Ordovician extinction – death caused by volcanism?


Researchers have found evidence that volcanism could possibly be one of the reason behind the end Ordovician extinction, as this event was called, was one of the five largest mass extinctions in Earth’s history.

In an article published in the journal Geology, scientists at Washington University in St. Louis and colleagues describe what happened when pulses of atmospheric carbon dioxide and sulfate aerosols were intermixed at the end of the Ordovician geological period more than 440 millions years ago. According to them when most of the planet north of the tropics was covered by an ocean and most complex multicellular organisms lived in the sea, 85 percent of marine animal species disappeared forever – the end Ordovician extinction.

Scientists blame gases injected into the atmosphere by massive volcanism as being responsible for this and while such an event will most likely never be exactly repeated, the team is of the opinion that the event reveals the potential instability of planetary-scale climate dynamics.

Volcanism coincides with mass extinctions with suspicious frequency. The event in question isn’t an isolated volcanic eruption, but massive eruptions that covered thousands of square kilometers with thick lava flows, creating large igneous provinces (LIPs). The most famous U.S. example of a LIP is the Columbia River Basalt province, which covers most of the southeastern part of the state of Washington and extends to the Pacific and into Oregon.

Volcanoes are plausible climate forcers, or change agents, because they release both carbon dioxide that can produce long-term greenhouse warming and sulfur dioxide that can cause short-term reflective cooling. In addition, the weathering of vast plains of newly exposed rock can draw down atmospheric carbon dioxide and bury it as limestone minerals in the oceans, also causing cooling.

Upon analysis of Ordovician age rock samples from south China and the Monitor Range in Nevada, scientist found anomalously high mercury concentrations. Some samples held 500 times more mercury than the background concentration. The mercury arrived in three pulses, before and during the mass extinction. So what could have caused this? It had to have been an unusual sequence of events because the extinction (atypically) coincided with glaciation and also happened in two pulses.

As the scientists began to piece together the story, they began to wonder if the first wave of eruptions didn’t push Earth’s climate into a particularly vulnerable state, setting it up for a climate catastrophe triggered by later eruptions.

The first wave of eruptions laid down a LIP whose weathering then drew down atmospheric carbon dioxide. The climate cooled and glaciers formed on the supercontinent of Gondwana, which was then located in the southern hemisphere.

The cooling might have lowered the tropopause, the boundary between two layers of the atmosphere with different temperature gradients. The second wave of volcanic eruptions then injected prodigious amounts of sulfur dioxide above the tropopause, abruptly increasing the Earth’s albedo, or the amount of sunlight it reflected.

This led to the first and largest pulse of extinctions. As ice sheets grew, sea level dropped and the seas became colder, causing many species to perish.

During the second wave of volcanism, the greenhouse warming from carbon dioxide overtook the cooling caused by sulfur dioxide and the climate warmed, the ice melted and sea levels rose. Many of the survivors of the first pulse of extinctions died in the ensuing flooding of habitat with warmer, oxygen-poor waters.

The take-home is that the different factors that affect Earth’s climate can interact in unanticipated ways and it is possible that events that might not seem extreme in themselves can put the climate system into a precarious state where additional perturbations have catastrophic consequences.