Can a supernova affect the Earth System?

The easy answer is yes, simply because chemical elements with a greater relative atomic mass than that of iron are thought to be created in supernovae when dying giant stars collapse under their own gravity and then explode. Interstellar dust and gas clouds accumulate their debris. If the clouds are sufficiently dense gravity forms clumps that may become new stars and the planets that surround them. Matter from every once-nearby supernova enters these clouds and thus contributes to the formation of a planet. This was partly proven when pre-solar grains were found in the Murchison meteorite, some of which are as old as 7.5 billion years (Ga) – 3 Ga older than the Solar System (see: Mineral grains far older than the Solar System; January 15, 2020). Murchison is a carbonaceous chondrite, a class of meteorite which likely contributed lots of carbon-based compounds to the early Earth, setting the stage for the emergence of life. It has been estimated that a near-Earth supernova (closer than 1000 light years) would have noticeable effects on the biosphere, mainly because of the effects on atmospheric composition of the associated high-energy gamma-ray burst. That would create sufficient nitrogen oxides to destroy the ozone layer that shields the surface from harmful radiation. There are reckoned to have been 20 nearby supernovae during the last 10 Ma or so from the presence of anomalously high levels of the isotope 60Fe in marine sediment layers on the Pacific floor. Yet there is no convincing evidence that they coincided with detectable extinctions in the fossil record. But supernovae have been suggested as a possible cause of more ancient mass extinctions, such as that at the end of the Ordovician Period (but see: The late-Ordovician mass extinction: volcanic connections; July 2017).

Diorama of an Early Devonian reef with tabulate and rugose corals and trilobites (Credit: Richard Bizley)

The Late Devonian is generally accepted to be one of the ‘Big Five’ mass extinction events. However, unlike the others, the event was a protracted decline in biodiversity, with several extinction peaks). In particular it marked the end of Palaeozoic reef-building corals. Some have put down the episodic faunal decline to the effects of species moving from one marine basin to another as global sea levels fluctuated: much like the effects of the ‘invasion’ of the coral-eating Crown of Thorns sea urchin that has helped devastate parts of the Great Barrier Reef during present-day global warming (see: Late Devonian: mass extinction or mass invasion? January 2012). Recently, attention has switched to evidence for ultra-violet damage to the morphology of spores found in the strata that display faunal extinction; i.e. to the possibility of the ozone layer having been lost or severely depleted. One suggestion has been sudden peaks in volcanic activity, hinted at by spikes in the abundance of mercury of marine sediments. Brian Fields of the University of Illinois, with colleagues from the USA, UK, Estonia and Switzerland, have closely examined the possibility and the testability of a supernova’s influence (Fields. B.D. et al. 2020.  Supernova triggers for end-Devonian extinctions. Proceedings of the National Academy of Sciences, v. 117, article 202013774; DOI: 10.1073/pnas.2013774117).

They propose the deployment of mass-spectrometric analysis for anomalous stable-isotope abundances in the sediments that contain faunal evidence for accelerated extinction, particularly those of 146Sm, 235U and the long-lived plutonium isotope 244Pu (80 Ma hal-life). They suggest that the separation of the extinction into several events, may be a clue to a supernova culprit. A gamma-ray burst would arrive at light speed, but dust – containing the detectable isotopes –  although likely to be travelling very quickly would arrive hundred to thousands of years later, depending on the distance to the supernova. Cosmic rays generated by the supernova, also a possible kill mechanism, given a severely depleted ozone layer, travel about half the speed of light. Three separate arrivals for the products of a single stellar explosion are indeed handy as an explanation for the Late Devonian extinctions. But someone needs to do the analyses. The long-lived  plutonium isotope is the best candidate: even detection of a few atoms in a sample would be sufficient proof. But that would require a means of ruling out contamination by anthropogenic plutonium, such as analysing the interior of fossils. But would even such an exotic discovery prove the sole influence of a galactic even?

Greening the Earth, Devonian forest fires and a mass extinction

Land plants begin to appear in the fossil record as early as the late Ordovician (~450 Ma), show signs of diversification during the Silurian and by the end of the Devonian Period most of the basic features of plants are apparent. During the Carboniferous Period terrestrial biomass became so high as to cause a fall in atmospheric carbon dioxide, triggering the longest period of glaciation of the Phanerozoic, and such a boost to oxygen in the air (to over 30%) that insects, huge by modern standards, were able to thrive and the risk of conflagration was perhaps at its highest in Earth’s history. Yet surprisingly, the first signs of massive forest fires appear in the Devonian when vegetation was nowhere near so widespread and luxuriant as it became in the Carboniferous (Kaiho, K. et al. 2013. A forest fire and soil erosion event during the Late Devonian mass extinction. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 392, p. 272-280). Moreover, Devonian oxygen levels were well below those of the present atmosphere and CO2 was more than 10 times even the post-industrial concentration (387 parts per million in 2013). Such atmospheric chemistry would probably have suppressed burning.

