Mass extinction 1: Ordovician-Silurian

The Ordovician–Silurian extinction events, also known as the Late Ordovician mass extinction (LOME), are collectively the second-largest of the five major extinction events in Earth's history in terms of percentage of genera that became extinct. Extinction was global during this period, eliminating 49–60% of marine genera and nearly 85% of marine species. Only the Permian-Triassic mass extinction exceeds the LOME in total biodiversity loss. The extinction event abruptly affected all major taxonomic groups and caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, echinoderms, corals, bivalves, and graptolites. This extinction was the first of the "big five" Phanerozoic mass extinction events and was the first to significantly affect animal-based communities. However, the LOME did not produce major changes to ecosystem structures compared to other mass extinctions, nor did it lead to any particular morphological innovations. Diversity gradually recovered to pre-extinction levels over the first 5 million years of the Silurian period.

The Late Ordovician mass extinction is generally considered to occur in two distinct pulses. The first pulse began at the boundary between the Katian and Hirnantian stages of the Late Ordovician Period. This extinction pulse is typically attributed to the Late Ordovician glaciation, which abruptly expanded over Gondwana at the beginning of the Hirnantian and shifted the earth from a greenhouse to icehouse climate. Cooling and a falling sea level brought on by the glaciation led to habitat loss for many organisms along the continental shelves, especially endemic taxa with restricted temperature tolerance. During this extinction pulse there were also several marked changes in biologically responsive carbon and oxygen isotopes. Marine life partially rediversified during the cold period and a new cold-water ecosystem, the "Hirnantia biota", was established.

The second pulse of extinction occurred in the later half of the Hirnantian as the glaciation abruptly recedes and warm conditions return. The second pulse is associated with intense worldwide anoxia (oxygen depletion) and euxinia (toxic sulfide production), which persist into the subsequent Rhuddanian stage of the Silurian Period.

Impact on life
The extinction followed the Great Ordovician Biodiversification Event, one of the largest evolutionary surges in the geological and biological history of the Earth.

At the time of the extinction, most complex multicellular organisms lived in the sea, and around 100 marine families became extinct, covering about 49% of faunal genera (a more reliable estimate than species). The brachiopods and bryozoans were decimated, along with many of the trilobite, conodont and graptolite families. Each extinction pulse affected different groups of animals and was followed by a rediversification event. Statistical analysis of marine losses at this time suggests that the decrease in diversity was mainly caused by a sharp increase in extinctions, rather than a decrease in speciation.

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. The Late Ordovician mass extinction had few of the long-term ecological impacts associated with the Permian–Triassic and Cretaceous–Paleogene extinction events. 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. Cambrian-type fauna such as trilobites and inarticulate brachiopods never recovered their pre-extinction diversity.

Trilobites were hit hard by both phases of the extinction, with about 70% of genera 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.Some geologists have argued that anoxia played a role in the first extinction pulse, though this hypothesis is controversial. In the early Hirnantian, shallow-water sediments throughout the world experience a large positive excursion in the δ34S ratio of buried pyrite. This ratio indicates that shallow-water pyrite which formed at the beginning of the glaciation had a decreased proportion of 32S, a common lightweight isotope of sulfur. 32S in the seawater could hypothetically be used up by extensive deep-sea pyrite deposition. 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. As a result, anoxia and euxinia would need to be common in the deep sea to produce enough pyrite to shift the δ34S ratio.

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. However, higher FeHR/FeT values are known from a few deep-water early Hirnantian sequences found in Nevada and China.

Glaciation could conceivably trigger anoxic conditions, albeit indirectly. If continental shelves are exposed by falling sea levels, then organic surface runoff flows into deeper oceanic basins. The organic matter would have more time to leach out phosphate and other nutrients before being deposited on the seabed. Increased phosphate concentration in the seawater would lead to eutrophication and then anoxia. Deep-water anoxia and euxinia would impact deep-water benthic fauna, as expected for the first pulse of extinction. Chemical cycle disturbances would also steepen the chemocline, restricting the habitable zone of planktonic fauna which also go extinct in the first pulse. This scenario is congruent with both organic carbon isotope excursions and general extinction patterns observed in the first pulse.

