Late Ordovician mass extinction
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.
[6][12] 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 and latitudinal range.
The second pulse was associated with intense worldwide anoxia (oxygen depletion) and euxinia (toxic sulfide production), which persisted into the subsequent Rhuddanian stage of the Silurian Period.
[25] 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.
[29] 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.
[42] Sponges thrived and dominated marine ecosystems in South China immediately after the extinction event,[43] colonising depauperate, anoxic environments in the earliest Rhuddanian.
[44] 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.
[45] 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.
[47] 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 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.
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.
[54] 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.
[56] 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.
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.
[63] 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.
[17][52] It may have also had a role in the first pulse of the Late Ordovician mass extinction,[70] though support for this hypothesis is inconclusive and contradicts other evidence for high oxygen levels in seawater during the glaciation.
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.
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.
[70] In the Yangtze Sea, located on the western margins of the South China microcontinent, the second extinction pulse occurred alongside intense euxinia which spread out from the middle of the continental shelf.
Like most global anoxic events, an increased supply of nutrients (such as nitrates and phosphates) would encourage algal or microbial blooms that deplete oxygen levels in the seawater.
[86][17] On the other hand, the occurrence of euxinic pulses similar in magnitude to LOMEI-2 during the Katian without ensuing biological collapses has caused some researchers to question whether euxinia alone could have been LOMEI-2's driver.
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.
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.
[111] 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.
[112][84] 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.
[117] 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.
[100][92] Marine bentonite layers associated with the subduction of the Junggar Ocean underneath the Yili Block have been dated to the late Katian, close to the Katian-Hirnantian boundary.
[2] A 2022 study argued against a volcanic cause of LOME, citing the lack of mercury anomalies and the discordance between deposition of bentonites and redox changes in drillcores from South China straddling the Ordovician-Silurian boundary.
[121] 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.