Kunio Kaiho of Tohoku University in Japan and colleagues from Japan, the US and Belgium analysed organic molecules in Belgian marine sediments from the time of the late-Devonian mass extinction (around the Frasnian-Famennian boundary at 372 Ma). A range of compounds produced by hydrocarbon combustion show marked ‘spikes’ at the F-F boundary. The thin bed that marks the extinction boundary also shows sudden increase then decrease in δ13C and total organic carbon, indicative of increase burial of organic material and a likely increase in atmospheric oxygen levels. Another biomarker that is a proxy for soil erosion follows the other biogeochemical markers, perhaps signifying less of a binding effect on soil by plant colonisation: a likely consequence of large widlfires. Unlike the biomarkers, magnetic susceptibility of the boundary sediments is lower than in earlier and later sediments. This is ascribed to a decreased supply of detrital sediment to the Belgian marine Devonian basin, probably as a result of markedly decreased rainfall around the time of the late-Devonian mass extinction. But the magnetic data from 3 metres either side of the boundary also reveal the influence of the 20, 40, 100 and 405 ka Milankovich cycles.

Juan Ricardo Cortes , a placoderm from the Dev...
Dunkleosteus, a giant (10 m long) placoderm fish from the Devonian, which became extinct in the late Devonian along with all other placoderms (credit: Wikipedia)

This set of environmentally-related data encourages the authors to suggest a novel, if not entirely plausible, mechanism for mass extinction related to astronomically modulated dry-moist climate changes that repeatedly killed off vegetation so that dry woody matter could accumulate en masse during the Frasnian while atmospheric oxygen levels were too low for combustion. A mass burial of organic carbon at the end of that Age then boosted oxygen levels above the burning threshold to create widespread conflagration once the wood pile was set ablaze. Makes a change from continental flood basalts and extraterrestrial impacts… Yet it was about this time that vertebrates took it upon themselves to avail themselves of the new ecological niche provided by vegetation to haul themselves onto land.

Late Devonian: mass extinction or mass invasion?

A hand made lookalike for User:Dragons flight'...
Image via Wikipedia

The later part of the Devonian (the Frasnian and Famennian Stages) once marked the second largest marine mass extinction (~375 Ma) of the Phanerozoic Eon: it was one of the ‘Big Five’. For the last decade the drop in marine biodiversity in that interval has come under scrutiny: partly because it may have involved several  events;  no well-supported extinction mechanism has emerged; and extinctions seem have been concentrated on three animal groups – trilobites, brachiopods and reef corals. Something large did happen, as reef-building corals almost disappeared and coral reefs only returned with the rise of modern (scleractinian) corals in the Mesozoic. While the end of the Devonian still figures widely as having experienced a mass extinction event, more detailed palaeontological work at the genus and species level suggests another possibility.

‘Officially’ a mass extinction event must exceed the background extinction rate throughout the Phanerozoic and be above that in immediately preceding and following stages: statistically significant, that is. They always give rise to a marked reduction in biodiversity, but another mechanism can do that without extinctions suddenly increasing. The rate at which new species emerge can fall below that of species extinctions, when the overall number of living species falls. As far as ecosystems are concerned both processes are equally severe, but the causes may be very different.

Hederelloids encrusting a Spiriferida brachiop...
Brachiopod from the Devonian of Ohio, USA. Image via Wikipedia

Reviewing detailed records of Devonian species of two genera of brachiopods and one bivalve genus (50 species in all) in five North American stratigraphic sequences, Alycia Stigall of Ohio University, USA noted apparently significant variations in the local assemblages (Stigall, A. L. 2012. Speciation collapse and invasive species dynamics during the Late Devonian ‘Mass Extinction’. GSA Today, v. 22(1), p. 4-9). Speciation overall fell in the Frasnian and the preceding Givetian, while rate of extinction barely changed. For the three studied genera ,speciation reached low values in the Frasnian and Famennian, but that was accompanied by an equally large fall in extinctions. In this narrow sample we seem to be seeing not an extinction crisis but one of biodiversity. Why?

The Late Devonian saw repeated ups and downs in sea level, which repeatedly connected and disconnected shallow- to moderate-depth marine basins. The fossil record shows repeated cases of species from one basin colonising another, each invasion accompanying rapid marine transgression.. One means whereby species arise is through prolonged isolation of separate populations of the ancestral species through independent genetic drift and mutation. The episodic connection of basins may have prevented such allopatric speciation. Interestingly, the invading species  were dominantly animals with a broad tolerance for environmental conditions.

Whether this mechanism applied to all three main animal groups whose diversity plummeted in Late Devonian times remains to be seen, and it begs the question ‘why didn’t it happen among other animal groups that were less affected by whatever the events were?’ One of the problems associated with decreasing biodiversity in modern marine (and terrestrial) settings is growth in the numbers of invasive species, so the work on 375 Ma fossils might help understand and mitigate current ecological issues. The only difference is that for many of the hyper-successful invader species the means of invasion has been provided by human activities. brachiopod brachioopod