El Salvador's Ilopango volcano
The sixth century was a rough time to be alive: Lower-than-average temperatures in the Northern Hemisphere triggered crop failure, famine, and maybe even the onset of bubonic plague. The ultimate culprit, scientists say, were two back-to-back volcanic eruptions — one in 536 C.E. and another around 540 C.E. The first likely happened in Iceland or North America. But the location of the second one has remained a mystery — until now.

Researchers studying ancient deposits from El Salvador's Ilopango volcano knew that a massive eruption had taken place there sometime between the third and sixth centuries. That event, dubbed Tierra Blanca Joven (TBJ), or "white young earth," sent a volcanic plume towering nearly 50 kilometers into the atmosphere.

To better pin down the date of this eruption, the scientists collected slices from three tree trunks embedded in TBJ volcanic ash 25 to 30 kilometers from the present-day lake that covers the caldera (above). The tropical hardwood trees likely died after being engulfed by the searing hot, gale-force winds containing the volcanic gases, ash, and pumice that would have swept outward after the eruption.

Back in the lab, the researchers estimated the ages of different parts of the slices by counting their rings and using carbon-14 dating. The multiple measurements yielded much more precise dates than could have been gotten from single measurements.

The three trees all died between 500 and 545 C.E., dates that suggest the TBJ eruption was the mysterious 540 C.E. volcanic event, the researchers report today in Quaternary Science Reviews. In fact, their dating may be even more precise than that: Based on atmospheric circulation patterns, the researchers estimate that the eruption actually occurred in the fall of 539 C.E. That would help explain the era's ongoing global cooling and famine — and could even shed light on a mysterious, temporary break in monument building by the Maya.