts of carbon dioxide into the Earth's atmosphere, causing profound global warming and ocean acidification, and discharged immense quantities of toxic mercury into the environment. Older hypotheses have proposed that gradual changes in climate and sea levels may have been the cause, or perhaps one or more asteroid strikes.
Research historyEdit
The earliest research on the TJME was conducted in the mid-20th century, when events in earth history were widely assumed to have been gradual, a paradigm known as uniformitarianism, while comparatively rapid cataclysms as causes of extinction events were dismissed as catastrophism, which had been associated with biblical creationism. Consequently, most researchers believed gradual environmental changes were the best explanation of the extinction; prominent vertebrate palaeontologist Edwin H. Colbert suggested gradual changes in the seasonality of rainfall and eustatic sea level rise that decreased the available land area above sea level were the culprit.<ref name=":3" /> In the 1980s, Jack Sepkoski identified the Triassic-Jurassic boundary drop in biodiversity as one of the "Big 5" mass extinction events.<ref name="JackSepkoski">Template:Cite journal</ref> After the discovery that the Cretaceous-Palaeogene extinction event was caused by an impact event, the TJME had also been suggested to have been caused by such an impact in the 1980s and 1990s.<ref name=":2">Template:Cite journal</ref><ref name=":4">Template:Cite journal</ref> The theory that the TJME was caused by massive volcanism in the Central Atlantic Magmatic Province (CAMP) first emerged in the 1990s after similar research examining the Permian-Triassic extinction event found it to have been caused by volcanic activity and the emplacement of the CAMP was found to have occurred around the time of the Triassic-Jurassic transition.<ref name=":5">Template:Cite journal</ref> Despite some early objections,<ref name=":6" /> this paradigm remains the scientific consensus in the present day.<ref name="blackburn2013" />
EffectsEdit
Marine invertebratesEdit
The Triassic-Jurassic extinction completed the transition from the Palaeozoic evolutionary fauna to the Modern evolutionary fauna that continues to dominate the oceans in the present,<ref>Template:Cite journal</ref> a change that began in the aftermath of the end-Guadalupian extinction<ref name="DeLaHorraEtAl2012">Template:Cite journal</ref> and continued following the Permian-Triassic extinction event (PTME).<ref>Template:Cite journal</ref> Between 23% and 34.1% of marine genera went extinct.<ref name="JackSepkoski" /><ref name="GrahamRyderBook">Template:Cite book</ref> Plankton diversity dropped suddenly,<ref name="PeterWard2001"">Template:Cite journal</ref> but it was relatively mildly impacted at the Triassic-Jurassic boundary, although extinction rates among radiolarians rose significantly.<ref>Template:Cite journal</ref> Early Hettangian radiolarian communities became depauperate as a result of the TJME and consisted mainly of spumellarians and entactiniids.<ref>Template:Cite journal</ref> Benthic foraminifera suffered relatively minor losses of diversity.<ref>Template:Cite journal</ref> Some opportunistic foraminifera such as Triasina hantkeni increased in abundance as they thrived in oxygen-depleted waters.<ref>Template:Cite journal</ref> Ammonites were affected substantially by the Triassic-Jurassic extinction and were nearly wiped out.<ref>Template:Cite journal</ref> Ceratitidans, the most prominent group of ammonites in the Triassic, became extinct at the end of the Rhaetian after having their diversity reduced significantly in the Norian, while other ammonite groups such as the Ammonitina, Lytoceratina, and Phylloceratina diversified from the Early Jurassic onward.<ref name="TannerLucas">Template:Cite journal</ref> Bivalves suffered heavy losses, although the extinction was highly selective, with some bivalve clades escaping substantial diversity losses.<ref>Template:Cite journal</ref> The Lilliput effect, a term coined to describe a phenomenon wherein organisms shrink in size following a mass extinction, affected megalodontid bivalves,<ref>Template:Cite journal</ref> whereas file shell bivalves experienced the Brobdingnag effect, the reverse of the Lilliput effect.<ref>Template:Cite journal</ref> There is some evidence of a bivalve cosmopolitanism event during the mass extinction.<ref>Template:Cite journal</ref> Additionally, following the TJME, mobile bivalve taxa outnumbered stationary bivalve taxa.<ref>Template:Cite journal</ref> Gastropod diversity was barely affected at the Triassic-Jurassic boundary, although gastropods gradually suffered numerous losses over the late Norian and Rhaetian, during the leadup to the TJME.<ref>Template:Cite journal</ref> Brachiopods declined in diversity at the end of the Triassic before rediversifying in the Sinemurian and Pliensbachian;<ref>Template:Cite journal</ref> the dielasmatoid, athyridoid, and spondylospiroid brachiopods experienced particularly severe declines.<ref>Template:Cite journal</ref> Bryozoans, particularly taxa that lived in offshore settings, had already been in decline since the Norian and suffered further losses in the TJME.<ref>Template:Cite journal</ref> Ostracods also suffered significant losses,<ref>Template:Cite journal</ref> although opportunistic ostracod forms thrived in the eutrophic conditions of the TJME.<ref>Template:Cite journal</ref> Conulariids seemingly completely died out at the end of the Triassic.<ref name="TannerLucas" /> Around 96% of coral genera died out, with integrated corals being especially devastated.<ref>Template:Cite journal</ref> Corals practically disappeared from the Tethys Ocean at the end of the Triassic except for its northernmost reaches,<ref>Template:Cite journal</ref> resulting in an early Hettangian "coral gap".<ref name="EarlyHettangianCoralGap">Template:Cite journal</ref> There is good evidence for a collapse in the reef community, which was likely driven by ocean acidification resulting from Template:CO2 supplied to the atmosphere by the CAMP eruptions.<ref name="GeologicalRecordOceanAcid">Template:Cite journal</ref><ref name="OceanAcidDepTime">Template:Cite journal</ref>
