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-236 as an oceanic circulation tracer based on depth profiles of U-236 and Cs-137 in the Japan/East Sea. The concentration of the predominantly anthropogenic U-236, measured with Accelerator Mass Spectrometry (AMS), decreased from (13 +/- 3) x 10(6) atom/kg in surface water to (1.6 +/- 0.3) x 10(6) 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-137 for the same water samples, but with a slightly lower ratio of Cs-137/U-236 below 2000 m. The cumulative inventory of dissolved U-236 in the water column was estimated to be (13.7 +/- 0.9) x 10(12) atom/m(2), which is similar to the global-fallout level (17.8 x 10(12) atom/m(2)) in Japan. Additional analyses of suspended solids (SS) and bottom sediments yielded negligible amounts of U-236. Our results suggest that U-236 behaves as a conservative nuclide in seawater, with potential advantages over other tracers of oceanic circulation. (C) 2012 Elsevier B.V. All rights reserved.