Late Ordovician mass extinction

Ecological impacts The Late Ordovician mass extinction followed the Great Ordovician Biodiversification Event (GOBE), one of the largest surges of increasing biodiversity in the geological and biological history of the Earth. At the time of the extinction, most complex multicellular organisms lived in the sea, and the only evidence of life on land are rare spores from small early land plants. At the time of the extinction, around 100 marine families became extinct, covering about 49% of genera (a more reliable estimate than species). The brachiopods and bryozoans were strongly impacted, along with many of the trilobite, conodont and graptolite families. Following such a major loss of diversity, Silurian communities were initially less complex and broader niched. Highly endemic faunas, which characterized the Late Ordovician, were replaced by faunas that were amongst the most cosmopolitan in the Phanerozoic, biogeographic patterns that persisted throughout most of the Silurian. Nevertheless, a large number of taxa disappeared from the Earth over a short time interval, eliminating and altering the relative diversity and abundance of certain groups. Additionally, brachiopods with higher abundance were more likely to survive. The extinction pulse at the end of the Katian was selective in its effects, disproportionally affecting deep-water species and tropical endemics inhabiting epicontinental seas. The Foliomena fauna, an assemblage of thin-shelled species adapted for deep dysoxic (low oxygen) waters, went extinct completely in the first extinction pulse. During the glaciation, a high-latitude brachiopod assemblage, the Hirnantia fauna, established itself along outer shelf environments in lower latitudes, probably in response to cooling. However, the Hirnantia fauna would meet its demise in the second extinction pulse, replaced by Silurian-style assemblages adapted for warmer waters. Overall, the brachiopod recovery in the late Rhuddanian was rapid. Brachiopod survivors of the mass extinction tended to be endemic to one palaeoplate or even one locality in the survival interval in the earliest Silurian, though their ranges geographically expanded over the course of the biotic recovery. The region around what is today Oslo was a hotbed of atrypide rediversification. Brachiopod recovery consisted mainly of the reestablishment of cosmopolitan brachiopod taxa from the Late Ordovician. Progenitor taxa that arose following the mass extinction displayed numerous novel adaptations for resisting environmental stresses. Although some brachiopods did experience the Lilliput effect in response to the extinction, this phenomenon was not particularly widespread compared to other mass extinctions. Trilobites Trilobites were hit hard by both phases of the extinction, with about 70% of genera and 50% of families going extinct between the Katian and Silurian. The extinction disproportionately affected deep water species and groups with fully planktonic larvae or adults. The order Agnostida was completely wiped out, and the formerly diverse Asaphida survived with only a single genus, Raphiophorus. A cool-water trilobite assemblage, the Mucronaspis fauna, coincides with the Hirnantia brachiopod fauna in the timing of its expansion and demise. Bryozoans Over a third of bryozoan genera went extinct, but most families survived the extinction interval and the group as a whole recovered in the Silurian. The hardest-hit subgroups were the cryptostomes and trepostomes, which never recovered the full extent of their Ordovician diversity. Bryozoan extinctions started in coastal regions of Laurentia, before high extinction rates shifted to Baltica by the end of the Hirnantian. Bryozoan biodiversity loss appears to have been a prolonged process which partially preceded the Hirnantian extinction pulses. Extinction rates among Ordovician bryozoan genera were actually higher in the early and late Katian, and origination rates sharply dropped in the late Katian and Hirnantian. Echinoderms About 70% of crinoid genera died out. Early studies of crinoid biodiversity loss by Jack Sepkoski overestimated crinoid biodiversity losses during LOME. Most extinctions occurred in the first pulse. However, they rediversified quickly in tropical areas and reacquired their pre-extinction diversity not long into the Silurian. Many other echinoderms became very rare after the Ordovician, such as the cystoids, edrioasteroids and other early crinoid-like groups. A change in abundance is recorded, however; clathrodictyids increased in abundance relative to labechiids. Sponges thrived and dominated marine ecosystems in South China immediately after the extinction event, colonising depauperate, anoxic environments in the earliest Rhuddanian. Their pervasiveness in marine environments after the biotic crisis has been attributed to drastically decreased competition and an abundance of vacant niches left behind by organisms that perished in the catastrophe. Sponges may have assisted the recovery of other sessile suspension feeders: by helping stabilise sediment surfaces, they enabled bryozoans, brachiopods and corals to recolonise the seafloor. ==Glaciation and cooling==

