One of the largest mass extinctions of animals 440 million years ago may have been caused by volcanoes.

Researchers from Washington University believe that pulses of atmospheric carbon dioxide and sulfate aerosols were intermixed at the conclusion of the Ordivician geological period and caused 85 percent of marine animal species at the time to become extinct during one of the five largest mass extinctions in history.

During this time, most of the planet north of the tropics was covered by an ocean, which housed most complex multicellular organisms.

David Fike, an associate professor of earth and planetary sciences in Arts & Sciences and a co-author on the study, explained that different factors affect the Earth’s climate can interact in unanticipated ways and events that may not seem extreme in themselves can put the climate system into a precarious state where additional perturbations can have dire consequences.

“It's something to keep in mind when we contemplate geoengineering schemes to mitigate global warming,” Fike said in a statement.

Scientists often have a difficult time explaining past mass extinctions because the ancient atmospheres and oceans have long since been altered beyond recognition, forcing scientists to work from proxies including variations in oxygen isotopes in ancient rock to learn about climates of the past.

Fike said most elements in rock participate in so many chemical reactions that a signal can often be interpreted in more than one way, making proxies difficult to use.

David Jones, an earth scientist at Amherst College, bypassed the issue with proxies by measuring the abundance of mercury, where the main source of mercury during the Ordivician period was volcanism.

Volcanoes are climate forcers because they release both carbon dioxide that can produce long-term greenhouse warming and sulfur dioxide that can cause short-term reflective cooling.

Samples of rock from the Ordivician age found in south China and the Monitor Range Nevada showed anomalously high mercury concentrations where some samples with more than 500 times more mercury than the background concentration.

According to the scientists, the mercury arrived in three pulses, before and during the mass extinction where the first wave laid down a large igneous province that then drew down atmospheric carbon dioxide. The climate then cooled, allowing glaciers to form in the southern hemisphere on the supercontinent of Gondwana.

The cooling may have also lowered the boundary between two layers of the atmosphere called the tropopause with different temperature gradients. A second wave of eruptions then occurring, injecting prodigious amounts of sulfur dioxide above the tropopause, leading to the largest pulse of extinctions.

Ice sheets then grew and the sea level dropped, causing many species to become extinct.

The second wave of volcanism also produce a greenhouse warming from carbon dioxide to overtake the cooling caused by the sulfur dioxide. This caused the climate to warm, ice to melt and sea level to rise, killing many of the survivors of the first pulse.