Tree rings help dating eruption of Icelandic volcano

Iceland is now mostly treeless and barren, but there was a time when the island was covered with forests. Early settlers, who arrived in the late 9th century, harvested most of the trees they found on the island to establish an agricultural-based society.

In 2003, a spring flood of the Thverá River exposed hundreds of birch trees which had been buried for centuries beneath layers of volcanic sediment. The so-called Drumbabót forest is the best-preserved prehistoric forest in Iceland, and had been buried by an eruption of the nearby Katla volcano.

Large volcanic eruptions can be the cause of prolonged lower temperatures, but only with a precise date of eruption can researchers definitively account for the variability in climate. Thee rings contain information which can be used to reconstruct past climate conditions,

The trees that were uncovered in 2003 were studied by a team of researchers[1]. They were able to pinpoint the eruption date of the Katla volcano between late 822 and early 823, decades before the earliest settlers arrived.

The same team have previously confirmed that in 775 a large solar flare caused a spike in radiocarbon levels in the Earth’s atmosphere, which would be stored in the wood of trees that were alive at the time[2]. By measuring the radiocarbon levels in one of the Drumbabót trees, Ulf Büntgen and his colleagues were able to pinpoint the year 775 in the tree rings, and measure outward to the bark to count the number of years to the Katla eruption, when the tree died. The outermost tree ring had completely formed and a new one had not yet started, meaning that the eruption occurred after autumn 822 and before spring 823, before the next year’s growth had begun. Iceland was not settled until around 870, so this particular forest was destroyed almost half a century before humans arrived.

The unique tree ring results were then linked with those of co-authors Professors Christine Lane and Clive Oppenheimer. Lane and Oppenheimer used independent lines of ash (tephra) and ice core evidence to detect fingerprints of the Katla eruption.

The team also involved historians who analysed written documentary evidence from Europe and Asia, and found that there was a severe cold spell consistent with the timing of the reconstructed Katla eruption.

[1] Büntgen et al: Multi-proxy dating of Iceland’s major pre-settlement Katla eruption to 822-823 CE in Geology – 2017
[2] Büntgen et al: Extraterrestrial confirmation of tree-ring dating in Nature Climate Change - 2014