Most evidence points to a relatively fast recovery from the mass extinction. Benthic ecosystems recovered far more rapidly after the TJME than they did after the PTME.<ref>Template:Cite journal</ref> British Early Jurassic benthic marine environments display a relatively rapid recovery that began almost immediately after the end of the mass extinction despite numerous relapses into anoxic conditions during the earliest Jurassic.<ref>Template:Cite journal</ref> In the Neuquén Basin, recovery began in the late early Hettangian and lasted until a new biodiversity equilibrium in the late Hettangian.<ref>Template:Cite journal</ref> Also despite recurrent anoxic episodes, large bivalves began to reappear shortly after the extinction event.<ref>Template:Cite journal</ref> Siliceous sponges dominated the immediate aftermath interval thanks to the enormous influx of silica into the oceans,<ref>Template:Cite journal</ref> a consequence of the aerial extent of the CAMP basalts that were exposed to surficial weathering processes.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> In some regions, recovery was slow; in the northern Tethys, carbonate platforms in the TJME's aftermath became dominated by microbial carbonate producers and r-selected calcitic taxa such as Thaumatoporella parvovesiculifera, while dasycladacean algae did not reappear until the Sinemurian stage.<ref>Template:Cite journal</ref>
Marine vertebratesEdit
Fish did not suffer a mass extinction at the end of the Triassic. The Late Triassic in general did experience a gradual drop in actinopterygiian diversity after an evolutionary explosion in the Middle Triassic. Though this may have been due to falling sea levels or the Carnian Pluvial Event, it may instead be a result of sampling bias considering that Middle Triassic fish have been more extensively studied than Late Triassic fish.<ref>Template:Cite journal</ref> Despite the apparent drop in diversity, neopterygiians (which include most modern bony fish) suffered less than more "primitive" actinopterygiians, indicating a biological turnover where modern groups of fish started to supplant earlier groups.<ref name="TannerLucas" /> Pycnodontiform fish were insignificantly affected.<ref>Template:Cite journal</ref> Conodonts, which were prominent index fossils throughout the Paleozoic and Triassic, finally became extinct at the T-J boundary following declining diversity.<ref name="TannerLucas" />
Like fish, marine reptiles experienced a substantial drop in diversity between the Middle Triassic and the Jurassic. However, their extinction rate at the Triassic–Jurassic boundary was not elevated. The highest extinction rates experienced by Mesozoic marine reptiles actually occurred at the end of the Ladinian stage, which corresponds to the end of the Middle Triassic. The only marine reptile families which became extinct at or slightly before the Triassic–Jurassic boundary were the placochelyids (the last family of placodonts), making plesiosaurs the only surviving sauropterygians,<ref>Template:Cite journal</ref> and giant ichthyosaurs such as shastasaurids.<ref>Template:Cite journal</ref> Some authors have argued that the end of the Triassic acted as a genetic "bottleneck" for ichthyosaurs, which never regained the level of anatomical diversity and disparity which they possessed during the Triassic,<ref>Template:Cite journal</ref> although analysis of ichthyosaurian and eosauropterygian disparity across the Triassic-Jurassic transition has shown no evidence for such a bottleneck.<ref>Template:Cite journal</ref> The high diversity of rhomaelosaurids immediately after the TJME points to a gradual extinction of marine reptiles rather than an abrupt one.<ref>Template:Cite journal</ref>
Terrestrial animalsEdit
Terrestrial fauna was affected by the TJME much more severely than marine fauna.<ref>Template:Cite journal</ref> One of the earliest pieces of evidence for a Late Triassic extinction was a major turnover in terrestrial tetrapods such as amphibians, reptiles, and synapsids. Edwin H. Colbert drew parallels between the system of extinction and adaptation between the Triassic–Jurassic and Cretaceous–Paleogene boundaries. He recognized how dinosaurs, lepidosaurs (lizards and their relatives), and crocodyliforms (crocodilians and their relatives) filled the niches of more ancient groups of amphibians and reptiles which were extinct by the start of the Jurassic.<ref name=":3">Template:Cite journal</ref> Olsen (1987) estimated that 42% of all terrestrial tetrapods became extinct at the end of the Triassic, based on his studies of faunal changes in the Newark Supergroup of eastern North America.<ref name=":2" /> In contrast to the end-Cretaceous extinction, the TJME substantially affected freshwater ecosystems, and it further differed from the former in that body size did not affect extinction risk.<ref name="ImpactEventBiologicalProcesses">Template:Cite book</ref> More modern studies have debated whether the turnover in Triassic tetrapods was abrupt at the end of the Triassic, or instead more gradual.<ref name="TannerLucas" />
During the Triassic, amphibians were mainly represented by large, crocodile-like members of the order Temnospondyli. Although the earliest lissamphibians (modern amphibians like frogs and salamanders) did appear during the Triassic, they would become more common in the Jurassic while the temnospondyls diminished in diversity past the Triassic–Jurassic boundary.<ref name=":2" /> Although the decline of temnospondyls did send shockwaves through freshwater ecosystems, it was probably not as abrupt as some authors have suggested. Brachyopoids, for example, survived until the Cretaceous according to new discoveries in the 1990s. Several temnospondyl groups did become extinct near the end of the Triassic despite earlier abundance, but it is uncertain how close their extinctions were to the end of the Triassic. The last known metoposaurids ("Apachesaurus") were from the Redonda Formation, which may have been early Rhaetian or late Norian. Gerrothorax, the last known plagiosaurid, has been found in rocks which are probably (but not certainly) Rhaetian, while a capitosaur humerus was found in Rhaetian-age deposits in 2018. Therefore, plagiosaurids and capitosaurs were likely victims of an extinction at the very end of the Triassic, while most other temnospondyls were already extinct.<ref name="KonietzkoMeierEtAl2018">Template:Cite journal</ref>
