Acid Rain and Ozone Depletion May Have Triggered Mass Extinction 250 Million Years Ago

Acid rain and ozone depletion may have caused the Earth's most severe known mass extinction, a new study published in the journal Geology suggests.

Some 250 million years ago, at the end of the Permian period, more than 90 percent of marine species and over 70 percent of terrestrial species died off, setting the scene for the day of the dinosaur.

The die-off was so extreme that, according to fossil records left behind, ecological diversity was not fully restored for another several million years following the height of extinction.

The cause of the mass exintinction is unclear, though one popular theory blames the eruptions of a series of volcanoes in Siberia. In order to test this theory, scientists led by Benjamin Black of the Massachusetts Institute of Technology used 3-D modeling techniques to predict the impacts the gas released from the so-called Siberian Traps would have had on the atmosphere at the time.

The effects would have been catastrophic, the researchers found.

The results indicate that releases of carbon dioxide and sulfur dioxide would have triggered highly acidic rain that could have sapped the soil of its nutrients. As this happened, plants would have suffered and died, and vulnerable terrestrial organisms with them. What's more, releases of halogen-bearing compounds like methyl chloride could have caused a global ozone collapse.

The volcanic activity was episodic, meaning the onslaughts of acid rain and ozone depletion would have been as well. Such drastic fluctuations in pH and ultraviolet radiation, as well as combined overall temperature increase from greenhouse gas emissions, would have been enough, the researchers concluded, to cause what may very well have been the largest mass extinction in Earth's history.