The first pulse of the Late Ordovician Extinction has typically been attributed to the Late Ordovician glaciation, which is unusual among mass extinctions and has made LOME an outlier. Although there was a longer cooling trend in Middle and Lower Ordovician, the most severe and abrupt period of glaciation occurred in the Hirnantian stage, which was bracketed by both pulses of the extinction. The rapid continental glaciation was centered on Gondwana, which was located at the South Pole in the Late Ordovician. The Hirnantian glaciation is considered one of the most severe ice ages of the Paleozoic, which previously maintained the relatively warm climate conditions of a greenhouse earth. Atmospheric and oceanic CO2 levels may have fluctuated with the growth and decay of Gondwanan glaciation. The appearance and development of terrestrial plants and microphytoplankton, which consumed atmospheric carbon dioxide, may have diminished the greenhouse effect and promoting the transition of the climatic system to the glacial mode. Heavy silicate weathering of the uplifting Appalachians and Caledonides occurred during the Late Ordovician, which sequestered CO2. and the continued weathering caused a significant and rapid draw down of CO2 coincident with the rapid and short ice age. A hypothetical large igneous province emplaced during the Katian whose existence is unproven has been speculated to have been the sink that absorbed carbon dioxide and precipitated Hirnantian cooling. Alternatively, volcanic activity may have caused the cooling by supplying sulphur aerosols to the atmosphere and generating severe volcanic winters that triggered a runaway ice-albedo positive feedback loop. In addition, volcanic fertilisation of the oceans with phosphorus may have increased populations of photosynthetic algae and enhanced biological sequestration of carbon dioxide from the atmosphere. Increased burial of organic carbon is another method of drawing down carbon dioxide from the air that may have played a role in the Late Ordovician. Other studies point to an asteroid strike and impact winter as the culprit for the glaciation. Other studies have even suggested that shading of the sun's rays by a temporary planetary ring formed from the partial breakup of a large meteor in the atmosphere may have caused the glaciation, which would also link it to the Ordovician meteor event. Two environmental changes associated with the glaciation were responsible for much of the Late Ordovician extinction. First, the cooling global climate was probably especially detrimental because the biota were adapted to an intense greenhouse, especially because most shallow sea habitats in the Ordovician were located in the tropics. The southward shift of the polar front severely contracted the available latitudinal range of warm-adapted organisms. Second, sea level decline, caused by sequestering of water in the ice cap, drained the vast epicontinental seaways and eliminated the habitat of many endemic communities. The dispersed positions of the continents, in contrast to their position during the much less extinction-inducing Pleistocene glaciations, made glacioeustatic marine regression especially hazardous to marine life. Falling sea levels may have acted as a positive feedback loop accelerating further cooling; as shallow seas receded, carbonate-shelf production declined and atmospheric carbon dioxide levels correspondingly decreased, fostering even more cooling. In the Yangtze Platform, a relict warm-water fauna continued to persist because South China blocked the transport of cold waters from Gondwanan waters at higher latitudes. This incurred a shift in the location of bottom water formation, shifting from low latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which was accompanied by increased deep-ocean currents and oxygenation of the bottom water. An opportunistic fauna briefly thrived there, before anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters. Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions. However, not all studies agree that cooling and glaciation caused LOMEI-1. One study suggests that the first pulse began not during the rapid Hirnantian ice cap expansion but in an interval of deglaciation following it. == Anoxia and euxinia ==

