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The connections that science can make are sometimes surprising. What connection can a lunar eclipse, volcanic eruptions that took place in the early 12th century, and the ice of Greenland or Antarctica have? Although it is unlikely, such a relationship has been revealed by a team of researchers coordinated by Sébastien Guillet of the University of Geneva.
The gate of hell
For years, the origin of the sulfates found in the cores of ice blocks in Greenland and Antarctica has been debated in the scientific media. A large volcanic eruption, which occurred in the 12th century, was the most widely accepted cause for the ejection of these sulfates into the atmosphere, which were later deposited on the ice and which, over the centuries, were buried deep in the frozen masses. But there was debate about which volcano could have been guilty of these eruptions.
The sulfates were originally thought to come from an eruption of the impressive Hekla volcano, one of the most active in Iceland. This volcano, which is part of a long volcanic chain, has been active since prehistory. The most recent one took place in the year 2000, but particularly violent was that of 1947, which projected fire and ashes up to 30 kilometers high . And an even much more violent eruption is documented in 1104 AD. After it, almost half of the island was covered by ejected volcanic materials, and populations up to 70 kilometers from the volcano had to be abandoned. This eruption made Hekla famous in medieval Europe, naming it 'the gate of hell'.
It therefore seemed highly probable that this large eruption was responsible for the sulfates found in the Greenland ice, but doubts remained about the transport of these sulfates to Antarctica. The appearance of the same sulfates near the two terrestrial poles pointed more towards an eruption from the tropics.
On the other hand, a study published in Nature in 2015 by climate specialist Michael Sigl from the University of Bern showed that something else did not fit the Hekla hypothesis. In fact, a very precise dating of the sulfates (carried out by chemical methods) indicated that their deposition began around 1108 and ended around 1113 AD. Thus, this research group suggested that the Hekla had nothing to do with these sulfates. and that it was more likely that the cause of the phenomenon was some other volcanic eruption (not yet identified) that could have taken place in the tropics around 1108-1109.
The help of the Moon
Guillet and his group then went looking for some other natural phenomenon that could have taken place in the early 12th century and that could help decide between the two hypotheses. They thought about the effects volcanic aerosols ejected into the atmosphere have on the brightness of lunar eclipses. Indeed, the appearance of the Moon can be very different from one eclipse to another, and the brightness of the eclipse is described by the Danjon scale that goes from 0 (Very dark moon) to 4 (Copper moon, brightest), scale which is illustrated in the image that heads this article.
Volcanic aerosols, which remain in the stratosphere after an eruption, are possibly the largest factor causing the dimming and discoloration of the eclipsed moon. In fact, the darkest eclipses since the 17th century are clearly related to well-known volcanic eruptions. These are the eclipses that occurred in 1601, 1642,1816, 1884, 1913, 1983 and 1992. The particular darkness of these eclipses is directly related to known eruptions of the Huaynaptunina, Parker, Tambora, Krakatoa, Katmai-Novarupta, El Chichón and Pinatubo, respectively, which took place shortly before those eclipses happened.
The attached image shows a drawing of the Moon during the eclipses of 1884 and 1888 that took place one year and four years after an eruption of Krakatoa. It is clearly seen that the first was much darker than the second . During the first there was a rare 'black moon' and during the second a much more common 'blood moon'.
Examining the eclipse tables and historical archives, Guillet's group came up with one that took place exactly on May 5, 1110 and which had long attracted astronomers' attention because of its darkness. Many historical sources refer to him, some relate that the eclipsed moon, instead of having its familiar blood red color, appeared completely black. For example, Guillet's article reproduces the following quote from The Anglo Saxon Peterborough Chronicle : "On the fifth night of the month of May the moon appeared shining at night, and little by little its light diminished, so that, as soon as Night came, it was completely extinguished, without light, orb, or anything like that was seen. And it continued like that until almost the day ... "
Interdisciplinarity
It is thus concluded that, in fact, it was not the Hekla, but another volcano that caused the sulfates deposited in Greenland and in Antarctica. Scientists speculate that the eruption suffered by Mount Asama in Japan in 1108 may have been responsible. But again, transporting the ashes to Antarctica through the tropics poses a problem. It seems more likely that another eruption (or a series of eruptions) had yet to be identified.
We therefore see how a good scientific culture is key to establishing bridges between different disciplines. The superspecialization to which current researchers are subjected does not help in carrying out these types of studies that are straddling different knowledge .
The article by Guillet et al., Entitled "Climatic and societal impacts of a" forgotten "cluster of volcanic eruptions in 1108-1110 CE", has been published in the journal Scientific Reports and can be consulted at this link.
Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.
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