Australia: The Land Where Time Began
The End-Permian Extinction Event
The end-Permian event that occurred 251 Ma was the biggest mass extinction of the past 600 My during which as much as 95 % of all species on Earth went extinct. Key questions concern what combination of environmental changes could possibly have had such a devastating effect, the scale and pattern of species that were lost, and the nature of the recovery. According to Benton in recent years the perspective of the end-Permian mass extinction has been dramatically changed by new studies on the dating of the event, contemporary volcanic activity, and the anatomy of the environmental crisis. Evidence of the cause of the event has been equivocal, with evidence suggesting either an asteroid impact or mass volcanism, though the volcanic activity hypothesis seems the most likely. The extinction model includes global warming by 6oC and a huge input of light carbon into the ocean-atmosphere system from the volcanic activity, though especially from gas hydrates, that led to a positive feedback loop that continued to worsen, the ‘runaway greenhouse’.
New evidence has suggested 4 main parallel themes, not necessarily in chronological order in this paper:
1. The Permo-Triassic (P-T) boundary has been dated more precisely to 251 Ma.
2. The Siberian Traps, huge volumes of flood basalt eruptions have also been dated more precisely than was possible previously, and their eruption peak matches the P-T boundary.
3. It has begun to be shown by extensive study of rock sections straddling the P-T boundary, as well as the discovery of new sections, there was a common pattern of environmental changes through the latest Permian and earliest Triassic, ⁓253-249 Ma.
4. A common story of environmental turmoil has been revealed by the study of stable isotopes of oxygen and carbon in those rock sections.
Taken together these themes appeared to point to a model of change in which the normal feedback mechanisms could not cope, which allowed the chemical and temperature balance of the atmosphere and the oceans to breakdown catastrophically. These 4 advances will be reviewed in this paper.
Dating and timing
Until recent years it had been difficult to define the end-Permian mass extinction event, though it was known for some time to be the largest extinction event in the history of life on Earth, and was much more significant than the far better known end-Cretaceous event, the KT event at 65 Ma. The timing of the end Permian event has been a key problem. Dates that had been stated for the end-Permian event included 225, 245 or 250 Ma, though these dates were based on interpolation from rocks that had been dated more precisely that were well above of well below the KT boundary. Because precise dating was lacking it was not possible to determine if the decline of life on Earth at this time had been a long drawn-out process or instantaneous. New rock sections and new dating techniques, however, allowed the dating of volcanic ash bands in Chinese sections by the use of the uranium/lead technique (Bowring et al., 1989) and to assign a date of 251 Ma to the P-T boundary.
Dating the shape of the extinctions was another problem. The global stratotype for the Permian-Triassic boundary is the classic Meishan section in southern China (Yang et al., 1995) which provided the means to do this because of its fossil richness and there are several ash bands scattered throughout the succession that are datable. In a recent study 333 species from 15 marine fossil groups that included microscopic foraminifera, fusulinids, and radiolarians; rugose corals, bryozoans, brachiopods, bivalves, cephalopods, gastropods, trilobites, conodonts, fish, and algae were identified (Jin Yugan et al., 2000). There were 161 species in all that became extinct below the beds at the boundary during the 4 My prior to the end of the Permian. In particular beds extinction rates amounted to 33 % or less. Immediately below the Permian-Triassic boundary, at the contact of beds 24 and 25, most of the remaining species disappeared, which gave a rate of loss of 94 % at that level. There were 3 extinction levels that were identified, labelled A, B and C. It was argued by Jin et al., that apparently died out at level A are probably artefactual records, that actually belong to level B (examples of the Signor-Lipps Effect, that axiom that the very last fossil of a species is rarely found by palaeontologists. The possibility that Level C is real suggests that after the huge catastrophe at Level B that some species survived through the 1 My to Level C, though during that interval most disappeared step-by-step. According to Benton it is not easy to scale up from the local rock sections to establish the global pattern, though it is shown by figures from other rock sections, such as northern Italy (Rampino & Adler, 1998) and East Greenland (Twitchett et al., 2001; Looy et al., 2001), appear to agree with both the magnitude and extinction rate (See Extinction magnitude).
