The history of our planet is rife with catastrophic and not yet fully understood extinction events. One such event, known as the end Devonian mass extinction or Hangenberg Crisis, is especially mysterious. Although some features of the event are widely accepted—including a dramatic drop in sea level and major losses of vertebrate biodiversity—there is still uncertainty about exactly what caused this event.
Now researchers from England’s University of Southampton have proposed a new mechanism for this extinction event 349 million years ago: ultraviolet radiation.
Researchers came to this conclusion by studying the spores of fernlike plants present in the sediments of ancient lake beds in eastern Greenland. Normally, these spores were yellow orange in color and had an outer layer covered with straight, regularly spaced, similarly sized spines. But the scientists noticed that around the time of the Hangenberg Crisis, the spores began to look strange.
Some were much darker in color. Others had spines that were clumped together or different sizes or crooked. These malformations were likely due to the plants’ DNA becoming damaged.
“At that point, it sort of punches you on the nose,” said palynologist John Marshall, lead author of the study, which appeared last month in Science Advances. “Because malformed spores are [also] typical in the Triassic-Jurassic as a mass extinction effect.”
So, Marshall reasoned, whatever was causing the spores to become malformed might be the “kill mechanism” for the Hangenberg Crisis.
The Kill Mechanism
So what can cause DNA damage and spore malformations? Some scientists have attributed spore malformations during the Triassic-Jurassic extinction, which occurred about 150 million years after the end Devonian event, to DNA-damaging mercury that came from intense volcanic activity.
But Marshall doesn’t think that volcanogenic mercury is responsible for the malformed spores in his samples from the Devonian because there isn’t a mercury anomaly that is coincident with the malformations.
Instead, Marshall believes that ultraviolet B (UV-B) radiation is the culprit. UV-B could be responsible for both the malformations and darkening color of the spores. The malformations are associated with DNA damage, and the darkening is associated with extra pigment that may provide some protection from UV-B.
Today, the ozone layer protects us from UV-B radiation, but Marshall suggests it might not have always been that way. “Maybe [the ozone layer] is a bit more fickle than that,” he said. “As a shield, it probably went down for eight or nine thousand years: It thinned, it was eroded, UV-B got in…it raises the possibility that it’s happened a lot of times and we’ve just never detected it.”
Not So Simple?
But not everyone thinks the explanation for the Hangenberg Crisis is so straightforward. “I think there’s more to the story than just one single thing,” said Sarah Carmichael, a paleogeographer at Appalachian State University who was not involved in the study.
Carmichael points out that there was a lot going on during this time period: Climate was rapidly shifting, glaciers were appearing and disappearing, sea level changed dramatically, and many areas of the world had consistently high levels of volcanism.
Although Carmichael doesn’t think that we should rule out UV-B radiation, she thinks it’s more likely that multiple factors were involved in the initiation of the Hangenberg Crisis. “You’ve got this combination of events all happening at the same time. On their own, they’re no big deal, but when you put them all together, things go south.”
Implications for Our Future
There’s also some uncertainty about what this historical extinction event could mean for our future.
Marshall and his coauthors hypothesize that the ozone depletion during the Hangenberg Crisis was driven by a warming climate. This warmer climate could have increased the amount of water vapor that reached the lower stratosphere, altering the chemistry of the stratosphere in a way that promoted ozone loss.
“We don’t live in the Devonian world anymore,” said Marshall. “But what this tells us is that this is a process which has acted in the past.” In other words, it doesn’t always take an asteroid impact or catastrophic volcanic event to cause a mass extinction.
In the study, the authors warn that “we should be alert for such an eventuality in the future warming world.”
But Carmichael isn’t convinced, at least not yet. She doesn’t think the evidence is strong enough to be able to make any predictions about the future. “I’m wary of making the extrapolation that this [ozone depletion] was a global thing,” she said. Indeed, she points out, history has shown us that ozone holes can be fairly localized. “I’m not saying it’s wrong, I’m just saying we need a lot more information.”
Carmichael said that we need to assess samples from multiple areas in the world to find out if this was a global or local phenomenon. “There’s nothing wrong with the sections from Europe and North America, but they represent a really weird paleoenvironment that I don’t think is representative of everywhere.”
She said that international collaborations, such as those fostered by the International Geoscience Programme, are key to expanding our knowledge about historically understudied regions and finding out what really caused the Hangenberg Crisis.
—Hannah Thomasy (@HannahThomasy), Science Writer