The late Devonian extinction is one of the five major extinction events to have occurred on Earth. It’s not unusual for scientists to argue over what causes a mass extinction event — of the five, the only one we’ve decisively pinned down is the Cretaceous-Paleogene, aka K-PG, aka The Littlest Dinosaur’s Terrible, Horrible, No-Good Very Bad Day. The Late Devonian, however, is weird even by the standards of other extinctions. It looks like a protracted decline in speciation that persists for about ten million years, with an opening extinction pulse (the Kellwasser event) followed by a closing pulse (the Hangenberg event) for the era about 358.9 million years ago. Now, a new theory suggests that the Devonian extinction might be tied to multiple supernovas going off in fairly close proximity to Earth.
This analysis, published in PNAS, focuses on the Hangenberg event at the end of the Late Devonian. One important distinction to keep in mind is that the Devonian extinction primarily affected ocean-dwelling creatures, while this analysis of the Hangenberg event focuses on what was happening on land. There was a terrestrial component to the Late Devonian extinction, but land-based life was less well established. Overall, however, 97 percent of all jawed vertebrates perished in this extinction event, between terrestrial and marine ecosystems.
There is a 15 million-year gap in the fossil record from 360M to 345M years ago known as Romer’s Gap, where few terrestrial fossils of plants or animals have been recorded. Wikipedia notes that recent finds in Scotland have begun to shed some light on this period, but multiple independent lines of analysis have confirmed that Romer’s Gap appears to be a real, worldwide phenomenon. This does not automatically mean that there was less life on Earth. One alternative hypothesis is that global conditions changed in ways that made fossilization less likely for some 15 million years.
This new work is based on the discovery of large numbers of burned plant spores in Greenland. This type of damage is typically caused by UV-B light — a type of light typically blocked by our ozone layer. In order to produce this kind of damage, however, the ozone layer would have had to take massive damage, and the damage would have needed to persist for a very long time. The ozone hole we opened in the atmosphere by the mid-1980s, for example, is nowhere near large enough, and it only stays open as a true “hole” for part of the year. Nothing like the damage in Greenland has been observed on Earth as a result of recent ozone depletion. Volcanism can produce gasses that damage the ozone layer, but there’s no sign of the global volcanic activity that would have been required to do this kind of damage, either.
A supernova — or multiple supernovas — going off at a distance of ~65 light-years, however, would do the trick. A supernova pointed at us from that distance could bathe the planet in ozone-destroying cosmic rays for 100,000 years. One of the significant findings regarding the Hangenberg event is that it began with an early phase of black shale and anoxia, followed by a simultaneous extinction in terrestrial fish (fish that spend part of their life cycle on land, also called amphibious fish) and land plants.
The current leading theory for the Devonian mass extinction posits that increased weathering of rock caused a worldwide extinction event. During the Devonian, the maximum height of plants went from 30cm to 30m (1.18 feet — 100 feet). Root systems evolved commensurately to support these trunks. If you’ve ever seen the roots of an old tree, you’ve observed how they can break and reshape the ground. The current leading theory for the Devonian extinction is that when this action began to occur, for the first time, it washed nutrients into streams and rivers that promote the growth of algae and other plants. When these huge mats of organic material died en masse, their decomposition would have consumed most or all of the oxygen in the nearby water.
This isn’t necessarily an either/or scenario, however. Imagine the growth of the first root systems exposes new material to weathering for the first time and results in increased nutrient levels in lakes and streams. This stresses a number of ecosystems but does not cause a mass extinction in and of itself. Once the supernova cosmic rays hit, however, plant life on land begins to die even more quickly than it did before — possibly in an initial quick pulse followed by further slow acceleration as fewer and fewer healthy specimens remain with each passing generation.
The authors note that if this theory is accurate, we should be able to find evidence of it. There are certain isotopes deposited by supernova eruptions that we should still be able to find in deep fossil deposits, even after so many hundreds of millions of years. The length of the Devonian extinction event is part of what makes it interesting — there are relatively few events that might plausibly produce this set of outcomes, and the supernova option appears to be an unexplored idea.
Best of all, the presence or absence of the relevant isotopes should be a clear sign of whether the theorized events actually happened. There is some evidence that a bolide impact might have been responsible for the beginning of the Late Devonian extinction event, and a supernova or supernovae in fairly short succession could have accounted for the ecological damage at the end of the time period. The chances of directly locating the remains of any such event are slim. Supernovas do not always leave a visible remnant, and after nearly 360 million years, even a supernova remnant that was visible initially could have cooled below the ability of our instruments to detect it, assuming we even knew where to look.
Feature image by NASA, in the public domain
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