Despite volcanoes’ ability to alter global climate and records of eruptions dating back thousands of years, some recent major eruptions have gone relatively unnoticed.
Researchers have now used ice core records to pin down the identity of a large volcanic eruption that occurred in the 19th century. The culprit is an edifice known as Zavaritskii on a remote island between Japan and the Russian Far East, the team deduced. This discovery resolves a long-standing volcanic mystery and highlights the importance of ice cores to volcanology.
“We really don’t have a great understanding of Earth’s volcanic record.”
In the early 1830s, temperatures in the Northern Hemisphere temporarily dropped by 0.5°C–1.0°C, according to tree ring and instrumental records. Such widespread cooling is a common signature of a large volcanic eruption because the sulfur dioxide belched by the eruptions catalyzes the formation of aerosols, which scatter sunlight.
But determining which volcano blew its proverbial top is difficult. There is no satellite imagery from the 19th century to mine, and historical observations of volcanic eruptions are generally limited to populated locales. “We really don’t have a great understanding of Earth’s volcanic record,” said Will Hutchison, a volcanologist at the University of St Andrews in the United Kingdom.
Hot Volcano, Cold Ice
To identify the volcano responsible for that eruption, Hutchison and his team turned to ice cores.
The precise timing of a volcanic eruption can be gleaned from the composition of air bubbles trapped in ice. An eruption’s chemical signature shows up in several ways, but “the key evidence is the sulfur spike,” Hutchison said. And there’s a lot of sulfur present starting around 1831 in ice cores extracted from both the Northern and Southern Hemispheres.
It’s not surprising that a large eruption detected in ice cores from disparate locations would have affected our planet’s climate, said Katharine Cashman, a volcanologist at the University of Oregon in Eugene who was not involved in the research. “It is these bipolar signals that are of particular interest because they are most likely associated with global climate impacts.”
Previous research showed that the sulfur peak around 1831 is more than 6 times stronger in the Greenland ice record than in the Antarctic ice record, however. “It’s quite a skewed signature,” said Hutchison, and that’s good evidence that the eruption occurred in the Northern Hemisphere.
Records in Glass
Another clue to the eruption’s provenance came in the form of tephra—bits of glassy volcanic ash—found in several Greenland ice cores. Ash from eruptions in the tropics is rarely transported to polar regions, so the eruption likely occurred in the midlatitudes, Hutchison said. But there are plenty of midlatitude Northern Hemisphere locales that are volcanically active. Alaska, Iceland, and Russia’s Kamchatka Peninsula all fit that bill. “These are all the kind of classic sources,” Hutchison said.
Hutchison and his colleagues analyzed the chemical composition of 45 pieces of tephra extracted from three Greenland ice cores. Each of those shards was only about one tenth the width of a human hair, but the researchers were able to precisely measure the concentrations of elements such as potassium and silicon.
“We started getting in touch with people who had been to those islands.”
They found that the tephra was a bit of an outlier, chemically speaking. “One of the distinguishing things about it is that it has very low potassium,” Hutchison said.
It did not chemically resemble tephra known from Alaska, Iceland, or Kamchatka. The team also ruled out Japan as the site of the eruption because there were no reports of large eruptions there in 1831.
But the Kuril Islands, a disputed archipelago between Japan and Kamchatka, are also volcanic. “We started getting in touch with people who had been to those islands,” Hutchison said.
New Life for Old Samples
One of those people was Breanyn MacInnes, a geologist at Central Washington University in Ellensburg. MacInnes visited the Kurils, which are about 700 kilometers (430 miles) northeast of Hokkaido, several times for her Ph.D. research from 2006 to 2010.
It was quite a journey to get there, MacInnes said. A flight would take her from the United States to Incheon, South Korea, and she’d then take another flight to the Russian island of Sakhalin. From there, she’d board a research vessel and sail for a day or two to reach the Kurils.
MacInnes collected sediment samples from coastal plains in the Kurils to better understand how tsunamis may have affected local populations. “We dug so many holes,” MacInnes said. Those sediment samples invariably included tephra, and MacInnes was more than happy to share.
When Hutchison and his colleagues analyzed MacInnes’s tephra samples, they found a remarkably good chemical match to their Greenlandic ice core tephra. In particular, a sample from a caldera known as Zavaritskii on Simushir Island stood out. “The fingerprints matched perfectly,” Hutchison said.
And there’s evidence that Zavaritskii erupted within the past few hundred years, Hutchison and his colleagues discovered. On Simushir Island, a layer of Zavaritskii tephra lies on top of Russian artifacts dating from the 1700s to early 1800s. That layering is stratigraphically consistent with an 1831 eruption of Zavaritskii.
A caldera that most of the world has never heard of—let alone could pinpoint on a map—is the likely cause of a measurable blip in Northern Hemisphere temperatures, the researchers concluded. These results were published in the Proceedings of the National Academy of Sciences of the United States of America.
These findings make sense, Cashman said. However, logical follow-on work would involve digging more into the geochemistry of the Zavaritskii tephra. In particular, it would be interesting to determine whether the magma was unusually high in sulfur, Cashman said. If so, the eruption might have had an outsized effect on the planet’s climate.
—Katherine Kornei (@KatherineKornei), Science Writer