Radiocarbon concentration in modern tree rings from Fukushima, Japan.

A 30-year-old Japanese cedar (Cryptomeria japonica), collected from Iwaki, Fukushima in 2014, was analyzed for the long-lived radionuclide 14C. Values of Δ14C varied from 211.7‰ in 1984 to 16.9‰ in 2013. The temporal Δ14C variation can be described as an exponential decline, indistinguishable from the general Northern Hemisphere Zone 2 (NH Zone 2) values in the atmosphere, until at least 1994. Values of Δ14C for 1999 and 2004 are slightly depleted compared with NH Zone 2 values, while from 1999 to 2013 the data suggest a clear depletion with a 2–8 ppmV additional CO2 contribution from a 14C-free (i.e. fossil carbon) source. This change coincides with local traffic increases since two nearby expressways were opened in the 1990's. In addition, the small but visible 14C pulse observed in the 2011 tree-ring might be caused by release from the damaged reactors during the Fukushima nuclear accident.