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 dumping of radioactive waste in areas of the Okhotsk Sea, NW Pacific Ocean and the Sea of Japan/East Sea has been performed. Two joint Japanese-Korean- Russian 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 ⁹⁰Sr, ¹³⁷Cs, ²³⁸Pu, ^239,240Pu and ²⁴¹Am 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 the dumping areas and they generally fall within ranges previously reported for non-dumping areas of the investigated seas. Estimates of sediment inventories indicated differences in radionuclide load between shelf/slope and basin type sediments as well as dependence on water depth. Except for the shallow areas, most of the inventories of ⁹⁰Sr, ¹³⁷Cs and Pu isotopes are still to be found in the water column. Total inventories (in water+sediment) show a surplus of ¹³⁷Cs 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 ²³⁸Pu/^239,240Pu 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 profiles of ⁹⁰Sr, ¹³⁷Cs and Pu isotopes in cores of the basin area indicate a typical delay compared to the input records of global fall- out.