Triassic–Jurassic extinction event
The Triassic–Jurassic (Tr-J) extinction event (TJME), often called the end-Triassic extinction, was a
Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a decrease in
Effects
This event vacated terrestrial
Marine invertebrates
The Triassic-Jurassic extinction completed the transition from the Palaeozoic
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.[40] 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.[41] In the Neuquén Basin, recovery began in the late early Hettangian and lasted until a new biodiversity equilibrium in the late Hettangian.[42] Also despite recurrent anoxic episodes, large bivalves began to reappear shortly after the extinction event.[43] Siliceous sponges dominated the immediate aftermath interval thanks to the enormous influx of silica into the oceans from the weathering of the CAMP's aerially extensive basalts.[44][45] Some clades recovered more slowly than others, however, as exemplified by corals and their disappearance in the early Hettangian.[37]
Marine vertebrates
Fish did not suffer a mass extinction at the end of the Triassic. The late Triassic in general did experience a gradual drop in
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
Terrestrial vertebrates
Terrestrial fauna was affected by the TJME much more severely than marine fauna.[50] 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.[15] 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.[17] More modern studies have debated whether the turnover in Triassic tetrapods was abrupt at the end of the Triassic, or instead more gradual.[7]
During the Triassic,
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.[17] 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.[7] 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.[52][53][54]
Terrestrial plants
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%.[55] 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.[8] 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.[56] 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.[57] 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.[58]
Ferns and other species with dissected leaves displayed greater adaptability to atmosphere conditions of the extinction event,
Possible causes
Gradual climate change
Gradual
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.[15]
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).[7] In addition, the climate may have become much more seasonal, with long droughts interrupted by severe monsoons.[69] 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.[70] 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.[71]
Sea level fall
Geological formations in Europe seem to indicate a drop in sea levels in the late Triassic, and then a rise in the early Jurassic. 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,
Extraterrestrial impact
Some have hypothesized that an impact from an asteroid or comet caused the Triassic–Jurassic extinction,[63] 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.
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 100 km (62 mi) 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.[17] 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.[16] 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[74] 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,[16] 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.[74] 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.[75] 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.[76]
Other Triassic craters are closer to the Triassic–Jurassic boundary but also much smaller than the Manicouagan reservoir. The eroded
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.[85] 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.[86]
Central Atlantic Magmatic Province
The leading and best evidenced explanation for the TJME is massive volcanic eruptions, specifically from the
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 its oldest basalt flows were estimated to lie around 10 m above the transition zone.[116] However, updated dating protocol and wider sampling has confirmed that the CAMP eruptions started in Morocco only a few thousand years before the extinction,[102] preceding their onset in Nova Scotia and New Jersey,[117][118][119] and that they continued in several more pulses for the next 600,000 years.[102] 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.[120] In addition, at some sites, changes in carbon isotope ratios have been attributed to diagenesis and not any primary environmental changes.[121]
Global warming
The flood basalts of the CAMP released gigantic quantities of
The catastrophic dissociation of
Global cooling
Besides the carbon dioxide-driven long-term global warming, CAMP volcanism had shorter term cooling effects resulting from the emission of
Metal poisoning
CAMP volcanism released enormous amounts of toxic mercury. 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.[142]
Wildfires
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.[143] 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.[144] Elevated wildfire activity is also known from the Junggar Basin.[145] 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.[146] Frequent wildfires, combined with increased seismic activity from CAMP emplacement, led to apocalyptic soil degradation.[147]
Ocean acidification
In addition to these climatic effects, 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.[23][148][149] 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),[150] which resulted in a coral reef collapse[38][39] and an early Hettangian "coral gap".[37] 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.[151] Global interruption of carbonate deposition at the Triassic-Jurassic boundary has been cited as additional evidence for catastrophic ocean acidification.[152][23] Upwardly developing aragonite fans in the shallow subseafloor may also reflect decreased pH, these structures being speculated to have precipitated concomitantly with acidification.[153] In some studied sections, the TJME biocalcification crisis is masked by emersion of carbonate platforms induced by marine regression.[154]
Anoxia
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.
In northeastern Panthalassa, episodes of anoxia and euxinia were already occurring before the TJME, making its marine ecosystems unstable even before the main crisis began.[162] This early phase of environmental degradation in eastern Panthalassa may have been caused by an early phase of CAMP activity.[163] 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.[164] During the TJME, the rapid warming and increase in continental weathering led to the stagnation of ocean circulation and deoxygenation of seawater in many ocean regions, causing catastrophic marine environmental effects in conjunction with ocean acidification,[165] which was enhanced and exacerbated by widespread photic zone euxinia through organic matter respiration and carbon dioxide release.[166] 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.[167] In what is now northwestern Europe, shallow seas became salinity stratified, enabling easy development of anoxia.[151] Reduced salinity, in conjunction with increased influx of terrestrial organic matter, enkindled anoxia in the Eiberg Basin.[168] The persistence of anoxia into the Hettangian age may have helped delay the recovery of marine life in the extinction's aftermath,[155][169] and recurrent hydrogen sulphide poisoning likely had the same retarding effect on biotic rediversification.[170][156]
Ozone depletion
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.[171][172] 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.[173]
Comparisons to present climate change
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
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External links
- Theories on the Triassic–Jurassic Extinction Archived 2017-08-01 at the Wayback Machine
- The Triassic–Jurassic Mass Extinction
- 200 million year old mystery BBC News story, 12-Oct-2011