Anthropogenic radionuclides in sediments in the NW Pacific Ocean and its marginal seas: results of the 1994-1995 Japanese-Korean-Russian expeditions

Assessment of contamination of anthropogenic radionuclides from past dumpin g of radioactive waste in areas of the Okhotsk Sea, NW Pacific Ocean and th e Sea of Japan/East Sea has been performed. Two joint Japanese-Korean-Russi an scientific expeditions were carried out in 1994-1995, where seawater and seabed sediments were samples from 22 sites. Results of sediment analysis are reported here, where concentrations of Sr-90, Cs-137, Pu-238, Pu-239,Pu -240 and Am-241 in surface layer and bulk sediments showed on large spatial variations, ranging between < 0.04 and 0.75 Bq kg(-1) dry wt., < 0.2 and 7 .2 Bq kg(-1) dry wt., < 0.001 and 0.08 Bq kg(-1) dry wt., 0.006 and 2.0 Bq kg(-1) dry wt., 0.03 and 1.8 Bq kg(-1) dry wt., respectively. However, the concentrations are comparable with those found in reference sites outside t he dumping areas and they generally fall within ranges previously reported for non-dumping areas of the investigated seas. Estimates of sediment inven tories indicated differences in radionuclide load between shelf/slope and b asin type sediments as well as dependence on water depth. Except for the sh allow areas, most of the inventories of Sr-90, Cs-137 and Pu isotopes are s till to be found in the water column. Total inventories (in water + sedimen t) show a surplus of Cs-137 and Pu-isotopes compared to expected integrated global fall-out deposition, which is consistent with previous observations in non-dumping areas in the seas investigated. Analysis of sediment Pu-238 /Pu-239,Pu-240 activity ratios showed values in accord with that of global fall-out. Analysis of radionuclide depth distributions in core samples from areas of the Sea of Okhotsk showed sedimentation rates of 0.2-0.4 g cm(-2) year(-1) and 0.03 g cm(-2) year(-1) for shelf and basin areas respectively , which is similar to values found in the Sea of Japan/East Sea. Depth prof iles of Sr-90, Cs-137 and Pu-isotopes in cores of the basin area indicate a typical delay compared to the input records of global fall-out. (C) 1999 E lsevier Science B.V. All rights reserved.