Katherine Kornei

Sulfur is also indicative of a past eruption. Sulfur dioxide, a gas commonly belched by erupting volcanoes, reacts with water in the atmosphere to create sulfate aerosols. These tiny particles can linger in the stratosphere for years, riding wind currents, but they, like tephra, eventually fall back to Earth.

The ice also carries a time stamp. Dr. McConnell and his colleagues look for variations in elements like sodium, which is found in sea spray that’s seasonally blown inland. By simply counting annual variations in these elements, it’s possible to trace the passage of time, Dr. McConnell said. “It’s like a tree-ring record.”

Dr. McConnell and his collaborators recently analyzed six ice cores drilled in the Arctic. In layers of ice corresponding to the early months of 43 B.C., they spotted large upticks in sulfur and, crucially, bits of material that were probably tephra. The timing caught the scientists’ attention.

Researchers have previously hypothesized that an environmental trigger may have helped set in motion the crop failures, famines and social unrest that plagued the Mediterranean region at that time. But until now, “There hasn’t been the kind of data that these scholars brought forth to really get those theories into the mainstream,” said Jessica Clark, a historian of the Roman Republic at Florida State University who was not involved in the research.

Gill Plunkett, a paleoecologist at Queen’s University Belfast, set out sleuthing. After extracting 35 pieces of tephra from the ice, she pored over the rock chemistry of likely volcanic suspects. Nicaragua’s Apoyeque. Italy’s Mount Etna. Russia’s Shiveluch.

But it was Okmok, a volcano in Alaska’s Aleutian Islands, that turned out to be the best match, at least on paper. Sealing the deal would require testing two tephra samples — one from the ice and one from Okmok — on the same instrument.

Dr. Plunkett arranged for a tephra handoff at a conference in Dublin. A colleague from the Alaska Volcano Observatory, Kristi Wallace, packed four bags of Okmok tephra in her carry-on luggage. The match was spot on, Dr. Plunkett said. “There are some events that are tricky. With Okmok, there’s nothing else that looks like it.”

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