Straddling the border between China and North Korea, the massive Mount Paektu volcano has long been an enigma. In 946 C.E., Paektu (called Changbai in China) erupted with a force matched by few volcanoes over the past 2000 years. Yet curiously, Paektu’s “millennium eruption” is not thought to have had the same devastating climatic effects as Indonesia's Mount Tambora, whose 1815 explosion poured 28 megatons of sulfur into the atmosphere, cooling the planet by 1°C and causing the famed "year without a summer." Now, a new analysis of rocks from Mount Paektu suggests its eruption actually put out 45 megatons of sulfur—far more than Tambora and more than 20 times past estimates based on ice cores in Greenland.
"The amount of potential sulfur is huge," says Kayla Iacovino, a volcanologist at Arizona State University in Tempe who led the study, a rare collaboration between North Korean, U.K., and U.S. scientists. The results, she adds, make Paektu’s apparently weak climactic effect all the more mysterious.
Researchers typically estimate the climate effects of ancient eruptions using a mix of rock, ice, and tree-ring records. Sulfur, which turns into sun-reflecting, planet-cooling sulfate particles once it hits the stratosphere, eventually settles around the globe, where it is often preserved in ice cores. But cores in Greenland corresponding to 946 C.E. don’t hold much sulfur, and there is little evidence in the cores and tree rings for a dip in global temperatures at the time. That has led many volcanologists to believe that, although Paektu packed a local punch, it left little in the way of a global legacy.
Iacovino and her co-authors returned to Paektu’s pumice for their new estimates. Scientists have done this elsewhere by measuring the gas content of globs of magma that are preserved, like time capsules, in crystals right before an eruption and comparing it with the gas content of rocks hardened out of magma following the blast. This technique allows scientists to estimate the amount of liquefied gasses—including sulfur—spewed into the air. But it also misses any sulfur that had already shifted into a gas phase in Paektu’s magmatic stew before the eruption. And volcanoes rich in silica like Paektu, which tend to have chunky, viscous magmas, seem to hold a lot of gas. "Imagine a ball of peanut butter, and trying to inject gas into it," Iacovino says. "It's going to be pretty easy for the peanut butter to hold on to it."
To estimate that missing sulfur, Iacovino and her co-authors had to model the crystallization of the magma as it cooled in its reservoir. Certain elements form crystals relatively easily, whereas others, like uranium, resist crystallization. After estimating sulfur's crystallization rate, they could then compare how much was preserved in the glassy blobs with the amount of uranium. The difference indicated how much sulfur gas had already percolated out—fully 42 megatons, as the team reports today in Science Advances.
Iacovino and her colleagues offer a few reasons why this huge amount of sulfur didn’t seem to cool the planet—or leave much of a mark in Greenland’s ice cores. Paektu is a high-latitude volcano, which means emissions from eruptions tend to stay within the same hemisphere and, thus, have a smaller global cooling effect. The sulfate particles from high-latitude volcanoes also tend to fall out of the stratosphere more quickly than those from tropical eruptions like Tambora. Finally, there is evidence that Paektu blew its top in winter, which would have damped its climatic effect because there would have been less sun for its sulfate particles to reflect.
These explanations aren't convincing, says Alan Robock, a climate scientist at Rutgers University in New Brunswick, New Jersey, who specializes in stratospheric sulfates. "With the large sulfur emissions they claim, there would certainly be deposition in Greenland even if the eruption was in the winter," he says. A simpler explanation might be that the sulfur slowly burped up before the eruption, leaving the emissions from the main blast closer to earlier projections, says Ralf Gertisser, a volcanologist at Keele University in the United Kingdom.
But if the study's higher estimate holds, it should prompt concern about how the evidence from ice cores is used to measure past eruptions, Iacovino says. Rather than automatically assuming that a core gauges an eruption's emissions, it might be better to take it as a measure of its climatic effect. "People modeling the ancient atmosphere of Earth should think about that."
The study, which resulted from a 2013 visit organized by AAAS (the publisher of Science), was no typical collaboration. With her North Korean colleagues, Iacovino collected pumice dated to before and after the 946 eruption—a time that saw the rise of the Goryeo dynasty, which united the Korean peninsula and gave the modern country its name. Iacovino knew little about Asia, let alone Paektu, at the time. But volcanology brought the groups together. "When we were together in the field, we were talking about the science," she says. "We forgot about everything else."
But when it came to drafting the paper, things were not so simple. If Iacovino had a question for Kim Ju-Song, her co-author at North Korea's Earthquake Administration, she couldn't just pick up the phone. Instead she'd send an email to James Hammond, a volcanologist at the University of London who coordinated the project. Hammond would email the Chinese nongovernmental organization that connected the scientists to North Korea, and this nongovernmental organization would in turn contact Ju-Song. But most of their work still got done in person, which included one trip to London for the North Korean researchers.
Although this first round of work is nearing its end, Iacovino is convinced it’s the start of a sustained collaboration with North Korea’s volcanologists. After all, Paektu’s not done either. Although it hasn’t erupted since 1903, it started grumbling again this past decade.