First study on U-236 in the Northeast Pacific Ocean using a new target preparation procedure for AMS measurements

摘要

We succeeded in obtaining the depth profile of U-236 for a sampling station in the Northeast Pacific Ocean using only one litre of seawater sample from each depth. For this purpose, a new procedure was developed that allowed for the preparation of accelerator mass spectrometry targets for trace uranium using only 100 mu g of iron carrier material. The U-236 concentrations in water samples from the Northeast Pacific Ocean showed large variations from (9.26 +/- 0.42) x 10(6) atoms/kg at 60 m depth to (0.08 +/- 0.02) x 10(6) atoms/kg at a depth of 3000 m. The high U-236 concentrations in surface water reflect the input of U-236 by global and local" fallout from nuclear weapons tests. The low U-236 concentrations in seawater from 1500 m and below are an indicator for'the low vertical diffusion of surface water to deeper layers in the North Pacific Ocean. The total inventory of U-236 on the water column was (8.35 +/- 0.23) x 10(12) atoms/m(2), which is lower compared to those of other ocean regions solely affected by global fallout on comparable latitudes. This study represents the first dataset for U-236 in the Pacific Ocean and shows the possibility of downsizing sample volumes which may help in future applications of U-236 as tracer for large ocean areas. (C) 2016 Elsevier Ltd. All rights reserved.