Another heavily-discussed factor in the Late Ordovician mass extinction is anoxia, the absence of dissolved oxygen in seawater. Anoxia is the most common culprit for the second pulse of the Late Ordovician mass extinction and is connected to many other mass extinctions throughout geological time. It may have also had a role in the first pulse of the Late Ordovician mass extinction, The Ordovician ocean also had very low levels of sulfate, a nutrient which would otherwise resupply 32S from the land. Pyrite forms most easily in anoxic and euxinic environments, while better oxygenation encourages the formation of gypsum instead. Thallium isotope ratios can also be used as indicators of anoxia. A major positive ε205Tl excursion in the late Katian, just before the Katian–Hirnantian boundary, likely reflects a global enlargement of oxygen minimum zones. During the late Katian, thallium isotopic perturbations indicating proliferation of anoxic waters notably preceded the appearance of other geochemical indicators of the expansion of anoxia. A more direct proxy for anoxic conditions is FeHR/FeT. This ratio describes the comparative abundance of highly reactive iron compounds which are only stable without oxygen. Most geological sections corresponding to the beginning of the Hirnantian glaciation have FeHR/FeT below 0.38, indicating oxygenated waters. and well-oxygenated waters on a global scale. Other trace elements point towards increased deep-sea oxygenation at the start of the glaciation. Oceanic current modelling suggest that glaciation would have encouraged oxygenation in most areas, apart from the Paleo-Tethys Ocean. Devastation of the Dicranograptidae-Diplograptidae-Orthograptidae (DDO) graptolite fauna, which was well adapted to anoxic conditions, further suggests that LOMEI-1 was associated with increased oxygenation of the water column and not the other way around. Deep-sea anoxia is not the only explanation for the δ34S excursion of pyrite. Carbonate-associated sulfate maintains high 32S levels, indicating that seawater in general did not experience 32S depletion during the glaciation. Even if pyrite burial did increase at that time, its chemical effects would have been far too slow to explain the rapid excursion or extinction pulse. Instead, cooling may lower the metabolism of warm-water aerobic bacteria, reducing decomposition of organic matter. Fresh organic matter would eventually sink down and supply nutrients to sulfate-reducing microbes living in the seabed. Sulfate-reducing microbes prioritize 32S during anaerobic respiration, leaving behind heavier isotopes. A bloom of sulfate-reducing microbes can quickly account for the δ34S excursion in marine sediments without a corresponding decrease in oxygen. Late Hirnantian anoxia The late Hirnantian experienced a dramatic increase in the abundance of black shales. Coinciding with the retreat of the Hirnantian glaciation, black shale expands out of isolated basins to become the dominant oceanic sediment at all latitudes and depths. The worldwide distribution of black shales in the late Hirnantian is indicative of a global anoxic event, δ98Mo, and εNd(t) excursions found in many different regions. At least in European sections, late Hirnantian anoxic waters were originally ferruginous (dominated by ferrous iron) before gradually becoming more euxinic. Mercury loading in South China during LOMEI-2 was likely related to euxinia. However, some evidence suggests that the top of the water column in the Ordovician oceans remained well oxygenated even as the seafloor became deoxygenated. On a global scale, euxinia was probably one or two orders of magnitude more prevalent than in the modern day. Global anoxia may have lasted more than 3 million years, persisting through the entire Rhuddanian stage of the Silurian period. This would make the Hirnantian–Rhuddanian anoxia one of the longest-lasting anoxic events in geologic time. At Anticosti Island, a uranium isotope excursion consistent with anoxia actually occurs prior to indicators of receding glaciation. This may suggest that the Hirnantian–Rhuddanian anoxic event (and its corresponding extinction) began during the glaciation, not after it. Cool temperatures can lead to upwelling, cycling nutrients into productive surface waters via air and ocean cycles. There were few clear patterns of extinction associated with the second extinction pulse. Every region and marine environment experienced the second extinction pulse to some extent. Many taxa which survived or diversified after the first pulse were finished off in the second pulse. These include the Hirnantia brachiopod fauna and Mucronaspis trilobite fauna, which previously thrived in the cold glacial period. Other taxa such as graptolites and warm-water reef denizens were less affected. Although this suggests that the second extinction pulse may have been a minor event at best, other paleontologists maintain that an abrupt ecological turnover accompanied the end of glaciation. There may be a correlation between the relatively slow recovery after the second extinction pulse, and the prolonged nature of the anoxic event which accompanied it. Early Rhuddanian anoxia Deposition of black graptolite shales continued to be common into the earliest Rhuddanian, indicating that anoxia persisted well into the Llandovery. A sharp reduction in the average size of many organisms, likely attributable to the Lilliput effect, and the disappearance of many relict taxa from the Ordovician indicate a third extinction interval linked to an expansion of anoxic conditions into shallower shelf environments, particularly in Baltica. This sharp decline in dissolved oxygen concentrations was likely linked to a period of global warming documented by a negative carbon isotope excursion preserved in Baltican sediments. == Other potential factors ==