It is suggested by the suddenness and magnitude of the mass extinction that there was a dramatic cause, possibly an asteroid impact or volcanism. Earth scientists have traditionally been slow to accept such catastrophic models (Benton, 2003). An example is that Meteor Crater in Arizona had been produced by an impact, and the impact model for the KT mass extinction following its announcement in 1980 (Alvarez et al., 1980) was also accepted only slowly. Both views are, however, now the standard, and evidence that the end-Permian mass extinction was also caused by an extraterrestrial impact has been searched for.
Evidence of impact
There are 3 key pieces of evidence for the KT impact (Hallam & Wignall, 1997), the candidate crater in Mexico, the iridium spike (the massive enrichment of iridium, a rare metallic element, which generally gets to the surface of the Earth only from space), and shocked quartz (a form of the mineral that is most common in rocks that have been subjected to intense pressure). All 3 phenomena were found in the Permian-Triassic beds in the 1980s and 1990s, and all 3 have been rejected or accepted, though with not much enthusiasm (Benton, 2003; Hallam & Wignall, 1997).
The presence of extraterrestrial noble gases, helium and argon, trapped in the cage-like molecular structure of fullerenes at the Permian-Triassic boundary in China and Japan was reported in early 2001 (Luann Becker et al., 2001). Fullerenes are comprised of 60-200 carbon atoms arranged in regular hexagons around a large hollow ball. Buckyballs, a nickname for fullerenes after the inventor of the geodesic dome, Richard Buckminster Fuller (1895-1983), because the natural structure of fullerenes mimics the form of his invention. It has been found that fullerenes can form in meteorites, forest fires, and in mass spectrometers that are used to study them.
The helium and argon found in fullerenes at the Permian-Triassic boundary proved to be isotopically identical to those that were derived from meteorites, it was argued that they must have come from a meteorite impact. These results have been widely criticised. It was reported that reanalysis of samples from exactly the same sites in China (Farley & Mukhopadhyay, 2001) using exactly the same lab procedures, failed to replicate the results of Becker et al. Also, it was argued (Isozaki, 2001) that the Permian-Triassic boundary is not present in the Japanese section that was studied by Becker et al., and their samples were obtained from at least 80 cm beneath the boundary. The helium and argon that were reported by Becker et al. came from rocks that contained fullerenes, though it was never demonstrated that those noble gases were actually trapped in fullerenes, which was a key claim by Becker et al.
It has been reported more recently (Kaiho et al., 2001) that grains of sediments that supposedly show evidence of impact compression, as well as geochemical shifts that they interpreted as the impact of a huge asteroid. According to Benton et al. their data are, however, far from conclusive, and have also been severely criticised by other geochemists (Koeberl et al., 2002). Though evidence for impact at the Permian-Tertiary boundary has been vigorously promoted recently (Becker, 2002), Benton et al. regard it as tenuous. The evidence for impact is much weaker and more limited than that for impact at the KT boundary, and it would therefore be premature to construct an extinction scenario that was based on such evidence.
Evidence for an eruption
Giant volcanic eruptions of flood basalt began in Siberia at the end of the Permian that totalled about 2 million km3 (Reichow et al., 2002) of lava, which covered 1.6 million km2 of eastern Russia to a depth of 400-3,000 m, which is equivalent to the area of the European Community. It is now widely accepted that these massive eruptions, confined to a time span of < 1My, were a significant factor in the crises at the end of the Permian.
In the 1980s suggestions to this effect were first made. The Siberian Traps are composed of basalt, which is a dark-coloured igneous rock, which is not generally erupted explosively from classic conical volcanoes, usually emerging sluggishly from long fissures in the ground, as can be observed in Iceland. Typically, flood basalts form many layers and can continue to build up for thousands of years, eventually reaching considerable thicknesses. A characteristic landscape is produced; trap scenery, in which the different lava flows erode back over time to produce a layered, stepped appearance to the hills. The word ‘trap’ is derived from a word in old Swedish, trap, which means a staircase.