However, data supporting deep-water anoxia during the glaciation contrasts with more extensive evidence for well-oxygenated waters. Black shales, which are indicative of an anoxic environment, become very rare in the early Hirnantian compared to surrounding time periods. Although early Hirnantian black shales can be found in a few isolated ocean basins (such as the Yangtze platform of China), from a worldwide perspective these correspond to local events. Some Chinese sections record an early Hirnantian increase in the abundance of Mo-98, a heavy isotope of molybdenum. This shift can correspond to a balance between minor local anoxia 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.

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.

A few studies have proposed that the first extinction pulse did not begin with the Hirnantian glaciation, but instead corresponds to an interglacial period or other warming event. Anoxia would be the most likely mechanism of extinction in a warming event, as evidenced by other extinctions involving warming. However, this view of the first extinction pulse is controversial and not widely accepted.

Metal poisoning[edit]
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.

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[edit]
Some scientists have suggested that the initial extinctions could have been caused by a gamma-ray burst originating from a hypernova in a nearby arm of the Milky Way galaxy, within 6,000 light-years of Earth. A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, exposing surface-dwelling organisms, including those responsible for planetary photosynthesis, to high levels of extreme ultraviolet radiation. Under this hypothesis, several groups of marine organisms with a planktonic lifestyle were more exposed to UV radiation than groups that lived on the seabed. 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. A gamma-ray burst could also explain the rapid onset of glaciation, since ozone and nitrogen would react to form nitrogen dioxide, a darkly-colored aerosol which cools the earth. 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.

Volcanism and weathering[edit]
The late Ordovician glaciation was preceded by a fall in atmospheric carbon dioxide (from 7,000 ppm to 4,400 ppm). The dip is correlated with a burst of volcanic activity that deposited new silicate rocks, which draw CO2 out of the air as they erode. A major role of CO2 is implied by a 2009 paper. Atmospheric and oceanic CO2 levels may have fluctuated with the growth and decay of Gondwanan glaciation. Through the Late Ordovician, outgassing from major volcanism was balanced by heavy weathering of the uplifting Appalachian Mountains, which sequestered CO2. In the Hirnantian Stage the volcanism ceased, and the continued weathering caused a significant and rapid draw down of CO2. This coincides with the rapid and short ice age.

The appearance and development of terrestrial plants and microphytoplankton, which consumed atmospheric carbon dioxide, thus, diminishing the greenhouse effect and promoting the transition of the climatic system to the glacial mode, played a unique role in that period. During this extinction event there were also several marked changes in biologically responsive carbon and oxygen isotopes.

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.

Metal poisoning[edit]
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.

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[edit]
Some scientists have suggested that the initial extinctions could have been caused by a gamma-ray burst originating from a hypernova in a nearby arm of the Milky Way galaxy, within 6,000 light-years of Earth. A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, exposing surface-dwelling organisms, including those responsible for planetary photosynthesis, to high levels of extreme ultraviolet radiation. Under this hypothesis, several groups of marine organisms with a planktonic lifestyle were more exposed to UV radiation than groups that lived on the seabed. 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. A gamma-ray burst could also explain the rapid onset of glaciation, since ozone and nitrogen would react to form nitrogen dioxide, a darkly-colored aerosol which cools the earth. 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.

Volcanism and weathering[edit]
The late Ordovician glaciation was preceded by a fall in atmospheric carbon dioxide (from 7,000 ppm to 4,400 ppm). The dip is correlated with a burst of volcanic activity that deposited new silicate rocks, which draw CO2 out of the air as they erode. A major role of CO2 is implied by a 2009 paper. Atmospheric and oceanic CO2 levels may have fluctuated with the growth and decay of Gondwanan glaciation. Through the Late Ordovician, outgassing from major volcanism was balanced by heavy weathering of the uplifting Appalachian Mountains, which sequestered CO2. In the Hirnantian Stage the volcanism ceased, and the continued weathering caused a significant and rapid draw down of CO2. This coincides with the rapid and short ice age.

The appearance and development of terrestrial plants and microphytoplankton, which consumed atmospheric carbon dioxide, thus, diminishing the greenhouse effect and promoting the transition of the climatic system to the glacial mode, played a unique role in that period. During this extinction event there were also several marked changes in biologically responsive carbon and oxygen isotopes.

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.