Distribution of plutonium and americium in the marginal seas of the northwest Pacific Ocean

Plutonium isotopes and 241Am were studied in seawater and sediment from the East China Sea, the Yellow Sea, the East Sea/Sea of Japan, the Sea of Okhotsk, and the northwest Pacific Ocean, collected between 1993 and 1996 with the aim to contribute to better understanding the behaviour of plutonium and americium in the marine environment. 239,240Pu concentrations in surface water varied from 2.3 to 13 μBq l−1 in the East China Sea and from 3.5 to 9.4 μBq l−1 in the East Sea. The 239,240Pu vertical profiles in water showed a broad subsurface maximum between 500 and 1000 m with a range of 30–40 μBq l−1, and gradually decreased from 1000 m depth down to the seafloor. 241Am concentrations in surface water showed values from 1.1 to 2.2 μBq l−1 in the East Sea, from 1.0 to 3.1 μBq l−1 in the Sea of Okhotsk, and from 0.68 to 12.0 μBq l−1 in the northwest Pacific Ocean. The activity ratios of 241Am/239,240Pu in seawater showed values similar to the global fallout ratio, which suggests that the source of these radionuclides in the northwest Pacific Ocean is global fallout. However, the 241Am/239,240Pu activity ratios in sediment were found to be much higher (1.0–1.9) than the global fallout ratio (0.37), confirming that 241Am is scavenged from the water column more rapidly than 239,240Pu. The 239,240Pu inventories in the water column of the East Sea were from 0.98 to 93 Bq m−2 depending on water depth and sedimentation rates. The 241Am inventory in the water column east of Kamchatka was 6.3±1.0 Bq m−2 and the sediment inventory in the Sea of Okhotsk was 16±2 Bq m−2. 239,240Pu concentrations in sinking particles in the southwest Japan Basin were from 3.7 to 5.2 Bq kg−1 (dry weight) with fluxes of 0.19–4.50 mBq m−2 d−1 and at the Ulleung Basin from 2.4 to 3.7 Bq kg−1 (dry weight) with fluxes of 0.77–1.10 mBq m−2 d−1. The mean residence time of 239,240Pu in the water column of the East Sea derived from sediment trap data was 140±20 years, 2–3 times less than in the Atlantic and Pacific oceans.