Fossils show plants were producing higher levels of sunscreen chemicals to protect against higher ultraviolet light levels at the end of the Permian period
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The ozone layer in the atmosphere shields Earth from ultraviolet
rays. Shutterstock |
It has long been suspected that the ozone layer high in the
atmosphere that protects life from ultraviolet light was largely destroyed
during the mass extinction at the end of the Permian period 250 million years
ago. Now, we have the first direct evidence of that.
Phil Jardine at the University of Münster in Germany and his
colleagues have shown that pollen grains and spores from this time contain
higher levels of “sunscreen” chemicals than those from earlier or later
periods.
“We only see this at one point, and that coincides with the
end-Permian extinction,” says Jardine.
Around 90 per cent of marine species and 70 per cent of land
ones died out at the end of Permian. The Great Dying, as some call it, is
thought to have been caused by a period of unusually high volcanic activity.
The lava flows heated up other rocks, including coal, releasing huge quantities
of gases such as carbon dioxide. This led to ocean acidification and global
warming, which in turn led to low oxygen levels in the oceans.
This explains why marine life was so hard hit, but it is
less clear why so many terrestrial species also died out. One possibility is
that halocarbons released by the eruptions destroyed the ozone layer in the
same way as the chlorofluorocarbons (CFCs) used as refrigerants in the 20th
century did, resulting in high levels of harmful ultraviolet (UV) light.
Previous studies have shown there was an increase in
abnormally shaped pollen grains and spores at this time, which could be due to
mutations caused by UV rays. But these abnormalities could also be a result of
the many other toxic substances released during the eruptions, says Jardine.
An Alisporites pollen grain from one of the samples analysed
in the study. Image Credit:Feng Lui |
So his team used a technique called Fourier transform
infrared microspectroscopy to measure the levels of UV-absorbing chemicals in
fossilised pollen grains and spores from rocks in Tibet. The oldest rock layers
were deposited a few hundred thousand years before the extinction, and the
youngest a few hundred thousand years afterwards, says Jardine.
The level of preservation of the sunscreen chemicals should
be similar throughout this rock sequence, so the increase at the time of the
mass extinction must be because plants were producing more of them, he says.
“This is the UV signal.”
Higher UV levels would have decreased the growth of plants
on land, leading to cascading effects on ecosystems as herbivores and predators
starved. The effects on marine environments would have been much smaller
because water blocks UV.
The same thing would be happening again today if countries
hadn’t agreed to phase out CFCs under the Montreal protocol in 1987.
“Modelling studies suggest that by the 2060s, had we not had
the Montreal protocol, we would have enormous ozone disruption and collapse,”
says Jardine. “Hopefully, we have avoided that. These kinds of agreements and
environmental protections really do matter.”
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