Early dating attempts of the Siberian Traps resulted in a very large array of dates, from 160-280 Ma, with a particular cluster between 230 and 260 Ma. In 1990, according to these ranges, it could only be concluded that the basalts might be any time between Early Permian to Late Jurassic in age, though probably spanned the Permian-Triassic Boundary. In more recent dating attempts (Bowring et al., 1998; Renne et al., 1995; Mundil et al., 2001) newer radiometric methods were used which yielded dates that were more exactly on the boundary, and the range from the bottom to the top of the lava pile was ⁓600,000 years, which highlighted that geologically speaking the event took place overnight. Also, this range of time duration for the eruptions matches the evidence that has come from China of rapid extinction.
As a consequence, as dating becomes increasingly precise, the eruptions of the Siberian Traps have been found to be part of a complex web of interacting processes, and not having only a minor role in the crises at the Permian-Triassic boundary (Erwin, 1993), and are now considered to be the most probable trigger for the catastrophe (White, 2002; Wignall, 2001). There are, however, still debates that have not been resolved concerning the accuracy of the new dates (Mundil, et al., 2001). It has recently been suggested by some scientists that the actual cause of the massive flood basalts was an impact of a giant extraterrestrial object which penetrated deep into the crust of that part of Siberia of the present (Jones et al., 2002). Doubt has been cast on such a model, however, and evidence has not been found that any volcanism on Earth, or on any other planet, was triggered by an impactor (Erwin et al., 2002; White, 2002).
Reading the environmental changes
According to Benton continuous rock sections, which are fossiliferous, through the Permian-Triassic crisis need to be studied in order to investigate the faunal, floral and environmental changes in more detail. There were not many such sections that were believed to exist in the late 1980s, and those that had been studied previously were believed to contain significant gaps at the crucial extinction interval. In the early 1990s reanalysis of these sections (Hallam & Wignall, 1997; Wignall & Hallam, 1992; Wignall & Twitchett, 1996; Wignall & Twitchett, 2002), resulted in the realisation that the records through the extinction event were much more complete than was previously believed.
There is a huge variety of fossil shells and skeletons in the rocks, which shows that in the latest Permian the seas teemed with life. The sediments, in particular, are intensely bioturbated, being full of burrows of a plethora of benthic animals living, feeding and moving through the sediment. The diverse communities were ecologically complex. The sediments that had been deposited immediately after the event in the earliest Triassic, in contrast, are dark-coloured, often black, and full of pyrite. To a large extent they don’t have burrows, and those that do occur are very small, and marine invertebrate fossils are extremely rare. In association with evidence from geochemistry it is suggested by these observations that a dramatic change had occurred in the oceanic conditions, from bottom waters that were well oxygenated to benthic anoxia that was widespread (Wignall & Twitchett, 1996; Wignall & Twitchett, 2002). The fauna of the ocean was differentiated into distinct biogeographical provinces that were recognisable prior to the catastrophe. A cosmopolitan fauna of bivalves with thin shell, that were opportunistic, such as the ‘paper pecten’ Claraia and the brachiopod Lingula, that was inarticulate, were spread throughout the world.
In the latest Permian life was also very diverse on land. Amphibian and reptile terrestrial faunas had reached a high degree of complexity, which Benton says were arguably as complex as modern mammalian communities (Benton, 2003: Retallack, 1999), with 4 to 5 trophic levels among carnivorous forms. Thick-skinned herbivores the size of the rhinoceros were the prey of the sabre-toothed gorgonopsians, and there were several ranks of smaller carnivores that preyed upon smaller animals. A diversity of habitats was provided by many plant groups (Retallack, 1999), with some floras being endemic, which is an indication of geographical differentiation relating to climatic zones. In South Africa (Smith & Ward, 2001), where it is indicated that the loss of taxa was rapid, the decline and loss of tetrapods has been documented in some detail. It is suggested that the timing of the loss of species on land with the loss of species in the ocean were coincident (Twitchett et al., 2001; Looy et al., 2001). (See the fungal spike).
