End-Ordovician mass extinction, faunal diversification, glaciation and true polar wander

Enormous events occurred between 460 and 435 Ma around the mid-point of the Palaeozoic Era and spanning the Ordovician-Silurian (O-S) boundary. At around 443 Ma the second-most severe mass extinction in Earth’s history occurred, which eliminated 50 to 60% of all marine genera and almost 85% of species: not much less than the Great Dying at the end of the Permian Period. The event was accompanied by one of the greatest biological diversifications known to palaeontology, which largely replaced the global biota initiated by the Cambrian Explosion. Centred on the Saharan region of northern Africa, Late Ordovician glacial deposits also occur in western South America and North America. At that time all the current southern continents and India were assembled in the Gondwana supercontinent, with continental masses that became North America, the Baltic region, Siberia and South China not far off: all the components that eventually collided to form Pangaea from the Late Silurian to the Carboniferous.

The mass extinction has troubled geologists for quite a while. There are few signs of major volcanism having been involved, although some geochemists have suggested that very high mercury concentrations in some Late Ordovician marine sediments bear witness to large, albeit invisible, igneous events. No large impact crater is known from those times, although there is a curious superabundance of extraterrestrial debris, including high helium-3, chromium and iridium concentrations, preserved in earlier Ordovician sedimentary rocks, around the Baltic Sea. Another suggestion, poorly supported by evidence, is destruction of the atmospheric ozone layer by a gamma-ray burst from some distant but stupendous supernova. A better supported idea is that the oceans around the time of the event lacked oxygen. Such anoxia can encourage solution of toxic metals and hydrogen sulfide gas. Unlike other mass extinctions, this one was long-drawn out with several pulses.

The glacial epoch also seems implicated somehow in the mass die-off, being the only one known to coincide with a mass extinction. It included spells of frigidity that exceeded those of the last Pleistocene glacial maximum, with the main ice cap having a volume of from 50 to 250 million cubic kilometres. The greatest of these, around 445 Ma, involved a 5°C fall in global sea-surface temperatures and a large negative spike in δ13C in carbon-rich sediments, both of which lasted for about a million years. The complex events around that time coincided with the highest ever extinction and speciation rates, the number of marine species being halved in a short space of time: a possible explanation for the δ13 C anomaly. Yet estimates of atmospheric CO2 concentration in the Late Ordovician suggests it was perhaps 8–16 times higher than today; Earth should have been a warm planet then. One probable contributor to extreme glacial conditions has been suggested to be that the South Pole at that time was well within Gondwana and thus isolated from the warming effect of the ocean. So, severe glaciation and a paradoxical combination of mass extinction with considerable biological diversification present quite an enigma.

A group of scientists based in Beijing, China set out to check the palaeogeographic position of South China between 460 and 435 Ma and evaluate those in  O-S sediments at locations on 6 present continents (Jing, X., Yang, Z., Mitchell, R.N. et al. 2022. Ordovician–Silurian true polar wander as a mechanism for severe glaciation and mass extinction. Nature Communications, v. 13, article 7941; DOI: 10.1038/s41467-022-35609-3). Their key tool is determining the position of the magnetic poles present at various times in the past from core samples drilled at different levels in these sedimentary sequences. The team aimed to test a hypothesis that in O-S times not only the entire lithosphere but the entire mantle moved relative to the Earth’s axis of rotation, the ‘slippage’ probably being at the Core-mantle boundary [thanks to Steve Rozario for pointing this out]. Such a ‘true polar wander’ spanning 20° over a mere  2 Ma has been detected during the Cretaceous, another case of a 90° shift over 15 Ma may have occurred at the time when Snowball Earth conditions first appeared in the Neoproterozoic around the time when the Rodinia supercontinent broke up and a similar event was proposed in 1994 for C-O times albeit based on sparse and roughly dated palaeomagnetic pole positions.

