BOMB-PRODUCED C-14 IN TREE RINGS

The C-14 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), DELTAC-14 values rose dramatically from 250 parts per thousand in 1961 to a maximum 909 parts per thousand in 1964, and then gradually decreased to 238 parts per thousand in 1983. We compared DELTAC-14 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 DELTAC-14 values in both northern and southern hemispheres. We observe that: 1) annual tree rings grown in the same latitude zone have the same DELTAC-14 values, reflecting rapid longitudinal mixing of the atmosphere; 2) atmospheric C-14 concentrations reached a global equilibrium distribution at the end of 1968, and tree ring C-14 content reflects atmospheric C-14 concentration; 3) 1976-1982 rings of the Dailing spruce show excessive C-14, likely due to the effect of 1976 and 1980 Chinese bomb tests; 4) DELTAC-14 decreases exponentially, halving every 17 yr.