The Late Devonian biodiversity crisis is characterized by a protracted decline in speciation rate occurring over millions of years, punctuated by an extinction pulse (Kellwasser event) followed about 10 million years later by a more moderate extinction (Hangenberg event) around the Devonian/Carboniferous Boundary, about 359 million years ago. It has recently been suggested that the Hangenberg event was associated with ozone depletion, in light of evidence such as malformations persisting in palynological assemblages on the order of many thousands of years. It has been argued that volcanic eruption and a large igneous province triggered ozone depletion, whereas other research has instead linked it to an episode of global warming not caused by a large igneous province.
The precise patterns prevalent during the Devonian/Carboniferous boundary are complicated by several factors, including difficulties in stratigraphic correlation within and between marine and terrestrial settings and the overall paucity of plant remains. However, a general consensus seems to be emerging that there was first a loss of diversity in spores and pollen followed, after about 300 000 years, by a pulse of extinctions of many Plants including proto-trees, armored Fish, Trilobites, Ammonites, Conodonts, Chitinozoans, and Acritarchs, possibly coeval with the Hangenberg Crisis; this seems to have largely left intact Sharks, Bony Fish, and Tetrapods with five fingers and toes. The fact that these species disappeared over multiple beds indicates that the extinction extended over at least thousands of years.
Several studies have also reported the discovery of spores from this episode with distinct morphologies including malformed spines and dark pigmented walls, features consistent with severely deteriorating environmental conditions, and UV-B damage following destruction of the ozone layer. However, more quantitative data are needed to study their variation during quiescent times in the fossil record.
One recent study proposed an ozone depletion mechanism involving increased water vapor in the lower stratosphere caused by enhanced convection due to higher surface temperatures. Water vapour contributes to a catalytic cycle that converts inorganic chlorine (primarily hydrochloric acid and chlorine nitrate) to free radical form (chlorine monoxide). The chlorine monoxide then participates in an ozone-destroying catalytic cycle. A similar set of cycles involving bromine contributes to ozone depletion, but to a lesser extent. Increased chlorine monoxide and decreased ozone following convective injection of water into the lower stratosphere has been verified by observation and modeling. This study also argues that a period of exceptional and sustained warming would lead to the loss of the protective ozone layer via this mechanism.
This mechanism is important for lower stratosphere ozone depletion, and may have consequences for ground-level UV-B exposure. More detailed study is warranted. Until then, it is unclear whether this change would be sufficient to cause an extinction. There are several reasons for this.
First, the vertical extent of this ozone depletion mechanism should be limited to the lower stratosphere (12 km to 18 km altitude) and does not overlap with the largest concentration of ozone, which occurs around 20 km to 30 km. So, while depletion may be significant in the lower stratosphere, the bulk of the ozone layer lies above this region and would not be affected. The total column density would be reduced, but not to the extent of a complete loss of the protective ozone layer.
Secondly, the duration of the effect should be relatively short, no more than a week, since the injected water vapor is photolysed and chlorine monoxide is converted back to hydrochloric acid and chlorine nitrate. Thus, unless convective transport of water vapor to the lower stratosphere, for example, by storms, is continuous (on week timescales), the ozone reduction will be episodic, not sustained. The effect is also seasonal, since strongly convective storms tend to be limited to the spring/summer. While this is likely detrimental to surface life, most organisms have repair mechanisms that can cope with some short-duration UV-B exposure.
Thirdly, the effect is likely to be limited geographically, since strongly convective storms are not uniformly distributed and the enhanced water vapor is likely only to spread over about 100 km horizontally.
Finally, there is significant uncertainty as to the ozone depletion level needed to induce aberrations in pollen morphology and, even more critically, large-scale extinction. While the anthropogenic ozone 'hole' over Antarctica has led to increased UV-B exposure, no crash in the ecosystem has resulted. This may partly be due to the seasonal nature of the change, as would be the case in the Devonian as well. Recent work has shown that short-term exposure to significant increases in UV-B does not result in large negative impacts on the primary productivity of ocean phytoplankton, and other organisms show a wide range of sensitivity. The amount of column depletion over a given location in those cases was about 50%. The depletion caused by the mechanism considered previously seems unlikely to be that large. Hence, the convective transport of water vapor to the lower stratosphere may not be sufficient to induce a substantial extinction. It is thus worth considering other mechanisms for global ozone depletion.
