Scientists have found evidence that acid rain was a major cause of the largest extinction on Earth 250 million years ago.

The Permian was a geologic period that ended more than 250 million years ago. During this time the Earth consisted of one super continent called Pangea. Forests flourished on it and a large ocean called Panthalassa teemed with life. At the end of this period the Earth had an environmental crisis that caused the largest mass extinction ever recorded. Scientists suggest that a series of massive volcanic eruptions that deposited rocks now known as the Siberian Traps, released gases causing planet-wide acid rain showers that damaged the biosphere. Until recently, there has been little fossilised evidence to support global acid rain.

Now researchers have analysed rock samples from the Dolomite Mountains in Italy and found evidence of an organic compound called vanillin, which confirms that the planet at the time of the end Permian extinction underwent successive acid rain showers. Their evidence suggests that the soils on land were very acidic, killing the ecosystem and the animals that depended on it.

Professor Sephton (background) and his colleague precariously perched on the side of the Dolomites in Italy

The study, published in the journal Geology, was carried out by researchers from Imperial College London, The University of Oklahoma, Utrecht University and the University of California-Berkeley.

Professor Mark Sephton, co-author of the paper from the Department of Earth Science and Engineering at Imperial College London, said: “The end of the Permian was a major catastrophe for Earth, which saw nearly all plant and marine life obliterated. Acid rain destroyed the delicate ecosystem on land, making the soils as acidic as lemon juice or vinegar, which killed off nearly everything.”

The rock samples the team analysed were taken from the Dolomite Mountains, but originally formed from marine sediments off the coast of Pangea. These marine sediments contained remains of plant matter in soil that was washed into the ocean at the time of the Permian extinction, compacting together to form the rocks.

The team analysed the ratio of vanillin to vanillic acid in the rock samples. Vanillin is an organic compound found in plants and a constituent component of vanilla. Following the death of plants, vanillin normally oxidises to vanillic acid. However, this oxidation process is very slow when conditions are too acidic.  The team found mostly vanillin preserved in successive layers in the rock samples, suggesting very acidic conditions in the soil. The team think that acid rain may have been the cause of this acidity.

The next step will see the researchers carrying out similar studies on rocks from around the world to determine in more detail the global extent of acidity in the environment at the end of the Permian period.