Mutated ferns shed light on ancient mass extinction

At the end of the Triassic around 201 million years ago, three out of four species on Earth disappeared. Up until now, scientists believed the cause of the catastrophe to be the onset of large-scale volcanism resulting in abrupt climate change. Now, new research suggest there might be several factors in play.

An international research team led by the Geological Survey of Denmark and Greenland (GEUS) show that increased concentrations of the toxic element mercury in the environment contributed to the mass extinction. They recently published their finds in Science Advances.

"By looking at fern spores in sediments from the mass extinction, it was evident that these ferns were negatively affected by the mercury levels. Since mercury is accumulated in the food chain, it seems likely that other species have suffered as well," says lead scientist Sofie Lindström.

"These results suggest that the end-Triassic mass extinction was not just caused by greenhouse gases from volcanoes causing global climate change, but that they also emitted toxins such as mercury wreaking havoc," she says.

The mercury-volcano link

One of the co-authors of the study, Professor Hamed Sanei from Aarhus University, has previously demonstrated increased mercury levels from volcanism in a Large Igneous Province (LIP) during the most severe mass extinction known, the end-Permian crisis, where perhaps as much as 95% of life on Earth disappeared. Volcanic activity in LIPs is thought to be responsible for four of the five largest mass extinctions during the last 500 million years.

"Prior to industrialism, volcanic activity was the major release mechanism of large amounts of mercury from the Earth's crust. That makes it possible to use mercury in sediments to trace major volcanic activity in the Earth's past and in extent tie the extinctions of fossil organisms to LIP volcanism," Hamed Sanei explains.

Other previous studies have shown elevated mercury concentrations in Triassic-Jurassic boundary sediments over a very large area stretching from Argentina to Greenland and from Nevada to Austria and that made the team curious about the impact on the end-Triassic event.

"We decided to examine whether mercury could have played a role," Hamed Sanei says.

Fern spores as indicators

When looking at fern spores from core samples dating from 201 million years ago at the end of the Triassic the team indeed saw a link between increased mercury levels and mutations in the spores.

"During the mass extinction the mutated spores become increasingly common, and in turn the mutations get more and more severe. In some of my counts I found almost only mutated spores and no normal ones, which is very unusual," Sofie Lindström explains.

This rise in mutations happened during a period of increased volcanic activity in a LIP called the Central Atlantic Magmatic Province (CAMP) leading to rising mercury levels. Since mercury is a mutagenic toxin, its' increased distribution from the volcanic activity could help to explain the sudden deterioration of the ecosystem. Therefore, the fern spores could serve as indicators of increased mercury poisoning.

"This could hint to that the whole food chain might have been negatively affected," says Sofie Lindström.

Previous studies have found increased amounts of malformed pollen during the end-Permian mass extinction 252 million years ago, which like the end-Triassic crisis is blamed on volcanism. These studies have suggested that the mutations during the end-Permian crisis were caused by increased UVB radiation, due to thinning of the ozone layer from the volcanism.

"This could also be a possible explanation for the mutations that we see during the end-Triassic crisis," explains co-author Bas van de Schootbrugge from Utrecht University. "However, in our study we found only low amounts of mutated pollen, and during the end-Permian crisis spores do not appear to exhibit the same types of malformations registered during the end-Triassic mass extinction. This may indicate different causes for the plant mutations at the two events."

Not a simple explanation

However, it is important not to lock on to just one cause when looking at a global crisis such as the end-Triassic event, says Sofie Lindström:

"Generally, we prefer simple explanations to mass extinctions such as meteorite impacts or climate change, but I don't think it's that simple. As our study suggests there could very well be a cocktail effect of CO2 and global warming, toxins like mercury, and other factors as well."

Most of the prehistoric mass extinctions have indeed come in the wake of LIP volcanism, causing climate change and emitting toxic substances, Sofie Lindström says.

"Still, it is very difficult to say how big the importance of one factor is, because mass extinctions like this are very likely very complex events. Our study shows that mercury affected the ferns and likely also other plants, and it may also have had an impact on the entire food chain."

Present pollution looks like past volcanism

The researchers point out that their study of the end-Triassic mass extinction in many ways draws parallels to the current global situation.

"Our global society emits a lot of the same substances and greenhouse gases as these huge volcanic provinces did during these mass extinctions. Therefore, studies in what happened back then might help us to prevent it from happening again," says Sofie Lindström.