What caused the mysterious “Hangenberg crisis” 360 million years ago? A temporary collapse of the earth’s ozone layer could have been responsible for this mass extinction. This is the result of studies of fossil plant spores that have been damaged by ultraviolet radiation. As the scientists explain, global warming at the end of the Devonian era may have caused ozone-depleting substances to reach the upper atmosphere. They see this as an indication that similar processes could also occur due to current climate change.
The development of life on our planet was characterized by a constant coming and going of species – but there were also drastic cuts in which an enormous number of animal and plant species disappeared in a short time. The most famous of these mass extinctions ended the age of the dinosaurs. While in this case an asteroid impact 66 million years ago is considered to be the cause, the other known extinction events seem to have been caused by increased volcanism: there are indications that violent eruptions have led to atmospheric and oceanic changes, with which a large proportion of living beings could not cope.
On the trail of a mysterious mass extinction
However, the cause of the mass extinction at the Devon-Carbon border is still unclear. In the course of this so-called Hangenberg crisis around 359 million years ago, a large part of the plants and especially the freshwater animals died out, finds from various places around the world. According to the researchers led by John Marshall from the University of Southampton, geological survey results show that volcanic activity with global consequences did not occur during this period. It seems, however, that the Hangenberg crisis coincides with a period after the rapid warming that ended the Devon’s last ice age.
As part of their study, Marshall and his colleagues have now investigated fossil spores of fern-like plants that have deposited in the period in question. They come from rock samples that the team collected during expeditions in East Greenland. As the scientists explain, there was a huge lake bed 359 million years ago, which at that time was still close to the equator. It was part of the land mass that was once formed from what is now Europe and North America. Back in the laboratory, the researchers dissolved the primeval sedimentary rocks in acid, releasing the remains of the plant spores, some of which were surprisingly well preserved.
Characteristic malformations
On microscopic examination, the scientists found that many of the spores had malformations of their typical spike structures on the surface. As they explain, these are typical signs that damage has occurred in the plant’s genetic makeup during its formation, such as is caused by ultraviolet radiation. This interpretation also supported another finding: many of the spores had conspicuously dark pigmented shells. Probably it was a kind of “browning” that the plants formed to protect against unusually high UV radiation, the researchers say. You see in the results a clear indication that living beings were exposed to a potentially fatal level of UV radiation during the Hangenberg crisis. The forest ecosystems in particular probably collapsed as a result, which explains the large loss of species that is emerging in the strata of the era.
Relation to climate change?
But what about the increased radiation? It could have been the result of a temporary collapse of the earth’s ozone layer, the scientists say. According to them, the climate at the end of the Devon probably warmed up very quickly after the ice sheets melted. The researchers explained that the heat could have pushed more naturally produced ozone-depleting substances into the upper atmosphere across the continents. Due to the erosion of the protective layer, the earth’s surface could then have been exposed to very intensive UV-B radiation for several thousand years, which would have put an end to many land plants and animals in the shallow waters.
In addition to this explanation for the geological extinction event, the researchers also see their results as a reference to the current developments on our planet. The extent to which climate change affects the development of the ozone layer is still largely unclear – but consequences are possible. “At the end of the Devon, the ozone protection layer apparently disappeared at the same time as the Earth warmed briefly and quickly. Against the background of the current climatic developments, it seems possible that a similar collapse of the ozone layer could occur, in which life on the surface and in the shallow sea would be exposed to lethal radiation, ”concludes Marshall.
Source: University of Southampton, technical article: Science Advances, doi: 10.1126 / sciadv.aba0768