Mercury revealed as hidden driver in mass extinctions

Bad news loves company. Researchers have discovered that it wasn’t just erupting volcanoes, massive amounts of carbon dioxide, oceans full of sulphuric acid, runaway global warming and a thinning ozone layer that caused the end-Triassic mass extinction 201 million years ago. It was also large quantities of lethal mercury causing plant life to mutate and die.

Four out of the five mass extinctions that occurred over the past 600 million years have been linked to huge and prolonged bursts of volcanic activity.

In the case of the Triassic event – which saw the end of an estimated 40% of land animal genera and 30% of ocean-dwelling groups – there is ample evidence that volcanoes sprang to life across an area known as the Central Atlantic Magmatic Province (CAMP) 100,000 years before the great dying began. It continued, sporadically, for another 700,000.

Such a profound upheaval causes substantial environmental disruption – including long-lasting spikes in carbon dioxide and sulphur combinations that have been regularly and reliably associated with high levels of animal and plant deaths.

Now, however, scientists led by Sofie Lindström of the Geological Survey of Denmark and Greenland have identified another nasty: pulsed elevated concentrations of mercury in the ocean and the soil.

Mercury, write the researchers, is “the most genotoxic element on Earth”. That means it doesn’t just eventually kill whatever plants or animals it comes into contact with. It also causes mutations within their genomes, resulting in offspring that do not survive.

To test the contribution of mercury to the end-Triassic mass die-off, Lindström and colleagues examined fossilised fern spores from the period and looked for irregular shapes.

A proportion of mutation within any species is expected – it is, after all, one of the main drivers of evolution. In the fern species this natural rate of genetic change runs between 3% and 5%.

The scientists discovered that in spores and pollen collected from some areas in the highly volcanic period leading up to the mass extinction as many as 56% appeared deformed, suggesting very high rates of mutation.

“As one of the most toxic elements on the planet, mercury can cause both visible injuries and physiological disorders in plants,” the researchers write.

They add that that today “mercury pollution from anthropogenic sources is known to seriously disturb growth and reproductive cycles in plants, causing long-term effects on soil fertility and subsequent severe health issues to animals and human population”.

Around 200 million years ago, of course, there were no anthropogenic sources of mercury – but there is ample evidence that volcanoes pumped out enormous amounts of it.

Its effect on life on Earth would have been to deliver a double blow. First, plant species would have been severely depleted, with many becoming extinct. Second, the loss of plant matter would have resulted in a very short time in an upstream decrease in the numbers of animals who normally dined on them.

If lava or atmospheric poisons didn’t kill them, it seems, hunger would.

The research is published in the journal Science Advances.

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A normal fern spore compared with mutated ones from the end-Triassic mass extinction event.

S Lindström, GEUS

Related reading: Volcano link to end to Triassic extinction

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