Terrestrial reptile faunas were dominated by archosauromorphs during the Triassic, particularly phytosaurs and members of Pseudosuchia (the reptile lineage which leads to modern crocodilians). In the Early Jurassic and onwards, dinosaurs and pterosaurs became the most common land reptiles, while small reptiles were mostly represented by lepidosauromorphs (such as lizards and tuatara relatives). Among pseudosuchians, only small crocodylomorphs did not become extinct by the end of the Triassic, with both dominant herbivorous subgroups (such as aetosaurs) and carnivorous ones (rauisuchids) having died out.<ref name=":2" /> Phytosaurs, drepanosaurs, trilophosaurids, tanystropheids, and procolophonids, which were other common reptiles in the Late Triassic, had also become extinct by the start of the Jurassic. However, pinpointing the extinction of these different land reptile groups is difficult, as the last stage of the Triassic, the Rhaetian, and the first stage of the Jurassic, the Hettangian, each have few records of large land animals; some paleontologists have considered only phytosaurs and procolophonids to have become extinct at the Triassic–Jurassic boundary, with other groups having become extinct earlier.<ref name="TannerLucas" /> However, it is likely that many other groups survived up until the boundary according to British fissure deposits from the Rhaetian. Aetosaurs, kuehneosaurids, drepanosaurs, thecodontosaurids, "saltoposuchids" (like Terrestrisuchus), trilophosaurids, and various non-crocodylomorph pseudosuchians are all examples of Rhaetian reptiles which may have become extinct at the Triassic–Jurassic boundary.<ref>Template:Cite journal</ref><ref name="Edgar">Template:Cite journal</ref><ref>Template:Cite journal</ref>
In the TJME's aftermath, dinosaurs experienced a major radiation, filling some of the niches vacated by the victims of the extinction.<ref name="MichaelJamesBenton">Template:Cite journal</ref> Crocodylomorphs likewise underwent a very rapid and major adaptive radiation.<ref name="ToljagićButler2013">Template:Cite journal</ref> Surviving non-mammalian synapsid clades similarly played a role in the post-TJME adaptive radiation during the Early Jurassic.<ref name="MichaelJamesBenton" />
Herbivorous insects were minimally affected by the TJME; evidence from the Sichuan Basin shows they were overall able to quickly adapt to the floristic turnover by exploiting newly abundant plants.<ref>Template:Cite journal</ref> Odonates suffered highly selective losses, and their morphospace was heavily restructured as a result.<ref>Template:Cite journal</ref>
Terrestrial plantsEdit
The extinction event marks a floral turnover as well, with estimates of the percentage of Rhaetian pre-extinction plants being lost ranging from 17% to 73%.<ref>Template:Cite journal</ref> Though spore turnovers are observed across the Triassic-Jurassic boundary, the abruptness of this transition and the relative abundances of given spore types both before and after the boundary are highly variable from one region to another, pointing to a global ecological restructuring rather than a mass extinction of plants.<ref name="BarbackaEtAlPPP2">Template:Cite journal</ref> Overall, plants suffered minor diversity losses on a global scale as a result of the extinction, but species turnover rates were high and substantial changes occurred in terms of relative abundance and growth distribution among taxa.<ref>Template:Cite journal</ref> Evidence from Central Europe suggests that rather than a sharp, very rapid decline followed by an adaptive radiation, a more gradual turnover in both fossil plants and spores with several intermediate stages is observed over the course of the extinction event.<ref>Template:Cite journal</ref> Extinction of plant species can in part be explained by the suspected increased carbon dioxide in the atmosphere as a result of CAMP volcanic activity, which would have created photoinhibition and decreased transpiration levels among species with low photosynthetic plasticity, such as the broad leaved Ginkgoales which declined to near extinction across the Tr–J boundary.<ref name=":02">Template:Cite journal</ref>
Ferns and other species with dissected leaves displayed greater adaptability to atmosphere conditions of the extinction event,<ref>Template:Cite journal</ref> and in some instances were able to proliferate across the boundary and into the Jurassic.<ref name=":02" /> The species Lepidopteris ottonis evolved a reliance on asexual reproduction amidst the environmental chaos of the TJME.<ref>Template:Cite journal</ref> In the Jiyuan Basin of North China, Classopolis content increased drastically in concordance with warming, drying, wildfire activity, enrichments in isotopically light carbon, and an overall reduction in floral diversity.<ref>Template:Cite journal</ref> In the Sichuan Basin, relatively cool mixed forests in the late Rhaetian were replaced by hot, arid fernlands during the Triassic–Jurassic transition, which in turn later gave way to a cheirolepid-dominated flora in the Hettangian and Sinemurian.<ref>Template:Cite journal</ref> The abundance of ferns in China that were resistant to high levels of aridity increased significantly across the Triassic–Jurassic boundary, though ferns better adapted for moist, humid environments declined, indicating that plants experienced major environmental stress, albeit not an outright mass extinction.<ref>Template:Cite journal</ref> In some regions, however, major floral extinctions did occur, with some researchers challenging the hypothesis of there being no significant floral mass extinction on this basis. In the Newark Supergroup of the United States East Coast, about 60% of the diverse monosaccate and bisaccate pollen assemblages disappear at the Tr–J boundary, indicating a major extinction of plant genera. Early Jurassic pollen assemblages are dominated by Corollina, a new genus that took advantage of the empty niches left by the extinction.<ref name="Fowell19942">Template:Citation</ref> The site of St. Audrie's Bay displays a shift from diverse gymnosperm-dominated forests to Cheirolepidiaceae-dominated monocultures.