Increases in 14C concentrations in tree rings could be attributed to cosmic-ray events1, 2, 3, 4, 5, 6, 7, as have increases in 10Be and nitrate in ice cores8, 9. The record of the past 3,000 years in the IntCal09 data set10, which is a time series at 5-year intervals describing the 14C content of trees over a period of approximately 10,000 years, shows three periods during which 14C increased at a rate greater than 3‰ over 10 years. Two of these periods have been measured at high time resolution, but neither showed increases on a timescale of about 1 year (refs 11 and 12). Here we report 14C measurements in annual rings of Japanese cedar trees from ad 750 to ad 820 (the remaining period), with 1- and 2-year resolution. We find a rapid increase of about 12‰ in the 14C content from ad 774 to 775, which is about 20 times larger than the change attributed to ordinary solar modulation. When averaged over 10 years, the data are consistent with the decadal IntCal 14C data from North American and European trees13. We argue that neither a solar flare nor a local supernova is likely to have been responsible.
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- Supplementary Information (489K)
This file contains Supplementary Text and Data 1-6, Supplementary Figures 1-2, Supplementary Tables 1-3 and additional references.