Bomb-Produced 14C in Tree Rings

Abstract

The 14C content in 1961-1967 and 1970-1983 tree rings of a spruce grown in Dailing, China (47 degrees N, 129 degrees E) was measured by liquid scintillation. As a result of USSR bomb tests at Novaya Zemlya (72 degrees N, 53 degrees E), Delta-14C values rose dramatically from 250 per mil in 1961 to a maximum 909 per mil in 1964, and then gradually decreased to 238 per mil in 1983. We compared Delta-14C values in the rings of an oak tree grown at 43 degrees N, 74 degrees W and that of a pine grown at 49 degrees N, 9 degrees E, and atmospheric Delta-14C values in both northern and southern hemispheres. We observe that: 1) annual tree rings grown in the same latitude zone have the same Delta-14C values, reflecting rapid longitudinal mixing of the atmosphere; 2) atmospheric 14C concentrations reached a global equilibrium distribution at the end of 1968, and tree ring 14C content reflects atmospheric 14C concentration; 3) 1976-1982 rings of the Dailing spruce show excessive 14C, likely due to the effect of 1976 and 1980 Chinese bomb tests; 4) Delta-14C decreases exponentially, halving every 17 yr.