Uranium-236 as a new oceanic tracer: A first depth profile in the Japan Sea and comparison with caesium-137.

We present a feasibility study for using U as an oceanic circulation tracer based on depth profiles of U and Cs in the Japan/East Sea. The concentration of the predominantly anthropogenic U, measured with Accelerator Mass Spectrometry (AMS), decreased from (13±3)×10 atom/kg in surface water to (1.6±0.3)×10 atom/kg close to the sea floor (2800 m). The profile has a smooth trend with depth and concentration values are generally proportional to that of Cs for the same water samples, but with a slightly lower ratio of Cs/U below 2000 m. The cumulative inventory of dissolved U in the water column was estimated to be (13.7±0.9)×10 atom/m, which is similar to the global-fallout level (17.8×10 atom/m) in Japan. Additional analyses of suspended solids (SS) and bottom sediments yielded negligible amounts of U. Our results suggest that U behaves as a conservative nuclide in seawater, with potential advantages over other tracers of oceanic circulation.