Colossal volcano behind 'mystery' global cooling finally found

By Michael Greshko, Published August 23, 2019

 https://www.nationalgeographic.com/science/2019/08/colossal-volcano-behind-mystery-global-cooling-found/

The placid Lake Ilopango is the caldera of the Ilopango volcanic complex, which in 539 or 540 A.D. produced one of the largest volcanic eruptions in the last 7,000 years. The lake was also present then; as lava boiled the water into steam, the eruption became even more explosive.

Photograph by Jon Arnold Images Ltd, Alamy Stock Photo

 

The eruption devastated local Maya settlements and caused crop failures around the world.

The ices of Greenland and Antarctica bear the fingerprints of a monster: a gigantic volcanic eruption in 539 or 540 A.D. that killed tens of thousands and helped trigger one of the worst periods of global cooling in the last 2,000 years. 

Now, after years of searching, a team of scientists has finally tracked down the source of the eruption.

The team’s work, published in Quaternary Science Reviews, lays out new evidence that ties the natural disaster to Ilopango, a now-dormant volcano in El Salvador.
 Researchers estimate that in its sixth-century eruption, Ilopango expelled the equivalent of 10.5 cubic miles of dense rock, making it one of the biggest volcanic events on Earth in the last 7,000 years. The blast was more than a hundred times bigger than the 1980 Mount St. Helens eruption and several times larger than the 1991 eruption of Mount Pinatubo. It dealt the local Maya settlements a blow that forever altered their trajectory.

“This is the largest eruption in Central America that human beings have ever witnessed,” says lead study author Robert Dull, a geologist at California Lutheran University. “The importance of the event is even greater, both how the Maya overcame it and how it impacted what happened next.”

The new work helps solve a longtime geologic mystery. Historical accounts that date to 536 describe a dark fog that dimmed the sun and ushered in a wave of crop deaths. Until recently, scholars were open to the idea that these clouds were the remains of an asteroid or comet. But modern data confirms that the event was volcanic—and that it was two volcanoes up to four years apart, not just one.

Ice cores from Greenland and Antarctica show spikes of sulfate, a byproduct of large volcanic eruptions, at 536 and either 539 or 540. The two volcanoes were so large and so violent, they launched sulfur gases and particles miles into the sky. Since this material reflected sunlight away from Earth’s surface, it triggered severe global cooling: One 2016 study found that the volcanoes decreased average global temperatures by as much as 3.6 degrees Fahrenheit. Crops in northern Europe and elsewhere failed, likely triggering starvation and disease. 

Though its ties to the volcanoes remain tenuous, the infamous Plague of Justinian—which killed tens of millions of people—started in 541, during the worst of the cooling. (Learn more about human-caused climate change, which will cause more than 3.6 degrees Fahrenheit of warming without rapid emission reductions.)

Geologists surmised that the 536 eruption was from a high-latitude volcano—perhaps one in Iceland or Alaska—and that the 539/540 eruption was in the tropics. But the identities of the volcanoes were unknown.

A date with destiny

In the past, some scientists had considered Ilopango a possible suspect for the 536 eruption. It had clearly erupted sometime between the third and sixth centuries A.D., depositing ash and rock across El Salvador to form a rock formation called the Tierra Blanca Joven (Spanish for “young white earth”). But precisely when Ilopango erupted—and how it affected local people—remained uncertain.

In the late 1990s, Dull was a Ph.D student attempting to study El Salvador’s past land use by looking at vegetation in sediment cores. But the more he looked, the more he noticed a whitish layer of ash: the Tierra Blanca Joven. “My advisor said, ‘Eyes on the prize, this volcano thing is a distraction.’ And I just couldn’t avert my eyes,” says Dull. “How many people were killed? How large of an area was affected?”

In 2001, Dull tried to estimate Ilopango’s effects on the local Maya, but he didn’t make further headway until 2008, when two major studies grabbed Dull’s attention. In one, geologists published new evidence that the historical “dust veil” of 536 was caused by a volcano. 
In the other, researchers announced that the Tierra Blanca Joven extended into marine sediments off the coast of El Salvador. The Tierra Blanca Joven eruption was even bigger than Dull and others thought.

However, pinning down when Ilopango erupted—and whether it could have caused the cooling in the mid-sixth century—became a huge challenge.

To pinpoint the eruption, researchers relied on radiocarbon dating, which takes advantage of the fact that living plants (and whatever eats them) absorb traces of radioactive carbon-14. 
 Dead plants and animals no longer absorb carbon-14, and the trapped carbon starts to decay like clockwork.
 By counting up the products of this decay, scientists can see when the plant or animal died, a proxy for the age of objects found nearby.

But background levels of carbon-14 naturally vary over time, which can lead to “plateaus”: time periods when organisms that lived decades apart look like they have the same radiocarbon age.
 Try as they might, earlier researchers couldn’t nail down the Tierra Blanca Joven eruption because it fell within a 120-year plateau spanning the fifth and sixth centuries A.D.

continued at:  https://www.nationalgeographic.com/science/2019/08/colossal-volcano-behind-mystery-global-cooling-found/

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