<ref name="BonisAndKurschner2012" /> The Danish Basin saw 34% of its Rhaetian spore-pollen assemblage, including Cingulizonates rhaeticus, Limbosporites lundbladiae, Polypodiisporites polymicroforatus, and Ricciisporites tuberculatus, disappear, with the post-extinction plant community being dominated by pinacean conifers such as Pinuspollenites minimus and tree ferns such as Deltoidospora, with ginkgos, cycads, cypresses, and corystospermous seed ferns also represented.<ref>Template:Cite journal</ref> Along the margins of the European Epicontinental Sea and the European shores of the Tethys, coastal and near-coastal mires fell victim to an abrupt sea level rise. These mires were replaced by a pioneering opportunistic flora after an abrupt sea level fall, although its heyday was short lived and it died out shortly after its rise.<ref>Template:Cite journal</ref> The opportunists that established themselves along the Tethyan coastline were primarily spore-producers.<ref name="BonisAndKurschner2012">Template:Cite journal</ref> In the Eiberg Basin of the Northern Calcareous Alps, there was a very rapid palynomorph turnover.<ref>Template:Cite journal</ref> The palynological and palaeobotanical succession in Queensland shows a Classopolis bloom after the TJME.<ref>Template:Cite journal</ref> Polyploidy may have been an important factor that mitigated a conifer species' risk of going extinct.<ref>Template:Cite journal</ref>
Possible causesEdit
Central Atlantic Magmatic ProvinceEdit
The leading and best evidenced explanation for the TJME is massive volcanic eruptions, specifically from the Central Atlantic Magmatic Province (CAMP),<ref>Template:Cite journal</ref><ref name="DeenenEtAl2010">Template:Cite journal</ref><ref>Template:Cite journal</ref> the largest known large igneous province by area, and one of the most voluminous,<ref>Template:Cite book</ref><ref>Template:Cite journal</ref> with its flood basalts extending across parts of southwestern Europe,<ref>Template:Cite book</ref><ref name="ChrystèleEtAl2007" /> northwestern Africa,<ref>Template:Cite journal</ref> northeastern South America,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite book</ref> and southeastern North America.<ref name="HamesRenneRuppel2000" /><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The coincidence and synchrony of CAMP activity and the TJME is indicated by uranium-lead dating,<ref>Template:Cite journal</ref><ref name="blackburn2013">Template:Cite journal</ref> argon-argon dating,<ref name="HamesRenneRuppel2000">Template:Cite journal</ref><ref name="ChrystèleEtAl2007">Template:Cite journal</ref> and palaeomagnetism.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name=":5" /> The isotopic composition of fossil soils and marine sediments near the boundary between the Late Triassic and Early Jurassic has been tied to a large negative δ13C excursion,<ref name="HuFuLinSongWangTian2019">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> with values as low as -2.8%.<ref>Template:Cite journal</ref> Carbon isotopes of hydrocarbons (n-alkanes) derived from leaf wax and lignin, and total organic carbon from two sections of lake sediments interbedded with the CAMP in eastern North America have shown carbon isotope excursions similar to those found in the mostly marine St. Audrie's Bay section, Somerset, England; the correlation suggests that the TJME began at the same time in marine and terrestrial environments, slightly before the oldest basalts in eastern North America but simultaneous with the eruption of the oldest flows in Morocco, with both a critical Template:CO2 greenhouse and a marine biocalcification crisis.<ref name="WhitesideEtAl2010">Template:Cite journal</ref> Furthermore, chemostratigraphic analysis in the Junggar Basin has shown that the negative δ13C excursions associated with CAMP volcanism corresponded in time to biotic turnovers in the palynomorph record, strongly suggesting a causal relationship between the two.<ref>Template:Cite journal</ref> Contemporaneous CAMP eruptions, mass extinction, and the carbon isotopic excursions are shown in the same places, making the case for a volcanic cause of a mass extinction.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The observed negative carbon isotope excursion is lower in some sites that correspond to what was then eastern Panthalassa because of the extreme aridity of western Pangaea limiting weathering and erosion there.<ref>Template:Cite journal</ref> The negative δ13C excursion associated with CAMP volcanism lasted for approximately 20,000 to 40,000 years, or about one or two of Earth's axial precession cycles,<ref>Template:Cite journal</ref> although the carbon cycle was so disrupted that it did not stabilise until the Sinemurian.<ref>Template:Cite journal</ref> Mercury anomalies from deposits in various parts of the world have further bolstered the volcanic cause hypothesis,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> as have anomalies from various platinum-group elements.<ref name="PlatinumGroupElementsCAMP">Template:Cite journal</ref> Nickel enrichments are also observed at the Triassic-Jurassic boundary coevally with light carbon enrichments, providing yet more evidence of massive volcanism.<ref>Template:Cite thesis</ref>
Some scientists initially rejected the volcanic eruption theory because the Newark Supergroup, a section of rock in eastern North America that records the Triassic–Jurassic boundary, contains no ash-fall horizons and because its oldest basalt flows were estimated to lie around 10 m above the transition zone,<ref>Template:Cite journal</ref> which they estimated to have occurred 610 kyr after the TJME.<ref>Template:Cite journal</ref> Also among their objections was that the Triassic-Jurassic boundary was poorly defined and the CAMP eruptions poorly constrained temporally.<ref>Template:Cite journal</ref> However, updated dating protocol and wider sampling has confirmed that the CAMP eruptions started in Morocco only a few thousand years before the extinction,<ref name="blackburn2013" /> preceding their onset in Nova Scotia and New Jersey,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> and that they continued in several more pulses for the next 600,000 years.<ref name="blackburn2013" /> Volcanic global warming has also been criticised as an explanation because some estimates have found that the amount of carbon dioxide emitted was only around 250 ppm, not enough to generate a mass extinction.<ref name=":6">Template:Cite journal</ref> In addition, at some sites, changes in carbon isotope ratios have been attributed to diagenesis and not any primary environmental changes.<ref>Template:Cite journal</ref>