Astrophysical mechanisms for biosphere damage include bolide impacts, solar proton events, supernova explosions, gamma-ray bursts, and neutron star mergers (kilonovae). Bolide impacts, gamma-ray bursts and solar proton events are essentially impulsive, and recovery of the ozone layer takes about ten years, which is likely to avert lasting biosphere destruction. Moreover, these events and kilonovae are unlikely to recur frequently.
In a paper published in the Proceedings of the National Academy of Science of the United States of America on 18 August 2020, Brian Fields of the Illinois Center for Advanced Studies of the Universe, Department of Astronomy, and Department of Physics at the University of Illinois, Urbana, Adrian Melott of the Department of Physics and Astronomy at the University of Kansas, John Ellis of the Department of Physics at Kings College London, the Theoretical Physics Department at the European Organization for Nuclear Research, and the Laboratory of High Energy and Computational Physics at the National Institute of Chemical Physics and Biophysics, Adrienne Ertel, also of the Illinois Center for Advanced Studies of the Universe and Department of Astronomy at the University of Illinois, Urbana, Brian Fry of the Department of Physics at the United States Air Force Academy, Bruce Lieberman of the Department of Ecology & Evolutionary Biology and Biodiversity Institute at the University of Kansas, Zhenghai Liu and Jesse Miller, again of the Illinois Center for Advanced Studies of the Universe and Department of Astronomy at the University of Illinois, Urbana, and Brian Thomas of the Department of Physics and Astronomy at Washburn University, consider the possibility that a supernova explosion might have been the cause of the End Devonian Extinction.
Supernovae are prompt sources of ionizing photons: extreme UV, X-rays, and gamma rays. Over longer timescales, the blast collides with surrounding gas, forming a shock that drives particle acceleration. In this way, supernovae produce cosmic rays, that is, atomic nuclei accelerated to high energies. These charged particles are magnetically confined inside the supernova remnant, and are expected to bathe Earth for 100 000 years.
The cosmic ray intensity would be high enough to deplete the ozone layer and induce UV-B damage for thousands of years. In contrast to the episodic, seasonal, and geographically limited ozone depletion expected from enhanced convection, ozone depletion following a supernova is long lived and global and is therefore much more likely to lead to an extinction event, even given uncertainties around the level of depletion necessary. (Fields et al. note that, as well as the induced UV-B damage, cosmic rays could also cause radiation damage via muons produced when they impact the atmosphere). The supernova blast itself is unlikely to wreak significant damage on the biosphere, but may deposit detectable long-lived nuclear isotopes that could provide distinctive signatures.
There are two main types of supernova: (1) massive stars (eight times as large as the Sun or larger) that explode as Core-collapse Supernovae and (2) white dwarfs that accrete from binary companions and explode as Type Ia Supernovae. These supernova types have similar explosion energies, and both produce ionizing radiation able to damage the biosphere. However, their different nucleosynthesis outputs lead to different radioisotope signatures.
Near-Earth Core-collapse Supernovae are more likely than Type Ia Supernovae. Fields et al. estimate the nearby Core-collapse Suprnova frequency using a Galactic rate of one event every 30 years, and placing the Sun at a radius of 8.7 kiloparsecs (28 375 light years) in a thin disk of scale radius 2.9 kiloparsecs (9460 light years) and height 0.1 kpc kiloparsecs (326 light years). This gives a Core-collapse Supernova rate of four per billion years within 20 parsecs (65 light years) of the Earth. Hence a Core-collapse Supernova at a distance 2 times the 'kill radius' of 10 parsecs (33 light years) is a plausible origin of the End-Devonian event(s). In contrast, the Type Ia Suprnova rate is an order of magnitude smaller, as these events are spread over the 8 times larger volume of the thick disk.
Massive stars are usually born in clusters (OB associations), and are usually in binaries with other massive stars. Thus, if one Core-collapse Supernova occurred near the Devonian/Carboniferous Boundary, it is likely there were others. This could explain the Kellwasser and other enigmatic Devonian events, in addition to the Hangenberg event.
A Core-collapse Supernova close enough to cause a significant extinction would also deliver supernova debris to the Earth as dust grains, micron or submicron-sized particles created early after the explosion. Grains in the explosion would decouple from the plasma (gas) and propagate in the magnetised supernova remnant until they were stopped or destroyed by sputtering during collisions.
