Re-assessment of tree-ring radiocarbon age series for the Millennium Eruption of Changbaishan / Paektu volcano in relation to the precise date of 946 CE

Volcanic eruptions that are not historically attested are often dated by wiggle matching sequential radiocarbon measurements of the rings of trees killed by the eruption against a calibration curve. Although evidence for contamination by exposure to 14C-free geogenic carbon is well documented, it is generally assumed that the radiocarbon ages are unbiased by such old carbon. The 'Millennium Eruption' of Changbaishan / Paektu volcano (China/DPR Korea) provides an opportunity to test this assumption, since several age estimates were made before the acceptance of a precise eruption year of 946 CE. Accordingly, we develop a methodology that enables testing of assumptions and evaluation of discrepancies between reported wiggle match series. We then use it to correct for contamination levels of up to 4.5% old carbon to eight wiggle match date series for the Millennium Eruption. Without the need to discard ring ages, we find agreement indices as high as, or higher than, those for the published dates. Five of the eight series yielded high-agreement-index eruption dates closer to 946 CE than the published dates. None of the five yielded a best result without recognising some degree of contamination. Differences between the eruption dates reveal a weak association with the direction of the sampled tree from the caldera, but no relationship with distance. Our results suggest that old carbon contamination can lead to over-estimation of eruption ages by years, decades, or more. In addition, our results show that anomalously younger ages can also be a problem in radiocarbon series and these cannot be easily overcome at present by this methodology.

Automated summary

Volcanic eruptions without historical records are often dated by comparing radiocarbon measurements of tree rings killed by the eruption with a calibration curve. However, contamination from geogenic carbon has been overlooked in these measurements. This study shows that contamination can lead to over-estimation of eruption ages and also highlights the problem of anomalously younger dates in radiocarbon series, which cannot be easily addressed at present.