NGDIR News Section-- The discovery of anomalously high levels of mercury in rocks from the Ordivician geological period has led to a new interpretation of the ensuing mass extinction. A sequence of disturbances may have led to catastrophic cooling by reflective sulfate aerosols injected into the atmosphere by massive volcanism. The finding is important since aerosol cooling is under consideration as a way to temper global warming.
Anyone concerned by the idea that people might try to combat global warming by injecting tons of sulfate aerosols into Earth's atmosphere may want to read an article in the May 1, 2017 issue of the journal Geology.
In the article, a Washington University scientist and his colleagues describe what happened when pulses of atmospheric carbon dioxide and sulfate aerosols were intermixed at the end of the Ordivician geological period more than 440 million years ago.
The counterpart of the tumult in the skies was death in the seas. At a time 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 Ordivician extinction, as this event was called, was one of the five largest mass extinctions in Earth's history.
Although the gases were injected into the atmosphere by massive volcanism rather than prodigious burning of fossil fuels and under circumstances that will never be exactly repeated, they provide a worrying case history that reveals the potential instability of planetary-scale climate dynamics.
Figuring out what caused the end Ordivician extinction or any of the other mass extinctions in Earth's history is notoriously difficult, said David Fike, associate professor of earth and planetary sciences in Arts & Sciences and a co-author on the paper.
But what happened? 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 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.