“And it came about during this year that a most dread portent took place. For the sun gave forth its light without brightness, like the moon, during this whole year, and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear nor such as it is accustomed to shed.” – the Byzantine historian Procopius in 536 CE.
Historical sources describing a fog covering the sun and reduced growth seen in tree rings suggest that erupting volcanoes caused a dramatic cooling in Europe after the year 536, coinciding with a time of general crisis and local catastrophes, such as the Plague of Justinian in 541-542. Geologists believe that two powerful volcanic eruptions, the largest in the last 2,000 years, are to blame. If an eruption is powerful enough to send volcanic ash and gases high into Earth’s atmosphere, the resulting haze can shield the surface from the sunlight, causing a drop in temperatures worldwide. It remains uncertain to this day which volcanos erupted in 536 and 540/541. Based on traces of sulfur preserved in ice-layers recovered from the ice shield of Greenland and Antarctica some geologists argued that the volcano was located somewhere along the equator. Possible candidates include the Tavurvur in Papa-Neuguinea, the Krakatau in Indonesia or the Ilopango in El Salvador.
However, now a team of scientists studying an ice-core recovered from a glacier covering the Signalkuppe or Punta Gnifetti, a 4,554-meter peak in the Pennine Alps on the border between Italy and Switzerland, argue that the eruption of 536 actually happened in Iceland. The 72-meter-long core covers more than 2,000 years and the single ice-layers record things like fallout from volcanoes, Saharan dust storms and human pollution of the atmosphere. The research describes also a layer of volcanic ash preserved in the ice, dated around 536. Using X-rays to determine the chemical composition of the recovered ash particles, the researchers discovered that the chemical composition matches exactly the chemical composition of volcanic rocks from Iceland. In this scenario, the volcano erupted in Iceland and atmospheric circulation transported the ash over the Atlantic and Europe into the Alps, very similar to what happened in 2010 with the Eyjafjallajökull volcano. As for the 540/541 eruption, recorded in the ice from Greenland and the South Pole, the volcano that caused ash to spread to the two poles of Earth still remains to be identified.