Deposition of 236U from atmospheric nuclear testing in Washington state (USA) and the Pechora region (Russian Arctic)

Stratospheric fallout-derived 236U has been detected by sector field ICPMS at two field locations for which our laboratory possessed available archived samples: A) four soil cores from Washington state (northwestern USA) and B) sediment cores from three small lakes in the Pechora region (Russian Arctic). Four Washington state soil cores exhibit 236U inventories of 8.1 ± 1.3, 11.1 ± 0.9, 18 ± 2, and 30.2 ± 3.9 Tatoms/m2; the respective 239Pu contents are 52.9 ± 3.5, 67 ± 3, 71 ± 2, and 151 ± 2 Tatoms/m2. A 236U/239Pu atom ratio of 0.19 ± 0.04 (1 SD) has been determined from the Washington state soil cores. The three Pechora region lake cores each exhibit coincident maxima in their 236U and 239Pu atom concentration profiles. The 236U/238U atom ratios are controlled by two independent factors; 236U is from fallout deposition and 238U concentrations are a property of the geochemical distribution of naturally occurring U. A 236U/238U atom ratio as high as 8.9 × 10−6 has been observed for acid-leached soils containing Pu solely derived from bomb-test fallout. Accordingly, a non-zero 236U background from stratospheric fallout must be recognized and taken into account when detectable 236U is used to infer specific local or regional influences of reactor-irradiated U.

► The isotope 236U, originating from atmospheric nuclear weapons tests, has been identified at two locations in the Northern Hemisphere.
► Peak 236U concentrations match peak 239Pu concentrations in sediment cores.
► The 236U/239Pu atom ratio is 0.19 ± 0.04.