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

摘要

We present a feasibility study for using ~(236)U as an oceanic circulation tracer based on depth profiles of ~(236)U and ~(137)Cs in the Japan/East Sea. The concentration of the predominantly anthropogenic ~(236)U, measured with Accelerator Mass Spectrometry (AMS), decreased from (13±3)×10 ~6atom/kg in surface water to (1.6±0.3)×10 ~6atom/kg close to the sea floor (2800m). The profile has a smooth trend with depth and concentration values are generally proportional to that of ~(137)Cs for the same water samples, but with a slightly lower ratio of ~(137)Cs/ ~(236)U below 2000m. The cumulative inventory of dissolved ~(236)U in the water column was estimated to be (13.7±0.9)×10 ~(12)atom/m ~2, which is similar to the global-fallout level (17.8×10 ~(12)atom/m ~2) in Japan. Additional analyses of suspended solids (SS) and bottom sediments yielded negligible amounts of ~(236)U. Our results suggest that ~(236)U behaves as a conservative nuclide in seawater, with potential advantages over other tracers of oceanic circulation.