Additional clues about the nature of environmental changes are given by geochemistry. There is a dramatic shift in δ18O ratio value of about 6 ppt (‰), which corresponds to a global temperature rise of about 6oC. It has been shown by climate modellers how circulation, as well as the amount of oxygen dissolved in the ocean, can be reduced by global warming shift to benthic anoxia (Hotinski et al., 2001). The types of sediment and ancient soil that was deposited on land also reflects the dramatic global rise in temperature (Retallack, 1999).
The runaway greenhouse
Is it possible for the evidence for oceanic anoxia, global warming, a catastrophic diversity and abundance reduction of life to be linked to the co-occurrence of Siberian flood basalt eruption in a coherent model of extinction? Further study of carbon isotopes may provide the key (see Carbon Isotope shifts). A sharp negative excursion during the Permian-Triassic interval is shown by the values of δ13C, which declined from a value of +2 to +4 ppt to -2 ppt at the level of the mass extinction (Hallam & Wignall, 1997; Wignall, 2001; Erwin et al., 2002; Wignall & Twitchett, 1996; Wignall & Twitchett, 2002). A dramatic increase in the light carbon isotope (12C) is implied by this drop, and atmospheric modellers and geologists have had difficulty identifying its source. According to Benton there are 3 things that occurred at the Permian-Triassic boundary:
1. the catastrophic destruction of life on Earth,
2. the subsequent flushing of 12C into the ocean,
3. the amount of 12C estimated to have reached the atmosphere from the eruption of flood basalt from the Siberian Traps.
None of these are sufficient to explain the observed shift (see Carbon isotope shifts). There is still something else that is required.
According to Benton a new source of 12C must be identified that is capable of overwhelming normal feedback systems of the atmosphere. The only option that has been identified so far is the release of methane from gas hydrates (see Carbon isotope shifts) an idea that has been accepted with alacrity (Wignall, 2001; Erwin et al. 2002; White, 2002; Berner, 2002).
The assumption is that the Siberian flood basalt eruptions caused the bodies of frozen gas hydrates to melt which released massive volumes of methane, that was rich in 12C, to rise to the surface of the oceans as huge bubbles, and it was this that triggered the initial global warming at the Permian-Triassic boundary. As a result of vast input pf methane into the atmosphere it caused more warming, and it was this that could have melted further gas hydrate reservoirs. This process continued in a positive feedback spiral known as the ‘runaway greenhouse’ phenomenon. Benton suggests that it is possible some sort of threshold, that was beyond where the natural systems that would normally have reduced atmospheric carbon dioxide levels can no longer operate effectively. The biggest crash in the history of life resulted as the system spiralled out of control.
Conclusions and perspectives
At this time 251 Ma life came close to being completely annihilation. About 5 % of species survived and an understanding of how these few taxa were able to recover from the most severe of evolutionary bottlenecks (Raup, 1979) is crucial to gaining an understanding of subsequent evolution of the biosphere. Global biodiversity at the family level took 100 My to return to pre-extinction levels (Hallam % Wignall, 1997). Ecological recovery was, however, somewhat more rapid, with complex communities such as reefs being re-established by the Middle of the Triassic, about 10 My after the Permian-Triassic boundary.
There are only 2 sites where it is known that details of the recovery of the marine system in the aftermath of the extinction event exist, northern Italy (Twitchett, 1999) and the western US (Sepkoski, 1992), both of which were located in tropical regions during the Early Triassic. Small, epifaunal suspension feeders, that were opportunists, were living in conditions of subtropical environments, with low oxygen and low food supply. Much of the seafloor was covered by microbial mats. There were low numbers of infauna of small vermiform animals that were deposit feeders, which burrowed feebly just below the surface of the sediment. This apparently lasted for many millions of years. When benthic oxygen restrictions eased and there was increasing food supply, larger communities that were more diverse arose slowly. As crinoids and bryozoans returned, beginning to rise upwards into the water column, epifaunal communities increased in complexity (Twitchett, 1999; Schubert & Bottjer, 1995). Infaunal communities saw the return of suspension feeders, and eventually crustaceans, the burrows increased in size and depth, back to the pre-crisis levels by the Middle Triassic (Twitchett, 1999).
