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Isotopic ratios of radioactive releases into the environment are useful signatures for contamination source assessment. Uranium is known to behave conservatively in sea water so that a ratio of uranium trace isotopes may serve as a superior oceanographic tracer. Here we present data on the atomic 233 U/ 236 U ratio analyzed in representative environmental samples finding ratios of (0.1-3.7)Á10 À2 . The ratios detected in compartments of the environment affected by releases of nuclear power production or by weapons fallout differ by one order of magnitude. Significant amounts of 233 U were only released in nuclear weapons fallout, either produced by fast neutron reactions or directly by 233 U-fueled devices. This makes the 233 U/ 236 U ratio a promising new fingerprint for radioactive emissions. Our findings indicate a higher release of 233 U by nuclear weapons tests before the maximum of global fallout in 1963, setting constraints on the design of the nuclear weapons employed.

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