Metal poisoning Toxic metals on the ocean floor may have dissolved into the water when the oceans' oxygen was depleted. An increase in available nutrients in the oceans may have been a factor, and decreased ocean circulation caused by global cooling may also have been a factor. Hg/TOC values from the Peri-Baltic region indicate noticeable spikes in mercury concentrations during the lower late Katian, the Katian–Hirnantian boundary, and the late Hirnantian. The toxic metals may have killed life forms in lower trophic levels of the food chain, causing a decline in population, and subsequently resulting in starvation for the dependent higher feeding life forms in the chain. Gamma-ray burst A minority hypothesis to explain the first burst has been proposed by Philip Ball, Adrian Lewis Melott and Brian C. Thomas, Under this hypothesis, several groups of marine organisms with a planktonic lifestyle were more exposed to more UV radiation than groups that lived on the seabed. It is estimated that 20% to 60% of the total phytoplankton biomass on Earth would have been killed in such an event because the oceans were mostly oligotrophic and clear during the Late Ordovician. This is consistent with observations that planktonic organisms suffered severely during the first extinction pulse. In addition, species dwelling in shallow water were more likely to become extinct than species dwelling in deep water, also consistent with the hypothetical effects of a galactic gamma-ray burst. A gamma-ray burst could also explain the rapid expansion of glaciers, since the high energy rays would cause ozone, a greenhouse gas, to dissociate and its dissociated oxygen atoms to then react with nitrogen to form nitrogen dioxide, a darkly coloured aerosol which cools the planet. It would also cohere with the major δ13C isotopic excursion indicating increased sequestration of carbon-12 out of the atmosphere, which would have occurred as a result of the nitrogen dioxide reacting with hydroxyl and raining back down to Earth as nitric acid, precipitating large quantities of nitrates that would have enhanced wetland productivity and sequestration of carbon dioxide. Although the gamma-ray burst hypothesis is consistent with some patterns at the onset of extinction, there is no unambiguous evidence that such a nearby gamma-ray burst ever happened. More recently, in May 2020, a study suggested the first pulse of mass extinction was caused by volcanism which induced global warming and anoxia, rather than cooling and glaciation. Higher resolution of species diversity patterns in the Late Ordovician suggest that extinction rates rose significantly in the early or middle Katian stage, several million years earlier than the Hirnantian glaciation. This early phase of extinction is associated with large igneous province (LIP) activity, possibly that of the Alborz LIP of northern Iran, as well as a warming phase known as the Boda event. However, other research still suggests the Boda event was a cooling event instead. Increased volcanic activity during the early late Katian and around the Katian–Hirnantian boundary is also implied by heightened mercury concentrations relative to total organic carbon. Volcanic activity could also provide a plausible explanation for anoxia during the first pulse of the mass extinction. A volcanic input of phosphorus, which was insufficient to enkindle persistent anoxia on its own, may have triggered a positive feedback loop of phosphorus recycling from marine sediments, sustaining widespread marine oxygen depletion over the course of LOMEI-1. Other papers have criticised the volcanism hypothesis, claiming that volcanic activity was relatively low in the Ordovician and that superplume and LIP volcanic activity is especially unlikely to have caused the mass extinction at the end of the Ordovician. Mercury anomalies at the end of the Ordovician relative to total organic carbon, or Hg/TOC, that some researchers have attributed to large-scale volcanism have been reinterpreted by some to be flawed because the main mercury host in the Ordovician was sulphide, and thus Hg/TS should be used instead; a finding bolstered by ∆199Hg measurements much higher than would be expected for volcanogenic mercury input. Asteroid impact A 2023 paper points to the Deniliquin multiple-ring feature in southeastern Australia, which has been dated to around the start of LOMEI-1, for initiating the intense Hirnantian glaciation and the first pulse of the extinction event. According to the paper, further research is necessary to evaluate the hypothesis. == See also ==