Fossilised tree helps date volcanic eruption 1,000 years ago

The ‘Millennium Eruption’ of Changbaishan (also known as Mount Paektu). The volcano is located on the border between China and North Korea. The eruption ranks among the largest medieval volcanic eruptions. It produced a widely-dispersed tephra layer. It was however unknown when this eruption precisely occured, with estimates spanning at least the tenth century CE.

Now, a team of scientists has analysed the partly fossilised remains of a tree killed by the eruption[1]. The tree was 264 years old when it was killed and buried by a flow of larva, hot ashand pumice. They knew that the tree must have been standing in 775 AD – a year that was marked by a burst of cosmic rays reaching the Earth – and this event was detected in the tree rings. Correlating ice core data made it certain that the eruption occurred in the last two or three months of 946 AD.

This secure date rules out the possibility that the 'Millennium Eruption' contributed to the collapse of the Bohai Kingdom (Manchuria/Korea) in 926 AD, as has previously been hypothesised. Further, despite the magnitude of the eruption, the data did not show a consequent cooling signal in tree-ring-based reconstructions of Northern Hemisphere summer temperatures.

Lead author, Clive Oppenheimer says: "The Millennium eruption has fascinated scientists and historians for decades because of its size, potential worldwide impacts, and the mystery surrounding when it actually happened. Lacking a clear historical record of the event, there have been dozens of attempts to date the eruption using conventional tree ring techniques. We got lucky thanks to the burst of cosmic radiation that bathed the Earth in the year 775. It was only recently recognised that this left a worldwide signature in trees alive at the time. Now we have a secure date for the eruption at last, we can be more confident in investigating the effects it has on the climate, environment and society."

The new date focuses attention on a chronicle from a temple in Japan that reports "white ash falling like snow" on the 3rd November 946 AD. This site is not near any of Japan's active volcanoes, and is close to where ash from the Millennium eruption has recently been identified in lake sediments. It may well pinpoint the actual date of the eruption since it would only have taken the ash clouds a day or so to reach Japan.

Changbaishan is a site revered by the Koreans. It is steeped in folklore and Koreans see it as their spiritual and ancestral home. Its eruption in 946 AD was one of the most violent of the last two thousand years and is thought to have discharged around 100 cubic kilometers of ash and pumice into the atmosphere - enough to bury the entire UK knee deep.

[1] Oppenheimer et al: Multi-proxy dating the ‘Millennium Eruption’ of Changbaishan to late 946 CE in Quaternary Science Reviews – 2017