Global warmingEdit
The flood basalts of the CAMP released gigantic quantities of carbon dioxide,<ref>Template:Cite journal</ref> a potent greenhouse gas causing intense global warming.<ref name="AnthropogenicScaleDegassing">Template:Cite journal</ref> Before the TJME, carbon dioxide levels were around 1,000 ppm as measured by the stomatal index of Lepidopteris ottonis, but this quantity jumped to 1,300 ppm at the onset of the extinction event.<ref>Template:Cite journal</ref> During the TJME, carbon dioxide concentrations increased fourfold.<ref>Template:Cite journal</ref> The record of CAMP degassing shows several distinct pulses of carbon dioxide immediately following each major pulse of magmatism, at least two of which amount to a doubling of atmospheric CO2.<ref>Template:Cite journal</ref> Carbon dioxide was emitted quickly and in enormous quantities compared to other periods of Earth's history, rate of carbon dioxide emissions was one of the most meteoric rises in carbon dioxide levels in Earth's entire history.<ref name="VolumeRateCO2">Template:Cite journal</ref> It is estimated that a single volcanic pulse from the large igneous province would have emitted an amount of carbon dioxide roughly equivalent to projected anthropogenic carbon dioxide emissions for the 21st century.<ref>Template:Cite journal</ref> In addition, the flood basalts intruded through sediments that were rich in organic matter and combusted it,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> as evidenced by low Δ199Hg values showing elevated levels of organic matter-derived mercury in the environment.<ref>Template:Cite journal</ref> The degassing of volatiles resulting from volcanic intrusions into organic-rich sediments further enhanced the volcanic warming of the climate.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Thermogenic carbon release through such contact metamorphism of carbon-rich deposits has been found to be a sensible hypothesis providing a coherent explanation for the magnitude of the negative carbon isotope excursions at the terminus of the Triassic.<ref>Template:Cite journal</ref> Global temperatures rose sharply by 3 to 4 °C.<ref name="McElwainBeerlingWoodward1999">Template:Cite journal</ref> In some regions, the temperature rise was as great as 10 °C.<ref>Template:Cite journal</ref> Kaolinite-dominated clay mineral spectra reflect the extremely hot and humid greenhouse conditions engendered by the CAMP.<ref>Template:Cite journal</ref> Soil erosion occurred as the hydrological cycle was accelerated by the extreme global heat.<ref>Template:Cite journal</ref>
The catastrophic dissociation of gas hydrates as a positive feedback resulting from warming, which has been suggested as one possible cause of the PTME, the largest mass extinction of all time,<ref name="BentonTwitchett2003">Template:Cite journal</ref> may have exacerbated greenhouse conditions,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> although others suggest that methane hydrate release was temporally mismatched with the TJME and thus not a cause of it.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Global coolingEdit
Besides the carbon dioxide-driven long-term global warming, CAMP volcanism had shorter term cooling effects resulting from the emission of sulphur dioxide aerosols.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="blackburn2013" /> The extremely voluminous emission of this gas caused sharp drops in Earth's albedo and induced severe volcanic winters.<ref>Template:Cite journal</ref> High latitudes had colder climates with evidence of mild glaciation during the extinction interval. Cold periods induced by volcanic ejecta clouding the atmosphere might have favoured endothermic animals, with dinosaurs, pterosaurs, and mammals being more capable at enduring these conditions than large pseudosuchians due to insulation.<ref>Template:Cite journal</ref>
Metal poisoningEdit
CAMP volcanism released enormous amounts of toxic mercury.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The appearance of high rates of mutaganesis of varying severity in fossil spores during the TJME coincides with mercury anomalies and is thus believed by researchers to have been caused by mercury poisoning.<ref>Template:Cite journal</ref> δ202Hg and Δ199Hg evidence suggests that volcanism caused the mercury loading directly at the Triassic-Jurassic boundary, but that there were later bouts of elevated mercury in the environment during the Early Jurassic caused by eccentricity-forced enhancement of hydrological cycling and erosion that resulted in remobilisation of volcanically injected mercury that had been deposited in wetlands.<ref>Template:Cite journal</ref>
WildfiresEdit
The intense, rapid warming is believed to have resulted in increased storminess and lightning activity as a consequence of the more humid climate. The uptick in lightning activity is in turn implicated as a cause of an increase in wildfire activity.<ref>Template:Cite journal</ref> The combined presence of charcoal fragments and heightened levels of pyrolytic polycyclic aromatic hydrocarbons in Polish sedimentary facies straddling the Triassic-Jurassic boundary indicates wildfires were extremely commonplace during the earliest Jurassic, immediately after the Triassic-Jurassic transition.<ref>Template:Cite journal</ref> Elevated wildfire activity is also known from the Junggar Basin.<ref>Template:Cite journal</ref> In the Jiyuan Basin, two distinct pulses of drastically elevated wildfire activity are known: the first mainly affected canopies and occurred amidst relatively humid conditions while the second predominantly affected ground cover and was associated with aridity.<ref>Template:Cite journal</ref> At the Winterswijk quarry in the Netherlands, a surge in wildfire activity has been suggested to correspond to and have caused the sudden decline in coniferous vegetation.<ref>Template:Cite journal</ref> Frequent wildfires, combined with increased seismic activity from CAMP emplacement, led to apocalyptic soil degradation.<ref>Template:Cite journal</ref>
Anoxia and euxiniaEdit
