A time period doesn't earn the nickname "The Great Dying" for nothing. The mass extinction event that occurred at the end of the Permian period is the worst in Earth's history, wiping out some 70 percent of land animals and a staggering 95 percent of sea life. The exact cause of the event is up for debate, but a huge volcanic region known as the Siberian Traps is a leading candidate. New research out of MIT and the US Geological Survey points the finger squarely at that region – but not in the way previously expected.
Today, the Siberian Traps is made up of about 2 million sq km (770,000 sq mi) of igneous rock, but back in the Permian period the region was an ocean of lava belching up from the depths of the Earth and flowing across the land. These extreme eruptions have long been thought to be a top contender for triggering the end-Permian extinction event, but the pieces don't quite add up.
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"One thing really stuck out as a sore thumb to me," says Seth Burgess, first author of the study. "The total duration of magmatism in most cases is about 1 million years, but extinctions happen really quickly, in about 10,000 years. That told me that it's not the entire large igneous province driving extinction."
Burgess believed that there was a more immediate trigger for the extinctions contained inside the larger period of flowing lava. To investigate, the team analyzed samples of ancient rock collected from the Siberian Traps and found that the magmatic period started about 300,000 years before the extinction event, and continued for 500,000 years afterwards.
That seems like a pretty clear smoking gun, but it was still too wide a window to account for the fairly sudden decline in species, and there was no evidence of climate change until the extinctions began, some 252 million years ago. But under the surface are layers called sills, which are narrow channels of rock that form when lava squeezes between existing layers. And coincidentally, these sills started to appear 251.9 million years ago.
"I realized the oldest sills out there correspond, bang-on, with the start of the mass extinction," says Burgess. "You don't have any negative effects occurring in the biosphere when you've got all this lava erupting, but the second you start intruding sills, the mass extinction starts."
Armed with those new observations, the team outlined a new timeline that might have led to the biggest extinction event on Earth. First, 252.2 million years ago, lava erupts and flows across the land for some 300,000 years or so, eventually cooling and forming a solid layer of rock at the surface.
Then, 251.9 million years ago, that solid layer began preventing any more magma from erupting onto the surface. Instead, it was forced out sideways, spreading underneath the rocky cap and forming the sills we see today. In doing so, the researchers say it likely heated up sedimentary layers full of carbon, which in turn belched their greenhouse gases into the atmosphere.
This excess of CO2 in the atmosphere is consistent with other hypotheses about the extinction event. Previous studies have attributed the mass dying to ocean acidification and even microbes that released methane, and while these may have been factors, it's likely that they were themselves symptoms of this magma pulse scenario.
"Large igneous provinces have always been blamed for mass extinctions, but no one has really figured out if they're really guilty, and if so, how it was done," says Burgess. "Our new work takes that next step and identifies which part of the large igneous province is guilty, and how it committed the crime."
The research was published in the journal Nature Communications.