At present not much is known of the recovery pattern from other parts of the ocean, though work is continuing. The herbivorous Lystrosaurus was virtually the only tetrapod on land for millions of years, which survived on the herbaceous plants that had survived. Until the Middle Triassic there were no forest communities (Looy et al., 2001). It seems clear that in the ‘post-apocalyptic greenhouse’ life was not easy (Retallack, 1999).
The runaway greenhouse would be a model that is worth exploring further, if it is correct, and possibly explains the biggest crisis on Earth in the past 500 My. It apparently indicates that there was a breakdown in global environmental mechanisms, in which normal systems that equilibrate the gases and temperatures in the atmosphere took hundreds of thousands of years to come into play. Benton suggests that a possible cause of other extinction events was a combination of global warming and anoxia from the release of gas hydrates. This scenario has recently been postulated for the mass extinction event as the close of the Triassic (Sephton, 2002) and smaller events in the Early Jurassic (Hesselbo et al., 2000), Cretaceous (Jahren & Arens, 1998), and the Tertiary (Dickens et al., 1995).
The current debate about global warming and possible medium-term consequences of it, are affected by models of ancient extinction events.
During the Late Permian major losses were suffered by many animal groups. Some groups such as the Fusulinid foraminifera disappeared completely, though much lower levels of extinction were suffered by other foram groups. Among groups that became extinct were the Rugosa and Tabulata corals from the Palaeozoic. Near complete extinction was suffered by the Stenolaemate bryozoans and articulate brachiopods. Severe bottlenecks were experienced by all of the extant echinoderm groups at this time, with only 2 lineages of crinoids and echinoids that survived the mass extinction event to be present in the Mesozoic. There were several groups of echinoderms, such as Blastoidea, that underwent complete extinction. Dominant and ecologically important groups had major losses which led to the collapse of many biological communities. Many millions of years were to pass before complex communities reappeared, in the oceans and on land (Twitchett et al., 2001; Looy et al., 2001).
It is difficult to estimate the severity of earlier mass extinction events. When long-term biodiversity changes are discussed by palaeontologists the focus is on genera or families because at the species level preservation becomes patchier and true biological species are hard to recognise from fossil remains. Estimates of losses during the Permian-Triassic event, based on 2 databases of family diversity over time (Sepkoski Jr, 1992; Benton, 1993) are 49% (Sepkoski Jr, 1996) or 48.6% (Benton, 1995) of families of marine animals, 62.9 % of continental organisms (Benton, 1995), and 60 % of all life (Benton, 1995).
At lower taxonomic levels the level of extinction was then estimated by Raup (Raup, 1979) by use of the rarefaction technique (McKinney, 1995). This is based on the intuitive idea that the loss of 50 % of families must equate to the loss of a much higher proportion of species: Every species in a family must go extinct for the family to go extinct. If 50% of families are lost it must mean that the remaining families are also hit hard, though if as few as a single species of 100 species in that family survives, that family is said to have survived. It is estimated from this method that 96 % of marine species were lost during the end-Permian extinction event (Raup, 1979). This calculation assumes, however, random extinction of species across all families, i.e. no selectivity against certain groups, which is not true (Benton, 1995). According to Benton the rarefaction technique might overestimate levels of species extinction by 10-15%, therefore the actual magnitude of the end-Permian event might be closer to 80% loss of species.
The Fungal spike
Fern spores and not much else were contained in terrestrial sediments from North America immediately following the impact event of the end-Cretaceous (Vajda, 2001). This ‘fern spike’ was interpreted as the initial stages of colonisation of a land surface, which was left barren after its vegetation was removed by the impact of the asteroid and the wildfires that followed it. Following eruption of volcanoes of the present similar pioneering communities of ferns are observed to begin appearing soon after the lava and ash deposited by the volcano have cooled sufficiently.
At the Permian-Triassic boundary a similar spike has been found: though it is a fungal spike instead of a fern spike (Visscher &Brugman, 1998). Fungal remains have been shown to comprise 10% of the pollen and spores just below and just above the extinction horizon by a study (Eshet et al., 1995) on sections in northern Italy and Israel, and increasing to almost 100 % of the assemblage at the extinction level. These fungi have been interpreted as terrestrial species representing the survivors of the catastrophic extinction event of standing vegetation and a sudden surge of decomposers responding to the large amount of dead plant material that had accumulated.