Anoxia was another mechanism of extinction; the end-Triassic extinction was coeval with an uptick in black shale deposition and a pronounced negative δ238U excursion, indicating a major decrease in marine oxygen availability.<ref name="JostEtAl2017" /> Additional evidence for anoxia during the TJME comes from pyrite framboids, which grow in anoxic conditions.<ref>Template:Cite journal</ref> Evidence of anoxia has been discovered at the Triassic-Jurassic boundary across the world's oceans; the western Tethys, eastern Tethys, and Panthalassa were all affected by a precipitous drop in seawater oxygen,<ref>Template:Cite journal</ref> although at a few sites, the TJME was associated with fully oxygenated waters.<ref>Template:Cite journal</ref> Positive δ15N excursions have also been interpreted as evidence of anoxia concomitant with increased denitrification in marine sediments in the TJME's aftermath.<ref>Template:Cite journal</ref>
In northeastern Panthalassa, episodes of anoxia were already occurring during the Rhaetian before the TJME, making its marine ecosystems unstable even before the main crisis began.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> This early phase of environmental degradation in eastern Panthalassa may have been caused by an early phase of CAMP activity.<ref>Template:Cite journal</ref> Anoxic, reducing conditions were likewise present in western Panthalassa off the coast of what is now Japan for about a million years prior to the TJME.<ref>Template:Cite journal</ref> During the TJME, the rapid warming led to the stagnation of ocean circulation in many ocean regions, enabling the development of catastrophic anoxia; in what is now northwestern Europe, shallow seas became salinity stratified, enabling easy development of anoxia.<ref name="OrganicWalledDisasterSpecies">Template:Cite journal</ref> Another factor contributing to anoxia was the increase in continental weathering driven by intense warming that delivered vast quantities of nutrients to the ocean surface and engendered eutrophication; this uptick in weathering is evidenced by positive δ56Fe excursions.<ref>Template:Cite journal</ref> A combination of negative δ66Zn excursions, positive δ26Mg excursions, and a lack of significant change in δ65Cu provides further evidence of increased chemical weathering resulting from increased temperature and humidity on land at high latitudes.<ref>Template:Cite journal</ref> Increased influx of terrestrial organic matter, in conjunction with reduced salinity, has been directly shown to have enkindled anoxia in the Eiberg Basin.<ref>Template:Cite journal</ref> Persistent low δ238U ratios indicate prolonged global oxygen depletion continued into the Hettangian,<ref>Template:Cite journal</ref> with 87Sr/86Sr values showing that high influxes of terrestrial nutrients likely continued to eutrophicate the oceans well after the Triassic-Jurassic boundary.<ref>Template:Cite journal</ref> The persistence of anoxia into the Hettangian age may have helped delay the recovery of marine life in the extinction's aftermath.<ref name="JostEtAl2017">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Euxinia, a form of anoxia defined by not just the absence of dissolved oxygen but high concentrations of hydrogen sulphide, also developed in the oceans, as indicated by findings of increased isorenieratane. The increase in concentration of this substance reveals that populations of green sulphur bacteria, which photosynthesise using hydrogen sulphide instead of water, grew significantly across the Triassic-Jurassic boundary.<ref name="RichozEtAl2012">Template:Cite journal</ref><ref>Template:Cite journal</ref> A meteoric shift towards positive sulphur isotope ratios in reduced sulphur species indicates a complete utilisation of sulphate by sulphate reducing bacteria.<ref>Template:Cite journal</ref> Off the shores of the Wrangellia Terrane, the onset of photic zone euxinia was preceded by an interval of limited nitrogen availability and increased nitrogen fixation in surface waters while euxinia developed in bottom waters.<ref>Template:Cite journal</ref> Recurrent hydrogen sulphide poisoning following the TJME had retarding effects on biotic rediversification.<ref>Template:Cite journal</ref><ref name="RichozEtAl2012" />
Ocean acidificationEdit
Oceanic uptake of volcanogenic carbon and sulphur dioxide would have led to a significant decrease of seawater pH known as ocean acidification, which is discussed as a relevant driver of marine extinction,<ref name="Hautmann 2004">Template:Cite journal</ref><ref name="Green 2012">Template:Cite journal</ref><ref>Template:Cite journal</ref> acting in conjunction with marine anoxia.<ref>Template:Cite journal</ref> Additionally, acidification was enhanced and exacerbated by widespread photic zone euxinia, which caused increased rates of organic matter respiration and carbon dioxide release.<ref>Template:Cite journal</ref> Evidence for ocean acidification as an extinction mechanism comes from the preferential extinction of marine organisms with thick aragonitic skeletons and little biotic control of biocalcification (e.g., corals, hypercalcifying sponges),<ref>Template:Cite journal</ref><ref name="Hautmann et al. 2008">Template:Cite journal</ref> which resulted in a coral reef collapse<ref name="GeologicalRecordOceanAcid" /><ref name="OceanAcidDepTime" /> and an early Hettangian "coral gap".<ref name="EarlyHettangianCoralGap" /> The decline of megalodontoid bivalves is also attributed to increased seawater acidity.<ref>Template:Cite journal</ref> Extensive fossil remains of malformed calcareous nannoplankton, a common sign of significant drops in pH, have also been extensively reported from the Triassic-Jurassic boundary.<ref name="OrganicWalledDisasterSpecies" /> Global interruption of carbonate deposition at the Triassic-Jurassic boundary has been cited as additional evidence for catastrophic ocean acidification.<ref>Template:Cite journal</ref><ref name="Hautmann 2004" /> Upwardly developing aragonite fans in the shallow subseafloor may also reflect decreased pH, these structures being speculated to have precipitated concomitantly with acidification.<ref name="SubseafloorCarbonateFactory">Template:Cite journal</ref> In some studied sections, the TJME biocalcification crisis is masked by emersion of carbonate platforms induced by marine regression.<ref>Template:Cite journal</ref>
Ozone depletionEdit
