| Summary: | The 14C content in 1961-1967 and 1970-1983 tree rings of a spruce grown in Dailing, China (47N, 129E) was measured by liquid scintillation. As a result of USSR bomb tests at Novaya Zemlya (72N, 53E), ∆14C values rose dramatic ally from 250 in 1961 to a maximum 909 in 1964, and then gradually decreased to 238 in 1983. We compared ∆14C values in the rings of an oak tree grown at 43N, 74W and that of a pine grown at 49N, 9E, and atmospheric ∆14C values in both northern and southern hemispheres. We observe that: 1) annual tree rings grown in the same latitude zone have the same ∆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) ∆14C decreases exponentially, halving every 17 yr.
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