Not all authors accept this interpretation. Some question the interpretation that they were truly terrestrial species as they are found only in shallow marine deposits (Wignall et al., 1996). It has been suggested by others that what appears to be a predominance of fungi could be an artefact of preservation, because the hyphae of fungi are tougher than plant tissues and therefore are more likely to survive in the environment for longer (Erwin, 1993).
A detailed study in Greenland of terrestrial vegetation through a complete Permian-Triassic section that was preserved very well, no fungal spike was found (Looy et al., 2001). The fungi that were present were always in low abundance. There were, however, spikes of other types of vegetation. Spores of heterosporous lycopsids, especially Selaginellales, briefly increased in abundance just after the collapse of the diverse woody gymnosperms of the Late Permian. As was the case of ferns following the end-Cretaceous event, these groups were opportunistic pioneers. The floral response to the Permian-Triassic event is, however, more complicated, as different groups, fungi and lycopsids responded differently in different regions.
Carbon isotope shifts
In the study of mass extinction events an important tool is measuring the ratio of stable isotopes 13C and 12C in geological specimens, such as limestone and fossil shells. 12C is the most common form of carbon in nature, though there are also minor, though measureable amounts of 13C. In the atmosphere the ratio of these 2 isotopes is the same as the ratios of them in the ocean surface waters. Plants take up 12C preferentially during photosynthesis to produce organic matter. If this organic matter is buried instead of being returned to the atmosphere the 12C:13C ratio will shift in favour of 13C, the heavier isotope. This ratio is conventionally expressed as δ13C, which is the difference between the 12C:13C in the tested sample and in a known standard, which is a belemnite fossil from the Pee Dee Formation in South Carolina that dates to the Cretaceous.
In the ocean system large amounts of organic matter are fixed at the surface during periods of high surface productivity, which makes the surface waters relatively enriched in 13C. Carbonate deposits are precipitated in shallow water from this seawater and record the 12C:13C ratio of the seawater without any preferential uptake of either isotope. Carbonates from shallow water therefore record a positive shift in the δ13C during times of high surface productivity.
A negative shift in δ13C, which is recorded in the carbonate deposits of all geological sections that have been studied so far, characterises the Permian-Triassic interval (Magaritz, et al., 1988; Sephton et al., 2002), including those on land (Retallack, 1995; MacLeod et al., 2000). Apparently, this should imply that there is massive decrease of biological production, as well as the burial of organic matter.
This picture is, however, more complicated than this. Initially, there is a short, sharp negative shift in δ13C that is almost synchronous with the actual extinction horizon. Between sections there is a variable amount of negative swing, though it is typically 4-6‰ (Twitchett et al., 2001; Magaritz et al., 1988; Sephton et al., 2002; Retallack, 1995; MacLeod et al., 2000). A swing back towards the heavier end of the scale follows in most sections. The δ13C values never swing back to pre-extinction levels, however, instead remaining higher by about 0.5-1.5‰. Low productivity in the aftermath of the extinction can explain this difference that is relatively small. The initial swing, that is shorter and sharper, needs another explanation.
The amount of negative swing, 4-6‰, has been shown by calculations to be too great to be explained by a lack of biological production (Wignall, 2001). According to Benton there is a requirement of an input of light carbon into the ocean system. It has been shown by calculations that carbon output from the Siberian Traps alone could not cause the shift in δ13C that occurred, as the carbon dioxide emitted by volcanoes has a δ13C signature of - 5‰ (Wignall, 2001). Only 20% of the isotope shift that is required would be produced even if all life had been extinguished instantaneously and the biomass that resulted was incorporated into the sediments. The methane that is trapped in gas hydrates is the only viable source of light carbon which has a δ13C signature of - 65‰ (Dickens et al., 1997). Enough methane would be released to cause the shift that is observed if these gas hydrates melted.
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