Research on the role of ozone shield deterioration during the Permian-Triassic mass extinction has suggested that it may have been a factor in the TJME as well.<ref name="EnvironmentalMutagenesis">Template:Cite journal</ref><ref name="DyingInTheSun">Template:Cite journal</ref> A spike in the abundance of unseparated tetrads of Kraeuselisporites reissingerii has been interpreted as evidence of increased ultraviolet radiation flux resulting from ozone layer damage caused by volcanic aerosols.<ref>Template:Cite journal</ref>
Gradual climate changeEdit
The extinctions at the end of the Triassic were initially attributed to gradually changing environments. Within his 1958 study recognizing biological turnover between the Triassic and Jurassic, Edwin H. Colbert's proposal was that this extinction was a result of geological processes decreasing the diversity of land biomes. He considered the Triassic period to be an era of the world experiencing a variety of environments, from towering highlands to arid deserts to tropical marshes. In contrast, the Jurassic period was much more uniform both in climate and elevation due to excursions by shallow seas.<ref name=":3" />
Later studies noted a clear trend towards increased aridification towards the end of the Triassic. Although high-latitude areas like Greenland and Australia actually became wetter, most of the world experienced more drastic changes in climate as indicated by geological evidence. This evidence includes an increase in carbonate and evaporite deposits (which are most abundant in dry climates) and a decrease in coal deposits (which primarily form in humid environments such as coal forests).<ref name="TannerLucas" /> In addition, the climate may have become much more seasonal, with long droughts interrupted by severe monsoons.<ref name=":7">Template:Cite journal</ref> The world gradually got warmer over this time as well; from the late Norian to the Rhaetian, mean annual temperatures rose by 7 to 9 °C.<ref>Template:Cite journal</ref> The site of Hochalm in Austria preserves evidence of carbon cycle perturbations during the Rhaetian preceding the Triassic-Jurassic boundary, potentially having a role in the ecological crisis.<ref>Template:Cite journal</ref>
Sea level fallEdit
Geological formations in Europe and the Middle East seem to indicate a drop in sea levels at the end of the Triassic associated with the TJME.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Although falling sea levels have sometimes been considered a culprit for marine extinctions, evidence is inconclusive since many sea level drops in geological history are not correlated with increased extinctions. However, there is still some evidence that marine life was affected by secondary processes related to falling sea levels, such as decreased oxygenation (caused by sluggish circulation), or increased acidification. These processes do not seem to have been worldwide, with the sea level fall observed in European sediments believed to be not global but regional,<ref>Template:Cite journal</ref> and with even some European sections showing no sign of sea level fall across the Triassic-Jurassic boundary,<ref>Template:Cite journal</ref> but they may explain local extinctions in European marine fauna.<ref name="TannerLucas" /> However, it is not universally accepted that even this local diversity drop was caused by sea level fall.<ref>Template:Cite journal</ref> A pronounced sea level change in latest Triassic records from Lake Williston in northeastern British Columbia, which was then the northeastern margin of Panthalassa, resulted in an extinction event of infaunal (sediment-dwelling) bivalves, though not epifaunal ones.<ref>Template:Cite journal</ref>
Extraterrestrial impactEdit
Some have hypothesized that an impact from an asteroid or comet caused the Triassic–Jurassic extinction,<ref name="GrahamRyderBook" /><ref name="Fowell19942" /> similar to the extraterrestrial object which was the main factor in the Cretaceous–Paleogene extinction about 66 million years ago, as evidenced by the Chicxulub crater in Mexico. However, so far no impact crater of sufficient size has been dated to precisely coincide with the Triassic–Jurassic boundary.<ref name="TannerLucas" />
Nevertheless, the Late Triassic did experience several impacts, including the second-largest confirmed impact in the Mesozoic. The Manicouagan Reservoir in Quebec is one of the most visible large impact craters on Earth, and at Template:Cvt in diameter it is tied with the Eocene Popigai impact structure in Siberia as the fourth-largest impact crater on Earth. Olsen et al. (1987) were the first scientists to link the Manicouagan crater to the Triassic–Jurassic extinction, citing its age which at the time was roughly considered to be Late Triassic.<ref name=":2" /> More precise radiometric dating by Hodych & Dunning (1992) has shown that the Manicouagan impact occurred about 214 million years ago, about 13 million years before the Triassic–Jurassic boundary. Therefore, it could not have been responsible for an extinction precisely at the Triassic–Jurassic boundary.<ref name=":0">Template:Cite journal</ref> Nevertheless, the Manicouagan impact did have a widespread effect on the planet; a 214-million-year-old ejecta blanket of shocked quartz has been found in rock layers as far away as England<ref name=":1">Template:Cite journal</ref> and Japan. There is still a possibility that the Manicouagan impact was responsible for a small extinction midway through the Late Triassic at the Carnian–Norian boundary,<ref name=":0" /> although the disputed age of this boundary (and whether an extinction actually occurred in the first place) makes it difficult to correlate the impact with extinction.<ref name=":1" /> Onoue et al. (2016) alternatively proposed that the Manicouagan impact was responsible for a marine extinction in the middle of the Norian which affected radiolarians, sponges, conodonts, and Triassic ammonoids. Thus, the Manicouagan impact may have been partially responsible for the gradual decline in the latter two groups which culminated in their extinction at the Triassic–Jurassic boundary.<ref>Template:Cite journal</ref> The boundary between the Adamanian and Revueltian land vertebrate faunal zones, which involved extinctions and faunal changes in tetrapods and plants, was possibly also caused by the Manicouagan impact, although discrepancies between magnetochronological and isotopic dating lead to some uncertainty.<ref>Template:Cite journal</ref>