Xianqing Jing and colleagues report a wholesale 50° rotation of the lithosphere between 450 and 440 Ma that would have involved speeds of about 55 cm per year. It involved the Gondwana supercontinent and other continental masses still isolated from it moving synchronously in the same direction, as shown in the figure. From 460 to 450 Ma the geographic South Pole lay at the centre of the present Sahara. At 445 Ma its position had shifted to central Gondwana during the glacial period. By 440 Gondwana had moved further northwards so that the South Pole then lay at Gondwana’s southernmost extremity.

Palaeogeographic reconstructions charting true polar wander and the synchronised movement of all continental masses between 460 and 440 Ma. Note the changes in the trajectories of lines of latitude on the Mollweide projections. The grey band either side of the palaeo-Equator marks intense chemical weathering in the humid tropics. Credit Jing et al. Fig 5.

As well as a possible key to the brief but extreme glacial episode this astonishing journey by a vast area of lithosphere may help account for the mass extinction with rapid speciation and diversification associated with the O-S boundary. While the South Pole was traversing Gondwana as the supercontinent shifted the ‘satellite’ continental masses remained in or close to the humid tropics, exposed to silicate weathering and erosion. That is a means for extracting CO2 from the atmosphere and launching global cooling, eventually to result in glaciation over a huge tract of Gondwana around 445 Ma. Gondwana then moved rapidly into more clement climatic zones and was deglaciated a few million years later. The rapid movement of the most faunally diverse continental-shelf seas through different climate zones would have condemned earlier species to extinction simultaneous adaptation to changed conditions could have encouraged the appearance of new species and ecosystems. This does not require the catastrophic mechanisms largely established for the other mass extinction events. It seems that during the stupendous, en masse slippage of the Earth’s lithosphere plate tectonic processes still continued, yet it must have had a dynamic effect throughout the underlying mantle.

Yet the fascinating story does have a weak point. What if the position of the magnetic poles shifted during O-S times from their assumed rough coincidence with the geographic poles? In other words, did the self-exciting dynamo in the liquid outer core undergo a large and lengthy wobble? How the outer core’s circulation behaves depends on its depth to the solid core, yet the inner core seems only to have begun solidifying just before the onset of the Cambrian, about 100 Ma before the O-S events. It grew rapidly during the Palaeozoic, so the thickness of the outer core was continuously increasing. Fluid dynamic suggests that the form of its circulation may also have undergone changes, thereby affecting the shape and position of the geomagnetic field: perhaps even shifting its poles away from the geographic poles …

Ordovician ice age: an extraterrestrial trigger

The Ordovician Period is notable for three global events; an explosion in biological diversity; an ice age, and a mass extinction. The first, colloquially known as the Great Ordovician Biodiversification Event, occurred in the Middle Ordovician around 470 Ma ago (see The Great Ordovician Diversification, September 2008) when the number of recorded fossil families tripled. In the case of brachiopods, this seems to have happened in no more than a few hundred thousand years. The glacial episode spanned the period from 460 to 440 Ma and left tillites in South America, Arabia and, most extensively, in Africa. Palaeogeographic reconstructions centre a Gondwanan ice cap in the Western Sahara, close to the Ordovician South Pole. It was not a Snowball Earth event, but covered a far larger area than did the maximum extent the Pleistocene ice sheets in the Northern Hemisphere. It is the only case of severe global cooling bracketing one or the ‘Big Five’ mass extinctions of the Phanerozoic Eon. In fact two mass extinctions during the Late Ordovician rudely interrupted the evolutionary promise of the earlier threefold diversification, by each snuffing-out almost 30% of known genera.

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L-chondrite meteorite in iron-stained Ordovician limestone together with a nautiloid (credit: Birger Schmitz)