A Core-collapse Supernova close enough to cause a significant extinction would also deliver supernova debris to the Earth as dust grains, micron or submicron-sized particles created early after the explosion. Grains in the explosion would decouple from the plasma (gas) and propagate in the magnetised supernova remnant until they were stopped or destroyed by sputtering during collisions.
The portion that reaches Earth would deposit in the atmosphere live (undecayed) radioactive isotopes. There is very little pre-existing background for radioisotopes whose lifetimes are much shorter than the age of Earth. Those with lifetimes comparable to the time since the event would provide suitable signatures. The discoveries of live iron⁶⁰ in the deep ocean, the lunar regolith, and Antarctic snow provide one such signal, which is interpreted as due to at least one recent nearby Core-collapse Supernova two to three million years ago at a distance of 50 to 100 parsecs (163-326 light years), which is compatible with the rate estimate made by Fields et al.
Possible relic supernova radioisotopes from the end-Devonian period with an age 360 million years include samarium¹⁴⁶ (half-life 103 million years), uranium²³⁵ (halflife 704 million years) and plutonium²⁴⁴ (half-life 80.0 million years). The most promising signature may be provided by plutonium²⁴⁴, which has also been discovered in deep-ocean crust and sediment samples deposited over the last 25 million years. Moreover, it is absorbed into bones and retained during life, whereas uranium is absorbed during fossilisation and samarium¹⁴⁶ is soluble. There is a significant uranium²³⁵ background surviving from before the formation of the solar system, with uranium²³⁵/uranium²³⁸ ratio equal to 0:721, so a significant detection above this background requires deposition raising the uranium²³⁵/uranium²³⁸ ratio by at least three parts in 100 000. Uranium/lead dating has been used to date the End-Devonian extinction, with an uncertainty in the uranium²³⁵/uranium²³⁸ ratio that is much larger than this target sensitivity, but even a few atoms of non-anthropogenic plutonium²⁴⁴ in End-Devonian fossils would be unambiguous evidence for the rapid neutron-capture process in a supernova explosion.
The journey of supernova-produced radioisotopes from explosion to seafloor is complex. Ejecta in dust most readily reaches Earth. The fraction of atoms in dust should be high for the refractory species of interest. Due to their high speeds, supernova dust grains will easily overcome the solar wind and reach Earth. The fallout on Earth favors deposition at midlatitudes; additional dispersion occurs due to ocean currents. Fields et al. estimate that the ration of supernova-produced uranium²³⁵ would not be identifiable above background levels. On the other hand, there is no natural background to the prospective plutonium²⁴⁴ signal, which may be detectable in fossiliferous material. Its detectability depends on the temporal resolution of the available geological sample, whereas the possible detectability of the prospective samarium¹⁴⁶ signal depends also on the degree of dilution due to its solubility. Finally, if more than one supernova occurred before the Devonian/Carboniferous boundary, then each of these could deposit radioisotope signals.
Some hundreds or thousands of years after the optical and ionizing outburst, the cosmic ray and dust bombardment of Earth would begin, with several possible effects.
Cosmic ray ionization of the atmosphere and accompanying electron cascades may lead to more frequent lightning, increased nitrate deposition, and wildfires. The increased nitrate flux might have led to carbon dioxide drawdown via its fertilisation effect, thereby cooling the climate. There is evidence for cooling during the first stage of the Devonian/Carboniferous boundary, although this occurred an estimated 300 000 years before the radiation damage attested by the data on pollen and spores. Any increases in soot and carbon deposits during the end-Devonian could have been generated by increases in wildfires.
Cosmic rays striking the atmosphere produce energetic muons that can penetrate matter to a much larger depth than UV-B radiation. The radiation dose due to muons at Earth’s surface and in the oceans at depths of up to 1 km could exceed, for many years, the current total radiation dose at Earth’s surface from all sources. Therefore, in addition to comparing the effects of muons and UV-B radiation at or near the surface, they could be considered in end-Devonian extinctions of megafauna living at depth.
Finally, if there was one Near-Earth Core-collapse Supernova at the Devonian/Carboniferous boundary, there may have been more, which may have been responsible for the Kellwasser and additional events. These could show evidence for ozone depletion and the other signatures above.
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