Other Triassic craters are closer to the Triassic–Jurassic boundary but also much smaller than the Manicouagan reservoir. The eroded Rochechouart impact structure in France has most recently been dated to Template:Val million years ago,<ref name="Schmieder">Template:Cite journal</ref> but at Template:Cvt across (possibly up to Template:Cvt across originally), it appears to be too small to have affected the ecosystem,<ref name="RSmith">Template:Cite journal</ref> although it has been speculated to have played a role in an alleged much smaller extinction event at the Norian-Rhaetian boundary.<ref>Template:Cite journal</ref> The Template:Cvt wide Saint Martin crater in Manitoba has been proposed as a candidate for a possible TJME-causing impact, but its has since been dated to be Carnian.<ref>Template:Cite journal</ref> Other putative or confirmed Triassic craters include the Template:Cvt wide Puchezh-Katunki crater in Eastern Russia (though it may be Jurassic in age), the Template:Cvt wide Obolon' crater in Ukraine, and the Template:Cvt wide Red Wing Creek structure in North Dakota. Spray et al. (1998) noted an interesting phenomenon, that being how the Manicouagan, Rochechouart, and Saint Martin craters all seem to be at the same latitude, and that the Obolon' and Red Wing craters form parallel arcs with the Rochechouart and Saint Martin craters, respectively. Spray and his colleagues hypothesized that the Triassic experienced a "multiple impact event", a large fragmented asteroid or comet which broke up and impacted the earth in several places at the same time.<ref name=":4" /> Such an impact has been observed in the present day, when Comet Shoemaker-Levy 9 broke up and hit Jupiter in 1992. However, the "multiple impact event" hypothesis for Triassic impact craters has not been well-supported; Kent (1998) noted that the Manicouagan and Rochechouart craters were formed in eras of different magnetic polarity,<ref>Template:Cite journal</ref> and radiometric dating of the individual craters has shown that the impacts occurred millions of years apart.<ref name="TannerLucas" />
Shocked quartz has been found in Rhaetian deposits from the Northern Apennines of Italy, providing possible evidence of an end-Triassic extraterrestrial impact.<ref>Template:Cite journal</ref> Certain trace metals indicative of a bolide impact have been found in the late Rhaetian, though not at the Triassic-Jurassic boundary itself; the discoverers of these trace metal anomalies purport that such a bolide impact could only have been an indirect cause of the TJME.<ref>Template:Cite journal</ref> The discovery of seismites two to four metres thick coeval with the carbon isotope fluctuations associated with the TJME has been interpreted as evidence of a possible bolide impact, although no definitive link between these seismites and any impact event has been found.<ref>Template:Cite journal</ref>
On the other hand, the dissimilarity between the isotopic perturbations characterising the TJME and those characterising the end-Cretaceous mass extinction makes an extraterrestrial impact highly unlikely to have been the cause of the TJME, according to many researchers.<ref>Template:Cite journal</ref> Various trace metal ratios, including palladium/iridium, platinum/iridium, and platinum/rhodium, in rocks deposited during the TJME have numerical values very different from what would be expected in an extraterrestrial impact scenario, providing further evidence against this hypothesis.<ref name="PlatinumGroupElementsCAMP" /> The Triassic-Jurassic boundary furthermore lacks a fern spore spike akin to that observed at the terminus of the Cretaceous, inconsistent with an asteroid impact.<ref name="BonisEtAl2010">Template:Cite journal</ref>
Comparisons to present climate changeEdit
The extremely rapid, centuries-long timescale of carbon emissions and global warming caused by pulses of CAMP volcanism has drawn comparisons between the Triassic-Jurassic mass extinction and anthropogenic global warming, currently causing the Holocene extinction.<ref name="AnthropogenicScaleDegassing" /> The current rate of carbon dioxide emissions is around 50 gigatonnes per year, hundreds of times faster than during the latest Triassic, although the lack of extremely detailed stratigraphic resolution and pulsed nature of CAMP volcanism means that individual pulses of greenhouse gas emissions likely occurred on comparable timescales to human release of warming gases since the Industrial Revolution.<ref name="VolumeRateCO2" /> The degassing rate of the first pulse of CAMP volcanism is estimated to have been around half of the rate of modern anthropogenic emissions.<ref name="AnthropogenicScaleDegassing" /> Palaeontologists studying the TJME and its impacts warn that a major reduction in humanity's carbon dioxide emissions to slow down climate change is of critical importance for preventing a catastrophe similar to the TJME from befalling the modern biosphere.<ref name="VolumeRateCO2" /> If human-induced climate change persists as is, predictions can be made as to how various aspects of the biosphere will respond based on records of the TJME. For example, current conditions such the increased carbon dioxide levels, ocean acidification, and ocean deoxygenation create a similar climate to that of the Triassic-Jurassic boundary for marine life, so it is the common assumption that should the trends continue, modern reef-building taxa and skeletal benthic organisms will be preferentially impacted.<ref>Template:Cite journal</ref> The end-Triassic reef crisis has been specifically cited as a possible analogue for the fate of present coral reefs should anthropogenic global warming continue.<ref name="PandolfiAndKiessling2014">Template:Cite journal</ref>
ReferencesEdit
LiteratureEdit
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- McHone, J.G. (2003), Volatile emissions of Central Atlantic Magmatic Province basalts: Mass assumptions and environmental consequences, in Hames, W.E. et al., eds., The Central Atlantic Magmatic Province: Insights from Fragments of Pangea. American Geophysical Union Monograph 136, p. 241–254.
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