A lesser-known feature of the Ordovician Period is a curious superabundance of extraterrestrial debris, including high helium-3, chromium and iridium concentrations, preserved in sedimentary rocks, particularly those exposed around the Baltic Sea (Schmitz, B. and 19 others 2019. An extraterrestrial trigger for the mid-Ordovician ice age: Dust from the breakup of the L-chondrite parent body. Science Advances, v. 5(9), eaax4184; DOI: 10.1126/sciadv.aax4184). Yet there is not a sign of any major impact of that general age, and the meteoritic anomaly occupies a 5 m thick sequence at the best studied site in Sweden, representing about 2 Ma of deposition, rather than the few centimetres at near-instantaneous impact horizons such as the K-Pg boundary. Intact meteorites are almost exclusively L-chondrites dated at around 466 Ma. Schmitz and colleagues reckon that the debris represents the smashing of a 150 km-wide asteroid in orbit between Mars and Jupiter. Interestingly, L-chondrites are more abundant today and in post-Ordovician sediments than they were in pre-Ordovician records, amounting to about a third of all finds. This suggests that the debris is still settling out in the Inner Solar System hundreds of million years later. Not long after the asteroid was smashed a dense debris cloud would have entered the Inner Solar System, much of it in the form of dust.

The nub of Schmitz et al’s hypothesis is that considerably less solar radiation fell on Earth after the event, resulting in a sort of protracted ‘nuclear winter’ that drove the Earth into much colder conditions. Meteoritic iron falling the ocean would also have caused massive phytoplankton blooms that sequestered CO2 from the Ordovician atmosphere to reduce the greenhouse effect. Yet the cooling seems not to have immediately decimated the ‘booming’ faunas of the Middle Ordovician. Perhaps the disruption cleared out some ecological niches, for new species to occupy, which may explain sudden boosts in diversity among groups such as brachiopods. Two sharp jumps in brachiopod species numbers are preceded and accompanied by ‘spikes’ in the number of extraterrestrial chromite grains in one Middle Ordovician sequence. One possibility, suggested in an earlier paper (Schmitz, B. and 8 others 2008. Asteroid breakup linked to the Great Ordovician Biodiversification Event. Nature Geoscience, v. 1, p. 49-53; DOI: 10.1038/ngeo.2007.37)  is that the undoubted disturbance may have killed off species of one group, maybe trilobites, so that the resources used by them became available to more sturdy groups, whose speciation filled the newly available niches. Such a scenario would make sense, as mobile predators/scavengers (e.g. trilobites) may have been less able to survive disruption, thereby favouring the rise of less metabolically energetic filter feeders (e.g. brachiopods).

See also: Sokol, J. 2019. Dust from asteroid breakup veiled and cooled Earth. Science, v. 365, pp. 1230: DOI: 10.1126/science.365.6459.1230, How the first metazoan mass extinction happened (Earth-logs, May 2014)

The late-Ordovician mass extinction: volcanic connections

The dominant feature of Phanerozoic stratigraphy is surely the way that many of the formally named major time boundaries in the Stratigraphic Column coincide with sudden shifts in the abundance and diversity of fossil organisms. That is hardly surprising since all the globally recognised boundaries between Eras, Periods and lesser divisions in relative time were, and remain, based on palaeontology. Two boundaries between Eras – the Palaeozoic-Mesozoic (Permian-Triassic) at 252 Ma and Mesozoic-Cenozoic (Cretaceous-Palaeogene) at 66 Ma – and a boundary between Periods – Triassic-Jurassic at 201 Ma – coincide with enormous declines in biological diversity. They are defined by mass extinctions involving the loss of up to 95 % of all species living immediately before the events. Two other extinction events that match up to such awesome statistics do not define commensurately important stratigraphic boundaries. The Frasnian Stage of the late-Devonian closed at 372 Ma with a prolonged series of extinctions (~20 Ma) that eliminated  at least 70% of all species that were alive before it happened. The last 10 Ma of the Ordovician period witnessed two extinction events that snuffed out about the same number of species. The Cambrian Period is marked by 3 separate events that in percentage terms look even more extreme than those at the end of the Ordovician, but there are a great many less genera known from Cambrian times than formed fossils during the Ordovician.

Faunal extinctions during the Phanerozoic in relation to the Stratigraphic Column.

