Plant pollen records ozone holes
Fossil measurements might reveals causes of mass extinction.
Fossilized plant spores could carry a record of ancient holes in the Earth's ozone layer. If so, they could test one theory about what caused our planet's biggest mass extinction 250 million years ago.
Plant spores contain natural sunscreen chemicals called para-coumaric acids, which protect their DNA from harmful UV-B radiation.
The ozone layer normally blocks UV-B radiation. When that layer thins and UV-B levels rise, "plants invest more in the screening compounds they put into the spore walls," explains Charles Wellman, a palaeobotanist at the University of Sheffield, UK. He is part of a British team studying fossilized spores to work out if the Permian extinction, some 252 million years ago, was linked to ozone loss.
To test their method, the scientists used the British Antarctic Survey's decades-old collection of spores from the moss Lycopodium magellanicum. These spores were collected on the island of South Georgia, in the South Atlantic, where the skies have seen a larger ozone hole appear each year.
Present-day spores have three times as much protective chemical as those from 40 years ago. The team believes this reflects the 14% decrease in average ozone levels above the island in that time. Spores from equatorial climes with no ozone holes showed no variation in sunscreen levels.
There are many ideas about the Permian extinction, when almost 90% of all species died. "So far, no-one's come up with a definitive mechanism," says Andrew Saunders, a geochemist from the University of Leicester who has researched the event.
Some scientists think a comet or meteorite struck the Earth. Others link the event to an enormous volcanic eruption that left a large mass of lava in Siberia.
This eruption would have released dust, sulphur and halogen compounds, disrupting the chemistry of the atmosphere and possibly eating a hole in the ozone layer.
Spores from the time show severe mutations, which may have been caused by ultraviolet radiation let through by a thinner ozone layer1. But at the moment there's no conclusive evidence of an ozone loss at that time, says Barry Lomax, part of the Sheffield team, who presented the team's results on 10 August at the Earth System Processes conference in Calgary, Canada.
"We hope this method will be the first independent test of ozone levels from that period," says Wellman. There is an excellent fossil record of pollen spores, and although the para-coumaric acids break down over time, they leave signature chemicals that should remain in fossils that have not been heated, he says.
"If it works, we should see a massive increase in pigments during the Permian," says Wellman. With a thinner ozone layer, more ultraviolet radiation would hit plants, forcing them to slather on more sunscreen.
"They might be onto something useful here," says Saunders. "If they find there was an ozone collapse, the next question is how it was triggered." The most likely cause would be the Siberian eruptions, he says.
"It's unlikely that we'd ever be able to find a single smoking gun for the extinction," cautions Lomax. But he thinks that studying fossilized spores may allow them to prove that ozone loss was part of a cocktail of causes.
The team have already collected fossilized spores dating from the period from around the world, says Wellman, who expects the first results by the end of the year.
- Visscher H., Proc. Natl. Acad. Sci., 101. 12952 - 12956 (2004).