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As part of the GEOTRACES Program, six laboratories participated in an intercalibration exercise on several anthropogenic radionuclides of interest. The effort was successful for 239,240Pu activity, 240Pu/239Pu isotope ratio, and 137Cs activity measured in filtered seawater samples from the Bermuda Atlantic Time Series station (BATS) and a site on the continental slope of the Northeastern U.S. A limited number of analyses were reported for 237Np, 241Am, 90Sr, and 238Pu in filtered seawater. Intercalibration of any of the isotopes of interest in filtered particulate matter was unsuccessful due to insufficient size of the samples distributed. Methods used were based on traditional radio-counting techniques and inductively coupled plasma mass spectrometry (ICP-MS). Although the majority of analyses were performed on samples ≥60 L, one lab demonstrated the ability to analyze several of the anthropogenic radionuclides on 10-20 L sample volumes using ICP-MS.

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 Atlant

Waters from the Skagerrak Sea entering the system of fjords inside Farsund in the southern tip of Norway (including Lyngsdal, Hellvik and Framvaren fjords) contain plutonium derived from European nuclear fuel reprocessing facilities in the United Kingdom and France, as shown by the 238Pu/239,240Pu activity ratios measured for water samples from Hellvik and Framvaren fjords. The shallow sills interconnecting this series of fjords, however, severely restrict water exchange. Thus, at the northernmost Framvaren fjord (connected to Hellvik fjord by a 500-m channel of 2 m depth), the 238Pu/239,240Pu signature for Pu derived from fuel reprocessing is found only in the surface layer (upper 20 m) while global fallout Pu ratios are observed in the permanently anoxic zone of this fjord. Plutonium oxidation state measurements in waters collected from Hellvikkand Framvaren fjords show that water exchange occurs between these two basins by a process of interleaving, with subsequent reduction of oxidized Pu species as these reach the anoxic zone. Plutonium serves as a useful tracer for water exchange between these fjords and the Skagerrak Sea.

Water column profiles of reduced Pu (III + IV) and oxidized Pu (V + VI) species at two stations in the southwestern Black Sea reveal that oxidized Pu predominates even at the suboxic zone, between 50 and 100 m. Reduction of these oxidized species occurs at the base of the suboxic zone, where we noted a shift to the predominance of reduced Pu forms. At anoxic depths (below 150 m), we found only reduced Pu forms, at concentrations significantly less than the total Pu levels we found in the upper 150 m samples. Our data show that Pu oxidation states are useful in interpreting lateral advective processes that occur within the upper 150 m of the Black Sea water column. An additional interesting result of our measurements is that water at the cold intermediate layer (CIL) of the Black Sea is labeled with predominantly oxidized Pu. We believe this feature can be used to trace the origin of CIL water.