Empirical coincidences between the precise timing of several mass extinctions with that of large igneous events – mainly flood basalts – suggest a repeated volcanic connection with deterioration of conditions for life. That is the case for four of the Famous Five, the end-Ordovician die-off having been ascribed to other causes; global cooling that resulted in south-polar glaciation of the Gondwana supercontinent and/or an extra-solar gamma-ray burst (predicated on the preferential extinction of Ordovician near-surface, planktonic fauna such as some trilobite families). Neither explanation is entirely satisfactory, but new evidence has emerged that may support a volcanic trigger (Jones, D.S. et al. 2017. A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia. Geology, v. 45, p. 631-634; doi:10.1130/G38940.1). David Jones and his US-Japan colleagues base their hypothesis on several very strong mercury concentrations in thin sequences in the western US and southern China of late Ordovician marine sediments that precede, but do not exactly coincide with, extinction pulses. They ascribe these to large igneous events that had global effects, on the basis of similar Hg anomalies associated with extinction-related LIPs. Yet no such volcanic provinces have been recorded from that time-range of the Ordovician, although rift-related volcanism of roughly that age has been reported from Korea. That does not rule out the possibility as LIPs, such as the Ontong Java Plateau, are known from parts of the modern ocean floor that formed in the Mesozoic and Cenozoic. Ordovician ocean floor was subducted long ago.

The earlier Hg pulses coincide with evidence for late Ordovician glaciations over what is now Africa and eastern South America. The authors suggest that massive volcanism may then have increased the Earth’s albedo by blasting sulfates into the stratosphere. A similar effect may have resulted from chemical weathering of widely exposed flood basalts which draws down atmospheric CO2. The later pulses coincide with the end of Gondwanan glaciation, which may signify massive emanation of volcanic CO2 into the atmosphere and global warming. Despite being somewhat speculative, in the absence of evidence, a common link between the Big Five plus several other major extinctions and LIP volcanism would quieten their popular association with major asteroid and/or comet impacts currently being reinvigorated by drilling results from the K-Pg Chicxulub crater offshore of Mexico’s Yucatan Peninsula.

How the first metazoan mass extinction happened

The end-Ordovician mass extinction was the first of five during the Phanerozoic, andthe first that involved multicelled organisms. It happened in two distinct phases that roughly coincided with an intense but short-lived glaciation at the South Pole, then situated within what is now the African continent. Unlike the other four, this biotic catastrophe seems unlinked to either a major impact structure or to an episode of flood volcanism.

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Artist’s impression of an Ordovician shallow-sea community (credit: drtel)

In 2009 Earth Pages reported the curious occurrence in 470 Ma (Darriwilian Stage) Swedish limestones of a large number of altered chondritic meteorites, possible evidence that there may have been an extraterrestrial influence on extinction rates around that time. In support is evidence that the meteorite swarm coincided with megabreccias or olistostromes at what were then Southern Hemisphere continental margins: possible signs of a series of huge tsunamis. But in fact this odd coincidence occurred at a time when metazoan diversity was truly booming: the only known case of impacts possibly favouring life.

Number One of the Big Five mass extinctions occurred during the late-Ordovician Hirnantian stage (443-445 Ma) and has received much less attention than the later ones. So it is good see the balance being redressed by a review of evidence for it and for possible mechanisms (Harper, D.A.T et al. 2014. End Ordovician extinctions: A coincidence of causes. Gondwana Research, v. 25, p. 1294-1307). The first event of a double-whammy mainly affected free-swimming and planktonic organisms and those of shallow seas; near-surface dwellers such as graptolites and trilobites. The second, about a million years later, hit animals living at all depths in the sea. Between them, the two events removed about 85% of marines species – there were few if any terrestrial animals so this is close to the extinction level that closed the Palaeozoic at around 250 Ma.

No single process can be regarded as the ‘culprit’. However the two events are bracketed by an 80-100 m fall in sea level due to the southern hemisphere glaciation. That may have given rise to changes in ocean oxygen content and in the reduction of sulfur to hydrogen sulfide. Also climate-related may have been changes in the vertical, thermohaline circulation of the oceans, falling temperatures encouraging sinking of surface water to abyssal depths providing more oxygen to support life deep in the water column. Sea-level fall would have reduced the extent of shallow seas too. Those consequences would explain the early demise of shallow water, free swimming animals. Reversal of these trends as glaciation waned may have returned stagnancy and anoxia to deep water, thereby affecting life at all depths. The authors suggest generalized ‘tipping points’ towards which several global processes contributed.

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