Acid rain plays role in Great Dying

Rain as acidic as undiluted lemon juice could have contributed to the Great Dying, a severe mass extinction event nearly 252 million years ago.

The cause of the massive extinction at the end of the Permian period is still a matter of scientific debate with hypotheses ranging from an asteroid collision, global loss of oxygen in the oceans and a cascade of environmental events triggered by massive volcanic eruption.

This latter hypothesis has been simulated by MIT researchers. They created global climate models of scenarios where repeated bursts of volcanism spewed gases – including sulphur – into the atmosphere.

Simulations showed such emissions were significant enough to create widespread acid rain in the Northern Hemisphere with a pH of 2 – as acidic as undiluted lemon juice. Such acidity could have disfigured plants and stunted their growth, contributing to their ultimate extinction.

“Imagine you’re a plant that’s growing happily in the latest Permian,” says Benjamin Black, a post-doc in the Department of Earth, Atmospheric and Planetary Sciences. “It’s been getting hotter and hotter, but perhaps your species has had time to adjust to that. But then quite suddenly, over the course of a few months, the rain begins to sizzle with sulphuric acid. It would be quite a shock if you were that plant.”

Evidence from the Siberian rock record suggests immense volcanism in short bursts beginning near the end of the Permian period, continuing for another million years.

The group simulated 27 scenarios based on estimates from chemical analyses and thermal modelling, and tracked water in the atmosphere and interactions among various gases and aerosols, to calculate the pH of rain.

Results showed CO₂ and volcanic sulphur could have significantly affected the acidity of rain. CO₂ could have increased rain’s acidity by an order of magnitude – add sulphur emissions to the mix and acidity may have peaked to pH 2.

After an eruption ended, researchers found pH levels in rain bounced back with acidity decreasing after a year. However, repeated bursts of activity caused swings in acid rain that could have greatly stressed terrestrial species.

While more difficult to model, researchers also included ozone depletion resulting from volcanic activity; the mixture of gases may have destroyed between 5 and 65% of the ozone layer, thus increasing exposure to UV radiation, particularly near the poles.

“It’s not just one thing that was unpleasant,” Black says. “It’s a whole host of really nasty atmospheric and environmental effects. The results made me feel sorry for end-Permian organisms.”