Almost a millennium ago, a major upheaval occurred in Earth’s atmosphere: a giant cloud of sulphur-rich particles flowed throughout the stratosphere, turning skies dark for months or even years, before ultimately falling down to Earth.
We know this event happened because researchers have drilled and analysed ice cores – samples taken from deep within ice sheets or glaciers, which have trapped sulphur aerosols produced by volcanic eruptions reaching the stratosphere and settling back on the surface.
Ice can thus preserve evidence of volcanism over incredibly long timescales, but pinpointing the precise date of an event that shows up in the layers of an ice core is still tricky business.
In this case, scientists had assumed the sulphurous deposit was left by a major eruption unleashed in 1104 by Iceland’s Hekla, a volcano sometimes called the ‘Gateway to Hell’. Since the thin strip of ice ranks among the largest sulfate deposition signals of the last millennium, it sounds plausible.
Only, what if the accepted timeline of an ice core turns out to be time-warped? A few years ago, one study concluded that a timescale called the Greenland Ice Core Chronology 2005 (GICC05) was off by up to seven years in the first millennium CE, and by up to four years early in the next millennium.
Those findings, according to research published in April 2020 – led by palaeoclimatologist Sébastien Guillet from the University of Geneva in Switzerland – mean Hekla couldn’t have been the culprit for the giant sulphate signal after all.
“A prominent discovery arising from this revised ice-core dating is a major and hitherto unrecognised bipolar volcanic signal with sulfate deposition starting in late 1108 or early 1109 CE and persisting until early 1113 CE in the Greenland record,” Guillet and his co-authors explain in their paper, noting that evidence for the same event can also be seen in a similarly revised Antarctic ice core chronology.
To investigate what might have been responsible for leaving these ancient tracks at both the top and the bottom of the world, the team combed historical documentation, looking for medieval records of strange, dark-looking lunar eclipses that could correspond to the stratospheric haze of major eruptive events.
“The spectacular atmospheric optical phenomena associated with high-altitude volcanic aerosols have caught the attention of chroniclers since ancient times,” the team writes.
“In particular, the reported brightness of lunar eclipses can be employed both to detect volcanic aerosols in the stratosphere and to quantify stratospheric optical depths following large eruptions.”
According to NASA records based on astronomical retrocalculation, seven total lunar eclipses would have been observable in Europe in the first 20 years of the last millennium, between 1100 and 1120 CE.
Among these, a witness to a lunar eclipse that occurred in May 1110 wrote of the exceptional darkness of the Moon during the phenomenon.
“On the fifth night in the month of May appeared the Moon shining bright in the evening, and afterwards by little and little its light diminished, so that, as soon as night came, it was so completely extinguished withal, that neither light, nor orb, nor anything at all of it was seen,” an observer wrote in the Peterborough Chronicle.
Many astronomers have since discussed this mysterious and unusually dark lunar eclipse. Centuries after it occurred, the English astronomer Georges Frederick Chambers wrote about it, saying: “It is evident that this [eclipse] was an instance of a ‘black’ eclipse when the Moon becomes quite invisible instead of shining with the familiar coppery hue”.