From the U-233/U-236 fingerprint towards an environmental tracer: Tracking down the sources of anthropogenic U-233

Recently, the atomic ²³³U/²³⁶U ratio was proposed as a superior oceanographic tracer as the ²³³U/²³⁶U signature allows to distinguish environmental emissions of civil nuclear industry from weapons fallout [1] and uranium (U) behaves conservatively in sea water. In this previous work, the ratios detected in representative compartments of the environment affected either by releases of nuclear power production or by weapons fallout differ by one order of magnitude and varied between 0.1·10-²and 3.7·10-². Significant amounts of ²³³U were only released in nuclear weapons fallout, either produced by fast neutron capture on ²³⁵U or directly by ²³³U fuelled devices. For tracer applications, a careful characterization of the principal sources of ²³³U including the contribution from natural production is required.

The ²³³U/²³⁶U ratios were analysed in samples from different locations, partly time-resolved or of well-known age to be able to narrow down the time span of the maximum ²³³U release. Samples comprised air filters from 1961-1965 collected in Austria, i.e. the period of maximum deposition of global fallout, two time-resolved sediment cores from the Baltic Sea and sediment from the urban layered archaeosphere in the underground of Vienna (Austria). In addition, a depth profile of ²³³U/²³⁶U in a water column located in the Northeast Pacific Ocean was also analysed. This sampling station is assumed to be less affected by tropospheric fallout from a suspected ²³³U fuelled device (Nevada Test Site, 1955). Whenever possible, the ²³³U/²³⁶U data were directly compared to other isotopic signatures and/or mono-isotopic makers, such as ²⁴⁰Pu/²³⁹Pu or ²⁴¹Am, respectively.

The Baltic Sea sediment core confirms a maximum deposition of ²³³U around 1954/1955 [2]. This finding is supported by the rather low ²³³U/²³⁶U ratios (below 0.5·10-²) on the air filters from Vienna indicating a comparatively low ²³³U deposition during global fallout maximum. Values of around 0.1 were measured for the maximum of the ²³³U/²³⁶U ratio in the Baltic Sea sediment. The isotope ratios including ²³³U/²³⁶U found in a layer of the Vienna underground material which was assigned to the 1960s, clearly point to atmospheric atomic bomb fallout. The ²³³U/²³⁶U ratios in the upper part of the Pacific water column showed very stable values of (1.33±0.13) ·10-² which are in good agreement with the published value for global fallout [1].

The new data confirms the previous result that the maximum releases of ²³³U happened before the global fallout maximum in 1963. The consistently high ratios found in the Pacific Ocean indicate at least a contribution from thermonuclear explosions to the global inventory of ²³³U. The unexpectedly high ²³³U/²³⁶U values in the Baltic Sea show the necessity to systematically identify the global and local ²³³U input sources. Nevertheless, our preliminary data indicate that the ²³³U/²³⁶U ratio serves as a potential marker for the on-set of the Anthropocene, even in the rather demanding urban environment.

References
[1] K. Hain, et al. Nat Commun 11(2020), 1275.
[2] M. Lin, et al. Environ. Sci. Technol. 55(2021), 13, 8918–8927