Darkness on the Earth

Researchers presented underwater volcanism as a new interpretation of why the sky mysteriously went dark for over a year beginning 536 A.D., with some parts of Europe and Asia seeing the Sun for only about four hours a day and accounts saying the Sun gave no more light than the Moon for 18 months. The dimming led to catastrophic global cooling, famine, and civil upheavals in Medieval times.

“People thought it was the end of the world,” said Dallas Abbott who studies paleoclimate and extraterrestrial impacts at Columbia University’s Lamont-Doherty Earth Observatory.

536 A.D. was also one of the worst periods to be alive, according to Harvard University medieval historian Michael McCormick, who is not part of the study.

“And it came about during this year that a most dread portent took place,” wrote Byzantine historian Procopius. “For the sun gave forth its light without brightness, like the moon, during the whole year.”

This period of darkness and cooling marked the beginning of a longer upheaval period as trees struggled to grow from 536 to 555 A.D., indicating that the dimming was extensive, and experts have no idea exactly why.

Abbott and her colleague John Barron from the US Geological Survey presented their analysis in a poster at the meeting of the American Geophysical Union.

Their study of a Greenland ice core pointed to underwater eruptions that carried sediments and marine microorganisms into the atmosphere, which helped dim the sunlight.

Volcanic eruptions are known to spew sulfur and other particles into the atmosphere that can block out sunlight. However, geological records only showed big eruptions in 536 and 541 A.D., which are not sufficient in explaining the nine-year downward spike in tree growth.

Moreover, it would require a lot of sulfur and ash to darken the sky, and some of that material should be present in rock layers and ice cores.

However, “the amount of sulfate that was deposited wasn’t as much as in other eruptions where they experience a similar amount of dimming,” Abbott explained.

Number of fossils from tropical areas deposited in the Greenland ice during the 6th century (blue line). Sulfate levels in the ice core (black line). Image credit: Dallas Abbott/LDEO

The researchers examined ice layers from an ice core named GISP2 that were laid down between 532 and 542 A.D and measured the chemistry of the meltwater. They also extracted microscopic fossils.

“We found by far the most low-latitude microfossils that anybody’s ever found in an ice core,” said Abbott.

The researchers were only able to identify one high-latitude species that were believed to be blown into the atmosphere by underwater volcanic eruptions near the equator.

These submarine eruptions around 536 and 538 A.D. would have vaporized seawater with the rising steam carrying calcium-laden sediments and microscopic sea creatures into the atmosphere.

Some of the particles would have eventually settled in the Arctic.

Equatorial volcanic eruptions can affect the entire planet. Once the white sediments and microorganisms were in the atmosphere, it would have been very good at reflecting sunlight back into space. These are also difficult to detect in sediment records.

There is a chance that space rocks hitting near the equator could have thrown the sediments and microfossils into the air, but the ice core chemistry and lack of cosmic dust in the layers make this less likely.

“If there were impact events, they would have to be relatively small.”

Abbott and her team said they would like to study another Greenland ice core next to see whether they can replicate the surprising findings.

Related:

“New ice core evidence for a volcanic cause of the A.D. 536 dust veil” – Larsen et al. – Nature – DOI: 10.1029/2007GL032450

Abstract

New and well‐dated evidence of sulphate deposits in Greenland and Antarctic ice cores indicate a substantial and extensive atmospheric acidic dust veil at A.D. 533–534 ± 2 years. This was likely produced by a large explosive, near equatorial volcanic eruption, causing widespread dimming and contributing to the abrupt cooling across much of the Northern Hemisphere known from historical records and tree‐ring data to have occurred in A.D. 536. Tree‐ring data suggest that this was the most severe and protracted short‐term cold episode across the Northern Hemisphere in the last two millennia, even surpassing the severity of the cold period following the Tambora eruption in 1815.

Featured image credit: jplenio/Pixabay

See also: https://blogs.ei.columbia.edu/2019/12/17/undersea-volcanism-medieval-darkness/

Starting in 536 A.D., the sky went dark for more than a year. In some parts of Europe and Asia, the sun only shone for about 4 hours a day, and “accounts say the sun gave no more light than the moon,” says Dallas Abbott, who studies paleoclimate and extraterrestrial impacts at Columbia University’s Lamont-Doherty Earth Observatory. The mysterious dimming of the sun brought on global cooling, famines, and civil upheavals; the Chinese reported eclipses that still can’t be explained today. Naturally, “people thought it was the end of the world,” says Abbott.

The world didn’t end then, of course, but this period of intense dimming and cooling was the start of a longer period of upheaval. Trees struggled to grow from 536 to 555 A.D., suggesting that the solar dimming was extensive, and scholars don’t know exactly why. Last week, in a poster at the meeting of the American Geophysical Union, Abbott and her colleague John Barron from the U.S. Geological Survey presented a new interpretation of the event. Their analysis of a Greenland ice core points to underwater eruptions that carried sediments and marine microorganisms into the atmosphere, where they helped to dim the sunlight.

Researchers discovered a high number of fossils from tropical areas (blue line) deposited in the Greenland ice during the 6th century. This indicates that underwater eruptions near the equator may have contributed to epic sky-dimming during 536-537 A.D. (The black line shows sulfate levels in the ice core, an indicator of another type of volcanic eruption.) Image: Dallas Abbott/LDEO

Volcanic eruptions have been known spew sulfur and other particles into the atmosphere that can block out sunlight. But geological records only indicate big eruptions in 536 and 541, which aren’t enough to explain the nine-year downward spike in tree growth. In addition, it would require a lot of sulfur and ash to darken the sky so much, and some of that material should be visible in rock layers and ice cores. However, says Abbott, “the amount of sulfate that was deposited wasn’t as much as in other eruptions where they experience a similar amount of dimming.”

That led her and Barron to suspect that perhaps impacts from space rocks could have thrown up enough dust to cause the dimming. But now, after analyzing a Greenland ice core, they have another theory.

From an ice core named GISP2, the scientists carefully analyzed ice layers laid down between 532 and 542 A.D. They measured the chemistry of the meltwater, and extracted microscopic fossils to study them under a microscope.

Surprisingly, the layers of the ice core contained 91 fossils of microscopic species that would have lived in warm, tropical waters. “We found by far the most low-latitude microfossils that anybody’s ever found in an ice core,” says Abbott. By comparison, they were only able to identify one high-latitude species in the mix.

A few examples of microfossils found in the Greenland ice core. These species would have lived in warm tropical or subtropical waters. So what were they doing in Greenland? Image: Dallas Abbott/LDEO

How did all those warmth-loving tropical and subtropical species get all the way up onto the Greenland ice sheet?

The team suspects they were blown into the atmosphere by underwater volcanic eruptions near the equator. Rather than emitting lots of sulfur, these submarine eruptions (in approximately 536 and 538 A.D.) would have vaporized seawater, the rising steam carrying calcium-laden sediments and microscopic sea creatures into the atmosphere. After floating around the atmosphere for a while, some of these particles would have eventually settled in the Arctic.

Equatorial volcanic eruptions in particular can affect the entire globe and, once in the atmosphere, the white sediments and microorganisms would have been very good at reflecting sunlight back into space. They’re also difficult to detect in sediment records, which explains why they hadn’t been noticed before.

There is still a slight possibility that space rocks striking near the equator could have thrown the sediments and microfossils into the air, but the ice core chemistry and lack of cosmic dust in the layers makes this hypothesis less likely. “If there were impact events, they would have to be relatively small,” says Abbott.

Next, she and her team would like to analyze another Greenland ice core to see whether they can replicate these surprising results.

****